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Introduction to Open Hole Logging APS sta ndoff

Caliper

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APS po rosity CNL po rosity Correcti on

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Schlumberger Wireline & Testing

May, 1996

Introduction to Open Hole Logging

Introduction to Open Hole Logging

Introduction to Open Hole Logging

Introduction to Open Hole Logging

Introduction to Open Hole Logging

May 1996

May 1996

May 1996

May 1996

May 1996

Schlumberger

Schlumberger

Schlumberger

Schlumberger

Schlumberger

Wireline & Testing

Wireline & Testing

Wireline & Testing

Wireline & Testing

Wireline & Testing

Contents A1.0 INTRODUCTION TO OPENHOLE LOG INTERPRETATION .................................. 1 A.1 USES OF LOGS ................................................................................................................................... 1

A.2 BASIC PETROLEUM GEOLOGY........................................................................................................ 2

A.3 BASIC LOG INTERPRETATION CONCEPTS ................................................................................... 4

A.4 RESISTIVITY AS A BASIS FOR INTERPRETATION—THE ARCHIE EQUATION............................ 5

A.5 DEFINITIONS ....................................................................................................................................... 7 a) Formation Porosity () ..................................................................................................................... 8 b) Formation Resistivity (R) ................................................................................................................. 8 c) Formation Factor (F) ........................................................................................................................ 8 d) Water Saturation: Sw ...................................................................................................................... 8 e) Hydrocarbons Saturation (Shy) ......................................................................................................... 9 f) Clean Formations ............................................................................................................................. 9 g) Shaly Formations ............................................................................................................................. 9 h) Key Formulas ................................................................................................................................ 11 i) Key Symbols ................................................................................................................................... 11

A.6 LOG SCALES AND PRESENTATIONS ........................................................................................... 12

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Introduction to Openhole Logging

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A1.0 Introduction to Openhole Log Interpretation A.1 USES OF LOGS A set of logs run on a well will usually mean different things to different people. Let us examine the questions asked—and/or answers sought by a variety of people. The Geophysicist: • Are the tops where you predicted? • Are the potential zones porous as you have assumed from seismic data? • What does a synthetic seismic section show? The Geologist: • What depths are the formation tops? • Is the environment suitable for accumulation of hydrocarbons? • Is there evidence of hydrocarbons in this well? • What type of hydrocarbons? • Are hydrocarbons present in commercial quantities? • How good a well is it? • What are the reserves? • Could the formation be commercial in an offset well?

The Drilling Engineer: • What is the hole volume for cementing? • Are there any keyseats or severe doglegs in the well? • Where can you get a good packer seat for testing? • Where is the best place to set a whipstock? The Reservoir Engineer: • How thick is the pay zone? • How homogeneous is the section? • What is the volume of hydrocarbons per cubic meter? • Will the well pay-out? • How long will it take? The Production Engineer: • Where should the well be completed (in what zone(s))? • What kind of production rate can be expected? • Will there be any water production? • How should the well be completed? • Is the potential pay zone hydraulically isolated? • Will the well require any stimulation? • What kind of stimulation would be best?

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Introduction to Openhole Logging

Log evaluation can be many things to many people. As the answers are sought each individual will possibly use the available data in a different manner. The common approach will be in reading the logs and understanding the various reactions produced by formation characteristics on our logging devices. The factors influencing log reading and the information they provide are what we wish to introduce to you in this course. A.2 BASIC PETROLEUM GEOLOGY In order to better understand log responses, we should first review the types of rocks that are found in the boreholes. Common sedimentary rocks are sandstone, siltstone, shale, limestone, dolomite and anhydrite In general, sedimentary rocks are deposited as either clastic sequences containing sandstone, siltstones and shales or carbonate sequences of limestone, dolomite, anhydrite and shale. (Figure A1). Clastic Deposition Clastic rocks are formed from rock fragments and weathered particles of preexisting rocks. These sediments are transported by wind and water and are usually deposited in rivers, lakes and oceans as relatively flat-lying beds. Current and wave action later sorts the sediments such that in high-energy environments coarse-grained sands are deposited and in low energy environments fine-grained silts and clays are deposited. The nature of the deposition is such that crossbedding struc-

(05/96) A-2

tures, channel patterns and gradational rock types are common. In areas of freshwater deposition coal beds may occur, indicating nonmarine conditions. After deposition and with deeper burial of the sequence, compaction occurs and the clastic grains can become cemented together to form sedimentary rock. Carbonate Deposition Carbonate deposition occurs in marine conditions by the precipitation of limestone from organisms as fine particles, shells or massive growths. Limestones are deposited either as flat-lying beds on the ocean floor or as mounds or pinnacle reefs. Barrier reef chains that grow in this manner may form restricted ocean basins landward, in which dolomite and anhydrite are precipitated by the evaporation of seawater. When limestones form near shore, there may be mixing of limestone and eroded clastic material. In deeper ocean basins, limestone and shale mixtures are common. After deposition, later burial may cause dolomitization of the limestone in which the actual composition of the rock is changed to dolomite. Because of their brittle nature compared with other sediments, limestones tend to fracture with deformation, which increases permeability and helps in the dolomitization process.

Figure A1: Clastic Deposition vs. Carbonate Deposition

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Introduction to Openhole Logging

In many parts of the world multiple sequences of clastic rocks overlie older carbonate sequences. Between each of the clastic and carbonate groups, erosional inconformities are common and the nature of deposition within each group is unique. A.3 BASIC LOG INTERPRETATION CONCEPTS Any given rock formation has numerous unique physical properties associated with it. Only those that can be measured and are useful will be considered in this course. They are a. b.

c.

porosity: the void space between grains that is generally filled with liquids or gases. Sw = water saturation: the percentage of the pore space filled with water (as opposed to hydrocarbons or air). R = resistivity: the resistance to electrical current flow presented by a unit volume of rock.

d.

e.

RW = water resistivity: the electrical resistance of the water filling the pore space in the rock. This value varies with water salinity and temperature. k = permeability: the ability of the rock to pass fluids through it.

Consider the following unit cubes (Figure A2): Cube A If the porosity () is filled with water then, by definition, the water saturation SW = 100%. Cube B If the porosity  is 70% filled with water and 30% hydrocarbons, then, the water saturation 70 SW =

% = 70% 70 + 30

and hydrocarbons saturation

Cube ìAî: porosity = waterfilled SW = 100%

Cube ìBî: porosity = hydrocarbons and water in SW = 70%



Figure A2

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Shy = 1 - Sw = 30% Therefore the percentage volume of water saturation =   Sw

The usefulness of resistivity logging rests on the facts that - water is a conductor (low resistivity) - hydrocarbons and rocks are insulators (high resistivity) Consider the following unit cubes (Figure A3):

For example: if  = 20% and Sw = 70%, then 14% of the bulk volume is water and 70% of the pore space is water filled. A.4 RESISTIVITY AS A BASIS FOR INTERPRETATION—THE ARCHIE EQUATION In the previous section we introduced a number of parameters used to evaluate rock formations. If we could build on the effects of resistivity in conjunction with the other parameters to develop a mathematical relationship, we would have an extremely useful tool for our work with potential hydrocarbon zones.

Cube C The resistivity Rt of the cube will vary with water resistivity Rw (i.e. as Rw increases, Rt increases and vice versa). Therefore: Rt Rw.

(1)

Cube D Replace 25% of the cube with rock (hence  = 75%) but maintain a constant Rw. Resistivity Rt increases with decreasing porosity  (i.e. as  decreases, Rt increases).

The remainder of this section is devoted to developing such a formula.

Cube ìC” - Constant Current - Porosity = 100% - Sw = 100%

Cube ìDî - Constant Current - Porosity = 75% - Sw = 100%

Cube ìEî - Constant Current - Porosity = 75% - Sw = 70%

Figure A3

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Introduction to Openhole Logging

Therefore: Rt 1/.

(2)

Rw Ro 

(5) 

Cube E Replace 30% of remaining porosity  with hydrocarbons. Resistivity Rt increases with decreasing water saturation Sw (i.e. as Sw decreases, Rt increases). Therefore: Rt 1/Sw.

(3)

By combining the above observations (1, 2 and 3), we can say 1 Rt Rw 

1

Now, let F = constant of proportionality defined as the formation factor. Therefore: Ro = FRw Ro or F =

(6) Rw

Returning to Equation 5 and introducing porosity as a variable, it is clear that

 

Now, let  = 1, then Ro  Rw .

Sw

1 F

or



Rw Rt 

(4)  Sw

To solve for the constants of proportionality let us first limit the equation as follows: Let Sw = 100% (i.e. there is no hydrocarbon present and the porosity is 100% water filled). Then, define Ro = Rt (ie: Ro is the wet resistivity of the formation for the condition Sw = 100%):

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This is intuitively obvious as the relationship between Ro and Rw is related to that particular unit cube of rock and its porosity characteristics. Through empirical measurements, it was determined that a F=

(7) 

m

where a = constant m = cementation factor

The cementation factor m relates to the porosity type and how it will transmit electrical current to the actual rock (also called tortuosity).

aRw or S

n w

=

(9)  Rt m

Using the above equations Recall Ro = FRw (Equation 6) aRw when Sw = 100% m

Rt = Ro = if Sw  100%, then aRw

1

Rt 

 

Sw

m

a) Formation Porosity () Defined as the fraction of total volume occupied by pores or voids, where

or Rt  Ro  Sw

pore volume

Ro (8) Rt Through laboratory measurements, it was found that this relationship (8) is dependent on the saturation exponent n as Ro S

n w

= Rt FRw

or Swn =

The remainder of this course is dedicated to measuring, evaluating and using porosity and resistivity to calculate water saturation and hence hydrocarbons reserves using the concepts of this equation. A.5 DEFINITIONS

1

or Sw 

Equation 9 forms the Archie relationship that is the basis for all conventional log interpretation techniques. Enhancements and refinements may be applied for the more complicated rock types.

=

 100% total volume

When the pore space is intergranular it is known as primary porosity. When the porosity is due to void space created after deposition, (e.g., vugs or fractures in carbonates), the porosity is known as secondary porosity. When shale is present, the pore space occupied by the water in the shale is included with the pore space in the rock to give total porosity (T). If only the rock pore space is considered in a shaly formation, the pore space is called effective porosity (e).

Rt

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Introduction to Openhole Logging

b) Formation Resistivity (R) Defined as the resistance offered by a formation to the flow of electrical current. It is expressed in ohm-meter2/meter. We use several terms to describe formation resistivity under various circumstances of fluid content. Rt: Describes the resistivity of a formation undisturbed by the drilling process. Ro: Describes a special form of Rt. It is the resistivity of a clean formation when all pore space is filled with connate water (Rw). Rw: Is the symbol for the resistivity of formation (connate) water.

For Porosity In a 1942 paper Gus Archie proposed that the relationship between formation factor and porosity could be described by the formula a F= m where a = empirical constant. m = cementation factor. Some recommended F and  relationships are 0.62 F=

(for sands) 

2.15

0.81 F= 

c) Formation Factor (F) For Resistivity An important relationship exists between the resistivity of a fully water saturated formation and the resistivity of the contained water. The ratio of these two values is called formation resistivity factor (or more commonly, formation factor) where: Ro F= Rw F is a constant for the formation under consideration. The value of F for any particular formation depends on: - formation porosity - pore distribution - pore size - pore structure. (05/96) A-8

(for sands) 2

1 F=

(for carbonates) 

2

Chart Por-1 (figure A4) in the Log Interpretation Chart book is based on several different F- relationships. d) Water Saturation (Sw) Defined as the fraction of pore volume filled with water where water filled pore volume  100%

sw = total pore volume

e) Hydrocarbons Saturation (Shy) Defined as the fraction of pore volume filled with hydrocarbons where:

g) Shaly Formations This describes formations where some of the formation void space (porosity) is filled with shale.

hydrocarbon-filled pore volume

Shale distribution is considered to be: - Laminated: The formation is built up of thin laminae of sand and shale. - Dispersed: The shale particles are dispersed in the pore space. - Structural: The shale replaces matrix.

 100%

Shy = total pore volume or

Shy = 1 – Sw.

f) Clean Formations The term clean formation refers to those that are shale free.

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Introduction to Openhole Logging

Formation Resistivity Factor versus Porosity

This chart gives a variety of formation resistivity factor-to-porosity conversions. The proper choice is best determined by laboratory measurement or experience in the area. In the absence of this knowledge, recommended relationships are the following: 0.62 For Soft Formations: Humble Formula: Fr =

2.15

0.81 or Fr =

2

0.62 For Hard Formations: Fr =

m

with appropriate cementation factor, m.

EXAMPLE:  is 6% in a carbonate in which a cementation factor, m of 2 is appropriate Therefore, from chart, Fr = 280. Chart Por-1

Figure A4 (05/96) A-10

h) Key Formulas FRw Archieís formula: S = Rt n w

Formation Factor: Ro a. From deep resistivity

F = Rw

where n is usually taken as 2

Rxo b. From shallow resistivity

F = Rmf a

c. From porosity

F = m

i) Key Symbols BHT -

Sxo

di

Shc

hRIDPH RIMPH RSFL Rm Rmf Rmc Rw Rwa Rt Ro Rxo Rsh F



Sw

bottom hole temperature in degrees Celsius - average diameter of invaded zone (Di) bed thickness in meters - resistivity from the deep phasor induction - resistivity from the medium Phasor induction - resistivity from the Spherically Focused Log - resistivity of the mud - resistivity of the mud filtrate - resistivity of the mudcake - resistivity of the formation water - apparent resistivity of the formation water - resistivity of the formation (uncontaminated zone) - resistivity of the formation when 100% water filled - resistivity of the flushed zone (close to borehole) - resistivity of the shales - formation resistivity factor - porosity in percent - water saturation, percent of pore space occupied by water in uncontaminated zone

K SSP

PSP k-

S D N T e 2 Vsh Pe

-

water saturation, as above, in flushed zone - hydrocarbons saturation as percent of pore space occupied by water - coefficient in the sp formula - static spontaneous potential - the maximum possible for a particular Rmf / Rw - pseudostatic spontaneous potential—the SP found in a thick shaly sand permeability in millidarcies pore volume porosity = 100%. total volume - sonic porosity - density porosity - neutron porosity N + D - total porosity  2 - effective porosity - secondary porosity - volume of shale - photoelectric index

A complete list of symbols and subscripts is included in Section J (Miscellaneous).

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Introduction to Openhole Logging

A.6 LOG SCALES AND PRESENTATIONS a) Well logs provide a continuous graph of formation parameters versus depth. Normal depth scales are - 1:240—1 m of log per 240 m of measured hole depth. Each line is 1 m, with heavy lines every 5 m, and heavier lines every 25 m for ease of reading. Depths are indicated every 25 m (Figures A5 and A6). - 1:600—1 m of log per 600 m of measured hole depth. Each line is 5 m, with heavy lines every 25 m. Depths are indicated every 25 m (Figure A7). - Other scales are available. These include 1:1200, 1:120, 1:48 and 1:5. - Log grids may be either logarithmic (resistivity logs—Figure A6) or linear (porosity logs— Figure A5). b) If a caliper device is present or the log being generated is a type of sonic log, event markers are placed on each side of the depth track integrating the quantity of hole volume or transit time recorded. 1. Integrated hole volume—requires caliper device (Figure A5) - placed on the left side of the depth track 3 - small marks indicate 0.1 m whereas large marks represent 3 1.0 m . 2. Integrated cement volume—Requires caliper device plus future casing size - placed on the right side of the depth track when space permits— and if sonic not present 3 - small marks indicate 0.1 m while large marks represent 3 1.0 m . (05/96) A-12

3. Integrated transit time—Requires sonic tool (Figure A5) - placed on the right side of the depth track - small marks indicate 1 msec whereas large marks represent 10 msec of time. If the log is recorded using logging-whiledrilling methods, event markers on both sides of the depth track (Figure A6) represent the conversion from time-based sampling to a depth-based presentation. The markers therefore indicate the number of data samples per unit depth. In other words, the larger the concentration of markers over a depth interval, the greater the number of data samples used to make the log. c) Logs also have headings and inserts. - Log headings provide such information as well depth, casing depth, mud params, maximum temperature and other comments pertinent to the evaluation of log data (Figures A8 and A9). - Inserts provide such information as curve scaling, coding, date/time of acquisition, data curve first-reading points and constants pertinent to the logging run following the insert. Curve coding on the log data indicates the deepest reading primary measurement (long dashed) to the shallowest reading primary measurement (solid) when two or more measurements are combined (Figure A10).

Figure A5: Linear Grid 1/240 Scale (05/96) A-13

Introduction to Openhole Logging

Logarithmic Grid 1/240 Scale Data Sample Event Markers for LWD Curves Figure A6

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Figure A7: Linear Grid 1/600 Scale

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Figure A8: Log Heading (page 1) (05/96) A-16

Figure A9: Log Heading (page 2) and Log Tail

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Introduction to Openhole Logging

Figure A10: Log Insert (05/96) A-18

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Contents

B1.0 THE RESISTIVITY OF THE FORMATION......................................................................................1 B1.1 INTRODUCTION ...................................................................................................................1 B1.2 FORMATION WATER RESISTIVITY RW ..............................................................................3 B1.3 FORMATION RESISTIVITY MEASUREMENTS .................................................................3 Chart Gen-9: Resistivity of NaCl Solutions .....................................................................4 B1.4 TO SUMMARIZE ...................................................................................................................6 B1.5 THE DRILLING PROCESS AND PERMEABLE BEDS ........................................................5 Invasion Profiles ................................................................................................................5 Chart Gen-3: Symbols Used in Log Interpretation .........................................................6 B1.6 SPONTANEOUS POTENTIAL (SP) CURVE ........................................................................8 Chart SP-1: Rweq Determination from ESSP (Clean Formations) ...................................13 Chart SP-2: Rw versus Rweq and Formation Temperature ............................................14

B2.0 MEASUREMENT OF RT BY INDUCTION PRINCIPLES .............................................................15 B2.1 INDUCTION LOGGING PRINCIPLES ................................................................................15 B2.2 SPHERICALLY FOCUSED LOG PRINCIPLES ................................................................16 B2.3 DUAL INDUCTION - SPHERICALLY FOCUSED LOG ......................................................17 B2.4 PHASOR INDUCTION SFL TOOL .....................................................................................23

B3.0 MEASUREMENT OF Rt BY LATEROLOG PRINCIPLES ...........................................................29 B3.1 DUAL LATEROLOG ..........................................................................................................29

B4.0 MEASUREMENT OF RXO BY MICRO-RESISTIVITY PRINCIPLES ............................................35 B4.1 INTRODUCTION ................................................................................................................35 B4.2 MICROLOG ........................................................................................................................36 B4.3 MICRO - SPHERICALLY FOCUSED LOG ........................................................................38

B5.0 WORK SESSION ..........................................................................................................................41

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B1.0

The Resistivity of the Formation

B1.1 INTRODUCTION The resistivity of a formation is a key parameter in determining hydrocarbon saturation. Electricity can pass through a formation only because of the conductive water it contains. With a few rare exceptions, such as metallic sulfide and graphite, dry rock is a good electrical insulator. Moreover, perfectly dry rocks are very seldom encountered. Therefore, subsurface formations have finite, measurable resistivities because of the water in their pores or absorbed in their interstitial clay. For the purposes of our discussions we will divide substances into two general categories, conductors or insulators. Conductors are substances which pass electrical current e.g. water, shales, mud. Insulators are substances which do not allow electrical current flow e.g. hydrocarbons, or rock matrix. The measured resistivity of a formation depends on: - Resistivity of the formation water. - Amount of water present. - Pore structure geometry. The resistivity (specific resistance) of a substance is the resistance measured between

opposite faces of a unit cube of that substance at a specified temperature. The metre is the unit of length and the ohm is the unit of electrical resistance. In abbreviated form, resistivity is R = r A/L, where R is resistivity in ohm-metres, r is resistance in ohms, A is area in square metres, and L is length in metres. (See Figure B1) The units of resistivity are ohm-metres squared per metre, or simply ohm-metres (ohmm). Conductivity is the reciprocal of resistivity and is expressed in mhos per metre. To avoid decimal fractions, conductivity is usually expressed in millimhos per metre (mmho / m), where 1000 mmho/m = 1 mho/m: C = 1000 / R. Formation resistivities are usually from 0.2 to 1000 ohm-m. Resistivities higher than 1000 ohm-m are uncommon in permeable formations but are observed in impervious, very low porosity formations (e.g., evaporites).

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Introduction to Open Hole Logging

Figure B1: Principles of Resistance and Resistivity

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B1.2

FORMATION WATER RESISTIVITY RW As previously indicated, formation matrices are insulators; thus a formation’s ability to conduct electricity is a function of the connate water in the formation. Several factors must be considered: - the volume of the water (porosity) - the pore space arrangement (type of porosity) - the temperature of the formation - the salinity of the water. a) Water Salinity As salinity increases, more ions are available to conduct electricity so Rw (water resistivity) decreases. b) Water Temperature As water temperature is raised, ionic mobility increases and resistivity decreases. Chart Gen-9 (Figure B2) in the Log Interpretation Chart Book, illustrates these relationships. c) Water Volume As water filled pore space in a rock is increased, resistivity decreases. If some water is displaced by hydrocarbons (insulators), water saturation decreases; resistivity increases.

B1.3

FORMATION RESISTIVITY MEASUREMENTS If we consider a formation whose pore space contains only water, its true resistivity is called Ro. We know that an important relationship exists between formation resistivity and the resistivity of the saturating water - Rw. The ratio of these two values, F, is called Formation Resistivity Factor, or more commonly Formation Factor, which is a constant; where: F = Ro / R w For example, if the salinity of the connate water increases, Rw will decrease. This will in turn allow current to flow more easily through the formation, thus lowering Ro and maintaining F at a constant value. This is what we should expect as F is an inherent formation characteristic. Formation factor can be related to formation porosity by the general formula: F = a / m where a = constant m = cementation factor

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Introduction to Open Hole Logging

Resistivity of NaCl Solutions

Chart GEN-9 Figure B2 (05/96) B-4

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B1.4 TO SUMMARIZE 1. Dry rock formation is an insulator. 2. Formations conduct current because of water in the pore spaces. 3. Knowledge of water resistivity (Rw) is essential for log interpretation. 4. Resistivity used rather than resistance. 5. Formation Resistivity Factor (F) is a porosity related formation characteristic. 6. Relationships: a. F = (Rt / Rw) = (Ro / Rw) 100% water saturated porous rock b. F = a / m 7. Symbols: Rw - resistivity of connate water. Rt - true formation resistivity. Rxo - resistivity of flushed zone. a - a constant. m - cementation factor. B1.5

THE DRILLING PROCESS AND PERMEABLE BEDS Before proceeding to a discussion of methods of obtaining formation resistivity, let us examine what happens to a permeable formation when it is penetrated by the drill bit. (Refer to Chart Gen-3 (Figure B3) in this section or the Log Interpretation Chart Book.) Under normal conditions the hydrostatic head of the mud column is greater than formation pressure. This differential pressure forces filtrate from the mud system into the formation pore spaces, leaving solid particles or mud cake build up on the borehole wall. Eventually this impervious mud cake will seal off further invasion (unless it is removed by some mechanical process e.g. removing the drill bit).

Mud Cake thickness is symbolized by hmc. Invasion Profiles: 1. Flushed Zone. Adjacent to the borehole the invasion process flushes out the original water and some of the hydrocarbons (if any were present). The resistivity of this zone is termed Rxo; the water saturation is called Sxo where: FRmf Sxo = 2

Rxo (for clean formations only) Plotting Rxo as a function of radial depth into the formation yields Figure B4. 2. Transition Zone. Further from the borehole the flushing action of the mud filtrate may create a variety of situations. If the flushing proceeds as a uniform front, we call this a step profile of invasion (Figure B5a). If the intermingling of formation fluids is very gradual, we would call this a transition zone (Figure B5b). Sometimes in oil or gas bearing formations, where the mobility of hydrocarbons is greater than the connate water, the oil or gas move out leaving an annular zone filled with connate water (Figure B5c). If Rmf > Rw, then the annular zone will have a resistivity lower than Rxo and Rt and may cause a pessimistic saturation calculation.

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Introduction to Open Hole Logging

Symbols Used in Log Interpretation

Chart GEN-3

Figure B3

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3. True Unaffected Zone. This is the zone which we wish to analyse - it is the formation undisturbed by the drilling process. Its resistivity is termed Rt,

water resistivity Rw, and water saturation Sw. Plotting Rxo, Ri and Rt as a function of invasion:

Figure B4: Invasion Process

(a)

(b)

(c)

Figure B5

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Introduction to Open Hole Logging

B1.6

SPONTANEOUS POTENTIAL (SP) CURVE

a) Introduction The SP curve is a continuous recording (versus depth) of the difference in potential between a moveable electrode in the borehole and a fixed (zero) potential surface electrode. Units used are millivolts. The SP was discovered quite by accident in the very early days of electrical logging. In some of the first test wells logged by Schlumberger using the point-by-point technique, it was noted that a small natural potential was present in the well even when the current source was turned off. This spontaneous potential is due to a combination of two phenomena: an Electrokinetic potential usually negligible, and an Electrochemical potential composed of a membrane potential and a liquid-junction potential. The membrane potential is about five times bigger than the liquidjunction potential.

Figure B6: Electrokinetic Potential of SP

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b) Electrokinetic Potential If a solution is forced, by differential pressure, to flow through a membrane, an electrical potential will appear across the membrane (Figure B6). A similar situation occurs when the mud filtrate flows through the mudcake because of the differential pressure between the mud column and the formation. This electrokinetic potential (Ekmc) is generally very small. In a very low permeability formation, where the mudcake is only partially built up, this electrokinetic potential may be as high as 20 mV. This situation is, however, very rare and in general the total electrokinetic potential can be neglected. c) Electrochemical Potential This potential is created by the contact of two solutions of different salinity, either by a direct contact or through a semi-permeable membrane like shales.

Figure B7: Electrochemical membrane potential of SP

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1) Membrane Potential An ideal cationic membrane due to its physico-chemical composition is permeable to positive ions (cations) only. Shales are ideal membranes as long as they are not too sandy or too limy. In a borehole, a shale section usually separates salty water (generally the connate water of the virgin zone) from a less salty liquid (generally the mud) (Figure B7). There is migration of the positive ions (Na+) from the salty water (formation) to the less salty water (mud). When an equilibrium is reached: - positive ions that have already crossed the shale membrane exert a repelling force on the positive ions in the mud. - negative ions left behind in the formation exert an attractive force on the positive ions which cannot travel any more into the shale.

where: amf and aw are the electro-chemical activities of mud filtrate and connate water. 2) Liquid Junction Potential The liquid junction potential takes place at the boundary between the flushed zone and the virgin zone. There is no shale separating the two solutions. Anions as well as cations can transfer from one solution to the other (Figure B8) because of the higher salinity of the formation water, both cations Na+ and anions Cl- will migrate towards the mud filtrate. The Na+ ion is comparatively large and drags 4.5 molecules of water. The Cl- ion is smaller and drags only 2.5 molecules of water. Hence, the anion Cl- will migrate more easily than the Na+ ions.

The difference of potential appearing between the two solutions is given by the formula: Em = K Log

amf aw

Figure B8: Electrochemical Liquid Junction Potential of SP

Figure B9: The SP Circuit Path

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Introduction to Open Hole Logging

The result is an increase of positive charges left behind in the formation water. These positive charges restrict the Cl- migration toward the flushed zone. A difference of potential appears at the boundary between the two solutions: amf Ej = K' Log aw d) The Spontaneous Potential or SP The total potential of the whole chain is thus the algebraic sum Em + Ej which is also called the Static Spontaneous Potential or SSP. Electrokinetic potential is neglected. The SP is the drop of potential measured across the current

lines in the borehole. Along its path the SSP current has to force its way through a series of resistances, both in the formation and in the mud (Figure B9). This means that the total potential drop (which is equal to the SSP) is divided between the different formations and mud in proportion to the resistances encountered by the current in each respective medium. The SP, which is the measure of the potential drop in the mud of the borehole, is only part of the SSP. In general, it is a large portion because the electrical resistance offered by the borehole is, in general, much greater than that offered by the formations.

Figure B10: The SP Deflection and its R mf-Rw Dependency (05/96) B-10

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So, we can write: SP  SSP = (K + K') Log

amf aw

The SP curve is generally presented in track 1, and usually recorded with resistivity surveys, assuming a conductive mud is in the borehole. Opposite a permeable formation, the SP curve shows excursions from the shale base line. In thick, clean beds the SP deflection tends to reach an essentially constant deflection defining a clean line. The deflection may be either to the left (negative) or to the right (positive) depending mostly on relative resistivity of the formation water and of the mud filtrate (Figure B10). The magnitude of SP deflections is always measured from the shale line and for a clean, water-bearing formation containing a dilute sodium chloride solution, is given by: SSP = -K log(Rmfe / Rwe) K, a constant, depends on the temperature and salt types in formation water. K = 71 @ 25 degrees Celsius for NaCl.

In practice, the SP is affected by a number of factors, all which tend to reduce its magnitude. The maximum available SP in a thick, clean, water-bearing zone is called Static Spontaneous Potential, or SSP (Figure B10). The SP is reduced by the shale in a shaly zone and the deflection is called the Pseudostatic Spontaneous Potential, or PSP. The ratio of these two values, termed  = PSP/SSP is occasionally used as a shale indicator in sands. An approximation of the SSP in a shaly sand is SSP = PSP / (1 - Vsh) where the volume of shale (Vsh) is estimated from the Gamma Ray deflection which will be discussed later. e) Uses of SP The SP can be used to: - detect permeable beds (a qualitative indication only). - determine Rw, formation water resistivity. - give an indication of zone shale content. - indicate depositional environment.

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Introduction to Open Hole Logging

f) Factors Affecting the SP - Bed Thickness*: SP decreases when bed thickness decreases. - Invasion*: Reduces SP - Shaliness: Shale reduces SP - Hydrocarbons: Hydrocarbons in slightly shaly formations will reduce the SSP - Mud Filtrate: The magnitude and direction of SP deflection from the shale base line depends on relative resistivities of the mud filtrate and the formation water. - Fresh Mud - negative SP (Figure 8). Rmf > Rw - Saline Mud - positive SP (Figure 8). Rw > Rmf Rw = Rmf - zero SP (Figure 8).

g) Solution of Rw from SP Because of its dependence on Rmf and Rw, the magnitude of SP deflection enables us to solve for the Rw of the formation when Rmf is known. This method, when applied in clean matrix, is generally accurate. 1. From log heading, get Rmf at surface temperature. 2. Convert Rmf to formation temperature using chart Gen-9 (Figure B12). 3. Convert Rmf at formation temperature to Rmfe using: Rmfe = .85 x Rmf. (approximation) If Rmf is below .03 ohm-metre or above 1.5 ohm-metre @ formation temperature, use chart SP-2 (Figure B12) to get Rmfe. 4. Calculate static SP from log at zone of interest. 5. Enter chart SP-1(Figure B11) with static SP, formation temperature and Rmfe to get Rwe at formation temperature. 6. Enter chart SP-2 (Figure B12) with Rwe and formation temperature to get Rw.

- Pyrite in the formation produces a positive SP * Corrosion Charts available to correct for these factors.

(05/96) B-12

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Rweq Determination from Essp (CLEAN FORMATIONS)

SP-1 Figure B11 (05/96) B-13

Introduction to Open Hole Logging

Rw versus Rweq and Formation Temperature

Gyp-base mud filtrates EXAMPLE: Rweq = 0.025 m at 120oC. From chart, Rw = 0.031 m at 120oC Special procedures for muds containing Ca or Mg in solution are discussed in Reference 3. Lime base muds usually have a negligible amount of Ca in solution; they may be treated as regular mud types.

SP-2m Figure B12 (05/96) B-14

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B2.0

Measurement of Rt by Induction Principles

We have two different types or classes of tools designed for the two most common borehole environments: 1.

Non-Conductive Boreholes - including Fresh Mud Systems, Invert Mud Systems and Air-filled holes. a. Dual Induction - SFL (No longer in service) b. Phasor Dual Induction - SFL c. Array Induction Imager

2.

Conductive Boreholes - including Saline to Salt Saturated Mud Systems a. Dual Laterolog

B2.1

INDUCTION LOGGING PRINCIPLES The induction logging tool was originally developed to measure formation resistivity in boreholes containing oil-base muds and in airdrilled boreholes. Electrode devices did not work in these nonconductive muds, and attempts to use wall-scratcher electrodes were unsatisfactory.

Experience soon demonstrated that the induction log had many advantages when used for logging wells drilled with water-base muds. Designed for deep investigation, induction logs can be focused in order to minimize the influences of the borehole, the surrounding formations, and the invaded zone. Principle Today’s induction tools have many transmitter and receiver coils. However, the principle can be understood by considering a sonde with only one transmitter coil and one receiver coil (see Figure B13). A high-frequency alternating current of constant intensity is sent through a transmitter coil. The alternating magnetic field created induces currents in the formation surrounding the borehole. These currents flow in circular ground loops coaxial with the transmitter coil and create, in turn, a magnetic field that induces a voltage in the receiver coil. Because the alternating current in the transmitter coil is of constant frequency and amplitude, the ground loop currents are directly proportional to the formation conductivity. The voltage induced in the receiver coil is proportional to the ground loop currents and, therefore, to the conductivity of the formation.

(05/96) B-15

Introduction to Open Hole Logging

There is also a direct coupling between the transmitter and receiver coils. The signal originating from this coupling is eliminated electronically. The induction tool works best when the borehole fluid is an insulator - even air or gas. The tool also works well when the borehole contains conductive mud unless the mud is too salty, the formations are too resistive, or the borehole diameter is too large.

B2.2

SPHERICALLY FOCUSED LOG PRINCIPLES The SFL device measures the resistivity of the formation near the borehole and provides the relatively shallow investigation required to evaluate the effects of invasion on deeper resistivity measurements. It is the short-spacing device used in the Phasor Induction - SFL tool. The SFL system differs from previous focused electrode devices. Whereas systems attempt to focus the current into planar discs, the SFL system establishes essentially constant potential shells around the current electrode.

Figure B13: Basic two-coil induction log system

(05/96) B-16

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The SFL device is able to preserve the spherical potential distribution in the formation over a wide range of wellbore variables, even when a conductive borehole is present. To accomplish this, the SFL device is composed of two separate, and more or less independent, current systems (Figure B14). The bucking current system serves to plug the borehole and establish the equipotential spheres. The io survey current system causes an independent survey current to flow through the volume of investigation; the intensity of this current is proportional to formation conductivity.

The first sphere is about 9 inches away from the survey current electrode; the other is about 50 inches away. A constant potential of 2.5 mV is maintained between these two spherical surfaces. Since the volume of formation between these two surfaces is constant (electrode spacing is fixed) and the voltage drop is constant (2.5 mV), the resistivity of this volume of formation can be determined by measuring the current flow. B2.3

DUAL INDUCTION SPHERICALLY FOCUSED LOG This is the most basic of induction devices and was the reference resistivity induction device for 20 plus years until its retirement in 1990. The tool supplies three focused resistivity curves: two Induction and a shallow investigating Spherically Focused Curve plus the Spontaneous Potential. Each curve has a different depth of investigation (Figure B15). Spherically Focused Log - a shallow reading device affected mainly by the flushed (Rxo) zone. (Radial Distance 30 cm.)

Figure B14: Electrode array of SFL tool and schematic representation of surveying current (io) lines (dashed) and focusing curent (io) lines (solid).

The SFL device consists of current-emitting electrodes, current-return electrodes, and measure electrodes. Two equipotential spheres about the tool’s current source are established.

Medium Induction (ILM) - depending on the invasion diameter and profile the ILM may be influenced by Rxo or Rt zones ... or both. (Radial Distance 60-80 cm.) Deep Induction (ILD) - is mostly affected by Rt, unless invasion is very deep. Either or both induction curves may be influenced if an annulus is present. (Radial Distance 1.21.5 m.)

(05/96) B-17

Introduction to Open Hole Logging

Figure B15 (05/96) B-18

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a) Log Presentation a. Logarithmic: A 1:240 scale is presented with resistivity curves on a logarithmic scale. This is the preferred presentation for log analysis. (Figure B15) b. Log-Lin: Here the 1:600 scale presents two resistivity curves, the SFL (averaged) and the ILD on the linear resistivity scale. Also included is the equivalent ILD conductivity curve. This presentation is primarily for correlation purposes. Both presentations are recorded simultaneously. b) Tool Characteristics and Applications 1. The Dual Induction SFL is most effective when used in holes drilled with moderately conductive mud, e.g. where Rmf / Rw > 2.5 . 2. Vertical Focusing is good, reliable values of Rt may be obtained where bed thickness is > 4.0 metres. 3. Since this tool actually measures formation conductivity and converts the values to resistivity, results are most accurate in zones of low resistivity. 4. The recording of three curves which investigate different amounts of formation volume enable us to study invasion profiles, and where invasion is deep, make correction to obtain Rt. 5. Since the two Induction devices produce their signals by inducing a magnetic field in the formation, they can be run in air drilled wells or wells drilled with non-conductive mud. (The SFL requires a conductive mud path to the formation and cannot be presented). A Gamma Ray curve is usually recorded in place of the SP.

Correction Charts are available for the influence of: - borehole (diameter and mud resistivity). - bed thickness - invasion c) Limitations 1. The logging of large diameter holes drilled with saline mud should be avoided, particularly in high resistivity formations. Large borehole signals will add to the formation signals producing anomalously low apparent resistivities. 2. In zones of high resistivity (low conductivity), e.g. in excess of 250 ohmm, errors in measurement can occur. The above problems can sometimes be minimized by a system of downhole calibration checks. A thick zero porosity zone, e.g. limestone, or anhydrite for this purpose. Thus if difficulties in producing a good DIL are expected, it is often advantageous to run a porosity - caliper log before the DIL. (It should also be noted that these changes were only made to DIL Logs and noted in the remarks section of the log heading). d) Log Responses (Figure B16) For wells drilled with fresh muds (Rmf/Rw > 2.5, Rxo/Rt>2.5) the following general conclusions can be reached by log inspection: - When SFL = ILM = ILD; Rt = ILD, this indicates zero or very shallow invasion. - When SFL > ILM = ILD; Rt = ILD, this indicates moderate invasion. - When SFL > ILM > ILD; and if Rxo = SFL, then Rt < ILD, this indicates deep invasion. (05/96) B-19

Introduction to Open Hole Logging

When SFL = ILM > ILD, and if Rxo = SFL we must use chart Rint-2c (Figure B17) to obtain Rt. This response indicates very deep invasion. In general, the closer the medium curve is to the SFL, the deeper the invasion. The result of correcting for invasion is to obtain an Rt which is lower than ILD. Hence, by using ILD without correction, you will obtain an optimistic Sw. e) Summary Benefits: 1. Dual Induction SFL can most effectively be used in holes filled with moderately conductive mud, nonconductive mud, and air drilled holes. 2. Vertical focusing is good and gives reliable values of Rt, for beds thicker than three metres.

(05/96) B-20

3. It measures low resistivities (less than ten ohm-metres) accurately. 4. Recording of three focused resistivity logs, which investigate different volumes of formation enables us to study invasion profile, and good Rt values in the case of deep invasion. Correction charts are available for: - Borehole - Bed thickness - Invasion Disadvantages: 1. Not reliable for resistivities > 250 ohm-m (use a Dual Laterolog) 2. Large hole and saline mud results in large borehole signals which give an unusually low apparent resistivity. (use DLL in this case).

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Figure B16

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Introduction to Open Hole Logging

DIL* Dual Induction - SFL* Spherically Focused Log ID - IM - SFL

Rint-2c Figure B17 (05/96) B-22

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B2.4 PHASOR INDUCTION SFL TOOL The Phasor Induction SFL tool (Figure B18) uses a conventional Dual Induction-SFL array to record resistivity data at three depths of investigation (see Chart B1 ). In addition to the usual in-phase (R-signal) induction measurements, the tool makes a high-quality measurement of the induction quadrature signal (X-signals). These measurements are combined with new advances in signal processing to provide an induction log with thin-bed resolution down to 60 cm (2'). Full correction for such environmental distortions such as shoulder effect and borehole effect are also performed.

Central to this development is a nonlinear deconvolution technique that corrects the induction log in real time for shoulder effect and improves the thin-bed resolution over the full range of formation conductivities. This algorithm, called Phasor Processing, requires the use of the induction quadrature signals, or Xsignals, which measure the nonlinearity directly. Phasor Processing corrects for shoulder effect and provides thin-bed resolution through Enhanced Processing down to 60 cm in many cases.

Since its introduction in the early 1960’s, the Dual Induction tool has evolved into the primary logging service for openhole formation evaluation in fresh and oil-based muds. Previous tools have, however, produced logs with response limitations. These limitations have usually required tedious hand correction. In extreme cases tool response limitations have produced features on logs that were mistaken for geological features. Although the distortions of the formation resistivity caused by resolution effect and shoulder effect are fully predictable from electromagnetic theory, automatic correction algorithms were not successful before now because of the nonlinearity of the R-signal measurement, which was the only measurement made in the older tools. New developments in electronics technology, work on computing the response of the induction tool in realistic formation models, and modern signal processing theory have combined to allow the development of a newer tool which is able to overcome the limitations of previous tools.

Figure B18: Schematic of the Phasor Induction SFL tool

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Introduction to Open Hole Logging

By adding borehole geometry measurements in the same tool string, borehole effect can also be corrected in real time. With these environmental effects removed, a real-time inversion of the data into a three-parameter invasion model can be done at the wellsite. The Phasor Induction design provides several additional advantages over existing tools. These include improvements in the calibration system, sonde error stability, SFL response, and a reduction of signal and cable noise. Each of these improvements contributes toward providing more accurate formation resistivity measurements over a wider range of resistivity and borehole conditions. a) Phasor Tool Description and Features The Phasor Induction SFL tool can be combined with other cable telemetry tools. Measurements returned to the surface include deep (ID) and medium (IM) R-signals, ID and IM X-signals, SFL voltage and current, SFL focus current, spontaneous potential (SP), SP-toArmor voltage, and array temperature. All measurements except SP are digitized downhole with high-resolution analog-to-digital converters, and all measure channels are recalibrated every 15 cm (6 inches) during logging. The operating frequency of the induction arrays is selectable at 10 kHz, 20 kHz, or 40 kHz, with a default frequency of 20 kHz. The tool also provides measurements of important analog signals and continuous monitoring of digital signals as an aid to failure detection and analysis. Depths of investigation and vertical resolution of the measurements are listed.

(05/96) B-24

b) Log Presentation The same presentation format is used for both generations of induction tools. The two logs can be identified by the following differences (Figure B19): 1. Deep Induction (IDPH) - the log inserts use the IDPH acronym to identify Phasor processing. 2. Medium Induction (IMPH) - the log inserts use the IMPH acronym to identify Phasor processing. 3. There is a hash mark up the right side of the depth track. c) Tool Characteristics, Improvements, and Applications 1. Phasor Induction - SFL can be most effectively used in holes filled with moderately conductive mud, nonconductive mud, and air drilled holes. 2. Vertical focusing is good and gives reliable values of Rt for beds thicker than 2.5 metres with no shoulder bed corrections required. 3. Measures low resistivities accurately. 4. Recording of three focused resistivity logs, which investigate different volumes of formation. 5. Reliable for resistivities up to 1000 ohm-m versus 250 ohm-m with normal Induction tool. 6. Gives accurate readings in boreholes up to 66 cm in diameter (Rt/Rm < 1000). 7. Operates at varying transmitter frequencies to improve signal to noise ratios. 8. Uses digital transmission techniques to improve accuracy of calibration and measurement.

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Correction charts are available for: - Borehole - Bed thickness - Invasion (Chart Rint-11a)

Phasor Induction - SFL Median Depth of Investigation 1. Above 100 ohm-m ID homogeneous for- IM mation SFL

Metres

Feet/Inches

1.58 0.79 0.41

62 inches 31 inches 16 inches

1.22 0.66 041

48 inches 26 inches 16 inches

2. At 0.1 ohm-m ho- ID mogeneous forma- IM tion SFL

Phasor Induction - SFL Vertical Resolution Vertical resolution bed thickness for full Rt determination no invasion

IDPH IMPH IDER* IMER IDVR# IMVR SFL

2.46 1.85 0.92 0.92 0.61 0.61 0.61

8 feet 6 feet 3 feet 3 feet 2 feet 2 feet 2 feet

* ER - Enhanced Resolution Phasor # VR - Very Enhanced Resolution Phasor Chart B1

(05/96) B-25

Introduction to Open Hole Logging

Figure B19 (05/96) B-26

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Phasor* Dual Induction-SFL Spherically Focused Log ID Phasor - IM Phasor - SFL

These charts (Rint-11) apply to the Phasor Induction tool when operated at a frequency of 20 kHz. Similar charts (not presented here) are available for tool operation at 10 kHz and 40 kHz. The 20 kHz charts do provide, however, reasonable approximations of R xo/Rt and Rt/RIDPH for tool operation at 10 kHz and 40 kHz when only moderately deep invasion exists (less than 100 inches). All Phasor* Induction invasion correction charts are applicable to Enhanced Resolution Logging (ERL*) and Enhanced Resolution Analysis (ERA*) presentation.

Rint-11a Figure B20 (05/96) B-27

Introduction to Open Hole Logging

(05/96) B-28

Schlumberger Schlumberger

B3.0

Measurement of Rt by Laterolog Principles

B3.1 DUAL LATEROLOG Broadly speaking, borehole fluids during drilling operations are broken into conductive and nonconductive categories. Each poses its particular challenges in measuring formation resistivities. The Dual Laterolog is a current emitting electrode device that performs best in saline muds (i.e. where Rt/Rm >>> 100, Rmf /Rw < 2.5). It is designed to extract Rt by measuring resistivity with several arrays having different depths of investigation.

a) Description and Features This resulted in the development of the Dual Laterolog-MicroSFL tool with simultaneous recordings. Figure B21 illustrates the focussing used by the deep laterolog device (left) and by the shallow laterolog device (right). Both use the same electrodes and have the same current-beam thickness, but have different focussing to provide their different depth of investigation characteristics.

Measurements responding to three appropriately chosen depths of investigation usually approximate the invasion profile well enough to determine Rt. For best interpretation accuracy, such a combination system should have certain desirable features: - Borehole effects should be small and/or correctable. - Vertical resolutions should be similar. - Radial investigations should be well distributed; i.e., one reading as deep as practical, one reading very shallow, and the third reading in between. Figure B21: Dual Laterolog Deep and Shallow Current Patterns

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Introduction to Open Hole Logging

The DLL tool has a response range of 0.2 to 40,000 ohm-m, which is a much wider range than covered by previous laterolog devices. To achieve accuracy at both high and low resistivities a constant-power measuring system is employed. In this system both measure current (io) and measure voltage (Vo) are varied and measured, but the product of the two, (i.e., power) Voio, is held constant. The deep laterolog measurement (LLD) of the DLL tool has a deeper depth of investigation than previous laterolog tools and extends the range of formation conditions in which reliable determinations of Rt are possible. To achieve this, very long guard electrodes are needed; the distance between the extreme ends of the guard electrodes of the DLL-Rxo tool is approximately 8.5 metres (28 feet). The nominal beam thickness of 60 cm (2 feet), however, insures good vertical resolution. Radial investigation is 1.2 to 1.5 metres (4-5 feet).

b) Log Presentation The DLL-MSFL presentation is very similar to the Phasor Induction. Differences include expanded resistivity scale (0.2-200,000 ohm-m) and the addition of Gamma Ray and Caliper (if MSFL is used). See log in Figure B23. c) Tool Characteristics and Applications 1. The Dual Laterolog performs most effectively in saline mud (high Rt/Rm ratios) or where Rmf/Rw < 2.5. (Figure B22) 2. The tool has an excellent resistivity range; by utilizing a unique design, resistivity resolution from 0.2 to 40,000 ohm-m is possible.

The shallow laterolog measurement (LSS) has the same vertical resolution as the deep laterolog device 60 cm (2 feet), but it responds more strongly to that region around the borehole normally affected by invasion. It uses a type of focusing called pseudolaterolog, wherein the focusing current is returned to nearby electrodes instead of to a remote electrode. This causes the measure current to diverge more quickly once it has entered the formations, thus producing a relatively shallow depth of investigation of 50 to 60 centimetres (20 to 24 inches). Figure B22: Preferred ranges of applications of Induction logs and Laterologs

(05/96) B- 30

Schlumberger Schlumberger

Figure B23

(05/96) B-31

Introduction to Open Hole Logging

3. Vertical resolution is excellent, Rt can be obtained in beds as thin as 60 cm (2 feet). 4. The LLd has very little borehole effect in large holes. 5. When combined with an Rxo measurement, the LLd, LLs curves may be used to study invasion profiles and compute a more accurate Rt. See Chart Rint-9 (Figure B24). 6. Assuming borehole conditions are suitable, the separation of the LLS, LLD curves may be used to give quick look indications of hydrocarbons; particularly in salt mud. In salt muds Rxo/Rt will be less than one so the better the zone, the greater the separation between LLs and Lld.

(05/96) B- 32

d) Limitations 1. The tools should not be used in fresh muds (Rmf/Rw > 2.5.) 2. The tools requires good centralization to minimize borehole influence on the LLs. 3. If invasion is deep, a good value of Rxo (e.g. from a Micro-Spherically Focused Log) is required to correct LLd for invasion influence to obtain an accurate value of Rt. Correction Charts are available for the influence of: - borehole (diameter and mud resistivity). - invasion. (Chart Rint-9b) - bed thickness.

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Dual Laterolog -Rxo Device DLT-D/E LLD - LLS - Rxo Device

Rint-9b Figure B24 (05/96) B-33

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B4.0

Measurement of Rxo by Micro-resistivity Principles

B4.1 INTRODUCTION As has been mentioned, a measurement of flushed zone resistivity, Rxo, is an important input when attempting to define invasion diameter. Since the flushed zone may only extend a few centimetres from the borehole, a shallow reading device is required. Such tools are the Microlog, Microlaterolog, Proximity log and the Micro-Spherically Focused Log. All are pad type devices which are pressed against the borehole wall to make their measurements. Today, the Microlog and Micro-Spherically Focused Log are completely combinable with all main logging services. The Microlaterolog and Proximity log have been discontinued due to their limitations in design, hence explanations of their measurements are not provided. Another service, the Electromagnetic Propagation Tool, also provides an excellent Rxo measurement. This service is an advanced device and will not be discussed in this book. For more information, refer to Schlumberger Log Interpretation Applications/Principles.

To measure Rxo, the tool must have a very shallow depth of investigation. Since the reading should be affected by the borehole as little as possible, a sidewall-pad tool is used. Currents from the electrodes on the pad must pass through the mudcake to reach the flushed zone. Therefore, microresistivity readings are affected by mudcake; the effect depends on mudcake resistivity, Rmc, and thickness hmc. Moreover, mudcakes can be anisotropic, with mudcake resistivity parallel to the borehole wall less than that across the mudcake. Mudcake anisotropy increases the mudcake effect on microresistivity readings so that the effective, or electrical, mudcake thickness is greater than that indicated by the caliper.

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Introduction to Open Hole Logging

B4.2 MICROLOG With the microlog tool, two short-spaced devices with different depths of investigation provide resistivity measurements of a very small volume of mudcake and formation immediately adjoining the borehole. Comparison of the two curves readily identifies mudcake, which indicates invaded and, therefore, permeable formations. a) Principle The rubber microlog pad is pressed against the borehole wall by arms and springs (Figure B25). The face of the pad has three small inline electrodes spaced 1 inch (2.5 centimetres) apart. With these electrodes a 1 by 1 inch microinverse (R1"x1") and a 2 inch (5.1 centimetres) micronormal (R2") measurement are recorded simultaneously. The currents emitted from these electrodes are totally unfocused and hence flow by the path of least resistance (Figure B26).

Figure B25: Microlog

(05/96) B-36

As drilling fluid filters into the permeable formations, mud solids accumulate on the hole wall and form a mudcake. Usually, the resistivity of the mudcake is slightly greater than the resistivity of the mud and considerably lower than the resistivity of the invaded zone near the borehole. The 2 inch micronormal device has a greater depth of investigation than the microinverse. It is, therefore, less influenced by the mudcake and reads a higher resistivity, which produces positive curve separation. In the presence of low-resistivity mudcake, both devices measure moderate resistivities, usually ranging from 2 to 10 times Rm. In impervious formations, the two curves read similarly or exhibit some negative separation. Here the resistivities are usually much greater than in permeable formations. (See Figure B27 - Microlog).

Figure B26: Microlog ML

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Figure B27 (05/96) B-37

Introduction to Open Hole Logging

Under favourable circumstances the microlog can be used to obtain Rxo but it is generally considered a good qualitative indicator of permeability, rather than an Rxo measurement.

This eliminates the need for a separate logging run to obtain Rxo information. See Figure B23 for a log example of MSFL with Dual Laterolog.

b) Microlog Limitations - Rxo/Rmc must be less than about 15. - Mudcake thickness < 1.2 cm - Depth of Flushing > 10 cm, otherwise the microlog readings are affected by Rt.

The second improvement is in the tools response to shallow Rxo zones in the presence of mudcake. The chief limitation of the Microlaterolog measurement was its sensitivity to mudcakes. When mudcake thickness exceeded about 3/8 inch, the log readings were severely influenced at high Rxo/Rmc contrasts. The Proximity log, on the other hand, was relatively insensitive to mudcakes, but it required an invaded zone diameter of about 100 cm in order to provide direct approximations of Rxo.

B4.3 MICRO - SPHERICALLY FOCUSED LOG The MicroSFL is a pad-mounted spherically focused logging device that has replaced the Microlaterolog and Proximity tools. It has two distinct advantages over the other Rxo devices. The first is its combinability with other logging tools, including the Phasor Induction, the Array Induction, and Dual Laterolog tools.

The solution was found in a adaptation of the principle of spherical focusing in a sidewall-pad device. By careful selection of electrode spacings and bucking-current controls, the MicroSFL measurement was designed for minimum mudcake effect without any undue increase in the depth of investigation. Figure B28 illustrates, schematically, the current patterns (left) and the electrode arrangement (right) of the MicroSFL tool. By forcing the measure current to flow directly into the formation, the effect of mudcake resistivity on the tool response is minimized; yet, the tool still has a very shallow depth of investigation.

Figure B28: Current Distribution of MicroSFL device (left) and Electrode Arragement (right)

(05/96) B-38

Synthetic microlog curves can also be computed from MicroSFL parameters. Since the measure current sees mostly the flushed zone and the bucking current sees primarily the mudcake, it is possible to mathematically derive micronormal and microinverse curves.

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a) MicroSFL Limitations - depth of flushing > 12 cm. - mud cake thickness < 1.2 cm. - radial investigation 10 cm. b) MicroSFL Applications - Identification of permeable zones. - An excellent value of Rxo from the MSFL provides a quick look over-lay technique for comparison with an Rt curve after being normalized in a 100% Sw zone. After normalization when curves separate, moved hydrocarbon is indicated. - Sw determination using Rxo and Rt values provide an independent lithology-free check on other methods. It should be noted that the use of this system in fresh muds where deep invasion is present, should be approached with caution.

An Rxo measurement is another method of finding Rw when a wet zone is available. F is found from Rxo and Rmf; Ro is found by obtaining RLLD and RLLS from the logs and then correcting for borehole and invasion. The Rw = Ro/F can be solved for. Also, knowing F,  can be calculated. Remember the reason for finding Rw is to allow you to solve for Sw2 = FRw/Rt in a possible pay zone elsewhere in the well. Correction charts are available for the influences of: - Mudcake (Chart Rxo-3) (Figure B29).

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Introduction to Open Hole Logging

MicroSFL* Mudcake Correction For Hole Diameter of 8 in. or 200 mm

Example:

RMLL = 9.0 ohm-m Rmc = 0.15 ohm-m at formation temperature hmc = 9.5 mm giving RMLL/Rmc = 9.0/0.15 = 60 Therefore, RMLLcor/RMLL = 2 and RMLLcor = 2(9.0) = 18 ohm-m

Rxo-3 Figure B29 (05/96) B-40

Contents C1.0 POROSITY MEASUREMENTS ............................................................................................................... 1 C2.0 POROSITY MEASUREMENTS FROM THE BHC SONIC TOOL.......................................................... 3 C2.1 INTRODUCTION ................................................................................................................................ 3 C2.2 POROSITY DETERMINATION .......................................................................................................... 4 C2.3 FACTORS AFFECTING SONIC INTERPRETATION: ....................................................................... 7 C3.0 POROSITY MEASUREMENTS FROM THE LITHO-DENSITY TOOL ................................................. 11 C3.1 INTRODUCTION .............................................................................................................................. 11 C3.2 PRINCIPLE....................................................................................................................................... 11 C3.3 POROSITY FROM A DENSITY LOG ............................................................................................... 13 C3.4 LITHOLOGY FROM THE PE MEASUREMENT ............................................................................... 17 C3.5 FACTORS AFFECTING DENSITY LOG:......................................................................................... 20 C4.0 POROSITY MEASUREMENTS FROM THE COMPENSATED NEUTRON TOOL .............................. 21 C4.1 INTRODUCTION .............................................................................................................................. 21 C4.2 PRINCIPLE ....................................................................................................................................... 21 C4.3 FACTORS AFFECTING CNL LOGS ................................................................................................ 23 C5.0 TOTAL POROSITY DETERMINATION ................................................................................................ 29 C6.0 GR LOG ................................................................................................................................................. 31 C6.1 INTRODUCTION .............................................................................................................................. 31 C6.2 PROPERTIES OF GAMMA RAYS ................................................................................................... 31 C6.3 NATURAL GAMMA RAY SPECTROMETRY TOOL ........................................................................ 34 C7.0 BOREHOLE GEOMETRY BY CALIPER MEASUREMENT ................................................................. 37 C7.1 PHYSICAL PROPERTIES ................................................................................................................ 37 Single-Arm Caliper Configuration .......................................................................................................... 40 Two-Arm Caliper Configurations ............................................................................................................ 40 Three-Arm Caliper Configurations ......................................................................................................... 41 Four-Arm Caliper Configuration ............................................................................................................. 41 C8.0 WORK SESSION ................................................................................................................................... 43

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Introduction to Openhole Logging

(05/96)

C1.0

Porosity Measurements

C1.1 INTRODUCTION Total porosity may consist of primary and secondary porosity. Effective porosity is the total porosity after the shale correction is applied. Rock porosity can be obtained from the sonic log, density log or neutron log. For all these devices, the tool response is affected by the formation porosity, fluid and matrix. If the fluid and matrix effects are known or can be determined, the tool response can be determined and related to porosity. Therefore, these devices are usually referred to as porosity logs.

For example, the formula for a density log measurement including all these variables can be written as be  Sw f +e(1 – Sw) hy + Vshsh + (1 – e – Vsh) ma. Solving for porosity in this case would not be easy because there are several unknowns and only one measurement. However, when we compare other porosity and log measurements, we can solve for these unknowns.

All three logging techniques respond to the characteristics of the rock immediately adjacent to the borehole. Their depth of investigation is shallow—only a few centimeters or less—and therefore generally within the flushed zone. As well as porosity, the logs are affected by - volume and nature (lithology) of matrix material - amount and nature of pore space contents (pore geometry, water, hydrocarbons) - volume and nature of shales.

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Introduction to Openhole Logging

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C2.0 Porosity Measurements from the BHC Sonic Tool C2.1 INTRODUCTION In its simplest form, a sonic tool consists of a transmitter that emits a sound pulse and a receiver that picks up and records the pulse as it passes the receiver.

The computer also integrates the transit time readings to obtain total traveltimes (see Figures C1 and C2).

The sound emanated from the transmitter impinges on the borehole wall. This establishes compressional and shear waves within the formation, surface waves along the borehole wall and guided waves within the fluid column. The sonic log is simply a recording versus depth of the time, tcomp, required for a compressional sound wave to traverse 1 m of formation. Known as the interval transit time, transit time, t or slowness, tcomp is the reciprocal of the velocity of the sound wave. (For the remainder of this document, tcomp is known as t.) The interval transit time for a given formation depends upon its lithology and porosity. This dependence upon porosity, when the lithology is known, makes the sonic log useful as a porosity log. Integrated sonic transit times are also helpful in interpreting seismic records. The sonic log can be run simultaneously with many other services. The borehole-compensated (BHC) tool transmitters are pulsed alternately, and t values are read on alternate pairs of receivers. The t values from the two sets of receivers are averaged automatically by a computer at the surface for borehole compensation.

Figure C1: Schematic of BHC sonde, showing ray paths for the two transmitter-receiver sets. Averaging the two t measurements cancels errors from the sonde tilt and hole-size charges. (05/96) C-3

Introduction to Openhole Logging

Sometimes the first arrival, although strong enough to trigger the receiver nearer the transmitter, may be too weak by the time it reaches the far receiver to trigger it. Instead, the far receiver may be triggered by a different, later arrival in the sonic wave train, and the travel time measured on this pulse cycle will then be too large. When this occurs, the sonic curve shows an abrupt, large excursion towards a higher t value; this is known as cycle skipping. Such skipping is more likely to occur when the signal is strongly attenuated by unconsolidated formations, formation fractures, gas saturation, aerated muds or rugose or enlarged borehole sections.

The sonic log is run with t presented on a linear scale in tracks 2 and 3 with a choice of two scales: 500–100 and 300–100 sec/m. A three-arm caliper curve representing the average borehole diameter and a gamma ray (GR) curve are recorded simultaneously in track 1 (See Figure C3). The gamma ray curve measures the natural radioactivity of potassium, uranium and thorium in the formation and is usually representative of the amount of shale present. This is because radioactive elements tend to concentrate in clays and shales. Later, we will use the GR to compute volume of shale (Vsh). C2.2 POROSITY DETERMINATION a) Wyllie Time-Average Equation After numerous laboratory determinations, M.R.J. Wyllie proposed, for clean and consolidated formations with uniformly distributed small pores, a linear time-average or weighted-average relationship between porosity and transit time (see Figure C4): tLOG = tf + (1 –)tma

(C1)

tLOG – tma or 

(C2) tf – tma

Figure C2: BHC Sonic—GR tool distances

(05/96) C-4

where tLOG is the reading on the sonic log in   sec/m tma is the transit time of the matrix material

Figure C3 : Borehole-Compensated Sonic Log (05/96) C-5

Introduction to Openhole Logging

tf is the transit time of the saturating fluid (about 620 sec/m for freshwater mud systems)   is the porosity or volume occupied by pores  is the volume of the matrix. Typical Values: Sand  tmatrix Lime  tmatrix Dolomite tmatrix Anydrite  tmatrix

= 182 sec/m = 156 sec/m = 143 sec/m = 164 sec/m

When the formations are not sufficiently compacted, the observed t values are greater than those that correspond to the porosity according to the time-average formula, but the  versus t relationship is still approximately linear. In these cases, an empirical correction factor, Cp, is applied to Equation 2 to give a corrected porosity, SVcor (Equation 3):

t - tma SVcor =

1 

tf - tma

(C3) CP

The value of Cp is given approximately by dividing the sonic velocity in nearby shale beds by 328. However, the compaction correction factor is best determined by comparing SV, as obtained from Equations 1 and 2, with the true porosity obtained from another source. b) Raymer-Hunt Over the 25 years since acoustic velocity well logging was introduced, deficiencies have been noted in the transform of transit time t to porosity . Based on extensive field observations of transit times versus porosity, the new empirical Raymer-Hunt transform was derived. The new transform equation is too complicated to be presented in this course. An approximation of the transform is given in Equation C4 and the exact transform is presented in the chart books as the red lines on all sonic charts. tLOG - tma sv = C

(C4) tLOG

Figure C4: Components of the Wyllie Time-Average Equation

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The value of the constant C has a range of 0.625 to 0.7 depending upon the investigator. Chart Por-3m (Figure C6) uses 0.7 for C: this was the value originally proposed. However, more recent transit time-to-porosity comparisons indicate that a value of 0.67 is more appropriate.

For the case of a gas-saturated reservoir rock, C becomes 0.6. It should be used when the rock investigated by the sonic tool contains an appreciable amount of hydrocarbon in the gassy (vapor) phase. Because of the shallow depth of investigation, this condition normally exists only in higher porosity sandstones (greater than 30%). From the example sonic log (Figure C3) at 593 m we read 352 sec/m. Given tma =182 sec/m we can solve for : Wyllie:

Raymer-Hunt (approximation): 5(352 - 182)  =

 30% 8(352)

Chart Por-3m (Figure C6) solves this equation graphically. Enter tlog of 352 sec/m on abscissa and project upward until the appropriate tma line is reached (Vma= 5500 m/sec). If different values of Vma are used, we get different values of . With a tlog = 250sec/m we would get

352 - 182  =  39% 620 - 182

Vma Vma (m/sec)

tma ( sec/m)

Sandstone

5486

182

Vma (m/sec) Range of Values 5486–5944

Limestone

6400

156

6400–7010

Dolomites

7010

143

7010–7925

Anhydrite

6096

164

6100

Salt

4572

219

4566

Casing (iron)

5334

187

5348

Fluid Transit Time: V1 = 1615 m/sec tf = 620 microsec/m for fresh muds = microsec/m for salt muds

Figure C5: Chart showing values used for common reservoir rocks

Sandstone (5500 m/sec) Limestone (6400 m/sec) Dolomite (7010 m/sec)

Wyllie F

RaymerHunt F

16% 21% 26%

18.5% 24% 28.5%

C 2.3 FACTORS AFFECTING SONIC INTERPRETATION Lithology Lithology must be known to obtain the correct Vma. An incorrect choice of Vma will produce erroneous calculations. Shale Shale content generally causes t to read too high for a porosity calculation because of the bound water in the shale. The sonic reads primary porosity, which may be affected by shale.

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Introduction to Openhole Logging

Porosity Evaluation from Sonic Svf = 1615 m/s

EXAMPLE:

t = 76 s/ft (249 s/m) SVma = 19,500 ft/s (5950 m/s) - Sandstone Thus,  = 18% (by either weighted average or empirical transform)

Sandstones Limestones Dolomites

SVma (ft/S) 18,000 - 19,500 21,000 - 23,000 23,000 - 26,000

tma (s/ft) 55.5 - 51.3 47.6 - 43.5 43.5 - 38.5

Por-3m Figure C6 (05/96) C-8

SVma (m/s) 5486 - 5944 6400 - 7010 7010 - 7925

tma (s/m) 182 - 168 156 - 143 143 - 126

Fluid Type The depth of investigation of the sonic is shallow; therefore, most of the fluid seen by the sonic will be mud filtrate. Oil Oil usually has no effect. Water There is usually no effect from water except where the drilling fluid is salt saturated, and then a different Vf should be used, usually 607 sec/m. Gas Residual gas causes tlog to read too high when the formation is uncompacted. The gas between the sand grains slows down the compressional wave resulting in a long t. In compacted sands, the wave will travel from one sand grain to another and the gas effect will be reduced. Compaction The value of tlog will read too high in uncompacted sand formations. Compaction corrections can be made if the compaction factor (Bcp) is known.

An approximate Bcp is obtained from the surrounding shales (Bcp = tsh/328). Bcp can also be obtained by comparing the porosity obtained from another source (core, density log, neutron log, computed log porosity) to that obtained from the sonic log in a clean water zone. (For example, if the neutron log in a clean water zone reads 20% and the sonic log reads 25%, then Bcp = 25%/20% = 1.25.) Secondary Porosity The sonic generally ignores secondary porosity. For example, in vugular porosity, the traveltime through the formation matrix is faster than the time through fluid in the vugs, because tf is about 3 to 4 times the value of tma. Borehole Effect The compensated sonic is unaffected by changing hole size except in the case of extremely rough, large holes where the formation signal is severely affected by the noise of the mud signal and formation damage. Mudcake Mudcake has no effect on the BHC sonic because the traveltime through the mudcake is compensated.

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Introduction to Openhole Logging

(05/96) C-10

C3.0 Porosity Measurements from the Litho-Density Tool C3.1 INTRODUCTION Litho-Density logs are primarily used for porosity and lithology measurements. Other uses include the identification of minerals in evaporite deposits, detection of gas, determination of hydrocarbon density, evaluation of shaly sands and complex lithologies, determination of oil-shale yield and calculation of overburden pressure and rock mechanical properties. C3.2 PRINCIPLE A radioactive source, applied to the borehole wall in a shielded sidewall skid (Figure C7), emits medium-energy gamma rays (662 keV) into the formation.

3 occur with respect to Litho-Density operation. These gamma rays may be thought of as high-velocity particles that collide with the electrons in the formation. At each collision, a gamma ray loses some, but not all, of its energy to the electron and then continues with diminished energy. This type of interaction is known as Compton scattering. The scattered gamma rays reaching the detector, at a fixed distance from the source, are counted as an indication of formation density. The number of Compton-scattering collisions is related directly to the number of electrons in the formation. Consequently, the response of the density tool is determined essentially by the electron density (number of electrons per cubic centimeter) of the formation. Electron density is related to the true bulk density b, which, in turn, depends on the density of the rock matrix material, formation porosity and density of the fluids filling the pores.

(GR energy > 1.02 MeV) (over entire GR energy range) (e)

Figure C7: Schematic Drawing of the Dual Spacing Litho-Density Logging Device

Classical GR interactions by energy level are shown in Figure C8. Because of the medium-energy GR emission, only points 2 and

(low-energy GR) (Z)

Figure C8: Classical GR— Matter Interactions by Energy Level (05/96) C-11

Introduction to Openhole Logging

In addition to the bulk density measurement, the tool also measures the photoelectric absorption index of the formation, Pe. Photelectric absorption can be related to lithology; whereas the b measurement responds primarily to porosity and secondarily to rock matrix and pore fluid, the Pe measurement responds primarily to rock matrix (lithology) and secondarily to porosity and pore fluid. At a finite distance from the source, such as the far detector, the energy spectrum may look as illustrated in Figure C9. The number of gamma rays in the higher energy region (region of Compton scattering) is inversely related only to the electron density of the formation (i.e., an increase in the formation density decreases the number of gamma rays). The number of gamma rays in the lower energy region (region of photoelectric effect) is inversely related to both the electron density and the photoelectric absorption. By comparing the counts in these two regions, the photoelectric absorption index can be determined.

The gamma ray spectrum at the near detector is used only to correct the density measurement from the far detector for the effects of mudcake and borehole rugosity.

7m 4.5 m

E (keV)

Figure C10: Basic SGT- CNT- LDT Tool Configuration Figure C9: Variations in Spectrum forFormation with Constant Density but Different Z

(05/96) C-12

This can be written as

 ma

 f



(1 –   b

Figure C11: Components of Density Porosity Calculation

C 3.3 POROSITY FROM A DENSITY For a clean formation of known matrix density ma, with a porosity  that contains a fluid of average density f,, the formation bulk density b, will be (Figure C11): b = f + (1 – ) ma (clean wet zone) where:  b is the measured bulk density (from Litho-Density tool)  ma is the density of the matrix  f is the density of the fluid  is the percent volume of pore space (1 – ) is the percent volume of matrix.

ma – b 

D = ma – fl

where:  ma depends on lithology  b is measured by the density log  fl depends on fluid type in pore volumes.   LOGequation for b can be proven matheThe matically, unlike the sonic equation, which is an empirical relationship. Values of b are used for common reservoir rocks (zero porosity) (Figure C12). From the example Litho-Density log (Figure C13) at 593 m we read b = 2180 kg/m3. Given f = 1000 kg/m3, ma = 2650 kg/m3, we can solve for D: D =

 = 28.5% 

Chart Por-5 (Figure C14) solves this equation graphically. For b = 2180 kg/m3 solving for porosity using other matrix values gives: ma = 2710 kg/m3

D = 31%

ma = 2870 kg/m3

D = 36.9%

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Introduction to Openhole Logging

b Values for Common Reservoir Rocks and Fluids Compound

Formula

Actual Density 

a (as seen by tool)

Quartz Calcite Dolomite Anhydrite Sylvite Halite

SiO2 CaCO3 CaCO3MgCO3 CaSO4 KCI NaCI

2654 2710 2870 2960 1984 2165

2648 2710 2876 2977 1863 2032

Compound

Formula

Actual Density 

a (as seen by tool)

Fresh Water Salt Water Oil Gas

H2O 200,00ppm n(CH2) C1.1 H4.2

1000 1146 850 g

1000 1135 850 1.325 g-0188

Figure C12

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Figure C13

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Introduction to Openhole Logging

Formation Density Log Determination of Porosity

Bulk density, b, as recorded with the FDC* or LDT density logs, is converted to porosity with this chart. To use, bulk density, corrected for borehole size, is entered in abscissa; go to the appropriate reservoir rock type and read porosity on the appropriate fluid density, f. scale in ordinate. (f is the density of the fluid saturating the rock immediately surrounding the borehole - usually mud filtrate.) EXAMPLE: b = 2.31 Mg/m3 in limestone lithology ma = 2.71 (limestone) f = 1.1 (salt mud) Therefore D = 25 pu

Por-5 Figure C14 (05/96) C-16

C3.4 LITHOLOGY FROM Pe MEASUREMENT The Pe curve is a good matrix indicator. It is slightly influenced by formation porosity and the presence of gas, but responds mainly to lithology (Figure C15). Hence, a safe interpretation of matrix lithology can be made for simple lithologies (one-mineral matrix). In conjunction with other log data, more complex mineral combinations can be analyzed.

Pe

Typical Litho-Density responses for common minerals are presented in Figure C16. The Pe measurement is used 1. alone as a matrix indicator (the lithology curve) 2. in combination with density b to analyze two-mineral matrices and determine porosity

t 0.5 0.4 0.3 0.2 0.1 0

Figure C15: Photoelectric Absorption Index as a Function of Porosity and Fluid Content

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Introduction to Openhole Logging

3. In combination with the density and neutron to analyse more complex lithologies (solutions to three-mineral matrices and porosity). A direct benefit from the more accurate description of the matrix is a more reliable distinction between gas and oil. In this section of the course, we use the Pe curve as a matrix indicator in simple lithologies. Using Pe for more advanced applications

(complex lithology identification and heavy mineral-detection) is covered in Section H, Porosity in Complex Lithologies. Examples of the direct use of the Pe curve for lithology identification are shown in Figure C17. In the case of an anhydrite, Pe is equal to that of limestone. Anhydrite is positively identified by the bulk density or density porosity values.

Figure C16: Typical Litho-Density Responses for Common Sedimentary Rocks

(05/96) C-18

Figure C17: Lithology Identification with the CNT, Litho-Density and Pe

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Introduction to Openhole Logging

C3.5

FACTORS AFFECTING THE DENSITY LOG

Lithology The correct ma must be known to get correct porosity. Shale The density of shale in sands can range from 2200 to 2650 but is usually close to 2650, the same as sandstone. In shaly sands, the density usually gives a good value of effective porosity regardless of the shale content. The shale appears as matrix to the density tool. b = f e + ma (1 – e – Vsh) + shVsh collecting terms: b = f e) + ma(1 – e) + Vsh (sh – ma) if sh = ma , the last term is zero. Fluid Type The depth of investigation is quite shallow: usually most of the formation fluid is flushed away from the wellbore and the density tool sees drilling fluid or filtrate in the pore space. Hence, the values of f to use is that of the drilling mud filtrate rather than the formation water density. Oil Residual oil will make density porosities slightly high, because oil is lighter than drilling mud filtrate.

(05/96) C-20

Water Water density is proportional to the amount of salt content. The value of f is selected in the computer for porosity determination. Gas The f of gas is 100–300 kg/m3. Porosity determination in gas zones may be high if there is residual gas near the borehole. Usually most of the gas is flushed and little effect is seen on the density log. Compaction The density tool is unaffected by lack of compaction. Secondary Porosity The density reads intercrystalline, vugular and fractured porosity. The porosity measured is therefore total porosity. Borehole Effect Density gives good values for smooth holes up to 381 mm in diameter. The tool compensates for minor borehole rugosity, but a rough hole causes the density to read too low densities (high porosities) because the skid-toformation contact is poor. Mudcake For normal mudcake thickness, there will be no effect because the tool automatically compensates for mudcake. However for a  correction of 100 kg/m3 and greater (i.e.,  > 100 kg/m3), the tool compensation may be insufficient and the b no longer representative of the formation density. In this case, the density should obviously not be used for porosity calculations.

Introduction to Openhole Logging

C4.0 Porosity Measurements from the Compensated Neutron Tool C4.1 INTRODUCTION Neutron logs are used principally for the delineation of porous formations and determination of their porosity. They respond primarily to the amount of hydrogen in the formation. Thus, in clean formations that have pores filled with water or oil, the neutron log reflects the amount of liquid-filled porosity. Gas zones can often be identified by comparing the neutron log with another porosity log or a core analysis. A combination of the neutron log with one or more other porosity logs yields even more accurate porosity values and lithology identification—even an evaluation of shale content.

C4.2 PRINCIPLE Neutrons are electrically neutral particles, each with a mass almost identical to the mass of a hydrogen atom. High-energy (fast) neutrons are continuously emitted from a radioactive source in the sonde. These neutrons collide with the nuclei of the formation materials in what may be thought of as elastic billiardball collisions. With each collision, the neutron loses some of its energy. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. A greater energy loss occurs when the neutron strikes a nucleus of practically equal mass (i.e., a hydrogen nucleus). Collisions with heavy nuclei do not slow the neutron much. Thus, the slowing of neutrons depends largely on the amount of hydrogen in the formation. Within a few microseconds, the neutrons have been slowed by successive collisions to thermal velocities, corresponding to energies of about 0.025 eV. They then diffuse randomly, without losing more energy, until they are captured by the nuclei of atoms such as chlorine, hydrogen or silicon. The capturing nucleus becomes intensely excited and emits a high-energy gamma ray of capture.

Figure C18: Schematic Drawing of the Dual Spacing Compensated Neutron Tool

(09.95) C-

22

When the hydrogen concentration of the material surrounding the neutron source is large, most of the neutrons are slowed and captured within a short distance of the source. On the contrary, if the hydrogen concentration is small, the neutrons travel farther from the source before being captured. Accordingly, the counting rate at the detector increases for decreased hydrogen concentrations and vice versa. Thus, the neutron tool responds to the hydrogen index of the formation. The hydrogen index is a measurement of the amount of hydrogen per unit volume of formation (HI of water = 1). Neutron logging tools include the GNT (Figure C19) tools series (no longer in use),

sidewall neutron porosity (SNP) tools (in limited use) and the CNL tool series, which includes the compensated neutron and DNL* Dual-Energy Neutron Log. The current tools use americium-beryllium (AmBe) sources to provide neutrons with initial energies of several million electron volts. 1) SNP - detects epithermal neutrons - utilizes a skid mounted single detector - can be run in open hole only, either liquid-filled or empty - most corrections are automatically applied during logging - limited availability.

Figure C19: Neutron Energy Travel History

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Introduction to Openhole Logging

2) CNL tool detects thermal neutrons - The CNL tool uses a two-detector system that depth and resolution matches each count rate before the ratio is computed. The ratio value is then converted to porosity on a linear scale (Figure C20), based on the matrix selected for the computation (limestone, sandstone or dolomite). - Conversion from one porosity assumption to another can be done using Chart Por-13b (Figure C22). Por13b converts curves labelled "NPHI" that are not environmentally corrected and also converts for curves labelled "TNPH" and "NPOR," which are environmentally corrected. - The CNL tool is especially designed for use in combination with other devices. - The CNL tool can be run in liquidfilled holes, either open or cased, but not empty holes (i.e., air- or gas-filled holes.) 3) DNL tool detects thermal and epithermal neutrons - The DNL tool incorporates two epithermal neutron detectors in addition to the two thermal neutron detectors. Two separate porosity measurements are obtained, one from each pair of detectors. - Improves the response to gas and enhances interpretation in the presence of thermal neutron absorbers. - In shaly formations containing a large number of thermal neutron absorbers, the porosity measured by the epithermal detectors reads lower and agrees

(05/96) C-24

more closely with density-derived porosity. - As with the CNL tool, the DNL tool is especially designed for use in combination with other devices. In addition, the DNL tool can be run in liquidfilled holes, air/gas-filled holes (epithermal porosity only) and open or cased holes. C4.3

FACTORS AFFECTING CNL LOGS

Lithology A single known matrix must be present to accurately determine porosities. Large errors can occur if the matrix selection is incorrect. Shale The presence of hydrogen in chemically bound water causes the CNL/DNL tool to read high porosities in shales or shaly formations. Fluid Type Water: Fresh water has no effects. Saline water has a reduced hydrogen content and the CNL/DNL tool will read low porosity; the correction is in the chart book. Liquid Hydrocarbons: If the hydrogen content is close to that of water, there is little or no effect. Gas: If the hydrogen concentration is low, the CNL/DNL tool reads low porosity. Compaction All neutron logs are unaffected by compaction.

Figure: C20 (05/96) C-25

Introduction to Openhole Logging

Secondary Porosity All neutron equipment measures total porosity (including primary and secondary). Borehole Effect The effects of rough hole are minimized by a large depth of investigation obtained by the use of a high-yield source and the twodetector system. When run in combination with the density tool, an automatic caliper correction system is accurate to [356 mm]. Normally there is zero standoff correction.

(05/96) C-26

Mudcake Corrections for mudcake, fluid (mud and formation) salinity, mud weight, pressure and temperature are in Charts Por-14(a) and 14(b), in the Log Interpretation Chart Book, but are not discussed in this course. The average net correction is usually between one and three porosity units. Hence, for calculations by hand, the correction is usually not done.

Neutron Porosity Equivalence Curves Sidewall Neutron Porosity (SNP), Compensated Neutron Log (CNL*)

When the SNP or CNL log is recorded in limestone porosity units, this chart is used to find porosity in sandstones or dolomites. For the SNP log, first correct for mudcake thickness. (Chart Por-15 is used for SNP mudcake corrections.) For the CNL log, simply enter the chart in abscissa with the apparent limestone neutron porosity; go to the appropriate matrix line, and read true porosity on the ordinate. (Chart Por-14 is used for CNL environmental corrections.) EXAMPLE: Sandstone bed Giving, hmc = 1/4 in. øSNP = 13 pu (apparent limestone porosity) øSNP = 11 pu (corrected for mudcake) Bit Size = 77/8 in. And, øSNP (sandstone) = 14 pu SNP caliper = 75/8 in. This chart can also be used to find apparent limestone porosity (needed for entering the various CP-crossplot charts) if the SNP or CNL recording is in sandstone or dolomite porosity units. This chart should be used for CNL values labeled NPHI—it should not be used for CNL values labeled TNPH or NPOR.

Por-13a Figure C21 (05/96) C-27

Introduction to Openhole Logging

Neutron Porosity Equivalence Curves Compensated Neutron Log (CNL*)

*Mark of Schlumberger

Por-13b Figure C22 (05/96) C-28

C5.0 Total Porosity Determination We have seen that porosity measurements are inferred from measurements of bulk density, hydrogen index and acoustic traveltimes. We have also seen that each measurement provides the necessary input to calculate porosity under the following conditions: – Porosity type is intergranular, not fractured or secondary (vuggy, moldic, etc.). – Matrix type is known and constant. – Rock is clean, (i.e., no shale present). – Porosity is filled with fluid. Violations of any of these conditions will cause the different porosity measurements to disagree in one fashion or another. This can be used to determine lithology, primary and secondary porosity and gas vs. liquid content. The question to be answered here is: Which porosity measurement should be used? In a sand-shale sequence, for initial computations,

a) if D is available, use TOTAL = D b) if N and t are available, use TOTAL = S with compaction corrections applied. In a carbonate, for initial computations (limestone matrix), a) if N and D are available in sandstone and limestone units, then use TOTAL:  N + D  T =   b) if only t is available, use TOTAL:    T = S + estimate VUGS. If gas is present in the reservoir, additional corrections to N and D must be applied, as discussed in Section F. Porosity calculations in complex lithologies shall are discussed in Section H.

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Introduction to Openhole Logging

Figure C23: Porosity Comparison between the LDT, CNT and SLT (05/96) C-30

C6.0 GR Log 6.1 INTRODUCTION The GR log is a measurement of the natural radioactivity of the formations. In sedimentary formations the log normally reflects the shale content of the formations. This is because the radioactive elements tend to concentrate in clays and shales. Clean formations usually have a very low level of radioactivity, unless radioactive contaminant such as volcanic ash or granite wash is present or the formation waters contain dissolved radioactive salts. "Clean" Formation Sands Limestones Dolomites

radioactive elements of the uranium and thorium series. Each of these elements emits gamma rays, the number and energies of which are distinctive for each element. Figure C24 shows the energies of the emitted gamma rays: potassium (K40) emits gamma rays of a single energy at 1.46 MeV, whereas the uranium and thorium series emit gamma rays of various energies.

GR Reading 15 to 30 API 10 to 20 API 8 to 15 API

The GR log can be recorded in cased wells, which makes it very useful as a correlation curve in completion and workover operations. It is frequently used to complement the SP log and as a substitute for the SP curve in wells drilled with salt mud, air or oil-base muds. In each case, it is useful for the location of shales and nonshaly beds and, most importantly, for general correlation. 6.2 PROPERTIES OF GAMMA RAYS Gamma rays are bursts of high-energy electromagnetic waves that are emitted spontaneously by some radioactive elements. Nearly all the gamma radiation that occurs in the earth is emitted by the radioactive potassium isotope of atomic weight 40 (K40) and by the

Figure C24: Gamma Ray Emission Spectra of Radioactive Minerals

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Introduction to Openhole Logging

In passing through matter, gamma rays experience successive Compton-scattering collisions with atoms of the formation material, losing energy with each collision. After the gamma ray has lost enough energy, it is absorbed, by means of the photoelectric effect, by an atom of the formation. Thus, natural gamma rays are gradually absorbed and their energies degraded (reduced) as they pass through the formation. The rate of absorption varies with formation density. Two formations with the same amount of radioactive ma-

(05/96) C-32

terial per unit volume, but with different densities, will show different radioactivity levels; the less dense formations will appear slightly more radioactive. (Figure C25). GR uses: 1. definition of shale beds 2. indicator of shale content 3. detection of radioactive and nonradioactive minerals 4. identification of formation tops.

Figure C25: Relative GR Response for Various Rocks/Formations (05/96) C-33

Introduction to Openhole Logging

6.3

NGS NATURAL GAMMA RAY SPECTROMETRY TOOL Like the GR log, the NGS Natural Gamma Ray Spectrometry tool measures the natural radioactivity of the formations. Unlike the GR log, which measures only the total radioactivity, this log measures both the number of gamma rays and the energy level of each and permits the determination of the concentrations of radioactive potassium, thorium and uranium in the formation rocks (Figure C27). Physical Principle Most of the gamma ray radiation in the earth originates from the decay of three radioactive isotopes: potassium (K40), uranium 238 (U238) and thorium 232 (Th232). Potassium-40 decays directly to the stable argon-40 with the emission of a 1.46-MeV gamma ray. However, uranium-238 and thorium-232 decay sequentially through a long

sequence of various daughter isotopes before arriving at stable lead isotopes. As a result, gamma rays of many different energies are emitted and fairly complex energy spectra are obtained, as Figure C26 shows. The characteristic peaks in the thorium series at 2.62 MeV are caused by the decay of thallium-208 and bismuth-214 respectively. It is generally assumed that formations are in secular equilibrium; that is, the daughter isotopes decay at the same rate as they are produced from the parent isotope. This means that the relative proportions of parent and daughter elements in a particular series remain fairly constant; so, by looking at the gamma ray population in a particular part of the spectrum it is possible to infer the population at any other point. In this way, the amount of parent isotope present can be determined.

Figure C26: Potassium, Thorium and Uranium Response Curves (NAl Crystal Detector) (05/96) C-34

Figure C27 (05/96) C-35

Introduction to Openhole Logging

Once the parent isotope population is known, the amount of nonradioactive isotope can also be found. The ratio of potassium-40 to total potassium is stable and constant on the earth, whereas, apart from thorium-232, the thorium isotopes are rare and so can be neglected. The relative proportions of the uranium isotopes depend somewhat on their environment, and there is also a gradual change because of their different half-lives; at present, the ratio of uranium-238 to uranium235 is about 137.

Applications: - identification of radioactive sands that may be misinterpreted as shales - identification of different types of shales/clays (see Figure C28) - depth correlation (same as GR) - complex lithology analysis.

Figure C28: Classification of Radioactive Minerals as a Function of the Th and K Values

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Introduction to Openhole Logging

C7.0 Borehole Geometry by Caliper Measure C7.1 PHYSICAL PROPERTIES The hole diameter is usually recorded in conjunction with the following surveys: - Sonic logs (BHC versions, ASI Array Seismic Imager, DSI Dipole Shear Sonic Imager) - Microresistivity logs (microlog, Micro-SFL, EPT Electromagnetic Propagation logs) - Litho-Density logs - Dipmeter logs (Dual Dipmeter Formation MicroScanner, FMI Formation MicroImager tools) - Borehole geometry log

The readings given by different calipers in the same hole may be different depending on the caliper design and the hole cross section. Figure C29 shows the characteristics of the different calipers:

No. of Arms

Phasing of the Arms (Degrees)

Sonic tool

3

120

16 in. [406 mm]

Microlog tool

1

0

20 in. [508 mm]

Micro-SFL tool (option A)

1

0

16 in. [406 mm]

Micro-SFL tool (option B)

4

90

22 in. [558 mm]

Density tool

1

0

Short Arm 16 in. [406 mm] Long Arm 21 in. [533 mm]

Dipmeters

4

90

FMS/FMI 22 in. [558 mm]

Borehole Geometry tool

4

90

Standard 30 in. [762 mm] Special 40 in. [1016 mm]

Dual Axis

2

180

16 in. [406 mm]

Caliper tool

Maximum Diameter

Remarks 3 arms coupled 1 reading 1 arm 1 reading 1 arm 1 reading 4 arms coupled 2 2 2 paired readings 1 arm 1 reading 4 arms coupled 2 2 2 independent readings 4 arms coupled 2 2 2 independent readings 2 arms coupled 1 reading

Figure C29: Caliper Specifications for Different Devices Stated on the Logs

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38

1) Mudcake is a good reason to have different calipers reading different values: - If the arm of the caliper is the blade type, it will cut into the cake and this arm will ignore the thickness of the mudcake. - If the arm is of the pad type, it will skid over the cake and the mudcake thickness will be taken into account. 2. Assuming no mudcake, the readings of different calipers in a perfectly round hole will be identical. But holes are not always round. In clearly ovalized holes, two- three- and four-arm calipers will read different hole diameter values, mostly because of the way these arms are coupled together. If the logging tool is fairly free to rotate inside the hole: -Two-arm calipers will ride using the larger diameter of the hole. -Four-arm calipers will ride with one pair of coupled arms using the larger diameter of the hole. 3) In deviated wells, calipers may partially collapse under their own weight and give readings that are too low. The following example (Figure C30) shows different calipers in an ovalized hole:

- The sonic caliper (three arms linked together) shows an average hole diameter. -The density caliper (one arm) is applied on the wall with strength. Its back-up arm will cut into the mudcake. If no small-axis hardware is used, it will orient itself to read the largest diameter. If small-axis hardware is used, the LithoDensity tool tracks the smoother, short axis of the hole (if ovality exists). -The microlog caliper (one arm) will probably orient itself to read the larger diameter. Its pad will skid on any mudcake. This is the case in the upper part and lower part of this section. - Most calipers are designed to record accurate hole diameters in cylindrical boreholes. When boreholes are noncylindrical and depending on caliper configurations, a tool string will orient itself in some preferential direction. This can effect both caliper readings and log responses. Using Figure C31, consider the caliper responses in a 200-  400-mm oval borehole for the various caliper types, configurations and preferred tool orientations. 100 m of 200-  400mm hole has a volume of 6.28m3.

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Introduction to Openhole Logging

Figure C30: Comparison of Various Caliper Responses (05/96) C-40

Single-Arm Caliper Configuration: • records one borehole diameter = 400 mm • calculated 100 m hole volume = 12.57 m3 (+100% error) • tool examples: - Litho-Density log (No short-axis hardware) - MicroSFL tool (option A) - EPT Electromagnetic Propagation tool. Two-Arm Caliper Configurations: a. Unidirectional • records one borehole diameter = 400 mm • calculated 100 m hole volume = 12.57 m3 (+100% error) • tool example: - MicroSFL tool (option B).

b. Bidirectional Long Axis • records one borehole diameter = 195 mm • records a second diameter = 195 mm • calculated 100 m hole volume = 2.9 m3 ( 53%).

c. Bi-directional Short Axis • Records one borehole diameter = 273 mm • Records a second diameter = 273 mm • Calculated 100m hole volume = 5.85m3 (7%). Figure C31: Caliper Responses Under Various Hole Conditions

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Introduction to Openhole Logging

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Three-Arm Caliper Configurations: a. Centered • records one borehole diameter = 260 mm • calculated 100 m hole volume = 5.31m3 (15%) • tool example: - sonic log. b. 90- Degree Offset • records one axis diameter = 200 mm • records a second diameter = 382 mm • calculated 100m hole volume = 6.00 m3 (4%) • tool examples: - CNL Compensated Neutron log - Litho-Density log (short-axis hardware applied). Four-Arm Caliper Configuration: • records one-axis diameter = 200 mm • records a second diameter = 400 mm • calculated 100-m hole volume = 6.28 m3 (0%) • tool examples: - borehole geometry log - Dual-Dipmeter tool - Formation MicroScanner - FMI Formation MicroImager.

Figure C31 (Continued)

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C8.0 Work Session 1a. For the example logs of Figures C32 – C34, calculate the following: (Formation = Sandstone) 581 m

600 m

a. RILD b. Rt c. t d. S e. D f. N

2. Using the sonic log of Figure C34, calculate the sonic porosity at 586 m. tf = 620 sec/m  tma = 182 sec/m t - tma s =

= tf - tma t - tma)

s =

= t

b. Using Chart Por-3m (Figure C6)   s Wyllie Time-Average =  s Field Observation = (05/96) C-44

3a. On the CNT–Litho-Density log of Figure C35, what effect is seen at 1941 to 1946 m?

b. Using the Pe, what is the lithology in this zone?

c. Convert the log readings (N and D) to equivalent sandstone values.

d. Explain the effect identified in question 3a.

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Figure C32 (05/96) C-46

Figure C33 (05/96) C-47

Introduction to Openhole Logging

Figure C34 (05/96) C-48

Figure C35 (05/96) C-49

Contents D1.0 BASIC QUICKLOOK INTERPRETATION .............................................................................................. 1 D1.1 QUICKLOOK METHODS ................................................................................................................... 1 D1.2 METHOD ONE: OVERLAY TECHNIQUE .......................................................................................... 1 D1.3 METHOD TWO: RWA TECHNIQUE ................................................................................................... 2 D1.4 METHOD THREE: DIRECT METHOD OF CALCULATING WATER SATURATION FOR CLEAN ZONES ................................................................................................ 5 D2.0 WORK SESSION ..................................................................................................................................... 9

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D1.0 Basic Quicklook Interpretation D1.1

QUICKLOOK METHODS

Quicklook methods of log interpretation can be classified as those used to identify possible producing intervals, usually at the wellsite. The requirements are to locate permeable beds, calculate bed thicknesses, porosities and saturations of hydrocarbon zones and predict producibility. These generally simplified techniques are not intended as a substitute for more comprehensive methods of interpretations. The methods covered here are 1) overlay technique 2) Rwa 3) direct method of calculating Sw. A note of caution, though, because there are some assumptions that should be considered when using quicklook techniques. The zone should have 1) 2) 3) 4) 5)

constant Rw thick, homogenous formation continuous clean lithology clean-water-bearing zone moderate invasion and of step profile.

D1.2

METHOD ONE: OVERLAY TECHNIQUE

a. Define the clean zones (no clay) on the log with the GR and SP. b. Find a clean, 100%-wet zone on the log: this should have a good SP deflection, low GR, good porosity and low resistivity. c. In the clean, wet zone found in Step (b), overlay the sonic t on the deep resistivity curve. (If no sonic is available use density porosity.) d. Keeping the logs parallel and in the same relative position, trace the deep resistivity curve on the sonic log for the zones found in Step (a). e. Any zone where there is high resistivity relative to sonic porosity (t) has hydrocarbon and should be evaluated further. f. Use the same 100%-wet zone found in Step (b), and overlay the sonic t on the neutron porosity curve. g. Trace the neutron porosity curve on the sonic log for the clean zones defined in Step (a). Make sure the neutron and sonic log stay parallel and in the same relative position.

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Introduction to Openhole Logging

h. In the hydrocarbon zones defined in Step (e), where the neutron porosity decreases and the sonic t increases the zone is gas bearing. All other hydrocarbon zones contain oil. i. On the density porosity log define a cutoff value of porosity based on test and production experience for the area. j. When the density porosity is above this value, the zone will produce fluid. Below the cutoff value, no production will occur. D1.3 METHOD TWO: Rwa TECHNIQUE This technique assumes that all zones are 100% wet, estimates a value for Rw, and subsequently studies the anomalies to the first assumption.

Rt Rwa = F This value will represent Rw for the formation if the assumption that all zones are wet is correct. If the zones are not all at Sw = 100%, the value of Rwa will vary depending upon the actual Sw of the formation. If we select the minimum value of Rwa and call it Rw, then we can make a comparison of all calculated Rwa values against this Rwa (minimum) value through substitution into Archie's equation as follows: FRw Given S

2 w

Consider Archie's equation: aRw S

2 w

=

Rt FRw

If Sw = 100%, then

= m Rt

Rt

Rt Rwa =

Assume: Sw = 100%

F

FRw then

=

or conversely, Rt = FRwa

=1 Rt Rt

Rearrange to solve for Rw: Rw = F Because we assume that all zones have Sw = 100%, we state

Substituting Rwa for Rt yields

(minimum)

FRwa(minimum) S

=

2 w

FRwa Rwa(minimum) or S

2 w

= Rwa

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for Rw, and FRwa

Hence, we can compare the minimum Rwa value against all other Rwa values calculated and compute Sw. To work effectively, this technique requires that we in fact have a zone at Sw = 100% and that Rt or  vary through the zones to be evaluated. Procedure for Rwa Analysis: Problem: Find: Sw given a resistivity log, plus either a sonic, neutron or density log. Solution: This interpretation method is generally suited to sands, where porosity plus resistivity logs are available (refer to Nomograph in Figure D1). - Logs must be zoned so that the formations to be evaluated have reasonably consistent matrix and Rw values. - Calculate a series of Rwa values in permeable zones. Check the Rwa values (see later comments). - When Rwa  3Rw, investigate the zone for possible hydrocarbon presence, because Sw < 58% where Rwa > 3Rw. - If Rw is known, Sw may be calculated by Sw2 = Rw/Rwa. - If Rw is unknown, choose a minimum Rwa value  Rw. Several points should be examined to establish a suitable Rw value (i.e., anomalously low Rw values should be avoided, because they may be due to calcareous streaks or other matrix influences, etc.).

- The general rule for indicating zones of potential hydrocarbons is when Rwa  3Rw (approximate Sw = 58%). When Rmf > Rw, such an Rwa calculation may be due to the influence of invasion on the Rt device in a water sand. To help resolve this problem, an apparent mud filtrate resistivity value (Rmfa) may be computed using a shallow investigation resistivity reading e.g., Micro-SFL, SFL tool and AT-10. R(shallow device) Rmfa = F Quality Checks on Rwa Values: Assuming that Rw< Rmf: 1. If Rmfa  Rwa  Rw, invasion is shallow and Rwa is correct. The zone is water bearing. 2. If Rmfa  Rmf, there is probably some residual hydrocarbon saturation in the flushed zone. This would confirm a hydrocarbon indication on the Rwa curve. 3. If Rmfa  Rmf and Rw  Rwa  Rmf, deep invasion may have occurred. Check favorable Rwa indications further. - Having checked Rwa values and selected an Rw value, proceed to calculate Sw for all zones where Rwa  3Rw (Sw2 = Rw/Rwa). Limitations Limitations of this technique are similar to those for crossplots. The influence of invasion, shale, gas and matrix changes for each device should be recognized.

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Introduction to Openhole Logging

Figure D1 (05/96) D-4

D1.4

All water saturation calculations are based on one form or other of Archie's saturation formula, where: FRw S

n w

PSP

METHOD THREE: DIRECT METHOD OF CALCULATING WATER SATURATION FOR CLEAN ZONES

= Rt aRw =

SSP = 1-Vsh where Vsh is from the GR. d. Water Catalog: This is a summary of DSTs and produced water samples. Some countries have logging societies that publish these catalogs. F - Formation Factor Formation factor may be obtained for Rxo measurements (e.g., Micro-SFL Focused Log, electromagnetic propagation resistivity [EPR]).

mRt By calculating suitable input parameters we can solve these equations for water saturation in shale-free zones. Rw - Formation Water Resistivity An accurate knowledge of Rw is essential but often difficult to obtain. Rw values can be obtained from: a. Production Water Samples: samples should be collected prior to any chemical treatment; measure resistivity and temperature of the sample. b. Drillstem Tests (DSTs): if possible, collect three samples, at top, middle and bottom of the tool. Measure all three resistivities and record temperatures. The sample with the lowest value should be most representative of Rw. c. SP Log: if necessary, bed thickness corrections, etc., should be made prior to calculating Rw. (When shale is present, the SSP may be estimated by PSP).

Rxo F =

Sxo2 Rmf

 - Porosity Porosity may be obtained from neutron, density, sonic or a combination of these devices. Rt - True Resistivity True resistivity may be obtained from ILD, IDPH or LLD; any borehole and invasion corrections should be applied to the raw readings to obtain Rt. Chart Sw-1a (Figure D2) is a convenient method of solving this formula. However, note that the F versus  relationship used is F = 1/2. If any other relationship is used, F must be calculated before entering the chart. Remember, knowledge of formation water resistivity is essential for making an accurate interpretation.

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Introduction to Openhole Logging

Saturation Determination (Clean Formations - Humble Relationship)

This nomograph solves the Archie water saturation equation Sw =

R0 = Rt

FrRw Rt

It should be used in clean (nonshaly) formations only. If R0 (resistivity when 100% water saturated) is known, a straight line from the known R0 value through the measured Rt value gives saturation, Sw. If R0 is known, it may be determined by connecting the formation water resistivity, Rw, with the formation resistivity factor, FR, or porosity, Ø Example:

Rw = Ø = Rt = Thus, Sw

0.05 Ω.m at formation temperature 20% (FR = 20) 10 Ω.m = 31.6% Chart Sw-1a Figure D2

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Saturation Determination (Clean Formations - m = 2)

This nomograph solves the Archie water saturation equation Sw =

R0 = Rt

FrRw Rt

It should be used in clean (nonshaly) formations only. If R0 (resistivity when 100% water saturated) is known, a straight line from the known R0 value through the measured Rt value gives saturation, Sw. If R0 is known, it may be determined by connecting the formation water resistivity, Rw, with the formation resistivity factor, FR, or porosity, Ø Example:

Rw = Ø = Rt = Thus, Sw

0.05 Ω.m at formation temperature 20% (FR = 20) 10 Ω.m = 31.6%

Chart Sw-1b Figure D3 (05/96) D-7

Introduction to Openhole Logging

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Contents

E1.0 SHALY FORMATIONS ............................................................................................................................ 1 E1.1 INTRODUCTION ................................................................................................................................ 1 E1.2 POROSITY IN SHALY FORMATIONS ............................................................................................... 3 E1.3 EVALUATION OF SHALE VOLUME (VSH) ......................................................................................... 4

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E1.0

Shaly Formations

E1.1 INTRODUCTION Shales are one of the most important common constituents of rocks in log analysis. Aside from their effects on porosity and permeability, this importance stems from their electrical properties, which have a great influence on the determination of fluid saturations. Archie's water saturation equation relating formation resistivity to water saturation, assumes that formation water is the only electrically conductive material in the formation. The presence of another conductive material (e.g., shale) requires changes to either Archie's equation or the model relating resistivity to water saturation. As well, the presence of clay in the formation complicates the concept of porosity. The water associated with the clays can represent a significant amount of porosity. However, this porosity is not available as a potential reservoir for hydrocarbons. To this point, we have dealt with tool responses from our porosity devices that yield total porosity T. At this time we have to introduce a new term, effective porosity, e, which is that portion of the formation porosity available to contain and produce fluids.

The presence of shale in formations generally affects the response of the logging devices. In our discussions we usually speak of shaly sands; however, the presence of shale in carbonates can often be treated in a similar manner. As briefly mentioned before, we categorize the distribution of shaly material in formations in three possible ways (see Figure E1): 1) Laminar Shale: occurs when shale exists in the form of laminae or thin layers between thin layers of sand. The shale streaks do not actually influence the effective porosity of the sand layers in the formation; however, as the bulk volume of shale increases, the overall formation porosity decreases. The presence of the shale may have considerable influence on the logging tool responses. 2) Structural Shale: is defined as the type of shale that exists as grains or nodules in the formation matrix. It is considered to have properties similar to laminar shale.

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3) Dispersed Shale: occurs where the shaly material is dispersed through the sand to occupy part of the intergranular space. Dispersed shale reduces the pore space available for fluid accumulation and also reduces formation permeability. The evaluation of shaly sands requires that we assume some distribution model. With the advent of computers we can analyze formations on the basis of sedimentation principles. Here we determine the silt and wet clay content of the shale; the former is a maximum near the main sand body (high-energy deposition) and the wet clay becomes predominant

as distance from the main sand body increases (low-energy deposition). When shales consist of wet clay and silt, the bulk volume fractions may be expressed as: Vsh = Vsilt + Vclay Another commonly used expression is the silt index (Isilt) where Isilt = Vsilt/Vsh also Vclay = Vsh (I – Isilt).

Figure E1: Forms of Shale Classified by Manner of Distribution in the Formation Pictoral Representations Above, Volumetric Representations Below

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E1.2

POROSITY IN SHALY FORMATIONS When a sand contains shale we cannot obtain an accurate value of effective porosity from one porosity log. The responses of the density and neutron logs to shale content in sands is considered to be the same as in nearby bedded shales, no matter what model of shale distribution is considered. On the other hand, sonic logs have quite a different response between laminated-structural and dispersed shales.

b) Neutron (CNL/SNP) Logs - Neutron tools respond to the amount of hydrogen in the formation. Because shales contain bound water, the porosity recorded by neutron devices in shaly sands is always higher than the effective porosity.

a) Density Logs - When shale and sand matrix densities are close to each other, the density log is least affected by shale and reads close to the effective porosity. - When the shale matrix density is less than 2650 kg/m3 the density log in shaly sands will record porosities higher than the effective porosity. - When shale matrix density is greater than 2650 kg/m3, the density log in the shaly sands will record porosities lower then the effective porosity. - The relationship for liquid-filled shaly sands can be written as

c) Sonic Logs - Sonic traveltime in shales rises because of the fluid content of the shales; hence, sonic porosities in shaly formations are always higher than the effective porosity. To further enable sonic porosity determination, we must also know what shale model is present, and also whether a compaction correction is necessary. - In compacted formations with shales present, a general sonic relationship may be written as

b = fe +ma (1 - e – Vsh) + sh Vsh or b = (1 – e)ma + ef + Vsh(sh – ma)

- In liquid-filled shaly sands, the neutron relationships may be written as N = e + Vsh(Nsh )

tlog = e – Vsh)tma + (Vlam + Vstr)tsh + e – Vdis)tf - In uncompacted zones, sonic porosities derived from this relationship must also be corrected downward for the lack of compaction.

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E1.3

EVALUATION OF SHALE VOLUME (Vsh) Basic methods of shale (clay) volume calculation use the following indicators: - Gamma ray - NGS tool - Spontaneous potential - N versus D crossplot N versus S crossplot a) Gamma Ray If the radioactivity of the shale content is constant and if no other mineral in the formation is radioactive, the gamma ray reading may be expressed as a function of clay content. The formula can be written as GRzone – GRclean †Vsh = GRshale – GRclean

b) NGS Natural Gamma Ray Spectrometry Tool By using only thorium and potassium components of the gamma ray signal, the radioactive uranium element not associated with shales will be eliminated. The same method is then applied to the NGS as that for a regular gamma ray. CGRzone - CGRclean †Vsh = CGRshale - CGRclean These formulae will not hold true for zones that contain radioactive matrix materials or radioactive waters (e.g., granite wash sands). Similarly, this method will not hold true where nonradioactive shales occur. Some typical values for formations are - Clean Sandstone: GR = 15–30 API - Clean Carbonates - Dolomite: GR = 10–20 API - Limestone: GR = 8–15 A.P.I. - Shallow Cretaceous Shale: GR = 100–140 API †Strictly speaking, all GR values should be corrected for borehole effect and formation density. However, this approximation is usually satisfactory.

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c) Chart Calculation The linear equations in (a) and (b) of this section are good first estimates of shale volume. Chart Vsh-1 (Figure E2) allows us to correct for the non-linear relationship between Vsh and the GR deflection denoted as “x”. Line (1) is generally used, yielding good interpretation results.

d) Spontaneous Potential (SP) In waterbearing sands of low to moderate resistivity, the ratio of SSP (static SP) to PSP (pseudostatic SP) is indicative of clay content, where  = PSP/SSP and Vsh = 1 -  If hydrocarbons are present,  will be decreased because of the further reduction of PSP by the hydrocarbons. Also, when using this method to calculate Vsh, suitable bed thickness must be present to obtain PSP and SSP.

Figure E2: Chart Vsh- 1: Shale Model Correction

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Introduction to Openhole Logging

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