12-petrophysics Basics [compatibility Mode]

Applied Reservoir Geology

Chapter 12 Basics of Wireline Logging & Interpretation

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Applied Reservoir Geology

What you will learn What logging means Different measurements we make Basic wireline tools …and what they measure Simple log analysis

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Applied Reservoir Geology The Early Years— 1912–1927

1912: Conrad conceives the idea for electrical measurements 1919: Marcel joins his brother–first work in Normandy 1921: Office opens in Paris, rue Saint–Dominique 1927: First electrical downhole log in Pechelbronn, France Copyright 2009, NExT, All rights reserved

Applied Reservoir Geology

First well logs recorded in 1927 The recording system

The cable winch

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The stationary pointby-point log

Applied Reservoir Geology Modern Logging Truck Modern Surface equipment : High powered computers Controls downhole logging Changes signal configuration to obtain acquisitions Includes surface database to optimise results and for well-to-well correlations Used also for forward-modelling Includes also all the well configurations- depth, casing, formations, etc..

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Applied Reservoir Geology Logging Tools

Modern Tools Sensors used in modern logging: Electrical Electromagnetic Magnetic Flux Induction Acoustic Ultrasonic Nuclear: Neutron Nuclear:

γ- Rays

Nuclear: Nuclear Magnetic Resonance Imaging (MRI) Every potential signal source have been used in modern-day logging Copyright 2009, NExT, All rights reserved

Applied Reservoir Geology

Modern Logs

1600

1700 1 50

-1 8 0

(M V )

2 00

FX N D 50

(P U )

0

Facies

S P (S P ) 0

R t from H A L S 180

1

5.0 0 7.7 5 12 .01 18 .62 28 .85 44 .72 69 .81 1 07 .43 1 66 .51 2 58 .08 4 00 .00

R X 18 1

1 00 0

R t from A IT H 1

(O H M M )

10 0 0

M ud R e s istiv ity from H A L S 1

10 0 0 90 M ud R e s istiv ity fro m A IT H

1

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A H TP R 10 0 0

(O H M M )

1 00 0

Invasion Profile

(G A P I)

Layering

0

Pad

1:2 20 F t

G am m a R a y (G R )

0

90

Modern logs have more measurements but the principle is the same Shading is often added to make the log curves easier to read. Additional outputs can be made: Invasion Profiles Facies Layering

Applied Reservoir Geology Open Hole Measurements : Wireline Logging. LWD (Logging While Drilling) Logging on Drill Pipe (TLC)

Wireline

LWD TLC

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology

• Lithology (reservoir rock?) • Resistivity (HC,water,both?) • Porosity (how much HC?) • What type of HC

• Formation mech. properties • Permeability / cap pressure • Shape of the structure • Geological information • Geothermal • Unconventional applications

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Why we log ?

Applied Reservoir Geology

Review of Basic Logging Tools

• Spontaneous Potential (SP) and Gamma Ray (GR) • Resistivity • Neutron • Sonic • Density

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Applied Reservoir Geology

Log Measurements

Type log

Direct Measurement Self-Potential (SP) mV

Indirect Measurement Shaliness

Gamma-Ray (GR)

API units

Shaliness

Caliper

Hole diameter

Acoustic

Travel time

Various corrections Porosity

Density

Bulk density

Porosity

Neutron

Hydrogen index

Porosity

Induction/laterolog Resistivity

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Water saturation

Applied Reservoir Geology

Spontaneous Potential (SP)

• Measures the electrical potential in the formation caused by the salinity difference between the drilling mud and the formation water • SP is generally an indicator of permeability The SP log measures the electrical potential in the formation. This is a relative measurement. The deflection on the SP log is measured from the shale to the sand. The amount of deflection that you see between the shale and the sand is a relative amount of deflection. The log analyst does not read the value of the SP log directly from the log. Rather, it is the difference between the shale reading and the sand

.

reading

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Applied Reservoir Geology

SP Log 001) BONANZA 1 GRC 0 SPC -160 MV

ILDC 150

0.2

10700

SP Log 10800

10900

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1.95

200

CNLLC 0.45 -0.15

SNC 40

0.2

16

0.2

ACAL 6

RHOC 200

MLLCF 200

2.95

150

DT us/f

50

Applied Reservoir Geology

GR – Gamma Ray

The GR Log GR is the measurement of the natural radioactivity of the formation In sedimentary formation; this reflects the presence of shale Radioactive elements tend to concentrate in shales. Clean (Shale-free) formations usually have low level of radiation

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Applied Reservoir Geology

GR – Gamma Ray Gamma Rays are bursts of high-energy electromagnetic waves that are emitted spontaneously by some radioactive elements. Nearly all the Gamma Radiation encountered on Earth is emitted by: Potassium (K) Thorium (Th) Uranium (U)

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Applied Reservoir Geology

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GR – Gamma Ray

Applied Reservoir Geology

Resistivity Resistivity Theory

Current can only pass through the water in the formation, hence the resistivity (Rt) depends on: Resistivity of the formation water (RW ) Amount of water present (Ø and SW) Pore structure (F) This defines the tortuousity and throat radii of the current path. Copyright 2009, NExT, All rights reserved

Applied Reservoir Geology

Resistivity

Increasing Oil Saturation

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Effect of decreasing Sw on the measured Resistivity

Applied Reservoir Geology

The Resistivity Log Bonanza #2 09/13/2003 3:57:45 PM

Resistivity Logs can be of two types:

DEPTH FT 0.

GR(GAPI)

ILD(OHMM) 150. 0.2

SPC1 (MV) -100.

SN(OHMM) 0. 0.2

CALI (INCH) 1:500 6.

1. Induction Logs (shown here) 2. Laterologs Both measure resistivity, but use different physical methods. Laterologs cannot be used in OilBased Muds Three measurements usually made: 1. Shallow (mud filtrate) 2. Medium 3. Deep (true resistivity)

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RHOB (GC3) 2000. 1.7

DT2 (US/F) 2.7 150.

50.

NPHILS (dec) 2000. 0.6

0.

MLL (OHMM) 16. 0.2

2000.

10700

10800

10900

25

Applied Reservoir Geology

Porosity

3 porosity logs - acoustic, density, neutron • All read the same if: – lithology known – shale free – 100% water • Porosity calculation is complex - must take into account lithology, shale, and fluid type • Calibrate with core data - note scale difference

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Applied Reservoir Geology

FDC – Density Log

The density logging tool measures the formation density and formation lithology. The effects of borehole, mud, poor pad-formation contact is compensated for digitally. Gamma rays lose their energy when they collide with electrons (Compton Scattering) By measuring the number of gamma rays and their energy levels at a given distance from the source, the electron density of the formation can be predicted.

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Applied Reservoir Geology

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Applied Reservoir Geology

CNL – Neutron Log Neutron Tools: Principles Neutron tools emit high energy neutrons and measure the response of these neutrons as they interact with the formation, or in many cases, the fluids within the formation. This measured response is affected by the quantity of neutrons at different energy levels and by the decay rate of the neutron population from one given energy level to another. A neutron interacts with the formation in a variety of ways after leaving the source, it is the aftermath of these interactions that is detected by the tool.

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Applied Reservoir Geology

CNL – Neutron Log

Example of standard CNL - NEUTRON LOG STANDARD DISPLAY OF COMPENSATED NEUTRON LOG (CNL) - Basic Quality Control: Neutron Porosity values should be taken with care in front of bad hole - washout - values might read too high. CNL is usually run in combination with LDT. Zones of poor density readings are usually identical with poor neutron porosity readings.

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CNL – Neutron Log Clean Sand Formation Porosity: Neutron Matrix Correction (Chart)

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31

Applied Reservoir Geology

CNL – Neutron Log

Typical NeutronDensity Response

Note: scale is LIMESTONE compatible Copyright 2009, NExT, All rights reserved

Applied Reservoir Geology

BHC – Sonic Log

Basics of sonic tool

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The sonic tools create an acoustic signal and measure how long it takes to pass through 1’ of rock.




By simply measuring this time we get an indication of the formation properties.




The amplitude of the signal will also give information about the formation. 33

Applied Reservoir Geology

BHC – Sonic Log

Wyllie time-average equation

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34

Applied Reservoir Geology

BHC – Sonic Log

Sonic Log measures interval transit time. The higher the number, the slower the time – and the more porous the formation (sound travels quicker through more dense materials – porosity will slow it down)

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Applied Reservoir Geology

Wireline Log Interpretation

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology

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Applied Reservoir Geology shales

Shale Distribution in a reservoir Structural shale : where the shale grains replace some of the sand grains. In this case the matrix density changes but the porosity does not alter. Laminar shale : Thin layers of shale in the matrix, replacing both matrix and porosity. There are hence changes in matrix density and the porosity. Dispensed shale : The clay mineral fills in the intergranular space i.e.. it changes the porosity leaving the matrix density untouched.

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Applied Reservoir Geology Clean (Shale-Free) Formation Water / Hydrocarbon

Porosity (φ )

Oil Water

Matrix

Usually Good Permeability Relatively: High Porosity Easy to interpret and model Copyright 2009, NExT, All rights reserved

Matrix (sand, Limestone, Dolomite, Mixture)

Applied Reservoir Geology Shaly Formation Water / Hydrocarbon

Porosity (φ ) Oil Water

Shale Shale Matrix

Usually Poor Permeability Relatively: Lower Porosity Difficult to interpret and model Shale disguises thin reservoir beds in shale beds Plays a critical role in producing the reservoir Copyright 2009, NExT, All rights reserved

Shale Matrix (sand, Limestone, Dolomite, Mixture)

Applied Reservoir Geology

Why bother computing Vsh?

50 0hm-m

Sw= 25%

Sw= 25% Copyright 2009, NExT, All rights reserved

Increasing Vsh

Sw= 25%

Effect of Increasing Vsh on the measured Resistivity

Applied Reservoir Geology

Shales and appearance on Logs

Shales have properties that have important influences on log readings: Shales have porosity- but no appreciable permeability. The porosity is filled with conductive water. Shales are often radioactive (Thorium and Potassium). Resistivity logs show shales as low resistivity zones.

The Gamma Ray reads the high value in the shale (usually). Resistivity logs react to the water filled porosity of the shale as well as the electrical properties of the rock. This gives a low resistivity value for this rock. Copyright 2009, NExT, All rights reserved

Applied Reservoir Geology

Shales and appearance on Logs

Neutron porosity logs exhibit shales as high porosity. Density and sonic logs react to the porosity and matrix changes (grains). Gamma ray logs react to shale radioactivity.

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Applied Reservoir Geology

Shale Corrections

The electrical properties of shales greatly influence the calculation of fluid saturations. A layer of water close to the clay surface is electrically charged. Archie's equation assumes that the formation water is the only electricallyconductive material in the formation. The clay layer requires an additional term in the saturation equation. Porosity tools can be corrected for the shale effect. An "effective porosity" Фe can be computed as compared to a "total porosity" Фt which includes the shale effect.

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Applied Reservoir Geology

Shales and appearance on Logs

Vsh = Copyright 2009, NExT, All rights reserved

GR (zone) - GR (clean) GR (shale) - GR (clean)

Applied Reservoir Geology

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Shales and appearence on Logs

Applied Reservoir Geology

The Invasion process

Progressive invasion Mudcake is formed from solids in mud This creates an impermeable barrier Although Phydraustatic > Pformation little no invasion will take place

Progressive filtrate invasion and mud-cake build-up Copyright 2009, NExT, All rights reserved

Applied Reservoir Geology

The Invasion process

Formation Resistivity

Rxo

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Rt Virgin Zone

Borehole mud

Invaded Zone Filtrate filled

Transition Zone

Mud

Mud cake

BOREHOLE

Applied Reservoir Geology

The Invasion process Resistivity of zone Resistivity of the water in the zone Water saturation in the zone

Mud Rm

Adjacent bed Rs

hmc Rmc

Flushed zone

dh

(Bed thickness)

Mudcake

h

Uninvaded zone Zone of transition or annulus

Rxo

Rt Rw Sw

Rmf Sxo di dj (Invasion diameters) ∆rj

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dh Hole diameter

Rs Adjacent bed

The invasion process creates a zone where the main water is filtrate This invaded zone also has less HC than the virgin zone This fluid displacement is an indication of fluid mobility

Applied Reservoir Geology

Determination of Water Saturation

Archie’s Equation (uninvaded formation)

  a R n w   Sw =  φm R  t  Sw = Water Saturation Rt = Formation Water Resistivity o = Porosity Copyright 2009, NExT, All rights reserved

m is the tortuousity factor controlling the passage of current in the formation. This usually varies in the range 1.2 to 6.0 Sometimes an “a” term is used. This is done indirectly to account for the variation in m n is the saturation exponent: this is a function of Wettability (high for oil-wet, lower for water-wet) Usually m = n = 2 is used

Applied Reservoir Geology

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Identifying Hidrocarbon zones

Applied Reservoir Geology

Water Sample Analysis

The water’s ability to conduct electricity is a function two major factors: V

Water Salinity Formation water or filtrate

I

As salinity increases, more ions are available to conduct electricity so Rw (water resistivity) decreases. The resistivity, and hence the salinity, can be measured at the surface if a water sample is available.

Water Temperature As water temperature is raised, ionic mobility increases and resistivity decreases. Copyright 2009, NExT, All rights reserved

Applied Reservoir Geology

Log Rt vs Log Porosity CrossPlot

Log Rt vs. Log Porosity Crossplot

Rt

10

5- Rw from Cross-plots Rt =

HC Direction

1

Rt =

Rw

φ2 .

Sw2

Rw

φm

Log Rt = Log Rw - m log

A cross-plot of the above equation, on a log-log scale will give the following: Rw= 0.021 A slope= m An intercept on the Rt axis is 0.01 equal to Rw (for100% porosity)

0.1

Best fit line In the South-West Direction

1 Copyright 2009, NExT, All rights reserved

10

φ

φ

100

Applied Reservoir Geology

Lithology

Lithology could fall in one of these categories: Single Rock Lithology Single Lithology + Shale Two or more Lithologies Two or more Lithologies with shale

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shale

Applied Reservoir Geology

Cross-plots and their Applications

This is a classical example of using z-axis plot The z-axis here is the Gamma Ray, which is an indicator of shaliness. Higher red-colour intensity signifies a higher value of GR on the z-axis, which in turn, indicates an increase in the volume of shale (Vsh).

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Applied Reservoir Geology

Density-Neutron Cross Plot A Density-Neutron cross-plot in a carbonate reservoir. The matrix is a Ls-Dol mixture. This example explains how to compute the porosity and then the lithology for every log point. 1.

Porosity= 17 pu Lithology: Vdol= 30% Vlim= 70%

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Porosity= 24 pu Lithology: Vdol= 80% Vlim= 20%

2. 3.

Draw equal porosity lines between SST-LST-DOL lines Plot points Estimate for red points – porosity and lithology %

Applied Reservoir Geology

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Steps to achieve a Quick evaluation