5a. Resistivity Logging

RESISTIVITY LOGGING

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CONTENTS • What is electrical logging? • Principle • Single Point resistance logs(SPR) • Conventional Resistivity Logs • Response of Potential and gradient Logs over thin and thick conductive and resistance formations • Limitations of conventional resistivity tools • Focused resistivity logs • Advantages of focused resistivity tools over conventional resistivity tools. 2

CROSS SECTION OF THE BOREHOLE

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TOP VIEW OF BOREHOLE

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WHAT IS RESISTIVITY LOGGING? • The electrical resistivity of a substance is its ability to impede the flow of electrical current through it. its unit is the ohm-m. It is a fundamental inherent property of substance(metal, fluid, mineral or rock). • The measurement of formation resistivity is of particular importance for the evaluation of hydrocarbon saturation, particularly in the virgin, non-invaded portion of the reservoir. 8

HOW ELECTRICAL RESISTIVITY IS MEASURED? • Playing with the number of electrodes and the spacing between them one can measure the resistivity : - either at great depth (up to several feet beyond the borehole) giving a measurement inside the virgin zone of the reservoir (called true resistivity, R,) not too much affected by the invasion of mud filtrate, - or close to the borehole (called flushed-zone resistivity ,Rxo) where mud filtrate has largely replaced the original pore fluid. 9

HOW ELECTRICAL RESISTIVITY IS USEFUL? • Combination of the resistivity data with porosity measurements and formation-water resistivity knowledge allows the computation of the water saturation in both shallow and deep zones. • Comparison of these two saturation values gives an idea of the fluid mobility and, consequently, allows the evaluation of the reservoir producibility. 10

TECHNIQUES USED TO MEASURE RESISTIVITY • There are several techniques in use for measurement of the resistivity. All are variations of a common basic system: • one (or several) emitter (electrode) sends a signal (electrical current) into the formation. • One (or several) receiver (electrode) measures the response of the formation to this signal at a certain distance from the emitter. • Generally, an increase in the distance between emitter and receiver (called spacing) results in an improved depth of investigation (and a reading nearer to RJ, at the expense of vertical resolution. • The depth of investigation is defined as the point at which half the signal comes from the invaded zone and half from the uninvaded zone (J = 0.5). 11

FUNCTION OF THE SPACING VALUE ONE CAN CONSIDER TWO MAIN CATEGORIES OF TOOLS: • Long-spacing devices or macro-devices,

• Short spacing devices or micro-devices or micro tools,

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LONG-SPACING DEVICES OR MACRO-DEVICES • These devices are one medium to deep readings, include:

- the conventional electrical survey (ES), with normal and lateral (or inverse) electrode arrays, practically no longer in use in modern logging acquisition techniques; - the laterologs which have replaced the ES tools. 13

LONG-SPACING DEVICES OR MACRO-DEVICES CONTD… • They constitute the focused devices as they have guard electrodes which focus the current emitted by the central electrode. They are less sensitive to the borehole influence. Depending on the spacing and the nature of the focusing, either , may make the predominant contribution to the measured signal, under average conditions of invasion. 14

SHORT SPACING DEVICES OR MICRODEVICES OR MICRO TOOLS • These tools measure shallow readings. • All are mounted on pads which are applied against the borehole wall by spring. They are designed to read , by virtue of their short spacing and their very shallow depth of investigation.

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SHORT SPACING DEVICES OR MICRODEVICES OR MICRO TOOLS CONTD.. • There is very little borehole fluid effect, but the mud cake contributes a small signal. The Schlumberger micro tools include: • ML - the microlog (micro-normal and micro-inverse); • MLL - the microlaterolog (not to be confused with the • PL - the microproximity log; • MSFL- the micro spherically focused log; • The vertical definition obtained with these electrode tools is much finer than with the longer spacing's. Like the ES, the ML is the only non-focused system in the group. 16

WHY COMBINATION OF THESE TOOLS IS IMPORTANT? • A combination of deep-, medium- and shallow reading tools enables us to evaluate

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MEASUREMENT OF RESISITIVITY • THE CURRENT WILL RADIATE UNIFORMLY IN ALL DIRECTIONS, AND THE EQUIPOTENTIAL SURFACES WILL BE CONCENTRIC SPHERES CENTERED ON A. IF THE POTENTIAL AT DISTANCE R FROM A IS V(R), THEN THE DIFFERENCE BETWEEN TWO EQUIPOTENTIALS APART IS:

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THE NORMAL CONFIGURATION. ON THE LEFT: SCHEMATIC VIEW OF THE NORMAL DEVICE PRINCIPLE. ON THE RIGHT: THE REAL CONFIGURATION.

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NON-FOCUSED LONG-SPACING TOOLS • This type of tool is no longer in use because of its drawbacks. But, it is important to review the basic measurement principle in order to better understand how they worked and the interest of the focused tools.

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THE NORMAL DEVICE PRINCIPLE • The measuring electrode M is situated close to the current electrode A, . A constant current I flows from A to the remote return B. The potential VM of M is measured with respect to a reference electrode N (at zero potential) by means of a voltmeter. Although, theoretically, N should be on surface (at "infinity"), inductive phenomena necessitate placing it downhole, but at a distance from M considerably greater than is A

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THE LATERAL CONFIGURATION. ON THE LEFT: SCHEMATIC VIEW OF THE LATERAL DEVICE CONFIGURATION. ON THE RIGHT: THE REAL CONFIGURATION

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THE LATERAL CONFIGURATION. ON THE LEFT: SCHEMATIC VIEW OF THE LATERAL DEVICE CONFIGURATION. ON THE RIGHT: THE REAL CONFIGURATION • In the lateral configuration , two measuring electrodes, M and N, are placed close together below A.The difference AV between the spherical equipotential surfaces on which M and N lie, is derived as follows: VM is the potential at electrode M, VN is the potential at electrode N. So, the potential difference is

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THE CURRENT PATH • The resistivity measured is not exactly that of the virgin formation because it is neither infinite in extent, nor homogeneous. Heterogeneity is introduced by the presence of a fluid-filled borehole, the invaded zone , and adjacent formations in the sedimentary series. • The current traverses these zones, the equipotential surfaces are no longer spheres, and eqs previously discussed are no longer strictly valid.

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THE CURRENT PATH • A general relationship still holds, where: , is the apparent (i.e. measured) resistivity; is equal to V for the normal array, for the lateral and inverse; and K is the geometrical coefficient, for the spacing and configuration used. • After correction for the effects of the borehole for instance), a good approximation to R, can be made using the concept of the pseudogeometrical factor ( J ), considering the invaded and virgin zones to be in series electrically:

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MEASURING POINT (DEPTH ZERO) The normal •The depth reference point is taken at the middle of the spacing. For the normal, this corresponds to the mid-point of AM. Thus for the 16" and 64" normals, which share a common A electrode, the measuring points differ. This is corrected optically on the film. The lateral and inverse : •The measuring point is at 0, the mid-point of MN (lateral) or AB (inverse).

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RADIUS OF INVESTIGATION The normal: In a homogeneous isotropic medium, it is easy to show that 50% of the potential drop towards zero occurs within a sphere of radius AM, 90% within a radius 10 AM. So for the 16"-normal, only 10% of the signal comes from the formation beyond 160" from A. •The radius of investigation (at which an arbitrary 50% of the potential drop has occurred) is therefore equal to twice the spacing AM (Fig). Similarly, the vertical resolution is 2 AM.

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RADIUS OF INVESTIGATION OF THE NORMAL A) THE ELECTRODE LAYOUT IN THE WELL-BORE. B) THE CONTRIBUTIONS FROM THE VARIOUS ZONES, AS A FUNCTION OF DISTANCE FROM THE SONDE AXIS

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RADIUS OF INVESTIGATION • Lateral and inverse: The radius of investigation of the lateral is approximately equal to AO, with most of the signal deriving from the farthest part of the sphere . For the inverse, it is MO

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RADIUS OF INVESTIGATION OF THE LATERAL. A) THE ELECTRODE LAYOUT IN THE WELL-BORE. B) THE CONTRIBUTIONS FROM THE VARIOUS ZONES, AS A FUNCTION OF DISTANCE FROM THE SONDE AXIS

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ENVIRONMENTAL CORRECTIONS

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THE SHAPE OF THE APPARENT RESISTIVITY CURVE As well as influencing the magnitude of the apparent resistivity, the environmental factors discussed above affect the shape of the log response to a bed of finite thickness.

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THE NORMAL • Thick resistive beds (h > AM) The curve is symmetric about the middle of the bed. However, the points of inflection (p and p') on the lower and upper slopes give an apparent bedthickness shorter than true by a distance equal to AM. The peak value of R, will depend on borehole and invasion effects (and, to a lesser extent, bedthickness unless h >>AM).

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THE NORMAL CONTD.. • Thin resistive beds (h < AM) The response resembles an apparently conductive bed, with two small resistive shoulders (c and d), . The shoulders are spaced h + AM apart. This inverted response is stronger for higher resistivities.

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THE NORMAL CONTD… Very resistive thick beds (h > AM) •This is the case of anhydrite, salt or tight carbonate beds. If reference electrode N is on the surface, the response is a symmetrical bell-shape . If N is downhole (on the bridle, 18'8" from M) the curve is triangular in form

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THE NORMAL CONTD… Conductive beds Apparent bed thickness (points of inflection) is h + AM.The response is symmetrical . The thinner the bed, the harder it becomes to distinguish it from the adjacent beds.

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THE LATERAL AND INVERSE Thick resistive beds (h > AO) •The curve is not at all symmetrical with respect to the bed and can take one of several rather complex forms . When M and N (lateral) enter the bed (zone I)only a small difference of potential is measured because most of the current is reflected into the adjacent shale. As A enters the bed, a resistivity reading quite close to true value (allowing for borehole and invasion effects) is obtained (zone 2). As M and N leave the bed, the potential difference falls rapidly (after a small increase) to a value a little higher than that of the adjacent bed (zone 3), until A has also passed through.

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THE LATERAL AND INVERSE Thin resistive beds (h
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THE LATERAL AND NORMAL • Very resistive thick beds (h rel="nofollow">> AM) shows the unsymmetrical triangular response. the upper boundary appears displaced downwards by a distance equal to AO

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THE LATERAL AND NORMAL Conductive beds The unsymmetrical response produces an apparent bed thickness too large by a distance A0

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NON-FOCUSING SHORT-SPACING TOOLS • The knowledge of the resistivity of the flushed zone, is important for several reasons. When the invasion of the reservoir is moderate to deep, the knowledge of allows the correction of the deep resistivity measurements for the influence of the invaded zone. • Some methods for computing water saturation require the knowledge of the ratio. In clean formation, a computation of the formation factor, F, and consequently of the porosity, , can be achieved from knowing assuming to be estimated or fixed to 100%. • To measure the tool must have a very shallow depth of investigation because the flushed zone may extend only a few inches beyond the borehole wall. Since the reading should not be affected by the borehole mud, a sidewall-pad tool is used. 44

PRINCIPLE OF NON-FOCUSING SHORT-SPACING TOOLS • Three electrode-buttons, spaced 1 in. apart, are mounted in line on the face of an oil-filled rubber pad With these electrodes a 1in. by 1in. micro-inverse and a 2-in. micro-normal recorded simultaneously. As drilling fluid filters into the permeable formations, mud solids accumulate on the borehole wall and form a mud-cake. Usually, the mud-cake resistivity is higher than the mud resistivity but much lower than the flushed- or virgin-zone resistivity,

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PRINCIPLE OF NON-FOCUSING SHORT-SPACING TOOLS • The pad-face is pressed against the borehole wall either by a hydraulically controlled spring pressure system in the oldest tools, or by a micro-focused device such as microlaterolog or Proximity pad. Newer microresistivity equipment of Schlumberger includes a microlog tool and a MicroSFL tool. In modern tools, the microlog can be mounted on the powered caliper device and run simultaneously with any combination logging devices: natural gamma-ray spectrometry, litho-density, neutron, electromagnetism. The vertical resolution is 2 to 4 in. for the micro-inverse,and 4 in. for the micronormal. The sampling rate is each 2 in. displacement.

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ENVIRONMENTAL EFFECTS • If the pad is in perfect contact with the hole wall, the borehole fluid has no effect on the reading. However, these very shallow measurements are very sensitive to the mud-cake, across which the current must flow opposite permeable, porous beds. • In rugose holes, the pad makes irregular contact. There may be mud between the electrodes and the uneven wall of the hole, causing erroneously low measurements. • The vertical definition is very fine, and adjacent beds will only affect the readings when bed thickness is less than a few inches.

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TOOL RESPONSE • In permeable reservoirs, as the 2-in. micronormal device has a greater depth of investigation than the microinverse, it is, therefore, less influenced by the mud-cake. When mud-cake is present, and , the 2-in. micronormal reads a higher resistivity, which produces a distinctive “positive” curve separation with the micro-inverse. These “positive” separations are indicative of a porous permeable zone . This is because the depths of investigation of the two tools (and therefore the mud-cake effect) differ. However, negative separations will be observed when or when invasion is very shallow (with 48

TOOL RESPONSE Shaly formations There is generally no mud-cake, and the separation is negative.

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TOOL RESPONSE • Tight formations In impervious formations, in the absence of mudcake and invasion, the two micro-normal and micro-inverse curves are, in principle, measuring the high R,. However, the readings are very sensitive to the presence of mud-filled fissures, fine conductive strata, and poor application, and any such anomalies will show up very clearly.

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TOOL RESPONSE

• They can sometimes exhibit some “negative” separation with much higher resistivities. A “mud-log” was usually recorded while running into the hole, with the pad arms retracted. Both measurements would jump erratically as the tool made intermittent contact with the hole wall on the way down. The minima on the shallow-reading micro-inverse (corresponding to when the tool was farthest from the wall) were taken as indicative of

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FOCUSING LONG-SPACING TOOLS • Focusing resistivity tools overcome, to a greater or lesser extent, the following short-comings of the ES-type tools. The latters can be greatly affected by the borehole and the adjacent formations. • In thin beds ( h of the same order as the spacing) the apparent resistivity is a poor estimate of the true value, because of the influence of the adjacent beds. - The borehole (mud-column) and invaded zone signals are often appreciable. - The available correction charts or equations go some way towards correction for the environmental effects, but are rarely 100% effective. - Bed boundaries are difficult to define precisely. • The current path of a focused tool is constrained to flow in a desired direction. This is achieved using guard electrodes as in laterolog or spherically-focused type tools. 52

PRINCIPLE • It consists in forcing the current I,, sent by the central electrode A, to penetrate perpendicularly into the formation by sending, through symmetrically arranged guard electrodes, a focusing current. • The guard electrodes emit focusing currents which constrain the A, current beam to flow perpendicularly out into the formation (the guard electrodes in fact establish equipotential surfaces coaxial with the tool, forcing the A, current to radiate perpendicularly to the axis). As a result, the borehole and adjacent bed signals, are considerably less than those of the normals and laterals. 53

PRINCIPLE

• All the currents return to an electrode located far away from Ao.

• Several devices, based on this principle, have been built and commercialized by service companies . They differ by the number of electrodes and their location or spacing

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LATEROLOG 3 • The LL3 device, introduced by Schlumberger in 1957, had three electrodes (Fig. 4-13). A, is the main current electrode, which emits a variable current I, to a remote return. A, and A, are two long guard electrodes (about 5- ft), connected together. Their potential Vg is maintained equal to an internal reference potential V, by a self-adjusting bucking current Zg which flows from A,-A',. The potential V, of A, was held equal to Vg by varying the main current I,. Al-&-A, was thus an equipotential surface, and current I, could only flow out perpendicularly, as a disc of thickness 0-0'.T he magnitude of Zo was measured; it was proportional to the formation conductivity, since: Unfocused

KVo = Rlo so that: I, = KV,C, where C is the conductivity. 55

DRAWBACKS OF LL3 • The LL3 tool had certain drawbacks. The current tended to escape either into the more conductive surrounding shoulder. This is the reason why it was replaced by other devices.

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DRAWBACKS OF LL3 • The LL3 tool had certain drawbacks. The current tended to escape into a thin intercalated conductive layer . This is the reason why it was replaced by other devices.

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LL7 • The LL7 device, introduced by Schlumberger in 1957,comprised a center electrode Ao, and three pairs of electrodes: M1 and M2; M1', and M2',; and A1 and A',

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LL7 • This is a shallow-reading laterolog, similar in design to, the laterolog 7, except that the spacings are smaller, and the current returns are much closer. The thickness of the I, current sheet is 14 in., and the distance between the two bucking electrodes is somewhat less than 40 in. The return current is located a relatively short distance from A,. The spacing factor (AqA',/0102) was equal to 3. This sonde was combined with the dual induction DIL (Tixier et a/., 1965). Its pseudo-geometrical factor is reproduced in Adjacent graph

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DUAL INDUCTION LOG

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ENVIRONMENTAL CORRECTIONS • The measured apparent resistivity R, is a function of a number of different parameters:

• The apparent resistivity (Ra)c obtained by correcting for the borehole and adjacent beds , is related to the true R, by:

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FOCUSING SHORT-SPACING MICRO-RESISTIVITY TOOLS • In order to reduce the borehole effect on shallow depth of investigation, services companies have developed several types of tools based on the same fundamental configuration (Table 4-3). One will describe the Schlumberger tools, the other tools being very similar in their concept

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MICROLATEROLOG-MLL Principle The principle of measurement is the same as that of the LL-7. The electrode array is mounted on an oil-filled rubber pad, as shown below. The central A, electrode is surrounded by three concentric rings of buttons, constituting the MI, M, and A,, electrode rings respectively. (This amounts to a circular LL-7 array, in fact.)

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MICROLATEROLOG-MLL • Measuring point: • The measuring point is at A,. The spacing is the diameter of a circle passing mid-way between MI and M,.Vertical definition is about 1.7". Depth of investigation is 1 -2". • Environmental effects: We can ignore the borehole and mud influence provided that pad contact is good. • Mud-cake influence: The mud-cake, however, cannot be ignored. The chart of Figure 4-58 is used to correct RMLL for mud cake thickness and resistivity. (RMLJc should be close to R,,. The greater the value of R,,I R,,, the greater the tendency for the current to escape through the mud-cake to the mud in the borehole. 64

MICROLATEROLOG-MLL Bed thickness influence: For beds thicker than 1.7", adjacent beds have no appreciable effect on R MLL.

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MICROLATEROLOG-MLL Invasion influence: The virgin zone does not effect if invasion is deeper than a few inches. Very shallow invasion will produce a reading somewhere between R,, and R,. In this case, we can compute R, by combining several different resistivity measurements, and using the step profile model.

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MUD-CAKE CORRECTION FACTOR

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THE PROXIMITY LOG (PL) Principle: •This type of device works on the same principle as the LL-3, but uses rectangular electrodes with a common center (Fig. 460), mounted on a solid rubber pad, wider than the MLL pad. The system is automatically focused by monitoring electrodes. Its vertical resolution is about 6 in. Corrections for shoulder bed influence must be achieved if the bed thickness is smaller than 1 foot 68

THE MICROSFL (MSFL) • Principle This is a small-scale SFL array, mounted on a flexible rubber pad. It has two advantages over the MLL and PL: - it is less sensitive to the mud-cake than the MLL, and reads shallower than the PL ; - it can be combined with other tools, such as the DLL and DIL, while the MLL or PL require a separate run (and therefore more rig-time, and risk of sticking). Synthetic microlog curves can be computed from MSFL parameters. Since the measure current sees mostly the flushed zone and the bucking current sees primarily the mud-cake, it is possible to mathematically derive a micro-normal and a micro-inverse curves. 69

FACTORS INFLUENCING THE RESISTIVITY MEASUREMENTS

• Environmental effects • Geological factors influencing resistivity • Rock composition • Rock texture -Tortuosity -permeability -microscopic anisotropy • Dips • Fractures • Sedimentarv structure, facies, depositional environnent,the geological sequence 70

FACTORS INFLUENCING THE RESISTIVITY MEASUREMENTS • Temperature • Pressure-Compaction

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• Resistivity logging

SUMMARY

• Measurement of resistivity • Techniques used to measure resistivity • Function of spacing value and considering categories of tools. • Why combination of these tools is important? • Measuring point • Non focused long spacing • Non focused short spacing • Focused long spacing • Focused short spacing • Factors influencing the resistivity measurements. 72