49 registros electricos

7/28/2019 49 registros electricos

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Chapter 49

Electrical Logging

M.P. Tixier, Consulting Engineer *Fundamentals

Well logging is an operation involving a continuousrecording of depth vs. some characteristic datum of theformations penetrated by a borehole. The record is calleda log. In addition, a magnetic tape is usually made.

Many types of well logs are recorded by appropriatedownhole instruments called sondes, lowered into thewellbore on the end of a cable. The winch of the loggingcable is generally brought to the well on a special log-ging truck (Fig. 49. l), which also carries the recorders,power sources, and auxiliary equipment. The parametersbeing logged are measured in situ as the sonde is movedalong the borehole. The resulting signals from the sondeare transmitted through electrical conductors in the cableto the surface, where the continuous recording, or log, ismade.

Electricalloggingis an important branch of well log-ging. Essentially, it is the recording, sections

in uncased

ofa borehole,of (or their reciprocals,

the resistivities theconductivities) of the subsurface formations, generallyalong with the sponfaneous potentials (SP) generated inthe borehole.

Electrical logging has been accepted as one of the most

efficient tools in oil and gas exploration and production.When a hole has been drilled, or at intervals during thedrilling, an electrical survey is run to obtain quickly andeconomically a complete record of the formationspenetrated. This recording is of immediate value forgeological correlation of the strata and detection andevaluation of possibly productive horizons. The informa-tion derived from the electrical logs may at the same timebe supplemented by sidewall samples of the formationstaken from the wall of the hole or by still other types ofborehole investigations that can be performed by usingadditional wireline equipment available for use with thelogging truck [deviation surveys, caliper (hole-diameter)

surveys, dipmeter surveys, temperature surveys,

radioactivity (gamma ray, density, neutron, and nuclear

Authors of rhe orlglnal chapler on this top!c in the 1962 edltion included fhts author.Ii G Doll, M. MarIm. and F Segesman.

spectrometry) surveys, acoustic surveys, wireline forma-tion tester, etc.].


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As explained later, several types of resistivity-measuring systems are used that have been designed toobtain the greatest possible information under diverseconditions-e.g., conventional devices (normals andlaterals), induction log (IL), Laterolog (LL),microresistivity devices, and electromagnetic propaga-tion logs. Table 49.1 gives the service companynomenclature for various logging tools.

The typical appearance of a standard electrical log isillustrated in Fig. 49.2. The left track of the log containsthe SP curve. The middle track contains a l&in. shortnormal (shallow-investigation resistivity curve), record-ed on both regular and amplified sensitivity scales assolid curves, and a 64-in. normal (medium-investigationresistivity curve, dashed curve). The right track containsan 1%ft 8-in. lateral (deep-investigation curve).

Logs recorded with other combinations of resistivity-measuring devices have a similar general appearance,although the corresponding devices differ in principleand performance. Microresistivity logs generally includea microcaliper curve (hole-diameter recording), which is

useful in the location of permeable zones. Of late, four-logarithmic tracks are often replacing the two-arithmetictrack mentioned previously.

The curves are recorded on the most appropriate ofseveral available sensitivity scales. The usual depths ofscales are 2 in. = 100 ft (regular) and 5 in.=100 ft(detail). Less frequently a scale of 1 in. = 100 ft is used.For cases where great detail is involved, as in microlog-ging and dipmeter logging, special expanded scales areavailable. In many parts of the world, metric depthscales are used instead of English scales.

Earth Resistivities

Formation resistivities are important clues to probablelithology and fluid content. With a few exceptions thatare rare in oilfield practice, such as metallic sulfides and


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PETROLEUM ENGINEERING HANDBOOK

CABLE TENSIONMEASUREMENT

CONTROL

Fig. 49-l-Setup for wireline logging operations in wells(schematic).

graphite, dry rocks are very good insulators but, whentheir pores are impregnated with water, they conductelectric current. Subsurface formations in general havefinite measurable resistivities because of the water con-tained in their pores or adsorbed on their interstitial clay.Formation resistivity also depends on the shape and theinterconnection of the pore spaces occupied by thewater. These depend on the formation lithology and, inthe case of reservoir rocks, on the presence of noncon-ductive oil or gas.

Units of Resistivity and Conductivity. In electrical log-ging, the resistivity is usually measured. An exception is

induction logging, in which the conductivity is recordedalong with its reciprocal, the resistivity. Measurementsmade with electromagnetic propagation are discussedlater.

The resistivity (specific resistance) of a substance tothe flow of electrical current, at any given temperature,is the resistance measured between opposite faces of aunit cube of that substance. In electrical-logging work,

TABLE 49.1-SERVICE COMPANY NOMENCLATURE

Schlumberger

Electrical LogInduction Electric Log (IEL)Induction Spherically Focused Log (ISF)Dual Induction Spherically Focused LogLaterolog.3 (LL3)Dual LaterologMicrolog (ML)Microlaterolog (MLL)Proximity Log (PL)Microspherically Focused Log (MSFL)Borehole Compensated Sonic LogLong Spaced Sonic LogCement Bond/Variable Density Log

Gamma Ray NeutronSidewall Neutron Porosity LogCompensated Neutron Log (CNL)Thermal Neutron Decay Time Log

Dual Spacing TDTCompensated Formation Density LogLitho-Density Log

High Resolution Dipmeter


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Formation Interval TesterRepeat Formation TesterSidewall SamplerElectromagnetic Propagation LogBore Hole Geometry ToolUltra Long Spacing Electric LogNatural Gamma Ray SpectrometryGeneral Spectroscopy ToolWell Seismic ToolFracture Identification Log

GearhartElectrical LagInduction Electric Log

Dual Induction-LaterologLaterolog-3Dual LaterologMicro Electrrc LogMicrolaterolog

Sorehole Compensated Sonic Log

Sonic Cement Bond System

Gamma Ray NeutronSidewall Neutron Porosity LogCompensated Neutron Log

Compensated Density Log

Four Electrode Dipmeter

Selective Formation TesterSidewall Core Gun

X-Y Caliber

Fracture Detection Log

Dresser AtlasElectrologInduction Electrolog

Dual Induction Focused LogFocused LogDual LaterologMinilogMicrolaterologProximity Log

Sorehole Compensated Sonic LogLong Spacing BHC AcoustilogAcoustic Cement Bond LogGamma Ray NeutronSidewall Epithermal Neutron LogCompensated Neutron LogNeutron Lrfetime LogDual Detector NeutronCompensated Densilog


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DiplogFormation TesterFormation Multi TesterCorgun

Caliper Log

SpectralogCarbon/Oxygen Log

WelexElectric LogInduction Electric Log

Dual Induction LogGuard LogDual GuardlogContact Log

F,R,, Log

Acoustic Velocrty Log

Microseismogram

Gamma Ray NeutronSrdewall Neutron LogDual Spaced Neutron Log

Density Log

DiplogFormation Tester

Sidewall Coring

Caliper Log


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ELECTRICAL LOGGING 49-3the meter was chosen as the unit of length; so the unit ofresistivity is taken as the (Q.m)/m, or more simply, theohm-meter, 52.m.Since conductivity is the reciprocal of resistivity(C=lIR), a possible unit of conductivity would bel/(Q.m), or G/m. However, since this unit wouldnecessitate extensive use of decimal fractions, a unitone-thousandth as large, the millimho/meter (mu/m), isemployed. Thus, formations having resistivities of 10,100, or 1,000 Q*rn have conductivities of 100, 10, or 1mu/m, respectively.Dependence of Water Resistivity on Salinity andTemperature. The resistivity of an electrolytic solutiondecreases as the amount of chemicals therein increases.At any given temperature the electrical conductivity of aformation water or a drilling mud will depend on theconcentration and nature of the dissolved chemicals.In most cases the predominant solute is sodiumchloride (NaCl); therefore, the NaCl conversion chart(Fig. 49.3) may generally be used to obtain resistivityfrom concentration. If other chemicals are present inrelatively large amounts, it is possible to convert the con-

centrations of such chemicals into equivalent concentra-tions of NaCl to find the resistivity. To make the conver-sion, apply the appropriate multipliers given in Table49.2 for the concentration of each separate ion [in partsper million (ppm) or (m 3 /m 3 ) by weight, or in grains pergallon (gr/gal) or (kg/m)], and add the products. Notethat concentrations expressed in milligrams per liter(mg/L) and in ppm may be appreciably different at high tt f t LATERALconcentrations. Below about 50,000 ppm, however,measurements at room temperature in the two units maybe used interchangeably without serious error.Fig. 49.2-Typical electrical log.CONCENTRATION

G/G-

RESISTIVITY OF SOLUTION OHM-METERS

Fig. 49.3~Resistivity vs. concentration for NaCl solutions at various temperatures.


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The resistivity of an electrolytic solution decreases asits temperature increases. This is of great importance,since temperature in the earth increases with depth.

Before the resistivity of the drilling mud (measured atsurface temperature) can be compared with that of a for-mation (measured at a much higher temperature in a deepwell) the resistivities must be converted to values thatwould have been observed at a common temperature.The temperature conversion is accomplished by meansof Fig. 49.3, which shows for NaCl solutions the effectsof both salinity and temperature on resistivity . Downholetemperatures may be estimated from a so-called bot-tomhole temperature (BHT) obtained by means of amaximum-reading thermometer inserted in the body ofthe sonde.

Resistivities of Formation Waters. Formation waterscan vary remarkably with geographic location, depth,and geological age. Shallow groundwaters are usuallyfresh (not saline), with resistivities sometimes exceeding20 to 50 !l. m at room temperature. They also may con-tain appreciable amounts of calcium and magnesium

salts, which make them hard. At great depths, forma-tion waters generally tend to be more saline. In deepwells, formation-water resistivities sometimes may cor-respond to complete saturation (0.014 O.rn at 200F).

A knowledge of R,., the formation-water resistivity. isimportant in electrical-log interpretation. R,,may be ob-tained from the readings of the SP curve (Eq. 9) or fromresistivity measurements on samples of formation waterrecovered from production or in drillstem tests. It alsomay be estimated from measurements of the resistivity ofthe permeable formations of interest when they are 100%water-saturated, Ro,if the porosity or formation factor is

known (Eqs. 1 and 2). R, may be computed, as has beenexplained, from analyses of formation waters. Resistivi-ty of formation waters is discussed further in Chap. 24.

Mud, Mudcake, and Mud-Filtrate Resistivities.

Resistivities of the mud, R,,the mudcake, R,,, and themud filtrate, R,,,f, in log interpretation.

are all importantR, is obtained by direct measurement on a mud sample.R,,,f are by direct measurements on

and R, obtainedfiltrate and mudcakes pressed from a sample of the mud,or they can be estimated from average statistical data onthe basis of mud resistivity. 2-4 Correction for the varia-tion of these resistivities with temperature is made by useof Fig. 49.3.

Formation Resistivity Factor. If R. is the resistivity ofa clean (nonshaly) formation completely saturated withwater of resistivity R,, the ratio Ro/R,will be a con-


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stant that depends on the lithologic structure of the for-

TABLE 49.2-CONVERSIONS FOR CATIONS AND ANIONS

Cations AnionsNa 1.0 Cl 1.0Ca 0.95 so4 0.5WI 2.0 co3 1.26HCO, 0.27


mation and not on the resistivity, R,,of the saturatingwater. This constant is the formation resistivity factor,FR,commonly called formation factor.

Ro

FR=- R, . . . . . . . . . . . . . . . .(I)

Dependence of Formation Factor on Porosity andLithology. The formation factor, F,, of a clean forma-tion can be related to its porosity, 6. by an empirical for-mula of the form F~=alc$'?', a m are con-

where and

stants. The exponent m,sometimes called the cementa-

tion exponent or factor, varies with the lithology.

In the construction of many graphs for log interpreta-tion, 2 the Humble formula proposed by Winsauer eta1.5 has been generally adopted:

0.62

FR= ~2.,5. . . . . . . . . . . . . . . . . I . . . (2)

An early formula proposed by Archie, which fits par-ticularly well for consolidated formations such as hardsandstones and limestones, is

FR=L. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...(3)4J2

Limestones often contain vugs, interconnected withfissures, which add their porosity to that of the matrix.When the vugs and fissures are spaced closely, com-pared with the spacings of the resistivity-measuring

devices, Eq. 3 often can be used as in the case of sand-stones or limestones with only granular porosity. Never-theless, it is sometimes advisable to use values of mgreater than two as required to fit local observations.

Shaly (Dirty) Formations. Shales and clays arethemselves porous and are generally impregnated withmineralized water. Therefore, they have appreciableconductivity, which is enhanced by ion-exchange con-duction through the shale matrix. (This shale conduction


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is sometimes, though not quite properly, referred to asresulting from conductive solids. ) On the other hand,the size of the shale pores is so small that practically nomovement of fluid is possible. Accordingly, shale,whether deposited in thin laminations or dispersed in theinterstices of the sand, contributes to the conductivity ofthe formation without contributing to its effectiveporosity.

The relation between formation resistivity and porositybecomes more complex for shaly formations than forclean formations. Because of the additional shale con-ductance, the ratio of formation resistivity to waterresistivity (i.e., the formation factor) is not constantwhen the resistivity of the impregnating water changes. 6Nevertheless, if the shale content is not too great, ex-perimental observations show that for low enough valuesof water resistivity this ratio is almost constant, asthough the conductance of the shale were then negligiblein comparison with that of the water; and a limiting for-mation factor is found, which is related approximately tothe effective porosity in the same way as the formationfactor of a clean sand.


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ELECTRICAL LOGGING

Relation Between Formation Resistivity and Satura-tion. When a part of the pore space is occupied by an in-sulating material such as oil or gas, the resistivity of therock, R,,is greater than the resistivity that it has when100% water-bearing, R,. The resistivity of such rock isa function of the fraction of the PV occupied by water.

For substantially clean formations, the water satura-tion, S,, is related to R, (resistivity of formation con-taining hydrocarbons and formation water, with a watersaturation S,) and R,J (resistivity of same formationwhen 100% saturated with the same water) b an em-pirical relation known as the Archie equation. 7

l/II

. . . . . . . . . . . . . . . . . . . . (4)

Empirically determined values of n range between 1.7and 2.2, depending on the type of formation. Experienceshows that n =2 should give a sufficiently good approx-

imation. Then, combining Eqs. 4 and 1 gives

SW=(+) 1/i =(F) I/?.. . . . . . . . . . .

The ratio RJR0 is sometimes designated as the resistivi-ty index, 1~; accordingly, S, =(ZR) -I.

The relation between formation resistivity and watersaturation is more complex when the formations containsome shale or clay because of the additional conductanceresulting from the interstitial shale. sv9

Ranges of Resistivity-Formation Classifications.

Clays and shales are porous, practically impervious for-mations and are often very uniform throughout theirmass. Their resistivity is comparatively low and prac-tically constant over wide intervals. Compact and imper-vious rocks, such as gypsum, anhydrite, densecalcareous formations, or certain kinds of coal, arehighly resistive because of their very small interstitialwater content.

Resistivities of porous and permeable formations, suchas sands, vary widely, depending on their lithology andfluid content. In electrical logging it is convenient to

classify reservoir rocks as follows.

SoftForma&ions. These formations are chiefly poorlyconsolidated sand/shale series. The porosity of the sandsis intergranular and exceeds 20%. Resistivities rangefrom 0.3 !2* m for saltwater-bearing sands to several fl. mfor oil-saturated sands.

Intermediate Formations. These are chieflymoderately consolidated sandstones but frequently


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limestones and/or dolomites. Reservoir porosity isgenerally intergranular, ranging from about 15 to 20 % .The reservoir formations are interbedded with shales andvery often with tight rocks. Resistivities range from 1 toabout 100 Q-m.

Hard Formations. These are chiefly limestones andiordolomites, and also consolidated sandstones. They con-sist mostly of tight rocks containing porous andpermeable zones, and shale streaks. The porosity ofreservoirs is less than 15 % . Most often, the porous and

permeable zones contain fissures and vugs. Resistivityrange is from 2 to 3 Q. m to several hundred. For thecompletely tight formations, such as salt and anhydrite,the resistivity may be practically infinite.

Anisotropy. In many sedimentary strata, the mineralgrains have a flat or plate-like shape with an orientationparallel to the sedimentation. Current travels with greatfacility along the water-filled interstices, which aremostly parallel to the stratification. These strata,therefore, do not possess the same resistivity in all direc-tions. Such microscopic is observed

anisotropy mostly inshales.

Moreover, in electrical logging, the distance betweenelectrodes or coils on the measuring devices is greatenough that the volume of formation involved in themeasurements very often includes sequences of interbed-ded resistive and conductive streaks. Since current flowsmore easily along the beds than perpendicular to them,the formation has macroscopic

anisotropy.

Both kinds of anisotropy may add their respective ef-fects to influence the apparent resistivity. Thelongitudinal, or horizontal, resistivity , RH, measuredalong the bedding planes is always less than the transver-sal, or vertical (perpendicular) resistivity, Rv.

Resistivity-measuring devices whose readings are notappreciably affected by the borehole [the deep inductionlog (IM), and under certain conditions, the laterolog(LL), and the long lateral when the ratio RHIR, is lowor moderate] will read RH.Because of the borehole ef-fect, the short-spacing-electrode devices usually read

values greater than RH. lo

Distribution of Fluids and Resistivities in PermeableFormations Invaded by Mud Filtrate. Inasmuch as thehydrostatic pressure of the mud is usually maintainedgreater than the natural pressure of the formations, mudfiltrate (forced into the permeable beds) displaces theoriginal formation fluids in the region close to theborehole. Solid materials from the mud deposited on thewall of the hole form a mudcake, which tends to impede


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and reduce further infiltration.

The thickness and the nature of the mudcake dependon the kind of mud and on the drilling conditions ratherthan on the formations. The thickness, is usually

h,,,,,between /s and 1 in. For water-based muds the mudcakeresistivity, R,, , is about equal to one or two times themud resistivity, R,. In some oil-emulsion muds, R,,may be somewhat greater.

Fig. 49.4a represents a schematic cross section of anoil-bearing permeable bed penetrated by a borehole. Fig.49.4b and 49.4~ show the corresponding radial distribu-tion of fluids in formation and resistivities.

As indicated in Fig. 49.4a, the zones of differentresistivity may be divided into the drilling mud withinthe borehole (of resistivity R,);the mudcake R,,,theflushed zone R,,;a transition zone; in some cases anannulus. R, (present only in certain oil-or gas-bearing formations); and the uncontaminated zone (ofresistivity R,).The invaded zone (of average

resistivity, Ri)includes the flushed zone and the transi-tion zone.

Invaded Zone. This zone is behind and close to thewall of the hole; it is believed that most of the originalinterstitial fluids have been flushed out by the mud


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rANNULU5 (Ran)

MUD CAKE (Rnxl

t k--HOLE WALL

Fig. 49.4-a. Horizontal section through a

permeable oil-bearing bed

(S, < 60%); b. radial distribu-

tion of fluids in formation

(qualitative); c. radial distribu-

tion of resistivities.

filtrate. This flushed zone, of resistivity R,,, is con-sidered to extend, under usual conditions of invasion, atleast 3 in. from the wall. Exceptions to this rule canoccur.

If the bed is water bearing, the pores in the flushedzone are completely filled with the mud filtrate, and forclean formations R,,is nearly equal to F,R,f; FR beingthe formation factor and Rmf the mud-filtrate resistivity

If the bed is oil bearing, the flushed zone containssome residual oil saturation, S,, . From Eq. 5, S,, , thewater saturation in the flushed zone is

%

or

FRRtnf

R,,=T, . . . . . . . . . . . . . . . . . . . . . . . . . . . .

s

x0

where S,=l-S,,.

Beyond the region of maximum flushing, R,,, there isa more or less extended transition region, the nature ofwhich depends on the characteristics of the formation,

the speed of invasion, and the hydrocarbon content. Theinvaded zone includes the flushed zone and the part ofthe transition zone invaded by filtrate. In the case ofwater-bearing sands and oil-bearing sands of high watersaturation, the invaded zone extends up to the uncon-taminated zone, R,.

There can be no exact definition of the depth of the in-vaded zone, but it is convenient to introduce a factor di,called the electrically equivalent diameter of inva-


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sion, corresponding to an average invaded zone ofresistivity Ri, which has the same effect as the actual in-


vaded zone on measurements made in the borehole. Thedepth of invasion is variable. It depends on the plasteringproperties of the mud, pressure differences between themud column and the formation, time elapsed since theformation was drilled, porosity of the formation, propor-tion and nature of the fluids (water, oil, gas) present inthe pores, reaction of any interstitial clays with the mudfiltrate, etc.

All other conditions being the same, the greater theporosity, the smaller the depth of invasion. With usualmuds, di seldom exceeds 2dh (dh =hole diameter) inhigh-porosity sands, but it may exceed 5dh and even1Odh in low-porosity formations such as consolidatedsandstones or limestones. In some cases, invasion can beextremely shallow in very permeable formations and ingas-bearing formations.

In very permeable beds, when there is an appreciable

difference between the specific gravities of the mudfiltrate and the salt-laden interstitial water, gravity-segregation effects may occur, with the fresher filtratetending to accumulate at the top boundary of the bed,resulting in a decrease in the depth of invasion in thelower part of the bed. I

In fissured formations, the permeability is quite oftenenormous because of the fissures-much greater than thepermeability of the matrix material surrounding them.Suppose that a formation is composed of a porous butrelatively impermeable material, broken by networks ofroughly parallel fissures. Mud filtrate penetrates the

fissures easily and deeply, driving out much of theoriginal fluids (oil and formation water). On the otherhand, the matrix itself may be penetrated hardly at all bythe filtrate. Since the l%.sures constitute a small part ofthe total PV, only a vety small portion of the totaloriginal fluids is displaced. As a result, R, is little dif-ferent from R,,and the ratio R,IR,,fis no longerrepresentative of the formation factor.

Annulus. When the formation contains hydrocarbons,the process of invasion is complex. The distribution offluids is then affected by the two-phase permeabilities,relative densities (gravities) and viscosities of the fluids,

capillary forces, etc.

When the initial water saturation is low (less thanabout 50%), one important feature is the existence of anannular region just inside the uncontaminated zone, con-taining mainly formation water and some residual oil.This annulus is explained as follows. The mud filtratepenetrates the formation radially, sweeping theremovable oil and formation water ahead of it. For largeoil saturation, the relative permeability to oil is ap-


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preciably greater than that to water. Therefore, the oilmoves faster, leaving a zone (the annulus) enriched information water behind it.

It seems likely that, because of the effects of diffusion,capillary pressure, gravity, etc., the existence of a well-defined annulus is a transitory phenomenon. Field logexperience nevertheless seems to show that the annulusdoes very often exist at the time the logs are run. Com-putations have shown that the presence of the annulushas a practically negligible effect on the response of thedevices with electrodes (normals, laterals, andlaterolog) It may have an effect on the induction log, butthis can be taken care of for practical purposes by meansof appropriate interpretation charts. 2


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ELECTRICAL LOGGING

Uncontaminated Zone. For clean formations, from

Eq. 5,

FRR,R,= s,2. . . . . . . . . . . . . I . .

In the usual case, R,,,f is 10 to 25 times as large as R w.Thus, comparing Eqs. 6 and 7 with usual values of S,and S,, , R,, even in oil-bearing formations, is often lessthan R,, as represented in Fig. 49.4~.

Apparent Resistivity. Since any resistivity measure-ment is affected in some degree by the resistivities of allthe media in the immediate vicinity of the sonde (i.e.,mud, different parts of the formation that vary inresistivity, adjacent formations if the bed measured isthin), any given device records an apparent resistivity.Each resistivity device is calibrated so that when thesonde is in a homogeneous medium (or in some other

condition appropriate to practice, specified for the par-ticular device) the apparent resistivity reading is equal tothe actual resistivity.

Requirements for and Types of Resistivity Devices.

Inspection of basic relations in Eqs. 1, 2, 5, and 6 showsthat a determination of S, and 4 requires a knowledge ofR, and R,, (or Ri, in certain cases where R,, is not easi-ly determined). Thus, for the reservoir-evaluation prob-lem, it is necessary to have resistivity-measuring deviceswith different depths of investigation to obtain values in-dicative of the resistivities of the invaded zone and the

uncontaminated zone. The readings of the deep-andshallow-investigation curves may often be used to cor-rect each other, through correction charts or departurecurves, to obtain better values of R, and Ri

Another function of resistivity recording is to providean accurate definition of bed boundaries, particularly ofpermeable beds. Finally, it is desirable that the readingsnot be influenced by the effect of the mud column or, incase of thin beds, by the adjacent formations.

These requirements are only partly satisfied with theconventional resistivity devices. The introduction of

microdevices and focused devices has brought about anappreciable improvement.

Currently used resistivity devices may be classified intwo categories.

1. Macrodevices, which derive their reading fromabout 10 to 100 cu ft of material around the sonde (usefulfor R, and Ri evaluation), and include unfocused-electrode devices, focused-electrode devices, and induc-


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tion logging devices.2. Microdevices (also called wall-resistivity devices),which derive their readings from a few cubic inches ofmaterial behind or close to the wall of the hole. Since theelectrodes are mounted on an insulating rubber padpressed against the wall of the hole, measurements areaffected only marginally by the mud column.Microdevices arc of unfocused and focused types.Resistivity devices that have electrodes may be used inholes filled with water or water-based drilling mud,which provides the electrical contact necessary betweenelectrodes and formation. The induction log can also beused in empty holes or in holes filled with nonconductiveoil-based mud. The various resistivity devices aredescribed later.

49-7

Spontaneous Potential (SP) Log

The SP log is a record of the naturally occurring poten-tials in the mud at different depths in a borehole. Themeasurement is made in uncased holes containing water-based or oil-emulsion muds between an exploratory elec-

trode on the sonde in the borehole and a stationaryreference electrode at the surface.

Usually the SP curve (Fig. 49.2) consists of a more orless straight baseline (corresponding to the shales) hav-ing excursions or peaks to the left (opposite thepermeable strata). The shapes and the amplitudes of theexcursions may be different, according to the forma-tions, but there is no definite correspondence betweenthe magnitudes of the excursions and the values ofpermeability or porosity of the formation.

The principal uses of the SP curve are to (1) detect the

permeable beds, (2) locate their boundaries (except whenthe formations are too resistive), (3) correlate such beds,and (4) obtain good values for R,, the formation-waterresistivity.

Origin of the SP. The character of the potentialsmeasured in the mud results from ohmic drops producedby the flow of SP currents through the mud resistance. Ifthe mud is extremely conductive, these ohmic drops maybe insignificant, and the variations in the SP curve maybe too small to be useful.*

The SP currents flow as a result of electromotive

forces (EMFs) existing within the formations or at theboundaries between formations and mud. Onephenomenon that could cause an EMF to appear acrossthe mudcake opposite a permeable bed is electrofiltra-hon.The mud filtrate, in being forced through the mud-cake, would tend to produce an EMF, positive in thedirection of flow. According to experiments, I2 the EMFacross the mudcake may be quite sizable, but there isalso an electrotiltration EMF generated across the adja-cent shales. Thus, the net effect of electrotiltration in


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causing variations of SP is small and in most casesnegligible for all practical purposes-a conclusionverified by field experience.**

Most important ate the EMFs of electrochemicalorigin, which occur at the contacts between the drillingmud (or its filtrate) and the formation water, in the poresof the permeable beds, and across the adjacent shales. I6In a clean sand lying between shale beds, all penetratedby a borehole containing conductive (water-based) mud,the total electrochemical EMF, E,, is produced in thechain (Fig. 49.5): Mud/mud filtrate/formation water/shale/mud. The EMF of the junction, mud/mud filtrate,is taken to be practically nil because, although theresistivities of the mud and its filtrate may differ, theirelectrochemical activities should be the same.

The part of the chain consisting of formationwater/shale/mud gives rise to the shale-membraneEMF, Em. The part mud ftltrateiformation watergives rise to the liquid-junction EMF, EJ. For NaCl(monovalent-ion) solutions, at 75F,

E,=59log,+

amf

*In such acase the gamma ray log. which distinguishesshales from nonshale beds,IS sometimes recorded as a subslltute for the SPFurther information on the electrof~ltralion EMF, or streaming potential, may befound in Refs. 13 through 15.


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MUD

I-tI-tINVADED

ZONE

Fig. 49.5--Schematic representation of

electrochemical chain and

SP current path at boundary

between permeable bed and

adjacent shale.

and

where a, and a,f are the chemical activities of the for-

mation water and mud filtrate, respectively (at 75 F),and EM and EJare in millivolts. The total E,is the sumof E,+,

and E,:

E,=K, log,,% . . . . . . . . , . . . . . . . . . . . (8)

Umf

where K, is the electrochemical coefficient and is equalto 71 at 75F.

Eq. 8 is general, provided that both formation waterand mud filtrate are essentially NaCl solutions of anyconcentration. The values of K,are directly proportionalto the absolute temperature. Thus, at 150F the coeffi-cient K,in Eq. 8 becomes 81 instead of 71, and at 300Fit becomes 101 (see Fig. 49.6).

From Eq. 8, in the usual case of a, greater than a,,f,E, is positive. However, if a,f is greater than a,, cor-responding to mud mom saline than formation water,then E,is negative and the SP deflections correspondingto permeable beds are then reversed on the log.

Effect of Invasion on Generation of the EMF. In theexplanation of the electrochemical potential, it has beenassumed that no shale-type potential is created by themudcake. In the normal case, mud filtrate bathes bothsides of the mudcake and no shale-type potential canarise. In some formations, there is only a little filtratebehind the mudcake. Such small amount of filtrate willbe contaminated easily by the formation water. In thiscase, one face of the mudcake is wetted by the filtrate inthe hole, the other face by contaminated filtrate of dif-


20/101

ferent activity. This will give rise to a shale-type poten-tial of the same polarity as the main shale potential, andthe SP curve will be decreased. This explains thedecreasing of the SP curve with time in very highlypermeable beds. I7 The filtrate is evacuated by gravity

STATIC SP (mvl

Fig. 49.6--R, determination from the SP. The inset chart of true

R, vs. R, applies to formation waters of average

composition.

segregating forces and the formation fluids tend to comeback toward the hole with time.

Conversely, an increase in SP with time is observedoften in low-permeability water-bearing formations.Very little filtrate invades the formation in a freshlydrilled hole and the filtrate is contaminated by the forma-.tion water. As the invasion proceeds, more and morefiltrate goes into the formation and the mudcake is wet-ted on both faces by the mud filtrate. When the mudcake

does not contribute any shale-type potential, the SPcurve, recorded on the front of a thick permeable sand, issaid to be fully developed.

Effect of Interstitial Shales on the SP. Increasingamounts of shale or clay in a permeable bed effectivelyresult in a reduction of the SP curve. At the limit, for100% shaliness, E,becomes zero; that is, the sand isthen all shale and indistinguishable from the surroundingshales.

The presence of oil in a shaly sand tends to enhancethe effect of the shale. All other conditions being the

same, the total E,of a shaly sand will be smaller if oilbearing than if water bearing.

The effect of interstitial shale is also greater in low-porosity formations. In these cases, only a small amountof shale reduces the SP deflection appreciably. Con-versely, the E, of shaly water-bearing sands of highporosity remains practically equal to the E, of a cleansand, as long as the shale content is reasonablylow-i.e., does not exceed a few percent.


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ELECTRICAL LOGGING

GeometricEffect Influencing the SP Curve

Circulation of the SP Current, The various EMFs addtheir effects to generate the SP currents, which followthe paths represented schematically in Fig. 49.7 (right)by solid lines. Each current line encircles the junction ofmud, invaded zone, and uncontaminated zone. In theusual case where the formation waters are saltier than themud, E, is positive and the current circulates in thedirection of the arrows. The potential of a point in themud column opposite the sand is negative with respect toone opposite the shale.

Along its path, the SP current forces its way through aseries of resistances, both in the ground and in the mud.Along a closed line of current flow, the total of theohmic-potential drops is necessarily equal to thealgebraic sum of the EMFs encountered. Moreover, thetotal potential drop is divided between the different for-mations and the mud in proportion to the resistance of

the path through each respective medium.

Static SP (Clean Formations) and Pseudostatic SP(Shaly Formations). It is convenient to use an idealizedrepresentation in which the SP current is prevented fromflowing by means of insulating plugs placed across holeand invaded zones, as shown in Fig. 49.7a (right).Under these conditions, a plot of the potential in the mudcolumn would appear as the dashed cross-hatched curveon the left of Fig. 49.7a, with a maximum negativedeflection opposite the permeable bed equal to thealgebraic sum of all the EMFs of various origins. This isthe maximum SP that could be measured. It is therefore

convenient to use this theoretical value as a reference. Inthe case of a clean sand, it is called the static SP, ESp. Ifthe sand is shaly, it is called the pseudostatic SP. Epsp.

For given values of the activities of mud and formationwater, the pseudostatic SP of a shaly sand is smaller thanthe static SP of a clean sand. The ratio E,rplEsp iscalled the reduction factor or ratio and is designated bythe symbol (Y sp.

The SP log records only that portion of the potential

drop occurring in the mud. When the bed is sufficiently

thick the amplitude of the SP deflection approaches the

static SP (or EpsP in case of shaly formations), hccause

then the resistance offered to the current by the bed itself

is negligible compared with the resistance of the path

through the mud in the borehole.


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Factors Influencing the Shape and Amplitude of SPDeflections. As seen in Fig. 49.7b, the current circulatesin the hole not only opposite the permeable formation butalso a short distance beyond its boundaries. As a result,although on the static SP diagram the boundaries of apermeable bed are indicated by sharp breaks, those onthe actual SP curve show a more gradual change inpotential. An analysis of the circulation of the currentshows that, for uniform resistivity in the formations, thebed boundaries are located at the inflection points on theSP log. This fact provides a means of determining thethickness of a bed from the SP log.

Both the shape of the SP deflection and its relativeamplitude (in fractional parts of the Essp or EpsP) are in-fluenced by four factors, which determine the conditionsfor the circulation of SP currents: (1) bed thickness, (2)resistivities of the bed, the adjacent formations, and the

------STATIC SP DIAGRAM--POTENTIAL IN MUDWHEN SP CURRENTS ARE PREVENTEDFROM FLOWING.

:SP LOG-POTENTIAL IN MUD WHEN SPCURRENTS ARE FLOWING.

Fig. 49.7-a. Static SP diagram (left) that would beobserved in hole when current IS preventedfrom flowing by means of insulating plugs(right); b. actual SP diagram (solid curve,left) and schematic representation of SPcurrent distribution in and around perme-able bed (right).

mud, (3) borehole diameter, and (4) depth of invasion.

All other factors remaining the same, a change of thetotal EMFs affects the amplitude but does not modifythe general shape of the SP log.

Influence of Mud Resistivity and Hole Diameter. Themud resistivity has a predominant influence on the SPcurve. If the mud is of about the same degree of salinityas the formation water, electrochemical EMFs aresmall. If the mud is more saline than the formationwater, the SP may be reversed (sand deflections towardthe positive side of the log). Moreover, the lower themud resistivity (compared with the formation resistivity)the broader the deflection above and below the

permeable bed and, because the ohmic drops in the mudare decreased, the smaller the amplitude of thedeflection.

An increase in hole diameter acts approximately likean increase in the ratio of formation resistivity to mudresistivity. It tends to round off the deflections on the SPlog and reduce the amplitude of the deflections oppositethin beds. A decrease in hole diameter has the same ef-fect as a decrease in the ratio of formation resistivity to


23/101

mud resistivity.

The SP log would also be influenced by a lack ofhomogeneity of the mud-a change in salinity of the mud


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SCHEMATIC REPRESENTATION SCHEMATICOF FORMATIONS AND SPLOG DISTRIBUTIONOF SP CURRENTS

m SHALE (IMPERVIOUS AND COMPARATIVELY

CONDUCTIVE 1

COMPACT FORMATON

fzl

(VERY HIGH RESISTIVITY)

PERMEABLE

m

(COMPARATIVELY CONOUCTIVE 1

Fig. 49.8-SP phenomena in highly resis-tive formations (schematic).

at a certain level would result in an SP baseline shift atthat level. However, it has been found in practice thatsuch changes in salinity are rare.

Effect of Invasion. Permeable beds in general arc invad-ed by mud filtrate. Because the boundary between mudfiltrate and interstitial water is somewhere inside the for-mation, a fraction of the SP current flows directly fromthe shale into the invaded zone, without penetrating themud column. As a result, the presence of the invadedzone has an effect on the SP log similar to that of an in-crease in hole diameter.

SP in Soft Formations. Theory and field experiencehave shown that the amplitude of the SP deflection ispractically equal to the static SP (of a clean sand) or tothe pseudostatic SP (of a shaly sand) when the permeablebeds are thick and the resistivities of the formations arenot too great compared with that of the mud. Moreover,the SP curves define the boundaries of the bed with greataccuracy. The amplitude of the deflection is less than thestatic SP or pseudostatic SP for thin beds, and the thinnerthe bed, the smaller the deflection.

On the other hand, when the resistivity of the forma-tion, R,, is considerably greater than that of the mud,

R,, the SP curves are rounded off, the boundaries aremarked less accurately, and all other conditions beingthe same, the amplitude of the peak is less than when theratio R,IR, is close to unity.

For the case of shaly sands, the SP curve may also beaffected by the presence of oil. A change in themagnitude of the SP deflection occurs very often whenpassing an oil/water contact in a shaly sand. This changeis not a positive criterion for the detection of oil because


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the same effect would be obtained if the salinity of theinterstitial water were reduced or if the percentage ofshale were increased.

SP in Hard Formations. Hard formations are highlyresistive except for permeable beds, whether oil-orwater-bearing, and shales, which are impervious. TheSP currents generated by the different EMFs flow intothe hole out of the shale sections and out of the hole into


the permeable sections. In between, they flow through

the mud rather than through resistive sections close to the

borehole, because of the large resistances the latter paths

offer. However, within the formation at a distance from

the borehole, where the paths through the resistive beds

have larger cross sections and hence lower resistances,

the SP currents can complete their circuits from

permeable beds to shale. They cannot return to the mud

through adjacent permeable beds because there they en-

counter EMFs opposing them.

Opposite a given resistive bed, the SP current in themud column remains essentially constant along theborehole. This means that the potential drop per unitlength of hole is also constant,-thus giving g constantslope on the SP log as shown by the straight-line portions

of the SP in Fig. 49.8. At the level of each conductivebed, some SP current will enter or leave the mud col-umn, thus modifying the slope of the SP log. For in-stance, the slope of the SP log changes at the level of thepermeable bed, P2, because part of the current leaves thehole and flows into the bed. *

As a general rule, in hard formations the permeablebeds are characterized on the SP log by slope changes orcurvatures that are convex toward the negative side ofthe log. Shales are characterized by curvatures that areconvex toward the positive side of the log. Highlyresistive beds correspond to essentially straight parts of

the SP log.

Determination of Static SP (SSP). The SP deflection ismeasured with respect to the shale baseline, a referenceline which can generally be traced along the extremepositive edges of the SP curve. Usually the shale line isstraight and vertical. *

In any given well, since the mud salinity is constant


26/101

and the interstitial waters may tend to be constant, there

is often a definite tendency for the maximum SP deflec-

tions to be the same for the same types of permeable for-

mations at comparable depths. Thus, it is usually possi-

ble to draw, parallel to the shale line, a sand line on the

log along the maximum negative deflections of the clean

sands of sufficient thickness.

It is very likely that, for all the beds where the SPpeaks reach the sand line, (1) the formation-waterresistivity is practically the same, (2) the beds are vir-tually free from shaly material, and (3) the amplitude ofthe deflection is equal to the SSP. For thin beds in caseswhere the SSP cannot be determined as above (or for athin shaly sand), the SP reading from the log must becorrected by means of appropriate charts in order to ob-tain the Essp or Epsp. 2

Determination of R, from SSP

Since the variations of electrotiltration potential fromsand to shale can generally be neglected, the SSP is takenin practice as equal to the corresponding value of -EC aslong as the SP is fully developed.

It is convenient to replace Eq. 8 by

REssp = -Kc log+ . . . . . . . . . . . . (9)R

we

field experience has shown that in certa#n regions there may be shifts of the shale

line. Sometimes rhese shifts are found systemattcatty at the ?.ame places in the

geologlcal column and can be used as markers.


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ELECTRICAL LOGGING

where R, is an equivalent formation-water resistivity .

The computation of R, is given in the chart of Fig.

49.6, and R, is derived from R, by means of the aux-

iliary chart at the lower right of Fig. 49.6. The solid

curves on this auxiliary chart correspond to highly saline

formation waters, where the presence of salts different

from NaCl is negligible in practice. They are derived

from the known activity/resistivity relationships for pure

NaCl solutions. The dashed curves correspond to forma-

tion waters of low salinity, where the presence of other

salts (calcium and magnesium chlorides, sulfates, and

bicarbonates) have an important bearing on the activity

values. These curves are derived from empirical obser-

vations and cover formation waters of average composi-

tion. I9 Note that, for intermediate salinities (0.08

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