DEWAN Modern Hopen Hole

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MODERNINTERPRETATIONOPEN-HOLELOG


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-QiOZLQ


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DEDICATION

Copyright (c) 1983byPennWrli Publishing Company1421 South Sheridan RoadiP.0. Box 1260Tulsa, Oklahoma 74101LiDrurg of Congress Caiaioging ti Yuhlicotioii DatoDewan, John T.Modcrri open-hole og ntrrpretaticiri

Al l right5 rr5er\rd. N o part of this book nia) bereproduced, stored i n a retrieval systcvn, ortranscribed in ariy form or by ariy means, electronicor mechanical, including photocopying and recording,without the prior written permission of the publisher.Printed i n the United Statesof America

This book s dedicated to my former colleagues at Schlumberger, whodid much to advance the science ofwell logging, and to my presentassociates and my family, who patiently endured its preparation.

3 4 5 87 86 85i v


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CONTENTSDEDICATION vINTRODUCTION x i

1 The Logging Environment 1The borehole 1Logging procedure 2The undis turbed reservoir 4Disturbance caused by drilling 8Summary 13References 15

2 Evaluation of Hydrocarbons 17Fundamental interpretation relations 17The basic interpretation procedure 22Impact of invasion on resistivity measurements 26Summary 31References 33

3 Permea ble Zone Logs 35Spontaneous Potential (SP) Log 35

Source of spontaneous potential 36SP behavior over a long log 39Shape of the SP curve 40Computation of R from the SP 43SP log in shaly sands 49SP anomalies due to vertical migration of filtrate 49SP anomalies due to noise 50

The Gamma Ray (GR) Log 50Basic GR logs 50Spectral GR logs 53Statist ca Iuc uat ons 57Summary 60 References 61


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v i i i C O N T E N T SC O N T E N T S i x

4 ResistivityLogs 63Classification an d Application 63

Electrical survey (ES) tools 65Fresh mu d tools 66Salt mu d tools 68Ranges of applicationof nduc tion logs and Laterologs 69The spherically focused og (SFL) 76Log presentation 77

Dual Laterolog-R,, Logs 82

Dual Induction -Spher ically Focused Logs 70

The DLL-MSFL tool 83Depth of investigation 86Characteristicsof the MicroSFL (MSFL) 88Log presenta tion 88Quick-look hydrocarbon indication 89Summary 93References 94

5 Porosity Logs 95The current trend in poroslty logging 95Recent developm ents 96

Compensated Density an d Litho-Density Logs 97The compensated density tool 98Porosity derivat ion from the density logT h e Litho-Density og 108Lithology interpretation with pb-P, curves 114Neutron tool evoiution 117The Compensated Neutron 117Combined Density-Neutron nterpretation 128The Dual Porosity Compensated Neutron log 136

Compensated Sonic and Long-SpacedSonic Logs 139The boreho le compensated log 141Porosity dete rmination from Sonic logs 146The Long-SpacingSonic log 158

105

Com pensated Neutron and Dual Porosity Neutron Logs 115

ElectromagneticPropagation-MicrologCombination 170The EPT-ML sensor array 172The Microlog 173The ElectromagneticPropagation Log 177Summary 191References 194

6 Clean Formation interpretation 199Resistivity-PorosityCrossplots 199

The Hingle plot 200The Pickett plot 207Range of uncertainty n calc ula ted water saturations 209The M-Nplot 21 1The MID plot 214The Litho-Density-Neutronmethod 217Trends in mul timinera l identi fication 223References 225

Multimineral dentification 211

7 Shaly Formation Interpretation 227The natureof shale 230Shale or clay distr ibution in shaly sands 231Shaiy sand interpreta tion models 236Cation exchange capacity 237Shale porosity and conductivity 243Application of the dual-water method to shaiy sands 247Summary of du al water interpretation 257Summary of earlier shaly sand interpretation methods 261References 265

8 Predictionof Producibility 267Flow relations 267Absolute. relative, and effective permeabilities 269irreducible water saturation 271Estimation of permeability from logs 279


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x i INTRODUCTION Chapter1

'I'lie third and current plinse, which bcgan abont 1'370, ma be called t h rlog procrssing era. \I'ith the advent of computers, it has beconie possible toanalyze n much great er detail the \vealth of da ta sent uphole by the loggingtools. Log processing centers, providing sophisticated interpretation o1digitized logs transmitted b y telephone a n d satellite , haire been set u p bservice companies i n strategic locatioiis. Logging trucks h a v e been fittedn,ith computers that perinit computation of quick-look logs at the wellsite.A t thesan ic time logging tools have been coiribined to tlie p i n t that a full setof logs can be obtained on a sirigle run.

Tlic present sta te of the ar t is that logs are adeqiiiite to determinehydrocarbons in situ in inediuni- to high-porosity formations but a re piisliedto their limits in low-porosit ., slial ,, rnixed-litliolog ~ situations. .\loreprecise deterniina tion of the niatris make up, including ainounts and t p sof c l a ~ .resent, is needed. Promising developrnt~its re uiider\vay.

Ad\.ances are being made i n predic ting the proclucibilit ~ ofliydrocarboiis found in place. but tlie critical factor: a contiriuoiisperriieabilit . log, is still lacking. Meanwhile, point-hy-point permeabilityand pressure \zaliies can be 01)tairied b repeat format ion testirig. atecliiiiqut. that i s finding increased use.Developments i n the testing stage promise t o provide more preciselithology informa tion. better movable oil determination, and additioiinlrnechaiiical properties of formations. Uriciiiestioria~)l ..an suw s obtairial>lefroin logs will continue to become more accurate and broader in scope.

THE LOGGING ENVIRONMENTeiotively little is learned about th e producing potential of a well as i tis being drilled. This is a surprise to the uni nitiat ed, who have visionsof early gushers. But the dr illing m u d actually pushes hydrocarbons,

if encountered, out of the way and prevents their return to the surface.Examina tion of returned cuttings indicates the general lithology being pen-etrated and may reveal traces of hydrocarbons, but it allows no estimates ofthe amount of oil or gas in place.Well logs furnish the data necessary for quantitative evaluation ofhydroc arbons in situ. Modern curves provide a wealth of information onboth the rock an d fluid properties of the formations penetrated. From thepoint of view of decision-making, logging is the most impor tant part of thedrilling and completion process. Obtaining ac curate and complete log datais imperative . Logging costs account for only about 5 % of completed wellcosts, so it is false econoniy to cut corners in this phase.THE BOREHOLE

MJhcn the l ogging engineer arrives a t the wellsite with his highly instru-mente d logging unit, he finds readj . to be surveyed a borehole tha t has thefollowing: characteristics:an aver age depth of about 6,000 f t but which may be anywherebetween 1,000 and 20,000 ftan average diameter of about 9 in. but which c an be between 5 in.and 15 in.a deviation from vertical that is usually only a few degrees on landbut typically 20-40" offshorea bottom-hole temperatur e that averages about 150F but may bebetween 100F and 350Fa mud salinity averaging abo ut 10,000 parts per million (ppm) butwhich can vary between 3,000 and 200,000 ppm; occasionally themud may be oil baseda mud weight averaging about 11lbigal but which can vary from 9to 16 lb/gala bottom-hole pressure averaging perhaps 3,000 psi but which c anbe as low as 500 and as high as 15,000 psi

4


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,

References 3449 Wellsite Computed Logs 317

Hard-rock,salt-mud oggingsuites 343Special situations 346References 349

index 351

INTRODUCTIONThe aim of this book is to present modern log interpretation as simplyand concisely as possible. The book is written for the geologist,petrophysicist, reservoir engineer, or production engineer who is familiar

him with computer-processed logs generated by the service companies at thewellsite.Accordingly, obsolete logging tools are mentioned only in perspective.Very brief descriptions of the instruments in common use indica te how theyapply to different logging conditions. Salient features of new tools,including Spectral Gamma Ray, Litho-Density, Dual Porosity Neutron,and Long-Spacing Sonic, emphasize how these tools fit into the everydayconcerned how a tool

logs is run in a liquid-filled open hole to locate hydrocarbons in place andwhere promising zones are then tested to evaluate their producibility.Abnormal situations such as empty hole, water well, and geothermal andmineral logging are not included.To provide a little perspective, well logging is in its third majordevelopment stage. The first 20 years, from 1925-1945, saw theintroduction and gradual worldwide acceptance of the so-called ES(Electrical Survey) logs. These logs were run with simple downhoie toolsand, while quite repeatable, were often difficult to interpret.The second phase, from 1945-1970, was a major tool development era,made possible by the advent of electronics suitable for downhole use.Focused electrical devices were introduced, having good bed resolution andvarious depths of penetration. A variety of acoustic and nuclear tools weredeveloped to provide porosity and lithology information. There was aprogression through second- and even third-gene ration tools of increasingcapability and accuracy. Simultaneously, much laboratory and theoretical

work was done to place log interpretation on a sound, though largelyempirical, basis.x i


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2 ESSENTIALSOF MODERN OPEN-HOLE LOG INTERPRETATION

a sheath of mud cake on all permeable formations that averagesabout 0.5 in. in thickness but may be as little as0.1 in. and as muchas 1 in.an invaded zone extending a few inches to a few feet from theborehole in which much of the original pore fluid has beendisplaced by drilling fluidsEven more severe conditions are occasionally encountered. In any caseit is a challenging environment from which to derive accurate informa-tion about the state of the formations as they were prior to any drillingdisturbance.

LOGGINGPROCEDUREAccustomed to the challenge, the logging crew proceeds to align thetruck with t he well, spool the logging cable through the lower an d uppersheave wheels, and connect the logging tools. The engineer performs thesurface checks and calibrations. After this, the logging array is dropped tobottom as quickly as practicable. Once on bottom the downhole calibra-tions ar e carried out, recording scales are set up, and th e crew comes uplogging (Fig. 1-1). Survey speed is maintained constant, between 1,800and 5,400 ftlhr, depending on the logging tools.The logging str ing is typically 3 in, in diameter and 20-50 ft long. Itusually consists of several different tools in tand em. Th e most imp ortant isthe tool tha t measures the electrical resistance of t he forma tion becauseincreased resistance occurs when water is replaced by hydrocarbons.

Accompanying the resistivity tool is at least one tool th at measures porosityand one that distinguishes permeable from no npermeable zones. The basiclogs may be obtained on a single run in the hole or may require two runs withdifferent logging tools. Operating power for th e tools is sent down one pairof insulated conductors inside the armored logging cable, and logging dataare transmitted to the surface on the remaining five conductors.In recent years the major service companies have been rep lacing oldersurface instrumentation with completely computer -controlled systems thatare much more versatile and easier for the engineer to operat e (Fig. 1-2).*Logging data are digitized and fed into the computer where they areDenoted Cyber Service IJnit (CSU) by Schlumberger, Computer Logging System (CIS) byDresser Atlas, Digital Logging System (DLS) by Welex, and Direct Digital Logging (DDL) byGearhart.

THE LOGGING ENVIRONMENT 3

processed and output to paper, film, and magnetic tape recorders. Theengineer controls the system almost entirely with commands froin the key-board. At the same time, he monitors the output on a screen wliich, during

_-_____L__-Fig 1-1 Wellsite s e t u p for logging [courtesyGearhart)


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Fig. 1-2 Computerized surface instrumentation courtesy Schlumberger)logging, continuously displays the last 100 ft logged. The options in cali-brating, depth shifting, averaging, computing, and scaling the logs arevirtually unlimited with the computer. Further, after the logging i s com-pleted, the taped information can be played back and the various logsedited, combined, and run through interpretation programs to providefully interpreted logs at the wellsite.THE UNDISTURBEDRESERVOIR

An idealized view of porous hydrocarbon-bearing reservoir rock isshown in Fig.1-3. The rock matrix consists of grains of sand, limestone,

T 5

dolomite, or mixtures of these. Between th e grains is pore space filled withwater, oil, and perhaps gas. The water existsas a film around t he rock grainsand as hour-glass rings at grain contacts; it also occupies the very finecrevices. The water forms a continuous path, although very tortuous,through the rock structure.&spaces.

The rock properties importan t in log analysis are porosity, wat er satura-tion, and permeability. The former two determine the qua ntity of gasor oilin place, and the latte r determines the rate a t which that hydrocarbon canbe produced.Porosty

011+ water Oil + gas + water

Fig. 1-3 Hydrocarbon-bearingock


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6 E SS EN TI AL S O F M O D E R N O P E N -H O L E LOG I N TER PR ETATI O NSLJ: / O Q * / . - 5d .

Tlie fraction of pore space contain ing water is termed w a t e r s a f u r a f i o n ,denoted S . Th e rema inin g fraction Containing oil or gas is termed hydr o-c a r b o n s a f u r a t i o t i , Sh , which of course, equals (1 - C,,,). The generalassumption is tha t the reservoir was initially filled with water an d tha t overgeologic time oil or gas that formed elsewhere migrated into the porousformation, displacing water from the larger pore spaces. However. themigrating hydrocarbons never displace all of the interstitial water . Ther e isan irreducible wa ter sattiratioti, S,,,, representing the water retained bysurface tension on grain surfaces. at grain contacts, and in the smallestinterstices. Its valiie varies from abou t 0.05 in very coarse formations withlow surface area to 0 .4 or more in very fine-grained formations with highsurface area . Th e irreducible \vater will not flow Lvhen the formation is puton production.

The fraction of total formation \.oliiine that is hydrocarbons is then 4Shor I$ (1 - S,v) , A major objective o f logging is to determine this qiian tity, Itcan vary from zero to a maximum o f 4 (1 - S,,,,).Permeability

Permeability. denoted k, i s the Elowability of the formation. It i s ameasure of t he rat e at which fluid will f low through a given area of porousrock under a specified pressure gradient. It is expressed in millidarcies(md); 1,000 riid is a high value and 1.0 md is a low value for producingformations.In cont rast to porosity, permeability depends strongly on absolute grainsize of the rock. Large-grained sediments with large pores have high perme-abilities, whereas fine-grained rocks with small pores and more tortuousflow paths ha\Te lo~vermeabilities.Table 1-1lists porosities and permeabilitiesof some well-known produc-ing fornia tions . Porosity varies only by a factor of 3, whereas permeabilityvaries by a factor of about 4,000. We can infer that the\Voodbine formationwith extremely high permeability is exceptionally coarse sand, whereas th eCtrawn formation of th e same porosity bu t low permeability is a ve ry fine-grained sandstone.

Hydrocarbon-BearingRocksHydrocarbon-bearing rocks are primarily sands (SiOp,). limestones(Caco3),and dolomites ( CaCO, . MgCO,). hlost sands are transported by

TABLE 1 - 1 POROSITIES AND PERMEABILITIESOF SELECTED OILSANDS

Sand Poroslty Permeabllity( I md)Clinch, Lee Co. VAWllcox, Okl a. Co. OKCut Bank, Glacier.Co. MTBartlesville, Anderson Co. KSOlympic. Hughes Co. OKWoodbine, Tyler Co. TXStrown, Cooke Co . TXNugget. FremontCo.WYO'Hern, Duval Co. TXEutaw, Choctaw Co. AL-

9.612.015.417.520.52222252830-

0.91 01112535

3,390811471301O0

and laid down from moving water. Tlie greater the water velocity (theenergy of the env ironment ), the coarser th e sand will be. Because of thismechanism, sands tend to have fairly uniform intergranular-type porosity.Limestones, on the other hand, are not transported as grains but ar e laiddown by deposition from seawater. Some is precipitation from solution;some is the accumulated remains of marine shell organisms. Or igina l porespace is often alt ered by subsequent redissolution of some of t he solid matter .Therefore, porosity tends to be less uniform t han in sands, with vugs andfissures, termed secondary porosity, interspersed with the pr imary porosity.Ilolomites are formed when magnesium-rich water circulates throughlimestones, replacing some of the calcium by magnesium. This processgenerally results in a reduction of the matrix volume. Therefore, dolomi-tization i s an impo rtant mechanism in providing pore space for hydrocar-bon accumulation.Formations containi ng only sands or carbonates ar e called clean f o r m a -t ions . They are relatively easy to interpret with modern logs. When suchformations contain clay, they are called d i r t y or shaly f o r m a t i o n s . Suchreservoir rocks can be quite difficult to interpret.Clay and Shale

Clays are common components of sedimentary rock. They ar e alumino-silicates of t he general composition AI,O3.S O 2 . OH),. Depending on theenvironment in which they are formed, they may be of several basic types:rnontmorillonitr, illite, chlorite, or kaolinite.


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Clays have very small particle sizes-1 to 3orders of magnit ude less thanthose of sand grains. Surface-to-volume ratios are very high, 100-10,000times those of sands. Thus, clays can effectively bind large quantities ofwater t hat will not flow but that d o contribute to log response.Shales are primarily mixtures of clay and silt (fine silica) laid down from

hand, sands or carbonates containing modest amounts of clay or shale maybe important hydrocarbon producers.Accounting for clay and shale when analyzing hydrocarbon-bearingformations substantially complicates log interpretation. Consequently, inchapters 2 through 6 we establish the principles of log interpretation forclean formations and i n chapter 7 take up the analysis of shaly format

Durin g the drilling process formations may erode or cave to diameterslarger than bit size, drilling fluid may invade permeable zones, and mudcakes may build up o n the same zones. invasion in parti cular causes loggingproblems.The Processof Invasion

Th e process of invasion is also illustrated in Fig. 1-4. Duringdriiling themud pressure in the annulus, P,, must be kept greater than the hydrostaticpressure of fluid in the fo rmati on pores, P,, to prevent a well blowout. Th edifferential pressure, P, - P,, which is typically a few hundred psi, forcesdrilling fluid into the formation. As this happens solid particles in thedriling mud plate out on the form ation wall and form a mud cake. Liquidthat filters through this mud cake-the mudfiltrate-passes into the forma-tion and pushes back some of the reservoir fluid there. An invaded zone isformed adjacent to th e borehole. Fig. 1-4 Invasionprocess


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I O ESSENTIALS O F M O D E R N O P E N - H OL E LOG I N TER PR ETATI O N

Invasion involves a mud spurt, dynamic filtration, and static filtration.As the bit penetrates a permeable bed, there is an initial spurt of drillingfluid into the freshly exposed rock. This lasts for a matter of seconds untilsmall solid particles in the downcoming mud stream (or formed by grindingaction of the bit) bridge the pore entrances in the rock. Bridging is mostrapid when the part icle size distribution in the mud is well matched to thepore entrance distribution in the rock.As the bit passes on, rnud cake begins to build o n the newly formedborehole wall. Invasion is rapid in the beginning but slows quickly as themud cake thickens and increases its resistance to flow. If condit ions werestatic , the mud cake would continue to build indefinitely with filtration rat edecreasing in accordance with l /& ,where t is the time following spurt.During drilling, however, flow of mud and cuttings plus abrasioncaused by the turning an d whipping of the drillstring continuously erode themud cake arid even the fo rmation itself. Once the formation ceases to erode,a dyn amic equilibrium condition is reached where the mud cake thicknessand the ra te of filtra tion become constant.When the drillstring is pulled to change the bit, the abrasive action i s nolonger present and mud cake resumes building at the permeable zones understatic filtration conditions . When the string is run in an d drilling is resumed,the soft outer miid cake just formed will erode away and dynamic eqiiilib-rium once again will be reached .Finally, when the drillstring is pulled for logging, static filtration willresume and soft mud cake will again build up. The additional buildup isoften evidenced by logging tools measuring hole diameter less than bitdiamete r in permeable zones near bottom. Mud cake is typically in. inthickness at th e time of logging.Fig. 1-5shows schematically the rate o f nvasion, mud cake thickness,and d epth of invasicin at a given permeable bed as a function of time sincethe bed was pene trated. The depth of invasion increases rapidly during thespurt and fo rmation erosion periods. I.,ater it slows because of dynamicequilibrium and because the rat e of increase in invasion depth , for a con-stant filtration rate, s inversely proportional io the invasion depth a lreadyreached.DepthOf InvasionAt Time Of Logging

The depth to which mud filtrate has penetrated a porous formation atthe time of logging depends on several factors, principal of whch are thefiltration characterist ics of t he drilling mud and the differential pressurebetween mud and reservoir. Static filtration rate of a niud is given as a water

I 1THE LOGGING E N V I R O N M E N T

loss figure on the log heading . This is the amount of f iltrate (in cc) passingthrough a filter paper in 30 min at 100 psi differential pressure and 76F n astandarizedAPI est. A typical figure s 12 cc; 30 cc is considered poor wallbuilding mud, and 4 cc is very good. Unfortunately, experiinents haveshown tha t the re is little correlation between s tatic filtration characteristicsat surface temperature and dynamic filtration a t borehole teniperatilres.Consequentlj., it is not possible to predict invasion depth from availablemud and drilling information. The analyst must infer this from logs.One can predict, nevertheless. how depth of invasion for a given rnudrelates to porosity. Once the mud cake has started to build. its permeabilitybecomes low relative to that of the average formation so that almost all ofthe pressure differential (P, -- P,) is across the mud cake and little s appliedto the formation . The mud cake therefore controls filtration rate. Conse-quently, in a given time the same volume o f fluid will invade differentforniations, regardless of thei r porosities or permeabilities (iiriless permea-bility is belmv about 1 O nid ). This means depth of invasion will be niini-mum at high porosity where plenty of pore space is available for invadingfluid and maximum at low porosity where little room is available. I t is

DeDth of invasion -Rate of invasion

Drill4 rip +Drill -Eud cake thicknessI I I I

I

10 1O 0 1,000 10 000 100.000

Fig. 1-5 Invusioneffects


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EVALUATION OF HYDROCARBONSconducts no electricity. All conduction is then via the fluid in the pores. Atdepths below 2,000 ft, the water found in f ormation pores is generally fairlysaline, which makes it quite conductive. Water-bear ing formations there-fore tend to have high electrical conductivity or, the equivalent, low electri-cal resistivity since resistivity is the reciprocal of conductivity.

clean formations with modern logs can often be performed without a singlechart.F U N D A M E N T A L I N T E RP R E T A T IO N R E L A T I O N S

To establish the relations for hydrocarbon saturation arid to clarifjr theterms involved, let us conceptually construct an oil-bearing formation andmeasure its electrical properties as we do so.Definitionof R,I

Visualize an open-top cubic tank one meter in all dimensions. It haselectrically nonconducting sides except for two opposite walls that a re metaland serve as electrodes.17


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18 ESSENTIALS OF M O D E R N O P EN -H O L E LOG I N TER PR ETATI O N

First, the tank is filled with water containing about 10 % sodium chlo-ride ( NaC1) by weight to simulate an average formation water. A low-frequency alterna ting voltage, \, is applied across the electrodes, and th eresilltirig cur ren t I1 s measured (Fig. 2-la). ?he ratio V U , (voltsiamperes)is R the resistivity of tlie formation wat er, in units of ohm-meters. Thisresistivity is an intrinsic property of the watcr a nd is a function of its salinityand t eniperatu re. Th e higher these two variables, the more conductive thewater will he and the lower its rcsistii-it)..

Fig 2 - 1 Llefinitionof rewtiviiiec

E V A L U A T I O N O F H Y D R O C A R B O N S 19

Definitionof RoNext, sand ic poured into the water-filled tank and tlie volume of waterexpelled is meawred. \ihen the sand is level with t he top, the result is a

porous. water-bear ing formation of one-meter dinieri~io nsAbout 0.6 cti rnof \vater will have been expelled, s o the porosity of the forniatiori will IF (1-0.6) o r 0.4 . Again the voltage is applied and a current I, is measured (Fig .2 - 1b). I2 will be less than I , siiice thcrc is less water to conduct elrctricity.The ratio \./I2 isR the resistivity of tlie water-hear ing forniation. It will belarger than H,, .Formation Factor

The resistivity, I{


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- -efinition of R, I11

oped by G.E. Archie of Shell and s ermed th e Archiewell-lo@@lg i ndus t ys

The whole' p o n thisequa t ion*

Eq. 2.8 shows that hydrocarbons in place can be evaluated if there areNow an appreciable fraction of the pore water i replaced by oil, result-ing in the situation depicted in Fig. 2-le. The same voltage, V , is applied,and c urrent I, is measured. It will be less than IL since even less water is

Water SaturationKnowing R, and R,,water saturation, S, , the fraction of pore spacecontaining wate r, can be calculated. Againon general principles there mustbe a relation of the form

F)'$ 5 .)-"-") L' KThi) nduct-p*Lutu? % CSri\\+mod:This is the basic equation of log interpretataon. It was initially devel-

2.8 n a nearby water sand (S, = 1)or from the SP log or from catalogs orwater sample measurements; R, s obtained from deep resistivity readings(Induction or Laterolog); and 4 is obtained from porosity logs (Density,Neutron, or Sonic).1i

R, = R,/C, Hydrocarbons n Place(2.5)

This relation can be used directly to calculate the water saturation of ahydrocarbon-bearing zone when an obvious w ater-bearing zone of thesatneporosity and having water of thesamesal ini ty isnearby. An example wouldbe a thick sand with an obvious water-oil contact in the middle.In genera l there will not be a nearby water sand to giveR,, soEq. 2.6 willnot apply. Replacing R , by Eq. .1 givess, =

Replacing F by Eq. 2.3 gives

where c= 1.0 for carbonates and 0.90 for sands.

formation volume factor B, a value slightly greater than unity, which takesinto account the shrinkage of oil volume, principally by gas evolution, as itcomes to the surface.For gas, the number of cubic feet in situ is

G = 43,560.4(1 - S , ) . h . A (2.10)'The general equation is S = (a/


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ESSENTIALS OF M O D E R N O P E N - HO L E L O G N T ER P RE T A T IO N

cs is the amount of gas at reservoir pressure and temperature. To-t to st an dar d cubic feet at 14 .7 psi and 6O0F, he number is miilti----===d

7 the quant i ty +a2 I -

= reservoir pressure, psi: if not kiiown it may he taken as 0.46d= formation emperature, OF, determinable from log information= deviati on factor of the gas at formation temper ature and prei-

where d is the vertical depth of the reservoir in feetsu re , obtainable from charts; it will be close to unity- aniount o f hydrocarbons actuall . recoverable as a fraction of theIS ty in p la ce will depend on the reservoir type. the initial hydrocarbon- i o n , a nd the prodiiction mechanism. A reasonable estimate for pri-m r o d u c t i o n would be 20% for oil and 70 for gas.-- SIC INTERPRETATION PROCEDURE

Te(' ba si c logs are required for adequat e formation evaulation. One is4 o show permeable zones, one to give reiirtivit of the undisturbed* t i o n and one to record porosity. An idealized set is shown in Fig 2 - 2 .- r m e a b l e zone log is in Track 1. the resistivitl- log i n Track 2, arid thes t y og in Track 3 . The permeable zone log is either Spontanroiis- - i a l or Gamma Hay , the resistivity i s either deep Indiiction or deep-log, ar id th e porosity log i s either Density, Neutron, or Sonic. C'I\en-~~__

i i et of logc, the problem is to cletermine- r e re the potential producing zonesw uch hydrocarbon (oil or gas) do they contain- =tion of ProductiveZones-h e i r s t step is to locate the permeable zones. This is done by scanningg in Trac k 1. It has a base line on the right arid occasional slvings to tlic~~ ~


Permeable zoneindication

I Resistivity Porosity

Fig.2-2 Idealized og set


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26 ESSENTIALS O F M O D E R N O P E N- H OL E L O G N T ER P RE T A TI ON

Reservoir pressure would be estimated as 0.46 x 7,300 or 3,360 psi andreservoir temperature as approximately 150'F. Assuming a gas deviationfactor of 1 O, the amount of gas in place in standa rd cubic feet would be, byEq. 2.11

14.2 X lo6 X (3,360114.7) X (5201610) = 2,770 X I O f iWith a recovery factor of 0.70, the producible amount is estimatedat 0.7 x 2,770 or 1,940 Mh4scf. A gas price of $31Mcf would yield a po-tential revenue of $5.8 million. Again, the well would be definitely worthcompleting.

IMPACT O F INVASION ON RESISTIVITY MEASUREMENTSIn the foregoing discussion the formation resistivity, R,, was assumed tobe tha t of the undisturbed reservoir beyond any invasion. The difficulties inmeasuring that resistivity may be appreciated by considering the distur-bance caused by invasion, as illustrated in Fig. 2-3.Immedia tely behind the borehole wall is a flushed zone of diameter d,containing only mud filtra te of resistivity Rmfand residual hydrocarbon.The resistivity of tha t zone is denoted R,, and the water sa turation is Sxo.Thethickness of this zone is of the order of 6 in. but can be much more or muchless. Behind th e flushed zone is the trans ition zone of di ameter d, , which mayextend several feet. Beyond that is the undisturbed formation with resistiv-ity R,, interstitial water resistivity R,", and water saturation C,.Th e existence of invasion has forced the development of resistivity log-ging tools t hat measure as deeply as possible in an effor t to read R, uninflu-enced by mud filtra te. However, no tool has been developed that can readdeeply enough under all circumstances and still maintain good verticalresolution. Consequently , the industry is gradually standardizing on run-ning three resistivity curves at the same time. One is a deep investigationcurve, one is a medium curve, and one is a shallow curve. With three curves

the reading of th e deep one can be corrected for invasion effects to providean R,value. As a side benefit the flushed zone resistivity and the diameter o invasion can also be estimated.


Mud Filiraie R e s i s t i v i t y ; Fresh Muds and Salt Muds'To apprec iate the difference in resistivity readings of shallow, medium,and deep curves, i t is necessary to consider the contrast between miid filtra teresisti\ity, R,,, and inter stit ial wa ter resisti\.it v. Rw . M u d filtra te resjsti\.ityis measured at the kvellsite by the logging engineer. He catches a sample ofmiid. preferably from the mud return line, places it in a mud filte r press thatforces filtrate through a filter paper and measures the resistivity of thefiltrate in a resistivity-measuring cell. The value of H,,,, along with tlictemperature at tvhich it is measured are included in the log heading.0 e s i s t i v i t y o f t h e z o n e0 e s i s t i v i t y of t h e w a t e r i n t h e z o n eA- a t e r s a t u r a t io n i n t h e z o n e

Fig.2-3 ldeuliiec invasion profile (courtesySchlurnberger)


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IS are drilled with fresh (low-salinity) muds that have

value. If it were deep, the IL,, would he very influenced by the m u d filtrateand would read close to LL,. Thus, the position of theIL,,,urve relative tothe others is a quick-look indicator of invasion depth . Finally, we can seethat the shales have no permeability because they have not invaded; allcurves read alike.In wells drilled with saturated salt muds, the positions o f the threeresistivity curves in water -bea ring intervals will reverse. Theshallow curvewill read lowest and the deep curve will read highest.Invasion Profiles

Fig. 2-5 shows three different resistivity profiles proceeding from theflushed to the undisturbed zone for the fresh mud case where R, > R,. Theassumption made i n deri ving a correction for R, with three resistivity curvesis a step profile-an abr upt change from R,, to R, at an equivalent diameter

29

d,. In actuality the resistivity will change gradually from the R value atdiameter df to the R, alue at diameter d,, asshown by the dashed curve of

with high hydrocarbon saturation wherein invading filtrate displaceshydrocarbon faster than the interstitial water. This creates an annulus orbank of formation water where the resistivity is temporarily lower thaneither R,, or R, (Fig. 2-5, dotted line). I t is a transient phenomenon, lastingonly a few days.?

Fig.2-4 Effectof invasion on resistivity measurements (CourtesySchlumberger)


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30 ESSENTIALS O F MODER N OPEN-HOLE LOG INTERPRETATION

Movable Oil CalculationInvasion has one redeeming feature: It can provide information onhydrocarbon prodiicibility through comparison of li .drocarbon satur ation

in the flushed and undisturbed zones. The difference between tlie t ivosaturations represents hydrocarbon that has been pushed back by invadingfluid an d therefore should be recoverable on production . at least hy a \vaterdrive simulating invasion in reverse.The wat er satu ration relations already developed can be applied to theflushed zone provided appropriate resistivity values are used for the fluid-filled rock, &,. and the pore fluid, Rmf.Following Eq. 2.8, the watersaturation in the flushed zone is(2.12)

The hydrocarbon saturation (1 - S,,) will be less than that in theiininkaded zone, (1 - S,,), by virtue of the displacement caued b y filtrate.Therefore, the movable oil saturation is the difference, which is SXq - S,,).By Eq5. 2.12 and 9.8

t2=._E??c.-E8U

ODistance from borehole wall -Fig 2-5 Resistivity profiles- step, gradual, and annulus

EVALUATION O F HYDROCARBONS 31

This is the fraction of pore fluid that constitutes moved oil. It reprcsents anupper limit of M hat might be recovered on production.As an example, assume a limeytone formation with C$ == O.18. R,, = 0.04,

I t , , , ,= 0 .50 , H t = 10, and R,,, = 25 o h m - n i

Tha t is. 43% of t he reservoir pore space constitutes movable oil. Alterna-tively, th e bulk \rolunie fraction of movable oil is d (Sxo- S,v)or 7 . 7 % .E q . 2.13 works well in salt-mud situations but tends to overestimate theamoun t of movable oil in fresh-mud conditions. Recent investigation has ledto the conclusion that all of th e connate water in tlie hydrocarboil-bearingzone niay not be replaced by mud filtrate; some may be shielded hy therrGdual oil and left in place. This iiicomplete replacement does not m atte r\vhen R,,, = R,,,. as i n tliecase of salt m u d . However for fresh miid where> R,v. R,, is too high a value to use for flushed-zone \vater rcsistivity. T heerror can be large when the oil is heavy arid residual oil satiiration is high.?lie same overestimation occurs if the invasion is so shallow that the R,,reading is influenced b the formation water.Consequently, the preferred met hod of estimating movable oil in freshrriud is with the electromagnetic propagation technique describrd inchapter 5 .SUMMARY

ARCHIE RELATIONS F O R WATER SATURATIONGeneral: S -= c . +,IR, / 6

R, = deep resistivity,o h m - mR = interstitialwater resistivity, ohm-m4 = porosity, fractionc = 1 O for carbonates, 0.9or sands

Specific: S = JROEJR = resistivity of water-bearing formation of sameporosity and R, as for R, forrnation

32 ESSENTIALS OF M O D E R N O PE N -H O L E L O G N T E RP R E TA T I ON E V A L U A T I O N O F H Y D R O C A R B O N S 33


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R E C O V E R A B L E O I L , S T O C K - T A N K B BLc= 7758.r. 4 e (1 - S . h . A/Br = recovery factor, =0.2h = thickness of producing formation, ftA = well spacing, acres5 =;- volume factor of oil,z 4 2

R E C OV E R A B L E G A S , MMscfr = recovery factor, = 0.7P. = reservoir pressure, psiT, = reservoir temperature, O FZ = gas deviation factor, 0.8-1.2

G = 1.54 . r * I?)or saline (Rmf< R* Movableoil can be calculatedS - S = c ( ti~, , i~, , ./Rwin,) 156 e)

R = mud filtrate resistivityat levelof interest, ohm-m

R = flushed zone resistivity, ohm-m.The calculation is more reliable with salt mud than fresh mud.REFERENCES

J t i V -ity of Brine-Saturated Sands in Relation to Pore Geometry, AAPG Bull. , Vol. 36(February 1952), pp. 253-277..E. Archie, The Electrical Resistivity Log as an Aid in Determining SorneReservoir Characteristics, S P E - A M E Transactions, Vol. 146 (1942), pp. 54-62.3M. Condouin andA. Heim, Experinientally Determined Resistivity Profilesin Invaded Water and Oil Sands for Linear Flows, SPE paper 712 presentedOctober 6-9, 1963.C Boleldieu andA. ibbit,A More Accurate Water Saturation Evaluationin The Invaded Zone,SPWLA Logging Symposium Transactions (June1981).


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Chapter3

PERMEABLE ZONE LOGShe first step in analyzing a set of logs. as outlined previously. is to pickout the permeable zones, lvliich may be sands or carbonates. anddiscard the impermeable shales. The logs used for this purpose a rethe Spontaneous Potential (SP) and tfic Cainnia Ray (GR).Thcy are alwaysrecorded in Track 1.The two logs distinguish shalesfrom nonshales by quit e different niecha-nisms. The SP is an electrical measurement an d the GR is a nucle ar iiieasure-inent . Sometimes the logs are virtually identical: sometimes the , are vastlydifferent. Fortunately, when one is poor, thc: other is usually good.Fig. 3-1 compares SI and GR logs in a t .pical soft rock sand-and-shaleseqiience. Both curves are good in tliis case and clearly distinguish the shaleson the right from thc perrneable sands on the left. Iri soft rock tlie SPgmerally gives a more black-and-white distinction bet\veen tlie shalps andthe sands than does thc G H . The latter shows more variability in both shale

and sand readings.By contrast, in har d limestone formations the SP niay be a lazy, poorlydeveloped curve that hardly resolves permeable and impermeable zones.The Gil is siiperior under these conditions, giving good shale-carlmnatedistinction and bed resolution.Both curves are tised to indicate th e shale content of a permeable zone forshaly formation interpretation (chapter 7 ) .The GI1 i s inore quantitativethan the SP in this respect. O n the other harid the SP may be used to give theformation water resisti\ity reqiiirrd for saturation calculations.

~

_ _ _ _ _ _ ~ - -S P O N T A N E O U S P O TE N TI A L (SP) L O G

The SP log is a recording of the difference in electrical potential bctwee ~ia fixed electrode a t t he surfac e and a movable electrode in the borehole. Theliok must be filled with conductive mud . N o SP can be measured in oil-basemud, ernpty holes, or cased holes. The scale of th e SI log i s in millivolts.There is no absolute zero; only cliarige~n potential are recorded.35

40 ESSENTtALS O F M O D E R N O P E N- H OL E LOG I N TER PR ETATI O N P E RM E A B L E Z O N E LOGS 44


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ior is observed. At very shallow depths where fo rmation water is fresh, theSP in sands is reversed, i .e., has positive polarity relative to the shale value.Slightly deeper, perhaps at about 1,000 ft, the SP goes to zero. As depthprogresses the formation water gradually becomes more saline and the SPincreases in magnitude (negatively). Values of 70-100 niv at depths of 8,000ft aFM 41 greatw ~ e ~ ~ she eo te water sornci'rimcs esagain in salinity, especially when overpressured formation5 are encoun-tered. In such cases the SP correspondingly reduces in magnitude. In unu-sual cases it may even reverse again, as Fig. 3-3 illustrates. The SP in thepermeable bed at 6,300 ft is normal; the SP at 9,100 f t is reversed.In typical salt muds the SP is often useless because the SP magnitudes atdepths of interest are small (Rf t , *= IR,) and because boundary definitionwith low-resistivity mud and high-resistivity formations is extremelypoor.This is explained in the following.SHAPE OF THE SP CURVE

The measured SP is really the potential change that occurs in the bore-hole asa result of SP-generated currents flowing through the resistive bore-hole fluid. This is illustrated in Fig. 3-4. Opposite a shale far from apermeable zone boundary, no current flows, so the potential is constant. Asthe boundary of a pe e Zone is approached encountered,which causes the pot go negative with res shale.Directly opposite the boundary, current flow is maximum so the poten-tial change per unit length of hole, which is the slope of the SP curve, i sgreatest. Beyond the boundary the current density decreases and graduallygoes to zero. If the bed is thick enough the potential becomes constant veryclose to the SSP value. This value is always measured relative to the shalebase line. As the other boundary with the shale is approached, the reversesituation is encountered and the potential returns gradually to the shalevalue.The sharpness of the SFcurve at a boundary, and therefore the verticalbed resolution, depends on the pattern of current flow at the boundary. Thisis a complicated function of the relative resistivities of the mud, permeablezone (invaded and noninvaded portions), and shale and to a lesser extent is afunction of hole diameter and depth of invasion. The overridingprinciple isthat currents seek the lowest resistance path in a loop. In a given formationthey will spread out to the point that the formation resistance, whichdecreases as I/a rea of flow, is negligible with respect to the borehole resis-tance, which increases linearly with length of current path in the mud.

Consequently, when the ratio of formation to mud resistivity is high, thecurren ts will spread widely. The re will be long flow paths in the borehole,and bed boundaries will be poorly defined. Conversely, when that ratio isI

RESiSTiYH M 5 k TYI - II I

, 2 0

-A

Fig.3-3 Example of normal and reversed SP deflections


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42 ESSENTIALS O F M O D E R N OP E N -H O L E L O G N T ER P RE T A T IO N

Uj U

P ER ME A B L E Z O N E L O G S 43

h v , 1)ounclaries will he sharply defined. Note that the boundaries areopposite the points of inflection (maximum slope) of the S P curve, notnecessarily opposite the Ii alfway point between shale and permeable-zoneSP levels.The more gradual the bound ary transition on the SP, the poorer the bedresolution \vil1 be . A rule of t hun ib is that t he bed resolution 11(ft) . defined astha t thickness requirecl for the SI' to reach 80 ";o of the thick-bed reading ( tlieSSP), i s given b y

(3.1)\vl iere R, is tlie resistivity of the iiivadcd zone (the shallow log reading) andR,, is the mud resis.ti\ it . ,4n even coarser approximation is

11 = li4J ( 3 . 8 )Tvhere 4 is the porosity (fractional). This indicates that pernieable bedresoliition varies from a h i t 3 ft at 30%3 porosity to 30 ft at 3 % porosity.

SP logs therefore d~l in ea te ermeable beds quite well in porous cand-and-shale sequences but resolve beds very poorly in tight formations. Inhard rock areas there may be massive low-porosity limestone or ~n hy dr it esof x.ery high resist ivity. These show l i p as long. straight lines (no current atall entering or leaving the hole opposite these formations) on tlie SP andmake interbedded permeable zones rt:cognizable only by clianges in thes lope of the lazy SP cur\'e. Fig. 3-5 is an illustration.Seni ce company charts are available to correct the SP response in a thinI ) e ~ l , ~ . ' "uch as zonc B of Fig. 3-6, to \chat it woiild b e in a thick twi f therearc no thick l>eds o indicate the SSP directly. IIo\vever, thcse charts a re riottoo precise. A correction factor greater tha n 1 . 3 has diibioiis accuracy.COMPUTATION OF R, FROM THE SP

E q . 3.6 is used exteiisi\dy to determine the formation water resistivitythat is required for \vater saturation calculations. First. the SSP is read fromthe log as the difference in millivolts t)et\veen th e shale level and th e thick-clean-water -said level nrar the zone of intercst. This i s illiistrated in Fig.-3-6, The shale line is taken as the maximum SPexcursion to the right. Thesand l ine is taken as the maximum deflection to h e left in zone A , which is a\\ ater-bexaring sand . as shoxvri \)y the l o ~ veading (1 ohrii-m) of the deepresistivity (dashcd) ci1n.e. The SSI' is read as - i8 ni\', thtl scale tjeiiig 1 0 nivpt'r chart di\.iaion as indicated i n the log heading.

48 ESSENTIALS O F M O D E R N O P E N -H O L E LOG I N TER PR ETATI O N P E RM E A B L E Z O N E LOGS 49


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In such cases the dashed lines in th e lo\\ er half of Fig. 3-8 should be used totransform R,,, to R,, These lines are for average fresh forniation waters;corrections can be larg e. For example, R,,,,, = 0.7 at 100F leads to R, =1 . 4 , a factor of two increase.Whenever possible the R,, value obtained from the SP should be checkedEq. 2 ,, -= 1 n therante or nearby

water sands. If there is good agreement, \vater saturation in hydrocarbonzones can be calculated with confidence. If there is substantial disagree-nient, t he value derived from Eq. 2.8 i \ preferred. However, the reason for0 0 1 5 0 0 FE K 400F

F i g . 3-8 Determination of R, from R (courtesy Schlumberger)

the discrepancr should be sought, particularly by checking he R,,, nieasure-ment and if possible obtaining a sample of format ion water for analysis.SP LOG IN SHALY SANDS'2s'3

\VI1clay particles create internal membra ne potentials that,\vhen added together, constitute a potential opposing the normal electro-chemical potential in th e adjacent shale. This reduces the SSP to a pseudo-static value called PSP. C'rider ideal conditions where the shale laniinationshave thr same resistivity as the \arid laminations (both invaded andiinivaded portions), the percent reduction in SSP equals the percentage ofshale by volume.I n the case where the sand is suttstantially more resistive than the shale,the percent reduction in SSP i s much greater than the percentage of shale.

oil-beating zone by virtue of it9 higher resistivity.The SP log is used as oneof the prirne indica tors of shale fraction, V,,, in ashaly sand. Computation of Vq * s descrihed in chapter 7. The valueobtained from the SP tends to be an iipper limit for the reasons justindicated.SP ANQMALIES DUE TO VERTICAL MIGRATION O F FlLTRATE ''

lien a her)' permeable sand containing salt \vater is invaded h y fresh-rtiud filtrate, the lighter mud filtrate will float up toward the upper bound-ar>'of he sand. After a few dn )s , inva\ion \vil1 be quit e deep just below thetipper boiiridar . and ver) shallow near the lo uer. T he SSP will be signifi-cant11 reduced at the bottom because the diffusion potential, E,, disappears(n th no invasion). Further, it is replaced b) an electrochemical potentialacrojs the m u d cake that now directly separates the borehole miid a nd theformation water that has reclaimed the originally invaded pores. Thispotential opposes the normal shale potential Es,,.oth effects reduce thenormal SP deflection at the bottom of thesand. At the top of thesand, the SPdeflection will be rounded due to the higher resistivity of the deeply invadedfresh filtr ate.


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50 E S SE N TI A L S O F M O D E R N O P E N -H O L E LOG I N T ER P R E T A T IO N P E RM E A B L E Z O N E L O G S 51

Furt her, if such a sand bed contains thin, in terhedded shale strcak5, theSPwill show negative an d positive undulations from t he normal level aboveand below each streak.SP ANOMALIES DUE TO NOISESpurious DC currents in the ground and in th e boreliolccan be causcd b ytelluric currents, i)imetallic potentials, cathodic protection del ices, leakyrig power sources, welding machines. e tc. Other sources of noise are magne-tizcrl cable drums and intermittent contact5 between casing and loggingcable. Occasionally these niay disturb the normal SP, biit iiTiially t h e y caiibe eliminated with appropriate nieasiires.

THE GA MMA RAY (GR)LOGThe basic CR log is simple arid easy to record and to understand. It wasroutinely recorded for many years with only minor improvements in instru-mentation and with limited quantitative use. In the past few years, ho\x.-ever, the introduc tion of the spectral CR has given the recording of earthradioactivity a new lease on life. It is exciting to see new uses for thetechnique unfold.

BASICGR LOGSThe basic GR log is a recording of the natural radioactivity of forma-

tions. The radioactivity arises from uranium (U), thorium (Th), nd potas-sium (I


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O 5@ 100 API unitsI 1L

Shaly sand

Shale

sand

lean limestonetDolomite

Shaie

Fig.3-9 G a m m a Ray response intypical formations

Occasionally the mud in a well contains excessive amounts of potassiumor ur anium, either because potassium chloride has been ad ded (to preventshale swelling) or because potash or u ranium beds have been p enetrated.Th e potassium or uran ium in th e mud will contribut e to the GR log, givingan abn ormally high background level (th at will vary with hole diameter) onwh mal k e Smethod of correction for this effect has been published."Shale Determination

Because uranium, thorium, and potassium are largely concentrated inclay minerals, the CR log is used extensively in shaly sand interpretation toestimate he fraction of shale by volume, V,,,, in thesand."This procedureisdescribed in chapte r 7. Basically, it is a mat ter of es timating the clean sandand 10070 hale levels on the log and interpolating between them to deter-mine V,, in a partially shaly interval. It is not a very precise technique, soother shale in dicators are used as well.SPECTRAL GR LOGS'8s'9

The radioactive elements uranium, thorium, and potassium emitgamma rays of different energies, as shown in Fig. 3-10. Potassiumsingle energy at 1.46 rnev (million electron volts). Thorium and u raemit gamma rays of various energies, th e major distinction being a pronii-nent thorium energy at 2.62 mev and a predominant uranium energy atabout 0.6 niev. In principle it is therefore possible to distinguish the threeemitters by anal?zing the energies of detected gamma rays.As gamma rays progres\ from the point of origin in the formation to thedetector i n the borehole, their energies suffer severe degradation. Furthe rsmear ing takes place in the detec tor itself. Nevertheless, it is possible withappropriate instrumentation and careful analysis of the spectrum of pulseamplitudes from th e detector to break down t he total G R log into its ura-nium, th orium, and potassium components and to generate spectral GR logsshowing directly the concentrations o f each of these elements in th e loggedformations.Fig. 3-11 is an example. Uranium and thorium are scaled in parts permillion (ppm) and potassium is scaled in percent by weight jwt%). Thethree elements contribute roughly equally to the total counting rate eventhoug h potassium ispresent in much greater concentration. Thespectral logshows large differences in the shales above and below the clean zone from


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O

u

i

N

N

u

O?

56 ESSENTIALS OF M O D E R N O P E N -H O L E L O G N T ER P RE T A T IO N P ER ME A B LE Z O N E L O G S 57


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eliminated ar e better indicators of shale content than the total G R activity.This is because uranium salts are soluble and can be transported by liquidmovement after p rimary deposition. They may be picked up in one locationand precipitated elsewhere, particularly where a pressure drop occurs. Fig.3-12shows a case where the standar d GR log indicates interval A to be threeSt? ed bthe thorium and potassium spectral curves clearly show the reservoir to be acontinuous unit. The shale streaks are simply permeable zones high inuranium con ten t. New wells in old fields often show such response oppositepermeable zones that are producing in adjacent wells.In other areas where abnormal amounts of potassium-bearing micas andfeldspars are associated with the sands (the North Sea, grani te wash areas ofthe southwestern U.S.), the thorium component alone is the best indicatorof shale content. I n any case, whichever shale indicator is used, thorium -potassium or thorium alone, the shale fraction is derived from the desiredspectral cur ve in exactly the same fashion asdescribed in chapter 7 for thetotal GR curve.Efforts are underway to determine from spectral GR logs the type as wellas the amount of clay encountered in formations. Fig. 3-13 is a proposed

I Ilo API UNITS 15 1 I KO o 5 co Th7 0 ppmlCD1 I

A

Fig. 3-12 Spectral GR log showing single reservoir instead of three T:interval A (Alberta) (couriesy Dresser)

I 917 /

POSSIBLE1000 K A O L I N I T E . l o o m IL L ITE P O I N T MONTMORILLONITE,20

i

O 1 2 3 4 5 6K(%)

Fig 3-13 1denttficat:onof clays and other minerais from potassium andthorium responses (courtesy Schlumberger, Q SPE)chart for identification of clay type from thorium and potassiuni logresporises.Approaches such as this will assist in determing formation pro-ductivity, since different types of clay have quite different surface-to-volume ratios and therefore .water-retention and permeability-reductioncapabilities.In an other application the urani um component of thespectral curves hasbeen used to indicate fr actured zones in tight carbonate?.. High uran iumcontent is taken ab an indication that fissures in the zones were permeableenough t o flow fluids in the past and therefore may still be able to conduct oilarid P ~ Sr o m rcrriote porosit .S T A T I ST I C A L F L U C T U A T I O N S

Ganima Ray logs never repeat exactly. Th e same is true for all types ofnuclear logs. Some of thes mall wiggles on the logs are statistical fluctuationsthat do not reproduce an d do not represent formation variations. In readingthe logs, averages over about 3-4 ft should be used. T he exception is a bedtha t is less than 3 ft thick, in which case the peak reading should be taken.Th e Source of statistical fl uctua tion is the random natu re of nuclearprocesses. The emission of any one gamma ray in a formation is in no waytime related to any other . Consequently, the pulses from the gamma raydetector appear as a random sequence. The number occurring in any given


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58 ESSENTIALS O F M O D E R N O P E N - HO L E L O G N T E RP R E TA T I ON

time interiaal \vil1 differ from that i n a succewive but identic.al tinw interval,eien thoiigh the detector is stat ionary. Iercciitage-\vise this difference willbe small if the timc interval is long enough tha t a large nilinher of p u l w s isciuntect. Conversely, it nil1 b e large if the averaging time is small . Ameasure of the percentage fiuctiiation is 1 0 0 i 6 where N is the atwageniiniber of piilses in the measiiring interval.Typical pulse rates for the basic CH log are 50-300 pulses per second.The iisual averaging time is 2 sec. This means the log will s h o w fluctiiationsof about 4 (j above and helow the mean reading in shales(N= 600) and 10 %either side of the mean in clean sands or carbonates (N= 00). ribsoliitemagnitudes will be in the range of k5-10 API units in shales and k 2-4 APIunits in clean forniatioris. The best way i o appreciate this is to overlayrepeat logs.Statistical fliictuations may he reduced by increasing the awragingtime, but the valrie employed sets a limit on the logging p e d . Fig. 3-14shows the approximate response (without statistical fluctuation) to a 4 ft bedui th a detector of 8-12 in. active length for a 2-sec averaging time and forlogging speeds of 600, 1,800, and 5,400 ftihr . At 600 ftihr the boundaryresponse is primarily determined by the length of t he counter; resolutionwill not improve at slower speeds. At 1,800 ftihr the bed is adeqiiatelyresolved, althoiigh it appears to be shifted about G in. in the direction oflogging; compensation is made in the recording process for this shift. At5,400 ftihr t he bed is quite distorted and its ampl itude is reduced us well asbeing shifted about 1s in.If the averaging time is doublcd, for example. logging speed must behalved to niaintain the same bed resolution. A Compromise must al\\,aq.sbemade bctween log acciiracy and logging speed. A reasonable logging speedis that whi rh nioves the detec tor its effective length (1 ft) in the averaginginterval. For 2-sec averaging thi s is 1:800 ftihr: most nuclear logs are rim atthis speed. If the speed varies, bed resolution will change but the iriaqriitutlesof statistical fluctuations will rcniain constant.With logging-truck computers, averaging over a fixed depth intervalrath er tha n time interval may be employed. Typically this is set at I ft for theGI1log. With loggingspeed of 1,800 fti hr, effective averaging time is again2 sec. However, if logging speed is doubled, averaging time is reduced by afactor of 2. Bed resolution remains fixed but statistical variations increaseb y a factor of v, .

The problem of statistical uncertainty is aggravated mith spectral CRlogs because the counting rates in the uranirini. tlioriiini. and potassiumchannels are 3-10 tinies lo\ver than t hat of the total GR, depending on the

P ER ME A B L E Z O N E L O G S 59

APlUO

jp\t5400 hr\

120

-Fig 3-14 Effects of GRweraging tirn61 and logging weed on bedresolulion

60 ESSENTIALS OF M O D E R N O P E N -H O L E L O G N T ER P RE T A T IO N P E RM EA B L E Z O N E L O G S


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particu lar inst rumentat ion. This means averaging times must be increasedand logging speed must be slowed. Typical values are 4-6 sec and 900-600ftih r. These a re painfully slow logging speeds, so the best technique i s to firstlog the well at normal speed while recording a total GR curve and then rerunthe sh ort sections of vital interest recording the spectral curves.SUMMARYS P O N T A N E O U S P OT E NT IA L L O G

0 A recording of voltage generated b y electrochemical andelectrokinetic action at the junction of a permeable zone andshale.

0 Distinguishes impermeable shales from permeable sands orcarbonates.

op with only a fraction of a miilidarcy, but there isbetween magnitudeand absolute permeability

0 Shales appear as excursions to the right, permeable forma-s are at points of inflection, not halfway points,good at high porosity, poor at low porosity.

0 Magnitude depends on contrast between R,, and R,. Thus, SPdelineates permeable zones well in fresh mud where R , , >=.Rbut poorly in salt mud where R = R,,.

0 I? obtainable from SP, but values must be chosen carefully,especially a t depths at less than 3,000 ft.0 Shaly sands reduce SPespecially when hydrocarbon bear ingMaximum value of shale content can b e computed from iog* Abnormal readings obta ined n pressure-depletedzones andin high vert ical-permeabil ity zones.

G A M M A R A Y L O G0 A measurement of gamma-ray ntensity in the borehole due to

U, Th, and K tend to concentrate in shales and occur least innatural disintegration of U, Th, and Kclean sands a nd carbonates.

Shales appear as excursions to right; clean formation to theGR curve is good in hard rock regions where SP is deficient.A prime indicator of degree of shalinessof formation. Shalinessestimated b y interpolating between clean formation line andSpectral GR logs separate total response into U, Th, andK contributions. Improves reservoir delineation and shaleestimation,

0 Do not repeat exactly. Statistical fluctuations limit speed to1,800 ftihr (vs 5,000 ftihr for Sonic or Electric).Also holds true forall nuclear logs. Do not read sharp peaks. Averages over 3-4ftshould be taken. Repeat run overlays indicate magnitude ofstatistical fluctuations.

left.

I

Depth of penetration-6 in.; vertical resolution- ft.REFERENCESH . G . Doll, The SPLog: Theoretical Analysis and Principlesof Interpreta- . R . J .Wyllie, A Quantitative Analysisof the Electro-Chemical Compo-h . , Vol. 1 (1949), p. 17.id h4.H. Waxman, An Investigation of the

B1.R.J. Wyllie, An Investigationof the Electrokinetic Component of thehl Gondouin and C. Scala, Streaming Potential and the SP Log, Jour

( M.R.J . Wyllie, A . J . deWitte, and J . E . iriarren, On the Streaming Poten-Ii. . Hili and A.E. Anderson, Streaming Potential Phenomena in SP LogV o y E . Althaus, Electrokinetic Potentials in South Louisiana Tertiary

tion, T?-ans AIME,Vol. 179 (1948).nerit ofElectro-Chemical Component of the SP Curve, Jour. Pet. Tech. (March 1962).Self-Potential Curve, Trans . A M E , Vol. 192 (1951), p.1.

3

P P t . TP4.h.August 1958).tial Problem, Traris. .41ME, Vol 213 (1958),p.409.Interpretation, Trans. A I M E , 101. 216 (1959),p.203.Sediments, The Log Analyst (May-July 1967), p.29. chfiimberger, Log Interpretation Charts (1979).

lo Dresser Atlas, Log Interpretation Charts (1979).I M . Gondouin, M.P. Tixier, and G.L. Simard, An Experimental Study onthe Influence of the Chemical Composition of Electrolytes on theSP Curve, ]our .Pet. Tech. (February 1957).H.G. Doll, The SPLog in Shaly Sands, Trans. A M E , Vol. 189 (1950).l3 L.J.M. Smits, SPLog Interpretation in Shaly Sands,Soc. of Pet. E n g . I . ,


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62 ESSENTIALS O F M O D E R N O P E N- HO L E LOG I N TER PR ETATI O N

l4 F. Segesman arid M.P. Tixier, Some Effects of Invasion on the SP Cunr:.J o u r . Yet . Tech. (June 1959).

l5 M.B. Belknap, J.T. Dewan, C.V. irkpatrick. \?.E. hlott, A.J. Pearson,a n d W.R. Rabson, API Calibratiori Facility For Nuclear Logs. Zlri. atid Prod .P rac . (Houston: API, 1959).l6 J,LV. Cox and L.L. Raymer, The Effoct of Potassium-Salt Muds o nCarnrna Ray and Spontaneous Potential hleasurenieiits. SZWLA L o g g i n g S y7t2po-. s iu r t i Transactions (June 1976).A. Heslop, Gamma Ray Log Response of Shaly Sandstones. Th c Logi inalys t (September-October 1974).

C.A. Lock and W . A . Hoyer , Natural Gamma-Ray Spectral Logging,Tlie Log Analyst (September-October 1971).Dresser Atlas, Spectralog (1080).M . Hassan, A. IIossin, arid A . Combaz. Fl.indanientals of the DiffermtialGamma Ray Log-Interpretation Technique, SPJCLA Logging Syrnposiiini Trans-ar t ion .?(June 1976).O. Serra. J.L. Baldwin, aridJ . A . Quircin, Theory and Practical Applica-tionsof Natural Gamma Ray Spectroscopy,SPULA L o g g in g Symposii ln~ ronsac-tiotis (July 1980).22JV.H.Fertl, and E. Frost, J r. , Experiences uith Natural Camnia RaySpectral Logging in North America, SPE 11145. New Orleans (September 19S2).J.A. Quirein, J.S. Gardner. a nd J.T. Watson, Combined Natural GammaRay SpectraliLitho-Density Measurements Applied to Complex Lithologies. SPE11143, New Orleans (September 1982).

17

18

23

24 Dresser Atlas, Spectralog, i b i d .

Chapter 4

RESISTIVITY LOGShe basic interpret ation relation in well logging, as dcvelopetl in chap-ter 2, is the water saturation relationT

The most important input to this equation (since its value can never beguessed) is the resistivity, R,, of the iiiiinvaded region of the formation inqiiestion.No resistivity-measuring tool has yet been designed that can reach deepenough to guarantee reading 11, under all possible invasion conditions whileretaining good bed resolution. Therefore, from early days resistivity logshave consisted of three curves: deep, medium, and shallow investigation.With these three measurements and t he assumption of a step invasion pro-file, correction can b e made to t he deep reading to obtain R,.Nevertheless, many logs have been run with only two curves, deep andshallow reading. These clearly show invasion effects but do not permit acorrection to the deep reading, which must be assumed equal to 11,. Theassumption is reasonable in high-porosity areas where invasion is shallowhut c an lead to significant errors in low-porosity regions where invasion maybe deep.Over the years there has been a continual succession of resistivity toolswith improved designs replacing older ones. It would \)e convenient toforget the obsolete versions, but we cann ot. Comp any files and lug li xaricsstill abound with old logs that are continually being reviewed for newdrilling or production prospects.

CLASSIFICATION AND APPLICATIONTable 4-1 is a classification of the major resistivity tools that h a w beenrivxi or are in use. Tlirl curves are categorized b y their radii of investigation,i.e , deep(3+ f t ) , mediiirn (1.5-3 f t ) , Challow(0.5-1.5ft), nclflushcdzone

63


32/176

Fu

Z

14n

Micoo(ML

Mino

Coa

PomiyPL

Micoaeoo

(MLFRx

MLoFOG

Mico

S

cy

,F

CLSSIFICTIONOFRESISTVITTOOLS 1 1 1

Dego

Commes

EE

ee

oee

IEE

IS

p

no

DILL

DIFlSG

DILSFo

ce

DlSF

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L7

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33/176

&a ESSENTIALS OF M O D E R N O P E N- H OL E LOG I N TER PR ETATI O N REStSTlVlTY L O G S 69


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with the Spherically Focused shallow log, the DISF, was introduced bySchlumberger in the mid 1970s. Gradually the Dual Induction tools arereplac ing the single Induct ion systems because of their invasion-correctingcapabilities.

indicates the presence or absence of mud cake in great vertical detail.Therefore, it is an excellent permeable-zone or sand-count indicator-thebest availab le. Under favorable circumstances it can give the flushed zoneresistivity, R,,, but is not really designed for that purpose. The Microlog isdiscuqsed in chapter 5 along with the Electromagnetic Propagation Log,with which it has been recently combined,The Proximity log is a focused curve tha t measures flushed zone resistiv-ity, R,,,, in the presence of thick mud cakes tha t can occur with fresh mud.The R,,value can be used for mnformation about the inrately from the Induction logs but can be run simultaneously with theMicrolog. It has been superseded by the MicroSFL, which can be runsimultaneously with the Induction,SALT MUD T O O L S

Two medium-investigation focused tools were introduced in the 1950'sfor salt mud surveying. They were the Laterolog-7 and the Laterolog-3, alsocalled the Guard log.' At the same time a flushed zone tool called theMicrolaterolog or FoRxo, was introdiiced.' I t could provide good R,, valuesfor mud cakes up to 34 in. thick. The medium and flushed-zone curves hadto be run separately and could only provide good water saturation andmovable oil values if invasion was not too deep.

As a corisequence the separ ate tools were succeeded in the 19705by DualLatero log systems comprising deep and shallow curves run simultaneouslywith a flushed zone log.'" Designations for Dresser and Welex are DLL-MLL and Dual Guard-FoRxo. In the case of Cchlumberger, the Dual Later -olog is combined with a Microspherically-Focused curve that can read R,,accura tely over a wider range of conditions than can the MLLlFoRxo curve.Th e combination is denoted DLL-MSFI,. Once again th e single Laterologtools are being phased out in favor of the dual systems with their invasion-correcting capability.

RANGES OF APPLICATIONOF NDUCTION LOGSAND LATEROLOGSThe Induction combination is the onesuitab le in the majority of cases. It

whenever R,T,,/R, is greater th an approximately 2 and t he format ion resistiv-ity, R,, oes not exceed abou t 200 ohm-m; the log is not accurate at higherresistivities. Resistivity increases as porosity decreases, so there is a low-

30

2 5

20

I5

IC

5

Fig. 4-3 Rangesof appiicaSchlumberger)


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70 E S SE N TI A L S O F M O D E R N O P E FI -H O L E L O G N T E R PR E T A T IO N 1 RESISTIVITY L O G S 71I

porosity limit. Th e cutoff corresponding to 200 ohm-m is dependent on theR, value, as indicated. Typically, R,. is about 0.05 and R,,,,/R, is approxi-mately 10 in fresh mud, in which case the low-porosity cutoff is 5 % .If the mud is conductive relative to forniatiori \vater, i.e.. R,,,/R,, < 2,tlie Laterolog should be run . I t is also the best in fresh mud when resistivitiesabove 200 olinis arcs encountered because it is accura tc a t high resisti\.iiies.In bo rderline cases, large boreholes ( > 12 in.) and decp invasion ( I >40 in.)favor t he Late rolog . This cominon1)- occurs in the Eastern Hemisphere.The reasons why the Induction and Laterolog are preferred under theindicated conditions will become apparent in the next two sections lvherethe principles and interpretat ion of the tools are presented. Th e discussion isconfined to the Schlumberger conihiriations. the D I I S F L on the one handand the DLL-MSFL on the othe r, since they are the most advanced tools ineach category. If these are iinderstood, the re is no difficult , i n interpretinglogs riiri with older or single-curve versions of Induction or Laterolog tools.

DUAL INDUCTION-SPHERICALLY FOCUSED LOGSThe Induction log inherently wnses the conduct ility of the form ation,which i s the inverse of its resistivity. In conimonly tised units

conductivity in rnmho/rn = I ,000iresistivity. ohm-niThe principle of the In duct ion system is illustratcd i n Fig. 4-4.. Aconstant current of 20 kHz frequency is fctl to a transmitter coil. Thisgenerates an alternating magnetic field tha t causes a circular current ( F o u -cault or eddy current) to flow in the surrounding medium. This current inturn creates a magnetic field that induces a voltage in the receiver coil. Theinduced voltage is approximately p ropor tional to tlie surroiiriding conduc-tivity. From this voltage the fo rmation conductivity and thence its resistiv-i y is derived for presentation 011 the log.With the single-transmitter, single-receiver system shown, coiitrihu-tions froni the borehole and invaded zone as well as from neighboring bedsabove and below the coil pair would constitute a significant port ion of th ereceiver signal. Practical Inductinri tools therefore iitilize an array of auxil-iary transmitter and rcceiver coils, spaced above and below the nia i i i ones.

to niinirnize tht.se contributions a n d to riiaxiiiize depth of penetration andvertical resolution. Typically, six or more coils with approsimatcly 40-in.spacing bet\veen tlie main tritnsniittt.r-receiver pair are used (Fig. $.-5) toobtain the d r e p d reading curve, denoted IL,i . Fewer coils are used toprovide the niediurn reading curve. denoted IT,,,,.The Induction log reqiiirts no condiictixve fluid in t h e borchole for its opera -tinn. It works very well--in fact, best- in holes filled w i t h air or gas or withoil-liase niud. N o other resistivity tool can be used under thcse circuni-stancesDepthof Penetration of InductionLogs

Fig. 4-6 shows the depth of penetration of the deep, IL,. and mcdium.IL,,,,arrays. The i t i tcgratet i gconietricaljnctorplotted is the relativi? weightthat the tool assigns to a cylindrical shell of t he surr ouiiding mediiiriiextcnding froni the surface of the sonde to any particiilar diarneter. Eachshell contribiitcs to the total conductivity signal in accordance with theproduct of its conductivity an d t s relative weight. For example, if C,, is the

_ -I.ig.4-4 Induction priricipie (courtesySchlumberger)

72 ESSENTIALS OF M O D E R N O P E N - H OL E LOG I N TER PR ETATI O N RESISTIVITY L O G S 73


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conductivity of the invaded zone, d, ts diameter, an d C, the conductivity ofthe undisturbed f ormatio n, then the apparent conductivity, C,, read by thetool (neg lectin g any borehole signal) is

c, = C,&, $- Ct(1 - GdJ (4.2)In te rms of resistivity

(4 .3)i R , = Gd,/Ryo+ (1 - G,,)/R,For homogenous formationsM ith no invasion, Fig. 3-0 (solid line) givesrelative signal contributions from different regions. For example, 50 70ofthe signal comes from within and 50 % comes from beyond a diame ter of 11f t for the IL,. Th e corresponding figure for the IL,, is5 ft, so i t has only halfthe de pth of investigation as does the IL,. Note that th e IL, assigns greatestart of the formathat part at 40

heir respective curves.d at about 60 in. dir, as shown by the

n

-Fig.4-5 Practical Induction array (courtesy Dresser]

For th e usual case of invaded formations, the above equations may beused to illustrate why the Induction works well in fresh mud and poorly insalt mud. E g a & y \ ~Consider a typica fresh mud case where R, = 1,R, = 10,R,, = 20, andd, = 65 in. The IL for 65 in. is 0.2. Eq. 4.3 ives1/R, = 0.2/20 + (1 - 0.2)/10R, = 11ohm-m

The L, therefore reads only 10% in error. Applying a small correctionfactor obtainable from the lL,, nd SFL readings is reasonable.The situation with salt mud is quite different. The same formation,drilled with salt mud of R,, =0.05 ohm-m, would have R, ,= 1.0. The IL,,reading would then be1/R, = (4.211.0 + (1 - 0.2)/10R, = 3.6 ohm-m

In this case the IL, eads far from t he correct value. It would be impossi-ble to obtain a sufficiently accurate correction factor from the IL, and SFLcurves.I - INTEGRATED RADIAL GEOMETRICAL FACTOR.-uWu

aOi-ULLa

10

O 8

0 6

04

-No Skin EffectSkin E f f e c t included: Case of2

O O 40 80 120 160 200 240 280 320 3I D I A M E T E R d i ( I N C H E S )Fig.4-6 Depth of penetrationof Inductionlog (courtesy Schlumberger]

74 ESSENTIALS OF M O D E R N O P E N -H O L E L O G N T E R P R ET A T IO N R ESI STI VI TY LO G S


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In reality the error in salt m u d conditions is even greater than illiistrateclbecause the geometrical factor curves of Fig. 4-6 tha t apply to qiiite resistiveformations shrink to srnaller diameters as resistivity decrcases. givinggreater weight to the invaded zone (dashed line). This pheiiomenorr is calledskin or propagation cyffrct.The important point is that t he Iridiiction log will be adversely affectedI) an invaded zone more conductive than the undisturbed formation. I tprefers in\,aded zones less conductive (rnore resistivt,) than the iininvadedformation.Borehole Effects

Borehole fluid contribiites a n iindcsircd sigiial to the Intliiction response.It is inconsequential when the n i i d is fresh. ihe hole size is small. or theformation conductivity is high. However, it can be significant iinder there\tr se conditions.Borehole correction is made by rneari:; of Fig. 4-7. For the exampleillustrated, IL, with 1.5-in. standoff in a I4.6-in.hole, th e borehole georriet-rica1 factor is 0.002. With R, = 0.35 ohm-m (2 ,857 mmhoim), the holesignal is (0.002 x 2,857) or 5.7 mmho/m, as indicated by the nomograph.This signal must be subtracted from the II,,, conductivity reading to obtainthe corrected forniation conductivity. The correction is negligible if t he II,,reading is less than 10ohm-ni i.e., greater than 100 nirnlioini) bu t signifi-cant at higher resistivities. For example, i f the IE,, read 50 ohm-in (20mmhoim), thecorrected resistivit).woiild be 1,000/(20- 5.7) or 7Oohrri-m.

The situa tion is aggravated Lvhen the miid is more conductive. For thesame conditions u s cited, except with R , , = O. 15ohm-ni. the hole signal is1 3 .3 mriihoini. An II,,, reading of 50 would correct to 149 ohm-rn, anexcessively large adju stment . In general. a correction of niore than 50% tothe indicated resistivity is unreliable because there is s ome uncertainty in thehole geometrical facto r, th e hole may be out-of-ro und. the standoff m a y h eaffected by mud cakeor holerugosity, and t hercm ay besoine i~ncertaintyirimud resistivity.Fig. 4-7 shoLvs two ad dition al impor tant points. First, borehole correc-tions are much greatcr with the Induction tool against the wall than stoodoff from the wall. A minimum standoff of 1. 5 in. should always be iw d .This is normal procedure for service companies bu t should be checked at the

wellsite. Standoff devices are somrtirnes omitted whcri there is difficulty ingetting the tool downhole.

Second, the hole corrections are much larger for the merliiirn-readingIJ+ than for th e JL0. In fact, corrections become escessivr Lvith hole diame-ters larger than 12 in ., even \t.ith fresh m u d . 'This limits the use o f the dualIiiduction tool in some areas.BedThickness EffectsThe Induction log has a vertical bed resolution of approsimately 1 ft, asdetermined primarily by the main coil spacing.Ther e arc, however, coni ribiitions f r o m beds well above and helow the4-ft section directly opposite the tool. These contributions arc negligible insoft rock but ar e significant in hard rock when t he shoulders are miicli moreconductive than the bed of interest. even when thc latter is iiiiich thicker

I I5

Fly 4-7 Boreholecorrections for I L d and ILm (courtesySchlumberger)

76 ESSENTIALS O F M O D E R N O P E N -H O L E L O G N T ER P RE T A T IO N R E S IS T IV I TY L O G S 77


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than 4 f t Such could be the case for example of a 10-ft hydrocarbon-bear inglow-porosity limestone sandwiched between two shales. The Inductioncould read too low by a factor of two in a worst case.Charts exist in service-company manuals for bed thickness correction.However, they presu me thick homogeneous shoulder beds, a condition notthickness corrections are ignored, either as unnecessary or inapplicable tothe conditions at hand .Sonde Error

If the Induction sonde is lifted high in the air with no conductive niate-rial i n its vicinity, there can still be a small signal on the order of a fewmmhoim due to residual coupling between transmitter and receiver coils orto imba lanc e in the receiver circuits, This is termed sonde error. Normallythe error is balanced out before the log is run. The log heading should soindicate. Ther e is no guarantee however that sonde error remains the sameunder high temperature and pressure condit

ole effect, is the reason themeasur ing formation conductivities less than 5mnihoim (resistivities above200 ohm-m).THE SPHERICALLY FOCUSED L O G SFL )

The SFL is the shallow-readin g resistivity curve of t he DIL-SFL combi-nation. It is obtained with a set of separate electrodes mounted on theInduction sonde. Fig. 4-3 illustrates the principle. Survey current i,,, f lowsfrom the center electrode, A 30. Correctioti charts are found in service company chartbooks but ar e not often necessary.bee CEt al imc;Signifia-

LOG PRESENTATIONFig. 9-10 shows a typical presentation of the 5-in.il00-ft DIL-SFL logwhen it is run in combination with the Sonic log. The SP curve, which isobtained simultaneously, i s recorded in Track 1 on a linear scale. Alsoshown is an R,, curve; this is described in chapter 9. The three resistivitycurves are recorded in Track 2 and half of Track 3 on a logarithniic scale

WFig.4-8 Principleof th e spherically focused log [courtesy Schlumberger)

78 ESSENTIALS O F M O D E R N O PE N - H OL E L O G N T ER P RE T A T IO N y R E Si S Ti V l TY L O G S 7 9


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wvcring 0.2-200 ohm-ni. The deep curve is always heavy dashed, thenediuni curve is light dashed, and the shallow curve is solid: the latter isinaveraged (SFLU) in this case. In the center of Track 3 is the caliper ciirvedotted) with the bit size (7% in.) shown dashed. The Sonic log, to beliscussed later, is recorded in the right-hand side of Track 3. Note the breaksn the right and left edges of the grid. These occur at 1-min intervals and,erve to indicate logging speed. In this c a e it W ~ I S 6 ftiriiin or 5 ,200 ftilir.

On th e 2-in.1100-ft log the resistivities are presented on a linear scale to'acilitate correlation with older linear scale logs. The SFL or IL 8 is aver-iged over3 ft to reduce its detail to that of the Induction curves. Further, thejeep Induction conductivity reading is presented in Track 3 on a scale froniig ht to left. Th is facilitates reading very low IL, values, such as may occurn salt-water sands. For example, ail IL,value of 0.55 ohm-m, difficiilt to-ead accurately o11 the resistivity scale, can be read easily as 1,820 mnihoimin the conductivi ty scale.In Fig. 4-10, which is a log of th e Travis Peak format ion in East Tcxas,:he SP curve indicates permeable zones at A , C. E, and G and relatively

1 o

Od (inches) -+-____

Fig.4-9 Depthsof penetrationof the shallow resistivity curves (courtesy

impermeable intervals a t 13. 13, arid i?. in thc perrneal>le zones the threercsisti\it), curt~(~shoxv wide separation indicating deep inva,;ioii. This i ss u p p o r t e d by the caliper ciirvc urherc i i i i i d cakcs u p t o ahout 2 i i i . i i ittiickncss are indicated (shaded sections). Zones L) a nd F slio\\ . i w ciirveseparation. indicating that they are foriiiationx. probably sha les . ;\,itti z e ropernieability. On the other hand interval n s1ion.s some separation 01 theS F L and Induction ciirvt"s hiit none between tiic two Induction logs. This

-ig 4-10 Typical presentcihonof Dual lnduciion StL (or LLS] log (courtes/khiiJrnberger)

80 ESSENTIALS OF M O D E R N OP E N - H OL E L O G N T ER P R ET A T IO NT H I C K BEDS,8-in t 2 0 3 m m t HOLE


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implies low but finite permeability with \hallow incasion. The section is ashaly sand or carbonate, not a pure shale.It is apparent from both the negative SP values and the R5FL> H l l nseparation i n the permeable zones tha t the well was drilled with fresh mud,with H,,,, >> R,, . However, it is riot evident from the resistivity iog5 alonee prc

for reference. C Z could calculate wafro m the Sonic lug b u t will reseme this until laterDeterminationof R,

The object of the interpietation procedure is t o utilize the three resi \ t i \ -i t ) values obtained m 2 given zurie to corrcct the deep rtJadiiig for i n \ ds101-tand thus to arrive at the best possibit R . \ alue. This I\ dorie b l i i i edm of thetornado chart in Fig. 4-11.The IL,, Ir,,,,, and SFL readings are first corrected for borehole anti bedthickness effects, if necessary, to give corrected H,,, R,,,, and R,, I 1 aluea

e location of this point, the \err the dotted curves. (R,Jest.) For example, i n the middle of zone C

ohm-m.Sacral poiiits are noteti orthy. First, the R , corrcction factor 15 I)eti\L-en1 O arid 0 . 6 . althougli sonrcch;irts qo to O 3 Thismean\ that (\en \ ith drq)in\ a w n he II,,, reads closr t o R,. Itool:, \\ ert s u c c w t u i ior i m n x bear c y r c ) 1 111 11, t rI i \ ia tes 3 olil) .I10 error IIIwatw \ d t u r 4 t i o n q tor e~ii iplc. hen t h e s,itiiratiori equat ion,Eq. 4.1. is applied.Second, the depth of ink asiori 15 eflected b j the relativevalue~ofhe t \ roInductiori readings. A ratio Rl,,/RIDgreater than 1.5 indicates quite deepitnwion (d,> 70 in.) and a value less than 1 I ndicates shallow invasion 1,< 40 in.). These ratios convert to constant separations between the curveson a logarithmic scaleso the d epth of invasion can b e eyeballed easily on the

-1)

logs.

CKiN EFFECT CORRECTEDR / R lo0D I S - E A OR E Q U I V A L E N T

Fig.4-1 1 Invasion correction chart for Dual Induction-SFL courtesySchiurnberger)

82 ESSENTIALS OF M O D E R N O P E N- H OL E LOG INTERPRETATION R ESI STI VITY L O G S 83


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Third , t he value of R,, derived from the chart, generally 1 . 5 to 2 timesthe SFL reading, is not sufficiently accurate for movable oil calculationbetxaiiseof approxirnations and uncertaintit35 in the char t. In an ' case inox a-ble oil calculation with fresh mu d is suipect, for reasons given in c1iaptc.r 2 .

D U A L L A T E R OL O G - R, , L O G S_ _. _ _______._I_ ___-_

Laterolog qystenis utilize a iniiltiple electrode ar ray to force survey cur-rent to t ra ~e laterall- across the mud and into the adjacent formation. Theadvantages that accriie arc the ability to operate in very salt- inrid. excellentbed dcfinition. and independence to neighboring bed resist\ ities.'There arc t u o basic types of focused-electrode 1,aterolog arrays. One isthe %electrode system, commonly called Guard Ing o r LL3. and t he other isthe 7 tn 9 electrode systvin. with designations such as J L 7 , IL8 , L,L,,, andLI+. Both syst~ms perate on much the same pnnc iplc , as illustrated i i i Fig.4-12.

L o l e r o i o p 7

Fig 4-12 Basic 1 aterolog a r r a y s (courtesy ScMumberger)

Cimsidering first the Li,3, survey current i,, is sent out rom the centerelectrode A and bucking curr ent i, is sent from the guard clectrodes A , a n dA:, Lvhicli are connected together. The bucking cur rent is adjusted to iiiain-tain zero voltage between A,,and the A,-& pair. There is then no ciiirentflowing iip or down the hole, which means the survey current is forced toflow i n a lateral sheet into the formation. The wid th of the A,,electrode,commonly 1 to 11 in., determines the vertical resolution of the tool. 'I'helength of the guard electrodes, usually 2.5 to 5 ft , and t he proximit . of thesurvey cur rent retu rn point determine the dept h of penetratio n. 'I'he longerthe giiards and the more remote the return point, the deeper the penetra-tion. Formation resistivity is proportional to Vi, where V is the commonelectrode potential relative to a far electrode.In the case of multielectrode systems, illustrated by the L,aterlog 7 ,survey curren t flows from th e A, electrode and bucking curren t flows fromthe A , and A, electrodes, which ar e connected together. Th e bucking cur-rent is adju

DEWAN Modern Hopen Hole

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