. Limitatio~~s of Electrical Logging t

ABSTRACT

The method of electrical logging is con~paratively new, and nlany of its users did not have opportunity to make a more thorough study of the technique of interpretation of a n ' electrical log. As a result there exists now in many instances a tendency to expect from the method a simple and definite answer to many com- plex and difficult questions. Such a trend niay lead to costly mistakes, and is unfair to the method itself.

Because the profile of an electrical log may be

affected by several factors which usually have no direct relation to these formational characteristics which are of primary interest in surveying a prospective oil well, the analysis of an electrical log requires considerable thought and study if serious errors of interpretation are to be avoided. The effect on an electrical log of salinity and weight of mud; of spacing of the electrodes; and of pressure, characteristics, and connate-water content of fortnatiol~s is discussed and illustrated.

INTRODUCTION I a large number of electrical logs of wells from different

In the brief period of a few years, since its intro- duction to the oil industry of this country, the method of electrical logging has nlet with general acceptance and is now one of the routine operations in oil-field esploitatioa. This acceptance was so rapid and so un1- versa1 tha t many of the present users of t,he method did not have the opportunity fully to learn the tecl~nique and the difficulties of interpretation. As a result of this fact there now exists in many instances a tendency to expect to'find in a n electric log a simple, definite, and coinplete answer to a number of conlples and diffi- cult questions. Such a tendency must lead inevitably to costly mistakes and is unfair to the method, a s mistakes nearly always mill be charged not against the wrong interpretation, but against the method itself.

The basic value of electrical logging lies in the fact tha t certain formational characteristics, of great im- portance in oil-field exploitation, a r e indicated in a n electric log a s easily recognizable electrical patterns. The basic limitation of the method, on the other hand, lies in the fact t h a t the electrical log also measures and records a nulnber of other factors which usually have no direct relation to those formational charac- teristics which a r e of primary interest in surveying a prospective oil well. Some of these factors a r e quite obvious, and their effect may be esplained easily by proper analysis of conditions of the survey. Other fac- tors, however, a r e not interpreted so easily, and they represent the inherent limitations of the method in i ts present stage of development. Even some of these fac- tors quite often may be recognized and isolated, if study of a n electrical log is combined with analysis of data furnished through other methods of investigating the formational characteristics.

Among the engineers and geologists affiliated with oil-producing companies o r conducting their ow11 con- sulting practice there a r e many who, having access to

* Rio Bravo 011 Co . I3~~11ston. Tusns f P r e s r ~ ~ t e d a t spring meeting. Sor~thmesteru District, Divl-

a~orl a~f l'roduct~un, l-iouston, Texns, hl i~r . 1940.

- oil-producing areas of the country, have learned to ascribe proper welght to factors affecting the profile of a n electrical log. F o r these men this paper will be only a review of the fundamentals. But there exists in the oil industry a large group of men who, although using electrical logging in their everyday work, d ~ d not have opportunity to make a more thorough study of the subject. F o r them the paper gives a summary of the most important interfering factors which may affect the profile of an electrical log, and in consequence may lead perhaps to wrong interpretation.

The basic principles of electrical logging a r e well- known and generally understood. They will be dealt with in this paper only very briefly.

Definitions and Principles

Electrical logging may be defined a s a method of determining the characteristics of formations penetrated by a drill, by measuring and recording certain electrical phenomena within the hole.

The electrical phenomena thus recorded a re : 1, elec- trical resistivity of formations and their fluid contents to the current transmitted from the surface through an electrode lowered into the well; and, 2, the electro- motive force generated within the fornlation a s a re- sult of electrofiltration and electrochemical action.

I n commercial practice electrical logging now is used for the purpose of: 1, determining the probable gas, oil, o r salt-water content of formations; 2, determining the characteristic markers which may be used in solv- ing the structural problems of a given field; and, 3, de- termining characteristics which may be used in local and long-range correlation.

ElectEical Resistivity

The electrical resistivity of fornlatioils is generally a function of the amount and characteristics of fluids contained within the formations. Saline waters a r e

good conductors of electricity; therefore, the formations containing saline water generally will have low re- sistivity. Gas and oil a r e electrically non-conductive, and porous forn~at ions containing oil o r gas usually will have high resistivity. On the other hand, forma- tions with no appreciable amounts of connate moisture- such a s granite, gypsum, rock salt, coal, and lignite- also wlll show high resistivity.

Self-potential

When a n electrolyte is caused to flow through a solid porous dielectric, a n electromotive force is generated which is proportional to the pressure and electrical resistivity of the liquid, and inversely proportional to i ts viscosity. The direction of the current is the same a s the direction of the filtration. The filtration of the water into the porous formations penetrated by a drill results In the occurrence of a n electromotive force, which is one of the components recorded 011 the self- potential diagram of a n electrical log. The other com- ponent 1s a n electromotive force resulting from electro- chemical action. When two electrolytes come in contact, a n electron~otive force is generated. I11 the case of a well the contact of drilling-mud water with saline water of formations results in occurrence of a spontaneous electromotive force.*

The electromotive force recorded by the self-potential curve of a n electrical log is a n algebraic sum of the two aforementioned electron~otive forces. Inasmuch a s the phenomena of electrofiltration and electrochen~ical action occur in front of porous formations, the self- potentla1 diagram often is referred to a s "porosity diagram."

Factors Affecting tlie Profile of an Electrical Log

The foregoing brief outline of the basic principles of electrical logging suggests the factors, o r combina- t ~ o n s of factors, which may bring cmsiderable changes in the profile of a n electrical log. F o r instance, because the electrolnotive force a s shown by the self-potential diagram is a n algebraic sum of electromotive forces due to filtration and due to electrochemical action, i t 1s theo- retically possible tha t conditions may a n s e under which these two forces would tend to neutralize each other.

The following is a discussion of more important fac- tors which should be taken into consideration in the interpretation of a n electrical log:

Salinity of Mud

Under ordinary conditions the concentration of salts in fol-;national waters is higher than in the drilling mud. The electric current, therefore, enters the forma- tion, and the self-potential diagram shows the well- known negative anomaly with respect to the shale-base line. Quite often, however, the inud is saltier than the formational watel--either due to the penetration of a sal t mass by a bore hole, o r due to the flow of salt water

from sands under pressure abnormally high for the . particular depth, or because the saline content of the mud purposely was increased to deal with certain drill: ing problems. I n such a case the self-potential diagram may show a positive anomaly, particularly a t shallow depths, where the flow potential may be small. An example of such a case is shown in Fig. 1. This figure shows the shallow sand section of a n electrical log of a well drilled i n Pierce Junction Field, Harr is County, Texas. This well encountered sal t a t the depth of 4,130 f t and was drilled 50 f t into the salt, which re- sulted in highly saline mud. An electrical survey of the well was made. The well then was plugged back a t 2,815 f t and was drilled directionally to the total depth of 4,947 f t , a t which depth another electrical survey was made. The second hole did not penetrate the salt mass. Flg. 1-A shows the log of the original well in which the sallne mud resulted in a positive anonlaly opposite the shallow sand. The same sand has a nega- tive characteristic in Fig. 1-B, which represents the log of a directional well with fresh-water mud. , At this particular depth the two holes were horizontally 8 f t apart.

This is, of course, a n extreme case. I t is easy to visualize a case in which a certain degree of salinity ,

of mud may cause only change in the amplitude of the self-potential diagram. If a n attempt is made to cor- relate such a log with logs of other wells drilled with fresh-water mud in the same field, these variations must be taken lnto consideration.

Another effect of saline mud on the self-aotential diagram is a complete masking of details of the profile. Fig. 2 shows sections of electrical logs of the same well drilled in Southwest Texas. The log in Fig. 2-A shows the survey made while fresh-water mud was used. The log in Fig. 2-B shows the survey a t the same depth af ter a change to brine inud had been made.

The foregoing two cases deal with the effect of brine mud on the self-potential diagram. The standard re- sistivity also may be affected, because filtration of the brine mud into the forn~at ion mill reduce the resistivity of the portion of formation immediately surrounding the hole bore. The effect will be noticed particularly on curves with the small radius of lateral investigation.

Factor of salinity of the mud should not be difficult to eliminate in the analysis of a n electrical log. Every log sho\~ys the resistivity of the mud d u r ~ n g the survey. If abnormally low resistivity of the mud is noted, this factor should be taken into consideration.

I t is difficult to say in general what resistivity value of the mud constitutes a limit below which the mud is saline enough to affect the self-potential curve. The value iv'ill be dlffesent fo r d~fferent areas.

Other Cliaracteristics of tlie Mud

The weight of the mud will affect the amplitude of the allonlalies of the self-potential curve. Field experi- ments have shown t h a t increasing the weight of mud will increase the peaks of the curves, and i n certain

Effect of Salinity of the Mud.

FIG. 1

Effect of Salinity of the Mud.

FIG. 2

cases also will have a tendency to shift the base line of the shales. Inasmuch a s the general pattern of the curve is not being changed, but only the amplitude of the peaks increased, this factor should not cause any undue difficulties when the logs a r e used f o r correla- tion purposes.

The phenomena of increased peaks of the self- potential curve can be noticed quite often in overlap surveys, when a well surveyed to a certain depth la ter was deepened and resurveyed.

Some chemicals used for controlling the viscosity may have the same effect on the self-potential and resistivity curve a s the increased salinity of the mud. Ordinarily, however, the ratio of the amount of chemical used to the total amount of mud is not sufficiently high to in- fluence the pattern of the curves.

Electrode Spacing

When the multi-electrode method of electrical logging is used, the spacing of the electrodes is a n important factor affecting the pat tern of the resistivity curve, because the spacing of the electrodes determines the depth of investigation. There is no need to illustrate this factor with any specific case, a s a glance a t any log havlng resistivity curves with different radii of lateral investigation, resulting from different electrode spacing, will show how different these curves may be.

I t is t rue tha t now the spacing of the electrodes is more or less standardized and uniform, but such was not the case in the past ; and when old logs a r e used, due consideration should be given to the possible in- fluence of thls factor. In general i t is a good practice to note the spacing of the electrodes during a survey.

For~llational Pressure

Formational pressures abnormally high f o r the given depth ]nay obliterate, through reversal of filtration, the normal effect of a porous formation on the self-poten- tlal curve. An example of such a case is shown in F1g. 3.

The well in question, drllled in Jefferson County, Tesas, showed a very h ~ g h pressure a t a depth of 7,900 ft. The u~eight of mud had to be increased con- siderably above the figure which would be normally sufficient to hold the pressure a t this depth. A mechani- cal core showed sand with some saturation. Casing was set, and the well-first on the drill-stem test and later on the open-hole flow test-made considerable gas and flowed some distillate w ~ t h a large amount of salt water.

The case is, of course, a r e r y unusual one, a s neither the self-potential nor the resistivity curve gave indica- tion of the porous body. There esists possibility of mechanical error in the instrument, or of faulty routine of the survey. But even if the foregoing example is only the one isolated case of such a type, i t supports the statenlent made in the introduction, which pointed out the value of combining t h e study of a n electrical

log with the analysis of data obtained through other exploitation methods, In this case the mechanical coring.

~ormational Cllaracteristics

Hardness and compactness, cleanliness and the type of cementation of the sand, all will have a n effect on the pattern of the curves indicating the sand.

Fig. 4 sho\vs a section of a n electrical log of a deep test in the Garwood area of Colorado County. The

TOTAL DEPTH 7930'

Electrical Log of a Well in Jefferson County, Texas.

FIG. ' 3

potential and .resistivity curves have a n appearance which may indicate a porous oil-bearing formation. Actually all the electrical-log shows were tested, and produced salt water. The well was abandoned i s dry. The section was cored mechanically and, although several cores were missed, a n analysis of some of the salnples is available. The sand was very hard. This type of electrical log is quite common for the Wilcos sand, and is probably a result of deep invasion of the formation. An adequate depth of investigation may cllsclose the actu'al fluid content of formation.

Fig. 5 shows a section of a n electrical log of a well

LIMITATIONS OF

drilled in F o r t Bend County, Texas. In order to deter- mine the salt-water level, the casing was perforated from 5,400 f t to 5,403 f t with 9 holes. On a 3-hour test

Electrical Log of a Well in the Garwood Area, Colorado County, Texas.

FIG. 4

ELECTRICAL LOGGING 25

the well produced a t a rate of 17 bbl of oil on &in. choke.

Proximity of a sand body only a few feet below the perforations suggests the possibility of a n error in measurements. This is, of course, possible; although the measurements were checked carefully. However, the mechanical core of the perforated section showed a very compact sand with saturation.

Quite often a statement is made that the existence of a "peak" on the self-potential curve indicates defi- nitely porous sand or limestone. Fig. 6 illustrates a case

5 5 5 0

5400

- - -

( A to R, perforated section.)

Elcctrical Log of a Well in Fort Bend Count?, Texas.

FIG. 5

which suggests tha t such may not always be the case. The section of a log shown is from a well drilled in Jefferson County, Texas. The log below 8,100 f t has a n appearance of a salt-water sand. Actually the sec- tion which was cored is a shale body without any sand. This type of log has been observed in several cases in the Vicksburg shales of the Gulf Coast, and the explana- tion may be that the formation is porous.

Thin lime streaks in a sand body sometimes will not be distinguishable, because they maintain the high re- sistivity in the resistivity curve of a n oil sand, and a t the same tirne they may not be registering clearly on the potential curve. Quite often, however, when the self-potential curve is amplified, the presence of these

lime streaks will be shown very definitely. If this pre- caution (the ampl~fy~ng of the self-potential curve) is not taken, and if no mechanical core is available, serious errors may result if-for the purpose, for in- stance, of estimation of reserves-the sand is cons~dered as a solid sand body.

Connate Water

The presence of connate water even in these oil-bear- ing strata, which are potential sources of production of clean 011, has a very decided effect on the resistivity curve of an electrical log, and, therefore, should be

~ i r s t qead,kg 8/45'

Electrical Log of a Well in Jefferson County, Texas.

FIG. 6

considered carefully-particularly in cases in which the log is used for the purpose of determining the probable fluid content of the formation. Fig. 7 shows a portion of an electrical log from a well in Fairbanks Field, Harris County, Texas. This well was completed a t the depth of 6,835 f t to 6,837 f t for the initial production of 130 bbl of oil on Q-in. choke, with tubing pressure of 750 Ib and casing pressure of 2,000 Ib. The core analysis showed a very high connate-water content a t this depth. 011 the basis of the electrical log alone this sand might be considered as a salt-water sand by a casual observer.

CONCLUSION

The foregoing discussion of factors which may affect the profile of an electrical log should not be interpreted a s an attempt to minimize the value of electrical log-

- -

ging. The method is one of the most important, if not the most important, contributions to,the science of oil- field exploitation. I t gives a continuous reliable log which is of incalculable value In incr,easing the knowl- edge of sub-surface conditions of the producing oil

Electrical )Log of a Well in the Fairbanks Field, Harris County, Texas.

FIG. 7

fields. The purpose of this paper is to focus the atten- tion of the users of the method on two facts: 1, that electrical logging is not a "cure-all" and is not meant to replace other esploitation methods; 2, that a casual glance a t an-electrical log is not sufficient for intelli- gent interpretation. Considerable thought and study are required if serious errors of analysis are to be avoided.

This paper, of necessity most general in its nature,

is written from tlie standpoint of a practical user of the method. Extensive research of the probleins con- nected w ~ t h electrical logging is being conducted by numerous corporations and individuals. I t is to be hoped tha t findings of this research will be publicized more freely in the fu ture ' than they were in the past, and tha t papers dealing in more detail with some of the questiohs considered in this paper will follow.

I I

ACKNOWLEDGMENT

Of great help in the preparation of this paper were information and data gathered during the meetings of the Houston Geological Society's first study group on electrical' logging, of which group the writer was a member. Specla1 thanks a r e due to E. J. Hanlner, of the Humble Oil and Refining Company; P. J. Fly, of the Davis 011 Company; James W. Kiesling, of the Alnerada Petroleum Corporation; E. F. Miller, of the Oliphant Oil Corporation; and H. C. Spoor, of the Union Producing Company-who discussed the subject with the writer, and furnished solne of the examples illustrated in the paper.

BIBLIOGRAPHY

1 II,oust,on C:eologic:~l Society S t u d r C:roul). "Elrctrical Well Lvgglng. Uirll d r t i Assuc. ~t'trolerrs; Gcol 23 [ 9 ] 1157 (103ll.l

2 \\'. J C;lll~ngl~;ln~ "JClectr~c:ll Logg~ng In the Apl~nlachl;in l?~c.lds." I'o~rr S t a t e 'Coll. Uttzcral I t~dr ls t r lcs Erpcr . ,St(!. 6'1111. 21 (1937)

3 . \ las:~r~cler 1Jeusst.n and,, Eugene G L ~ o n a r d o n . "Electrical Exl~lorntiun of Ijrll l IIoles, Drrllltrg (111d P r o d r l c t ~ o ~ ~ Pructlce, -li (1385)

'Alesandcr Ijeusaen and Ellgene C; Leonarclon, "Use of Elcctric;~l I,ogs tor Oa#rrel:lt~a~n In the (711lf 1:uast of Texas a n d L O I I ~ ~ I ~ I I I ; I . " l ' v ~ ~ c Al ' I 16 [IV] (P rod Ur!!l. A70 :?I61 1s (19331.

S I T ' . 3 c::~llingl~;ln~ nnrl t V U S t ~ ~ r n r r l . . I~ r l , l~cn t~on of Electri- cal-Logg~ng Methods t o West Texas Problrms." L)rrllrtrg tr~rrl l'rotlttctruti Pmctlce, 97 (193s) , also I'rt~-ul~rcrri. Etiyr I S [?I 52 and I S [8] 84 i l938) .

E L W. Qti~r ln . "Electrical Logging I'mctices on the C:ulf Co?st," 017 011s .I 35 1381 1-16 (l!+37)

bl AIartin. 1:; H AInrrar. and m7. J G~ll ingl~aru. "Determina- tion of t he k 'o tent~al l'roc?!ictlrity of Oil-Bmrlug Formations bg Res i s t~v i tg RIeart~~renrents, <feoplr!isics 3 [3] 25s (l!JZS1.

8 Ralph J Schiltllnis. "Connate Water in Oil and Gas Sands." An1 I t ~ s t . ~ I I I I I I I ~ Met. E11grs Tcch P u b NO 669, Oct (1957)

DISCUSSION

Ralph J. Schilthuis (Humble Oil and Refining Com- pany, IIouston, Texas) (written) : There is no question that electrical l o g g ~ n g is one of the most important advances yet made in the science of petroleum devel- opment. The application of the method was so immedi- ately obvious, so important, and so successful t h a t i t s development and almost universal acceptance came about very rapidly. The electrical log has established itself a s a most useful tool fo r general correlation, and a s a reliable 111eans for mapping formations when sufficient information has been developed to establish a knowledge of the character of the logs with respect to the formations being studied.

The successful use and ordinary interpretation of a n electrical log is a fairly simple matter i n many in- stances; yet, as Mr. Zaba very properly points out, a n electrical log is affected by a number of highly compli- cated factors, w h ~ c h may alter i ts character materially

and make a proper interpretation in some cases a very difficult matter. The usual ease and success of appli- cation have a tendency to cause one to overlook t h e limitations. Casual interpretations frequently a r e seri- ously in error ; and a careful study based upon a knowl- edge of the fundamental nature of a n electrical log, with full understanding of i ts peculiarities and limita- tions, is essential if costly mistakes a r e to be avoided.

A t the present time electrical-logging-service com- panies and oil operators actively a r e pursuing coopera- tive study and research on the technical proble~ns in- volved in electrical logging. Conti~lued advance in t h e utility of electrical logging can be espected to evolve

- froin such work.

W. D. Mounce (Humble Oil and Refining Company, Houston, Texas) (written) : The widespread general acceptance of the electrical well-logging method, to- gether wlth i ts often invaluable usefulness, has blinded many operators to the esistence of I ~ n ~ i t a t i o n s which a r e inherent in the method. However, I wish to take ex- ception particularly to Mr. Zaba's statement tha t : "The basic principles of electrical logging a r e well-known and generally understood." He should have said tha t the basic pr~nciples of electrical logging a r e much more well-known than understood. Even the physicists, chemists, and engineers who devote most of their time to the more fundamental research in well logging do not have a comprehensive understanding of these funda- mentals, but they have learned t h a t much of the com- monly accepted theory is either specious or irrelevant.

It must be borne in mind tha t electrical logs a r e electrical measurements made inside a bore hole filled w ~ t h a fluid, and tha t actually we measure the effect of the material surrounding the bore hole on the electrical properties of this fluid. Correspondeilce between geo- logical s t ra ta and electrical ano~nal ies i s fortuitous, but not essential.

In most of the literature 'S 4. the measured "self-po- tential" is assumed to be the sum of two potentials: a, the flow potential:

m = a constant associated with the inaterial R = the resistivity of the fluid V = the viscosity of the fluid P = the pressure difference, and

b, the electrochemical, or liquid, junction potential:

k = a constant C1 and Cz= the "concentrations" of the two fluids in

contact.

Gillingham2 h a s shown good correlation between measured potential and permeability of sands, but his

a Figures refer t o bibl~ogrnphy on p. 29

measurements were made with clear pure water in the hole. In most of the oil fields of the world, however, the drllling fluid is a mud which forms a filter cake on the walls of all permeable sections. Measurements of the permeability of this filter cake show i t to range from 0.01 to 0.00001 md (millidarcys), whereas any useful sands range from 5 to 1,000 nld, o r more. Hence we may see: tha t with a permeability ratio of the order of 1,000 to 1 esisting between the filter cake and the sand, essentially al l of the pressure drop and conse- cluently all of the flow potential between the well fluid and formation occurs across the filter cake; t h a t under these conditions the flow potential observed in a n electrical log is independent of the permeability or porosity of the sand within estremely wide ranges.

Experiments made in the Gulf Coast and i n Cali- fornia, in which the potential was observed a s a func t~on of pressure, indicate t h a t a t 5,000' f t , fo r esample, not more than about 20 per cent of the observed potential d~fference between sands and shale base reasonably can be ascribed to flow potent~al. Hence i t appears tha t the greatest par t of the observed potential n ~ u s t be due to some other cause-probably t o some kind of chemical reaction.

I t may be said that , whenever a difference of po- tential exists in any homogeneous conducting medium, there must be a n electric current flowing i n the medium to maintain this potential. Because variations of potential a r e observed a t certain depths in a well even after repeated circulatioll of the mud, i t follows t h a t the mud is essentially ho~nogeneous in the region, and tha t there nus st be electrical currents flowing through the mud whenever differences of potential a r e observed. F o r example, a t a typical sand-shale interface this current must flow from the shale to the drilling fluid to the sand and back through the ear th to the shale. This ~nvolves a system of electrolytes in which the sun1 of the junction potentials is not zero. Obviously, a t least three electrolytes must be involved; however, the necessary and suffic~ent conditions fo r such currents to flow in a system of this kind a r e not included in current electrochemical theory.

Any expression for the liquid-junction potentials C which obeys a n additive law, such a s E = k l o g C,

always will add up to zero around the circuit. F o r esample, if the 3 electrolytes have "concentrations" of 1,100 and 10,000, the net potential around the circuit will be +2k +2k -4k=O. Hence we can see t h a t a n espression of this kind in 110 way can esplain the observed conditions of potential difference existing in the mud and is, therefore, in no way pertinent to electrical logging.

Concerning the resistivity logs, one should bear in mind t h a t we a r e not able to nieasure the actual resistivity of any ear th material, but a r e nleasuring a n apparent resistivity with our measuring system in the drilling fluid. This value is affected by the re- sistivity of the surrounding earth. I f we have a measuring system surrounded by homogeneous ear th

material whose th~ckness is nluch greater than the dis- tance penetrated by the drilling fluid, this apparent resistivity will be close t o the resistivity of t h a t ea r th material, provided t h a t a sufficiently wide "spread of electrodes" is used. However, when the spread is in- creased, we do not simply increase the "lateral ~nvest i - gation," but the vertical investigation a s well. Inas- much a s the ear th is seldom homogeneous, i n practice we measure a n average resistivity of a large volunie made up of various ear th materials. This average has no simple relation to the resistivit~es of the colnponent parts, and there is no single point of reference within the electrode spread for depth measurements. A s is illustrated i n the Humme1 article,' when a three-elec- trode resistivity tneasuring system passes a n inter- face-such a s a shale-sand contact-three distinct de- flections a r e recorded, each corresponding to the passage

POlLNllPll

QOTLNTIAL

LRCNCC FOR

CUURLNT

FIG. 1 (MOUNCE)

0

of a n electrode across the interface [Fig. 1 (Mounce)]. This means tha t the indicated values of resistivity always a r e nlisleading for a distance equal t o the electrode spread on each side of a n interface, even if we ignore the effect of the mud and if there is no mud invasion. When the spread is small, 1 f t or 2 f t , this limitation is of no grea t consequence; but fo r the wider

I , 2

spreads used for "increased lateral investigation," the recorded values a r e likely to be most confusing or misleading, particularly in a sand section of which the

- I ;

L-. -. Rer , r l ~w /y e.?

thickness is conlparable to the electrode spread em- ployed. I n such a case the recorded values of resistivity show a sharp drop near what appears to be the middle of the sand [Fig. 2 (Mounce) 1. One would be likely t o postulate a n oil-salt-water contact if he tacitly assumed

- Relohvr

Rer,Jhr,Iy

ACTUAL RESISTIVITY +

7 R l W R D t D

/'arslsrlvlr~ I I I

I

t h a t the electrode spread was small. The diagrams herein [Fig. 1 and 2 (Mounce)] il-

lustrate the extreme importance of considering the electrode spacing before attempting any detailed in- terpretation. Furthermore, i t must be remembered also t h a t logs made with different electrode spacings or con-

figurations cannot be compared in detail, even though they a r e made in the same well. Operators who do their own interpretation of logs should insist that the log- ging-service companies include the configuration and spacings of the electrodes used to obtain the several curves on the heading of the log.

POTENTIAL ELECTROD15

POINT R [ I ERLNCt FOR DEPTH UltAlURiYUl 3 C U R U I N T LLfCTRODE

Res,st!vlty

ACTUAL RLSlSlIVllV

FIG. 2 (MOUNCE)

The ultimate usefulness of electrical logging will not be approached until these basic fundamentals a re in- vestigated rigorously, and such investigation seriously will be retarded if the users of logs smugly assume tha t they know all that they need to know about them.

Bibliography

1 J [ , ~ I I S ~ ~ I I I C:ci)logicnl Society Stlltly (;1.0111). "l':l(~ctrir:~l Wpll L ( y l r l g . ' ' IIrrll. d n i . ..lsxoc. Pf'trolf'itrrr Gcol. 2:) [9] 1237 (1!):3!)~.

\\I. .I. (;illi~~ghniii. "IClcctrical Loggiilg ill tilt- L \ ~ ) g a l : ~ c l ~ i a l ~ b'irltls." l'orn. Statc. Coll. lllineral Ii~flrr.~tr~iru Czl.prr. S'ta. U~rl l . !!I (1!!37).

, - . I . N. IIummel n i ~ d 0. nul l i~ . . " T ~ I I . A l ~ l ! ; ~ r r r ~ t S)~rbrific ltr- s i s t r ~ t ~ c r i l l Bore Holes." lIcitr. zrrr frrtyeic. Gcol.plrus., Leipzig, 6. lrt~rt, 1 (1:I:Ki I .

4 c . u ~ r d 31. Schlumbergrr : ~ n d 1%:. G . 1.ro1111rtlo11. "Elrc~lrio:rl ( 'or i l~g : A 3Icthod of IJetern~ining I{ottoni-llt~lts IJnta by ISIcctri- c i ~ l . \ l e n s ~ ~ r e ~ n ( ~ n t s , " l1run8. AWL. Ir18t. JIitrin!j J I c t . J . ; I I ~ ~ R . 110, 227 (19:{4).

I?. ant1 31. Schlr~mbcrger and 1,:. (:. I,rol~nl.don. ".\ X(.nr Coiilrihutio~l of Sub-surf:~cc~ Studir~s~,!;y Al\lcnns L I ~ IClcctricnl AIrasurt~mrnts in JJrill Ilolc~s." ibitl., - I . { .

Chairman H. M. Vance (A. and M. College of Texas, College Station, Texas) : Referring to Fig. 5, it is indicated tha t the casing was perforated in order to determine the salt-water level, and I wondered what influenced the operator to perforate a t that particular point.

Mr. Zaba: I have no comments to t h a t question, and why it was perforated there I cannot tell you.

There a r e two points in Mr. Mounce's discussion of my paper which I should like to answer.

The first one pertains to Mr. Mounce making excep- tion to the statement I used: "The basic principles of electrical logging a r e well-known and generally un- derstood." I t occurs to me t h a t the point is a highly academic one. Per definition, "basic" means funda- mental. Now, the fundamental principles of electrical logging a r e contained in the fact tha t the method is based on measurement of spontaneous potential occur- ring in a well, and of apparent resistivity of fluid content of formations. What causes the spontaneous self-potential; what, if any, is the relation between geological characteristics and electrical anomalies; what is the relation of average resistivity to resistivity of component parts, a r e not questions which constitute the "basic" principles of electrical logging. They belong in the realm of research, and my statement cannot be interpreted a s suggesting that these questions a r e well understood.

The other point is the sentence Mr. Mounce uses in the end of his discussion, viz: ". . . . such investiga- tion seriously will be retarded if the users of logs smugly assume tha t they know all tha t they need to know about them." In using this sentence Mr. Mounce either agrees with the main theme of my paper-in which case his discussion is quite i r r e l e v a n t o r he misses completely the main point of the paper.

The majority of the practical users of electrical logging fully realize how little is known about the method. To them electrical logging is a wonderful new tool, and they a re anxious to learn a s much a s they can about this tool. They hope, a s I tried to point out in my paper, tha t findings of the research now conducted on problems of electrical logging will be publicized more freely in the future than they have been in the past.