Electric Log Interpretation Research Articles

A New Nuclear Log for the Determination of Reservoir Salinity

Abstract The salinity log was developed as a means for determining the presence and degree of salinity in formations surrounding the borehole. Being a radiation- or nuclear-type log permits its application in wells that have been cased and in wells where air or other gases may have been the drilling fluid. Two curves ("chlorine" and "hydrogen") are recorded simultaneously with a single detector, and these curves are compared in the salinity determination. The two curves have been independently calibrated to show their response to changes in salinity and borehole size. The salinity log has been run successfully in various oil-producing areas of the United States and Canada. Because of changes in the invaded zone with time, the log is most useful when run several days after cementing casing in the well. Introduction Everyone familiar with the interpretation of electric logs recognizes that in many cases these logs do not provide positive answers. This is particularly true of the logs that were recorded five or more years ago. To at least partially satisfy the needs created by these limitations of the electrical jog approach, the salinity log was developed. In addition to not having some of the open-hole limitations of the electric log, the radioactive-type salinity log can be recorded through casing. It is believed that the invaded zone undergoes changes subsequent to the setting of casing in a well. This change may be effected by fluid migration, diffusion of formation fluid and drilling filtrate, gravity segregation, etc., and the rate of change will be influenced by the types of fluids involved, formation permeabilities and pressure differentials. Therefore, the maximum value from the salinity log probably will be realized by running it after sufficient time has elapsed subsequent to setting casing to allow the invaded zone to have effectively dissipated.

Electric Log Interpretation in Exploring for Stratigraphic Traps in Shaly Sands

Two quantities which can be calculated from conventional electric logs of shaly sands provide useful and reliable information on the reservoir rock and the fluid it contains. These quantities are the following. 1) Shaliness - a measure of the amount of disseminated clay material in the formation. This quantity measures the ease with which the rock gives up fluids and also makes an excellent mappable attribute for the construction of subsurface lithofacies maps. The agreement between this log-derived property and the results of cation exchange capacity measurements is good. 2) Saturation ratio - the ratio of mud filtrate saturation in the invaded zone to the interstitial water saturation in the non-invaded zone. The magnitude of this ratio is indicative of the amount of displaceable hydrocarbons in the formation. The values of shaliness and saturation ratio when viewed together are related to the performance of the formation under production tests. Definite ranges in the values of these quantities are associated with a) formations which produced hydrocarbons readily, b) formations which produced hydrocarbons in commercial quantities only when some form of artificial stimulation, such as sand fracturing, was applied, and c) formations which produced water along with the hydrocarbons. A single favorability criterion is developed which is a joint function of shaliness and saturation ratio. This criterion based on electric log-derived quantities is a numerical estimate of the production performance of a formation. Its use in exploration is demonstrated by maps of its variation in shaly sand reservoirs of several oil fields. It may have value for detecting from dry-hole data the proximity to good oil production.

Electric Log Interpretation in Exploring for Stratigraphic Traps in Shaly Sands

Electric Log Interpretation in Exploring for Stratigraphic Traps in Shaly Sands

Reverse-Wetting Logging

Published in Petroleum Transactions, AIME, Volume 213, 1958, pages 304–309. Introduction The most common method of identifying hydrocarbon-bearing strata in a well that penetrates many different formations involves measurement and interpretation of the electrical properties of the formations as determined by electrical logs. Even though this method is used extensively, and even though in a great many instances it is capable of indicating presence of oil or gas, situations arise for which it is extremely difficult, if not impossible, to deduce the presence of hydrocarbons. These situations may involve the following. A thin formation, bounded by highly resistive formations, in which it is impossible to obtain the actual resistivity of the uninvaded zone with existing logging devices. A formation in which invasion has been so extensive that a value for the uninvaded zone resistivity cannot be obtained. A very shaly formation in which the resistivity index, I, is lower than that usually associated with productive formations. Laminated formations comprised of thin productive sands separated by thin shale streaks in which the individual sand and shale streaks are too thin to permit measurement of uninvaded-zone resistivity with existing logging devices. Productive formations in which the water saturation is high. To extend the utility of electric log interpretation to identification of hydrocarbons in all types of formations, there is strong incentive to find a method not subject to these limitations. Some time ago, in connection with research on the wettability of reservoir rock, an investigation was conducted in which the resistivities of cores were measured shortly after they were removed from a core barrel, and again after they had been extracted and restored to their original oil and brine saturation. The resistivities after extraction were generally lower. Other tests made on the cores indicated that they were more nearly water wet after they were extracted; thus, it was assumed that the observed changes in resistivities were due to a change in wettability of the cores. Other experiments have shown that resistivities of rock samples are sharply dependent on wettability. These experiments have shown that oil-wet samples are more resistive than water-wet samples.

Effect of Clay and Water Salinity on Electrochemical Behavior of Reservoir Rocks

Published in Petroleum Transactions, Volume 207, 1956, pages 65–72. Abstract In quantitative interpretation of electrical logs the presence of clay minerals introduces an additional variable which further complicates an already complex problem. Although recognizing the difficulties introduced as a result of the heterogeneity of natural sediments and despite the present incomplete state of knowledge regarding electrochemical behavior of shades, disseminated clay minerals and concentrated electrolytes, it was felt that useful empirical correlations might be obtained from experimental investigation. Six typical sandstone formations, having a wide variety of petrophysical properties, were selected for the study. Approximately 45 samples from each formation were selected to satisfactorily represent the range of pore size distribution within the particular formation. As a matter of general interest, four limestone formations were also included in the investigation. Previously proposed equations relating to resistivity, SP and interrelationship of the two phenomena have, where possible, been tested with data obtained in this investigation. These equations do not satisfactorily describe experimental behavior of samples through all degrees of shaliness or throughout the range of brine solution resistivities normally encountered in logging practice. An empirical equation has been developed which quantitatively relates formation resistivity factor to saturating solution resistivity, porosity, and "effective clay content." This relation is indicated to be uniformly applicable to clean or shaly reservoir rocks. It is shown that both the SP and resistivity phenomena of shaly samples are related to the sample cation exchange capacity per unit pore volume. The independent chemical determination of this parameter is thus a means of determining the "effective clay content" of samples. Some implications regarding theory and electric log interpretation of shaly sands are discussed. Introduction The use of electrical resistivity logs as a means for estimating formation porosity is based upon the original work of Archie.

A Study of Electric Log Interpretation Methods in Shaly Formations

Published in Petroleum Transactions, AIME, Volume 204, 1955, pages 103–110. Abstract A review is given of the principles on which recently proposed methods of electric log interpretation in shaly sands are based and of the evidence brought up in support of the theoretical derivations. It is pointed out that the qualitative techniques suggested to date have a tendency to be too pessimistic in the prediction of the presence of commercial hydrocarbon accumulation. In the quantitative treatment, a new concept is introduced, namely that of strongly reduced activity of the double layer counter ions which are present near the negatively charged rock surfaces. This lemma when applied to the calculation of the equilibrium concentrations of interstitial waters yields a set of very simple relations. The resulting expressions for electrochemical potentials across rock samples appear in satisfactory agreement with laboratory experiments. The formulae obtained for the electrical conductivity of shaly formations are of the same form as those arrived at empirically by previous workers. Combination of the expressions for potentials and conductivities gives a direct proof of the Tixier relation which states that for shaly water sands the product of apparent formation water resistivity and apparent formation factor equals the resistivity of the sand 100 per cent saturated with formation water and which was verified by the work of Wyllie and Perkins and their co-workers. The relations for the resistivities and spontaneous potentials have been extended to the case of hydrocarbon bearing shaly formations, thus laying a formal basis forthe quantitative interpretation of electric logs in shaly oil sands. Introduction The presence of disseminated clays in porous rocks saturated with electrolytic solutions has a strong influence on the transference of ions taking place under electrical or chemical potential gradients. Both the electrochemical emf's in evidence on the SP curves of electric logs and the electrical conductivity measured on resistivity logs are directly dependent on the ionic transference in the interstitial waters of the formations traversed by boreholes.

BOOK REVIEW

Book review in this articleL. W. Sorokin. – Lehrbuch der Geophysikalischen Methoden zur Erkundung von Erdölvorkommen.M. R. J. Wyllie. – The fundamentals of electric log interpretation. Academic Press Inc. – New York, Sept. 1954. Price $ 3.60.

MICROLATEROLOG VERSUS MICROLOG FOR FORMATION FACTOR CALCULATIONS

A comparison of the MicroLaterolog and MicroLog for determining formation resistivity factors indicates that the MicroLaterolog is significantly more accurate. Data supporting this conclusion are presented comparing measured formation resistivity factors from cores with calculated values from the MicroLaterolog and MicroLog in both hard‐ and soft‐rock country. The use of the MicroLaterolog for quantitative electric log interpretation will increase the value of electric logging as an exploration tool.

An Experimental Investigation of the S.P. and Resistivity Phenomena in Dirty Sands

Abstract The importance of the so-called "dirty sand" or "conductive solids" problem in electric log interpretation was first stressed in a paper presented in 1949. Since that time the problem has been generally recognized as a most serious one. Several research papers bearing on the theory of the effects of clay contaminants on reservoir rock resistivities have appeared. These papers, while contributing to the theory of the problem, have not offered any practical solution. In the present paper an experimental investigation has been made of the effects of ion-exchange materials on the electrical properties of natural and synthetic porous media. The method used to make synthetic dirty sands is entirely novel and has proved a valuable guide to a better understanding of the properties of dirty sands generally. The effects of ion-exchange materials on both the resistivity and self potential (S.P.) logs have been examined. From the data obtained it has been possible to formulate a simple, practical method whereby electrical logs of dirty sands can be qualitatively interpreted. The method is in all essentials identical to that presently in use for interpreting clean sands. New light has also been thrown on the significance of the term formation factor when it is applied to dirty sands. Introduction In 1949 a paper was presented which showed that the presence of so-called "conductive solids" introduced considerable complexities into the analysis of the resistivities of dirty sands, i.e., permeable formations containing clay. The reduction of the self potential (S.P.) resulting from the presence of interlaminated shale or interstitial clay in permeable formations has been discussed by Doll. Noteworthy papers which dilate further upon the effects of shale and clay on the resistivity of permeable formations are those of deWitte, Berg, Wyllie, and most recently, Winsauer and McCardell. The last authors have endeavored to elucidate the chemistry of the resistivity phenomena in dirty sands in terms of an adsorption hypothesis. Specifically, they attribute the electrolytic conductivity of shales and clays within a permeable formation to an ionic double-layer effect and, for this reason, regard the term "conductive solids" as a misleading one. Some consideration has been given to the effect of dirty sands on the S.P. by Wyllie and by McCardell, Winsauer and Williams.

The Effect of Temperature on the Density and Electrical Resistivity of Sodium Chloride Solutions

Abstract This paper is a study of the effect of temperature on the electricalresistivity of sodium chloride solutions based on the published resistivity anddensity data in the International Critical Tables. This temperature effect isof considerable importance in quantitative interpretation of electric logs, where it is frequently necessary to correct mud resistivities measured undersurface conditions to temperatures existing at the bottom of the bore hole. Anempirical approximation formula was developed for this conversion which hasproved to be very useful and seems to provide results well within the range ofaccuracy of the data involved. Introduction Research by various investigators during the past 20 years has clarified thephysical principles of the phenomena which cause the natural or spontaneouselectric potential in rotary drilled bore holes. In order to calculate the resistivity of the connate water Rw and toestimate its "equivalent salinity" (as NaCl) from an electric log bymeans of this SP relationship from a knowledge of the static self potential andof the resistivity of the mud filtrate Rmf, it is necessary to first correctthe resistivity of the mud filtrate measured under surface conditions to theresistivity of this same mud filtrate in the well bore at a differenttemperature. In order to make this temperature conversion for the mud filtrate thevariation of the electrical resistivity of sodium chloride solutions withtemperature is commonly used. Since some discrepancies were found in the charts.ordinarily used for this purpose, it was felt desirable to make a new study ofthis relationship based on the published measurement data. Because of thefundamental nature of these relationships and as an aid to other workers inthis field, an attempt has been made in this study to show every step in thecalculations. T.N. 195

Formation Factors of Unconsolidated Porous Media: Influence of Particle Shape and Effect of Cementation

Abstract The literature reveals that scant attention has been paid to the systematic experimental determination of the formation factors of unconsolidated porous media. No experiments appear to have been made on the effect of increasing cementation on the formation factor of an initially unconsolidated porous medium. Measurements have accordingly been made of formation factors as a function of porosity for aggregates of spheres in the porosity range 12–56 per cent and of cubes, cylinders, discs and triangular prisms in the porosity range 30–45 per cent. The results are examined in the light of the theoretical equations of Clerk Maxwell, Fricke and Slawinski. Aggregates of unconsolidated spheres and beach sands have been artificially cemented with silica and the formation factor-porosity relationships determined. A theory is outlined which seeks to explain the results obtained and which postulates that formation factor and porosity data for consolidated sandstones may be used to determine the original porosity of the unconsolidated sands from which the consolidated sandstones were derived. Introduction All schemes of electric log interpretation implicitly or explicitly require the use of formation resistivity factors or, as they are now generally called, formation factors. The originator both of the concept of formation factor and of the use of formation factor data in electric log interpretation was Archie. More recently it has been suggested that formation factors are of value in describing fluid flow in porous media. A full discussion of this problem together with adequate references has given by Wyllie and Spangler. In spite of the theoretical and practical importance of the subject, there appear to be remarkably few basic formation factor data in the petroleum literature. A wider search reveals that this situation is not confined to the literature of the petroleum industry. At various times over the last century, attempts have been made to compute the theoretical relationship between the formation factor and porosity of simple systems of unconsolidated particles. However, the formulae derived are not all identical and the experimental evidence adduced in support of each is not wholly convincing. Most of the experimental work has been carried out on systems of unconsolidated spheres, although some attention has been given to systems of spheroids and cylinders. Other simple shapes have received neither theoretical nor experimental investigation. The writers feel that a better understanding of the formation factors of complex systems of practical importance requires a firm foundation of reliable experimental data applicable to simple systems. Accordingly, they have reviewed the relevant literature and have determined experimentally the relationship between the formation factor and porosity of unconsolidated aggregates of particles of regular geometrical shapes. Some experiments have also been carried out to investigate the effect of cementation on the formation factor-porosity relationship.

The Quantitative Aspects of Electric Log Interpretation

Abstract While intensive research continues to promote a more complete understandingof the potential and resistivity measurements that comprise the electric log, it is believed that consideration should also be given to translating thesenumerous and often widely separated findings into a coordinated and readablebody of fundamental facts designed specifically for the petroleum engineer andgeologist. Although provision is made through publication for a ready exchangeof new theoretical concepts, it is also desirable to provide reviews andappraisals of the more established techniques and methods from the operatingstandpoint so that an economic and practical application may be realizedconcurrently with the theoretical progress. With these basic premises as aguide the author reviews the present state of electric log interpretation. The paper is directed not so much to the logging or research specialist asto the petroleum engineer and geologist to whom the electric log is only one ofthe many tools which he employs. Frequently, these persons do not have the timeto follow in detail the many specialized contributions that appear and, as aconsequence, are not in a position to place these contributions in properrelation to each other, or to the art as a whole. The paper reviews the basic steps in making quantitative determinations fromthe electric log of the amount of oil or gas present in subsurface formationsand also discusses the degree of reliability of these determinations undervarious conditions. The paper also indicates the trend of future developmentsin electric logging systems and methods of interpretation. Introduction The electric log has been used about 20 years as a means for studying theformations penetrated by a well bore. The first half of this period ischaracterized by the development of suitable logging techniques and equipment.Although progress in this direction is continuing at a satisfactory rate, thelast ten years are characterized more by an increasing interest in methods ofelectric log interpretation. During this period, a large number of fundamentalpapers have been published, expounding various logging techniques andparticular phases of the interpretation problem. Many of these papers representimportant contributions, and a few are classic. T.P. 3280

The Calculation of Water Resistivities From Chemical Analyses

Abstract A method of calculating formation water resistivities from chemical analysesis presented which is somewhat faster and more accurate than previouslydescribed methods. For 26 formation water samples taken from wells in the TexasGulf Coast, the average deviation of calculated from measured waterresistivities was 3.3 per cent, with an average bias of + 1.1 per cent in thecalculated values. This compares with average deviations of 4.6 per cent and7.8 per cent, with corresponding average biases of +3.2 per cent and -6.3 percent, for the two methods previously described in the literature. All threemethods are sufficiently accurate for most purposes; the main advantage of themethod presented is its speed and convenience. Discussion It is frequently desirable to calculate the formation water resistivity fromthe chemical analysis of the water, as for example, in electric loginterpretation. Two methods have previously been given in the literature foraccomplishing this. Both methods require that the chemical analysis be given inthe Palmer system, as primary and secondary alkalinity and salinity. Thisconversion of the values of the ppm of the various ions to primary andsecondary salinity and alkalinity is time consuming, and many laboratoriesreport only the concentrations of the various ions in parts per million.However, all laboratories report the individual ionic concentrations, whetheror not the Palmer system of reporting is also used. Hence it was thought that amethod that would permit resistivity calculations directly from these datawould be useful. In the method developed, the various ions reported in the chemical analysisare converted to an equivalent amount of NaCl. The sum of the variouscontributions to the NaCl (actual or equivalent) can then be used inconjunction with a plot of NaCl concentration versus resistivity to obtain theresistivity of the solution in question. T.N. 67

A New Technique for the Measurement of the Formation Factors and Resistivity Indices of Porous Media

The importance of formation factor, F, not only in electric logging but as afundamental rock parameter has recently been stressed. The desirability ofinvestigating the range of variation of the resistivity index exponent, n, inthe relationship I = Sw-n, where I is the resistivity index and Sw the watersaturation as a fraction of the void volume of a porous medium, has also beenurged. The magnitude and variation of n with saturation and rock texture is asubject not only of theoretical interest but also one of prime importance inthe interpretation of electric logs. A simple technique has recently been developed which enables both F and n tobe measured with high accuracy and which may also find acceptance as aconvenient method for the determination of irreducible saturation attainment inthe restored state method of core analysis. Experience has taught that reproducible measurements of F are possible onlyif the resistance measuring electrodes are so arranged with respect to a planeface on a porous medium that they are able to make electrical contact withsubstantially all entry pores in that plane. In practice this may be achievedby using a platinized-platinum gauze electrode backed by some absorbentmaterial (such as felt) which has been saturated with a fluid identical withthat used to saturate the porous medium. Application of pressure to theelectrode and absorbent material then forces the gauze against the plane faceof the porous medium and simultaneously squeezes saline solution through themeshes of the gauze. By this means the electrode is in continuous aqueouscontact with all pores and satisfactory and reproducible low resistance contactwith the porous medium is achieved. Clearly this method, although satisfactory for measurements of F, cannot beapplied to the making of continuous resistance measurements on a porous mediumwhile capillary pressure desaturation is being carried out. However, acceptingthe principle that for satisfactory results electrical contact must be madebetween a measuring electrode and all pores adjacent to that electrode, methodsof bringing electrodes into intimate contact with the surfaces of porous mediawere investigated. T.N. 65

THE CASE FOR GRAVITY DATA FROM BOREHOLES

The immediate urge for gravity meter surveys in boreholes is the desire to improve the gravity method itself but there are other potential uses for data so obtained. Borehole gravity surveys, through the influence of density, would be related to numerous important rock properties such as the velocity of seismic waves, the coefficient of reflection, the electrical resistivity, lithology, porosity, and the kinds and states of fluids in the rock. Therefore, in addition to improvement in gravity prospecting, the data would bear upon seismic techniques and, depending on accuracy and detail, upon the interpretation of lithologic and electric logs. The more obvious of the possible uses are indicated and some of the problems and inherent limitations holding up development are discussed.

The Presence of Conductive Solids in Reservoir Rocks as a Factor in Electric Log Interpretation

Introduction In the quantitative interpretation of electric log data it is essential toknow the formation factor, defined as the ratio of the resistivity of theformation 100 per cent saturated with brine to the resistivity of the brine.This concept implies that the formation factor for any given rock is constantirrespective of the resistivity of the brine or solution with which it issaturated. It is apparent that this can be true only when the rock matrix iswholly non-conductive. As reservoir rocks may contain clay or shale having anappreciable conductance one may expect these constituents to influence thedetermination of true formation factor. The formation factor may be theoretically derived from log data by dividingthe resistivity of the mud filtrate invaded formation by the resistivity of themud filtrate. Such a method of calculation has implicit in it the assumptionthat the fluids in the invaded formation are wholly replaced by mud filtrate, that the solid matrix is non-conducting, and that the formation factor of theinvaded zone is identical with the formation factor of the uninvaded zone.Since the validity of the first two basic assumptions regarding the insulatingcharacter of porous matrices and the displacement of the interstitial fluidsin porous media do not appear to be fully established, the formation factors of anumber of cores and clay slurries have been measured under conditions suitablefor verifying the assumptions and the extent of any errors inherent inthem. Core Studies Apparatus and Procedure Resistivity measurements were made on short cylindrical permeable coreswhich had been previously mounted in lucite. The cores were clamped by means ofinsulated bolts between brass plates about 5 cm square and 1 cm in thickness.The plates were provided with a ring seal to prevent leakage between them andthe core mount, and had a centrally located orifice so that fluids could beflowed through the mounted core. The brass mounting plates were used aselectrodes and any current flow between them necessarily passed only throughthe constituents of the intervening core. Fluid saturation of the core, and fluid flow through the core, were effectedby a centrifugal pump connected to the outflow end of the core, thus providinga differential pressure across the core of approximately one atmosphere. T.P. 2797

The Relation Between Electrical Resistivity and Brine Saturation in Reservoir Rocks

In connection with electric log interpretation it is important to know thevalue of the saturation exponent. For example, if in a given reservoir it isfound that the resistivity is three timcs the resistivity observed when thereservoir is 100 per cent saturated with water, this fact would be interpretedas indicating a water saturation of 33 per cent if the saturation exponent were1 and a water saturation of 64 per cent if the saturation exponent were2.5. Experimental Method In the work to be described it was assumed that reservoir conditions aremost nearly obtained when core plugs are desaturated by the capillary pressuretechnique referred to in numerous places in the literature, as for example, inBruce and Welge's paper. In this technique the core, saturated 100 per centwith brine, is placed in contact with a ceramic disc permeable to brine but notto the displacing medium for the displacement pressures used. Pressure is thenapplied to the displacing medium and brine forced out of the core through theceramic disc. Fig. 1 shows the core plug in place in the cell in whichresistivity and saturation measurements are made. Fig. 2 shows the schematicelectrical diagram used to make resistivity measurements on the core plug. Afour-electrode type circuit is used, employing a Hewlett-Packard model 400A, ACvacuum tube voltmeter. The 60-cycle AC current through the core is adjusted to1 milliampere and measured by noting the voltage drop across the calibrated100-ohm resistor. The voltages appearing at probes 1, 2, 3, and 4 are thensuccessively measured. Voltage drops across the top, center, and bottomportions of the core are obtained by subtracting the voltages appearing atsuccessive probes. This technique avoids any polarization or other high contactresistance phenomena which may develop at the current input electrodes.Resistances which may develop between the core and the probes, and which aresmall compared to the 1-megohm input impedance of the vacuum tube voltmeterwill obviously not affect the measurements appreciably. Any very appreciableresistances which may develop at any of the probe wires are detected andallowed for by inserting a 1-megohm resistor in series with the voltagemeasuring probe. If the probe resistance is actually zero, the new voltagemeasured after insertion of the 1-megohm resistor will be approximatelyone-half of that previously measured- since the input impedance of the vacuumtube voltmeter is itself 1 megohm. If any appreciable probe resistance hasdeveloped, the new voltage is found to be appreciably greater than one-half ofthe previously measured voltage. T.P. 2711

Analysis of Decline Curves

Abstract Since production curtailment for other than engineering reasons is graduallydisappearing, and more and more wells are now producing at capacity and showingdeclining production rates, it was considered timely to present a brief reviewof the development of decline-curve analysis during the past three or fourdecades. Several of the commoner types of decline curves were discussed in detail andthe mathematical relationships between production rate, time, cumulativeproduction and decline percentage for each case were studied. The well-known loss-ratio method was found to be an extremely valuable toolfor statistical analysis and extrapolation of various types of curves. Atentative classification of decline curves, based on their loss ratios, wassuggested. Some new graphical methods were introduced to facilitate estimationof the future life and the future production of producing properties wherecurves are plotted on semilogarithmic paper. To facilitate graphical extrapolation of hyperbolic-type decline curves, aseries of decline charts was proposed, which will make straight-lineextrapolation of both rate-time and rate-cumulative curves possible. Introduction During the period of severe production curtailment, which is now behind us, production-decline curves lost most of their usefulness and popularity inprorated areas because the production rates of all wells, except those in thestripper class, were constant or almost constant. While production-decline curves were thus losing in importance forestimating reserves, an increasing reservoir consciousness and a betterunderstanding of reservoir performance developed among petroleum engineers.This fact, together with intelligent interpretation and use of electric logs, core-analysis data, bottom-hole pressure behavior and physical characteristicsof reservoir fluids, eliminated a considerable part of the guesswork inprevious volumetric methods and put reserve estimates, based on this method, ona sound scientific basis. At the same time, a number of ingenious substituteswere developed for the regular production-decline curve, which made it possibleto obtain an independent check on volumetric estimates in appraisal work, eventhough the production rates were constant. With the now steadily increasing demand for oil to supply the hugerequirements of this global war, proration for reasons other than prevention ofunderground waste is gradually disappearing. More and more wells are, or willbe, producing at capacity or at their optimum rates, as determined by soundengineering practice. T.P. 1758

Effects of Certain Gums and Starches on Filtration of Salt-water Muds at Elevated Temperatures

Abstract The wall-building properties of salt-water muds can be maintained at hightemperatures by additions of gum karaya, gum ghatti, gelatinized starch, orcommercially prepared starch. A germicide should be used to preventfermentation. Introduction The behavior of drilling mud has received special attention from thepetroleum industry within the past few years in numerous research projectsdealing with such subjects as chemical treatment, gelation, and filtrationcharacteristics. The importance and control of filtration, or water loss, is readilyapparent. It affects the analysis of side-wall cores and interpretation ofelectric logs; it controls to a large extent the work of primary cementing, reduces the amount of expensive fishing, and permits the running of longstrings of casing. It has been found that among the many factors that have an adverse effect onfiltration, the presence of electrolytes, in general, and of salt, inparticular, is outstanding. The effect is one of flocculation of the colloidsdue to neutralization of the negative charges on the clay particles. Thisreaction is further increased on the Gulf Coast because high temperaturesreduce dispersion to a minimum, causing increased flocculation in both fresh-water and saltwater muds. The problem of reducingwater loss in salt-water muds by the addition of gums and starch at atmospherictemperature has been investigated, and many successful field tests in WestTexas indicate its solution. From the work of Byck it is concluded that noexisting method will permit an approximate determination of the filtration rateat high temperatures from data taken at room temperatures. Filtration should bemeasured at the temperature actually anticipated in the well, or at least at anumber of lower temperatures and extrapolated over a small range. A laboratory study has been made of the chemicals available for thefiltration characteristics of salt-water muds at high temperatures, and the useof germicides to prevent fermentation. T.P. 1551