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By A. R. Jr. Amos

LOG washers have been used for many years in the washing of clay iron ores, phosphate rock and manganese ores, but not until the past 15 years have they been employed to any extent in the preparation of aggregate materials and fluxing stone. For a machine of such importance where tenacious clay or soft rock must be eliminated, there are few data generally available. The purpose of this paper is to set forth the results obtained from several operations, together with a few suggestions for planning a specific installation. Where the material to be washed out consists of loose surface soil, silt, or small amounts of quickly soluble clay, there is little competition with other forms of washers, because greater power and maintenance are required for the operation of log washers, but where the foreign matter is a stiff clay, such as occurs with high-calcium limestones and in marginal gravel deposits, the ability of other kinds of washers to eliminate com-pletely the hard clay balls is at least uncertain. Then the log washer comes into its own field, for with the breaking and churning action of the logs it will clean out the toughest clay. Even with material from the hydraulic dredge, if it contains tough clay, the log washer is desirable unless the gravel is subjected to a costly soaking and rehandling process. Recently there has been a considerable development in other types of machines to accomplish these results but it is not within the scope of this paper to make comparisons between competitive types, but to present the best obtainable data on log washers only. From the primitive design that gave it its name, a long log hewn from a tree, with paddles forged to shape from strap iron and attached by lag screws, the log washer has gradually been developed to the present all-steel type. As now manufactured, the log washer consists essentially of two built-up logs on which are mounted a series of paddles, carried by shafts fitted at the ends of the logs on bearings fastened outside the enclosing steel box, or trough, which is set at an angle of about 5° to 10° and into which the material is fed. Stuffing boxes at the places where the shafts pass through the box at the lower end are equipped with water-

Jan 1, 1936

By Robert E. Bush, E. S. Mardock

The objective of this paper is to present practical methods of applying radioactivity logs to problems of interest both to those engaged in evaluating fundamental reservoir data as well as to those engaged in subsurface structural work. The basic theories underlying the operation of radioactivity logging methods are briefly discussed along with some of the problems arising in the interpretation of the log. Relative methods of quantitative interpretation are described, with a discussion of relative reference lines and specific examples are given showing the application of these relative quantitative interpretation methods. To show the variation of the methods described, specimen wells penetrating formations widely divergent in point of geologic age were selected to demonstrate the application. A relative reference line is determined for the Neutron Curve and by comparison to known petrophysical data such as porosities from diamond cores, a direct quantitative relationship is developed. The application of this relative calibration curve to logs of unknown wells will permit a reasonably accurate determination of porosity in limestone and dolomite formations. Specific examples are given showing the application of these relative methods of interpretation to logs of the Smackover limestone in Louisiana, the Edwards limestone and dolomites of South Texas, and Permian dolomites of West Texas. Radioactivity logs have been used extensively by the petroleum industry for a number of years and recognition of their value has grown steadily, not only as tools to aid in the solution of specific stratigraphic problems, but also as potential sources of the basic data necessary for the estimation of net pay and reserves. The value of radioactivity logs for stratigraphic work has been pointed out in the literature,1,2,4,5,6 and now has long been recognized by the industry, but the quantitative application of the logs has advanced but little beyond the stage of a strong conviction that radioactivity logging curves will some day provide the information on such important quantities as the porosity and the liquid saturation of the formations. During the past few years the staffs of the Lane-Wells Company and Well Surveys, Incorporated, have devoted a great deal of time, thought, and experimentation to the relationships existing between radioactivity curves and the lithologic conditions encountered, and this paper is, in a manner of' speaking, intended as a progress report of this work and includes suggestions which the authors have found useful in the interpretation of radioactivity logs. The section on theory is not intended as a complete exposition of the physical processes involved, but rather as a brief introduction to the reader of these processes so that he may acquire a better understanding of the behavior of the radioactivity curves under the varied conditions encountered in normal well logging. Three relative methods for using neutron logs for the quantitative estimation of porosity in limestone or dolomite are presented. These methods are based on the comparison of logs and porosity data from wells in Louisiana, South Texas, West Texas, and New Mexico. These areas were selected because not only were there more core data available from these areas but also the majority of these data were from limestone or dolomitic zones. The correlation between core data and logs is simpler for limestones and dolomites than it is for sandstones due to the fact that sandstones are more complex than limestones and differ more widely among themselves as to composition of cementing materials, siltiness, etc. When data applicable to the various types of sandstones are available, the authors hope to present a discussion similar to this paper covering sandstones or possibly a more general discussion covering all types of reservoir rocks. The relative methods for estimating porosity which are presented in this paper fall short of the absolute method sought. However, these methods possess the advantage that they can be applied to almost any neutron log as long as the range of application is held within the limits imposed by the local lithologic conditions which prevailed during calibration, i.e., these methods are for short range application only. THE THEORETICAL BASIS FOR RADIOACTIVITY LOGGING The two redioactivity logging curves commercially available today are the

Jan 1, 1950

By M. H. Waxman, M. Gondouin, H. J. Hill

Streaming-potential experiments were conducted within the Muddy- and Dakota-sandstone interval of a Denver basin well. Analysis of the data shows that, for this case, streaming potentials opposite sands were higher than those opposite shales when the mud was fresh. Streaming potentials opposite shales, however, were of sufficient magniiIlde to be important in SP interpretation. They were linearly dependent upon pressure differential, and they increased with mud filtrate resistivity. The magnitude of streaming potentials opposite sands was influenced by the characteristics of the original mud cake formed at the time of drilling. Use of mud samples taken at the time of logging may not always be satisfactory for determining streaming-potential corrections. Both the streaming and electrochemical potentials were found to be affected by the previous history of the borehole system. Following a change in mud characteristics, several days were required for SP stabilization. This was, in part, due to the mud cake acting as an imperfect shale membrane. INTRODUCTION Laboratory investigations by Wyllie,1 Salisch,2 Moore," Sen Gupta and Bannerjee,0 and Hill and Anderson' have shown that streaming potentials developed across mud cakes deposited on permeable filter beds are of sufficient magnitude to be critically important in SP interpretation. Streaming potentials across shales have been reported by Schenck,' Gondouin and Scala; and Hill and Anderson." The magnitude of the shale streaming potentials obtained by Schenck and by Gondouin et a1 led these authors to conclude that in many cases the shale streaming potential would approximately equal the mud-cake streaming potential. Hill and Anderson, however, obtained somewhat lower magnitudes of shale streaming potential. The field experiment described in the present paper was a cooperative project carried out by the respective operations and research departments of Schlumberger Well Surveying Corp., Shell Development Co. and Shell Oil Co. The primary objective of the experiment was to determine in the borehole the streaming-potential behavior of both shales and mud cakes under conditions approaching, as nearly as possible, those normally encountered in logging practice. METHODS AND EQUIPMENT WELL A Muddy- and Dakota-sandstone vie11 in the Denver basin was chosen for the experiment. Both sandstones were water-bearing and, since reservoir pressures in these sandstones are abnormally low (gradient of approximately 0.25 psi/ft), it was expected that studies of streaming potential could be conducted from low-pressure differentials (approximately 200 psi) to about 27000 psi by bailing mud from the hole, filling up and then applying wellhead pressure. The selected well encountered the Muddy sand-

By A. J. de Witte

INTRODUCTION The importance of determining dips of subsurface formations in wells, especially in wildcat wells, is self-evident and need not be enlarged upon. Various dipmeter instruments have been described in technical literature1,2,3,4 over the years. The introduction of the continuous dip-meters3,4 has made the earlier types obsolete. Continuous dipmeter logs are run by at least two different major service companies. Included as part of the service are a limited number of dip calculations—usually 10—made at different levels from the log. Very frequently this is the total result of a dip survey. It is believed that dipmeter surveys would be of considerably more advantage if geologists and engineers were able to make their own calculations from the original records. Determination of dips from the dipmeter record starts with the correlation of three log curves taken at different sides of the borehole. It cannot be over-emphasized that the quality of the results hinges primarily on this correlation. Nobody should be better qualified to correlate logs than a subsurface geologist or field engineer. The next requirement is to have an easy yet accurate way of computing the dip from the correlated points. Service companies use a mechanical device for this purpose. Directions for a graphical method were given in a recent article by B. 0. Prescotts. This paper describes a different graphical technique which, in our experience, is simpler, faster and at least as accurate. The entire manipulation is carried out on the stereonet in a number of easy moves. The paper is divided into two parts. This first part contains the introductory sections and full explanation of the geometry involved. The second part reviews the reading of data from an actual field log and summarizes the practical rules for computation of dips, the whole process being illustrated by an example. Following the outlined procedure, one could calculate a good many levels in a relatively short time, thus being able in particular to check suspected anomalies such as fault zones or unconformities in as close detail as the log will permit. GENERAL EXPOSITION OF THE METHOD Prnciple of the Dipmeter The position of a plane in space is defined by the location of three points, not on a straight line. Referring to Fig. I, if a borehole traverses a bedding plane, we can determine the strike and dip of the bed by finding three points which are not in line along this plane. Such points as A, B, and C are obtained with the dipmeter. The device consists of three electrodes (or caliper arms)1, 2, and 3 spaced at 120 angles, as schematically indicated, whose readings are separately recorded (Curves 1, 11, 111, to the right). When electrode 1 passes the boundary of the bed, Curve I will kick, etc. From these curves, we know the relative distances of points A, B, C, measured along the axis of the hole. If A is at zero, then B and C are at levels d1 and d2 up the hole,

Jan 1, 1957

By H. J. Beach, W. C. Goins

The laboratory development and field use of oil well cement compositions* with high resistance to drilling mud contamination are described. Resistance is obtained by adding a I-lb mixture composed of three parts paraformaldehyde to two parts sodium chromate tetra-hydrate for each sack of cement. This mixture neutralizes the cement-retarding properties of quebracho and other tannins, starch, sodium carboxymethylcellu-lose, Ignite, and monovalent salts of lignosulfonic acid. The retarding properties of calcium lignosulfonate are unaffected. The decontaminating mixture may be used in common cement (designated Type I by ASTM specifica-tions,' Class A by API Std. 10A2) or in a slow set slurry prepared from common cement by addition of calcium lignosulfonate. No changes in slurry viscosities or thickening times are caused by the protecting reagents. The method has been used in 185 field jobs where an improved success ratio has been obtained in plug setting. INTRODUCTION Field experience and laboratory investigations have established contamination by drilling mud as a major factor in oil well cement failures. During 1955, 12 attempts were required to set six open-hole cement plugs in the Timbalier Bay field, La. Eighteen to 30 hours elapsed between placing and testing the plugs in preparation for sidetracking operations. Considerations of rig time expense required immediate resetting where soft cement was drilled after a reasonable waiting period. Sea water drilling mud was used in each well and daily additions of thinners, usually lignites, had been required. Sodium carboxymethylcellulose had been added regularly to control fluid loss. The 50 per cent plug failures were attributed primarily to contamination by the mud. Similar experiences can be found in other fields along the Texas and Louisiana coasts where drilling muds normally contain large quantities of organic treating materials. Cement contamination was the subject of a survey by the Mid-Continent District Study Committee on Cementing Practices and Testing of Oil-Well Cements of the API.3 Data collected from member laboratories and released in Sept., 1951, reported the effects of a number of drilling mud treating chemicals on cement. The study revealed that many of the additives, notably the organic materials in relatively small quantities, had marked effects on the properties of cements tested. Severe losses in strength and erratic thickening time resulted from small additions of mud treating chemicals such as quebracho, starch, sodium carboxymethylcellulose, and lignite. The report included no recommendations of methods for overcoming the problem. One investigation of the contamination problem reported the successful use of activated charcoal in setting cement plugs after conventional slurries had failed.4,5 The investigation indicated that an optimum concentration of 5 per cent activated charcoal was effective in combating contamination from 16.7 volume per cent of treated mud. Laboratory and field muds were included in tests and contained varied quantities of ode or more of the following: quebracho, caustic, starch, lime, and calcium lignosulfonate. The function of the charcoal appears to be physical, removing the organic materials by adsorption. The method may be employed in API Class A or Class D (unretarded slow set) cements. The chemical approach presented here is based on

Jan 1, 1958

By R. H. Winn

A method for the selection of the most suitable corrosion inhibitor for a particular system is given. The method involves the evaluation of surface passivity by means of copper ion displacement after the metal surface has been subjected to the corrosive atmosphere. The application of the method to the selection of suitable inhibitors for use in producing wells is described. Correlation with recognized laboratory tests has beet1 found to be good within the limitations of the test. The use of the test to evaluate inhibitor persistencc is described. INTRODUCTION Corrosion as it is associated with the industry shows wide variations in its qualitative as well as its quantitative aspects. This complicates the work of the corrosion engineer. In the past few years the efforts of the corrosion engineer have been further taxed by the large numbers of products being marketed as corrosion inhibitors by various manufacturers. These products are submitted to him together with a limited amount of data to support their use as inhibitors. The data are usually authentic and reliable yet all too often when these products are applied to an actual corrosive system the results are disappointing due to the ineffectiveness of the inhibitor and the corrosive system. The corrosion engineer must select a comparatively limited number of inhibitors for field trial from the large number available. The selection is usually made on the basis of a standardized laboratory test such as that adopted by the National Association of Corrosion Engineers or by field experience in an area; however, testing of this type is discouraging on large numbers of compounds, particularly in view of the specificity of environment. It has been an object of this work to devise a method which will allow the corrosion engineer to select the best inhibitor for a specific environment under field conditions. The test would of necessity be rapid and inexpensive. It would preferably eliminate the need for expensive and delicate instruments. STATEMENT OF THEORY The test as developed makes use of the fact that metal ions will be displaced from solution by metals having a more positive electrode potential. Thus, if an active iron surface is exposed to a solution of a copper salt, free copper will plate spontaneously on the iron surface as it is displaced from the salt solution by iron ions. If the iron surface is passified, as by coating with an insulating-type organic corrosion inhibitor, it is no longer capable of displacing the copper ions due to its own limited ionization. The result will be a change in the amount of copper plated upon exposure to a neutral copper salt solution. In practice it has been found possible to measure the magnitude of the alteration of the cell potential by determining the copper deposited on a standard iron surface under controlled conditions. This means that, as the film builds up, the resulting shift in the electrode potential tends to cause the iron surface to act more like a noble metal, resulting in a reduction in the cell potential to a point that copper will no longer plate spontaneously. Obviously, the salts of other metals such as silver, gold or platinum might be used for this operation; however, copper was selected because of the color difference of the plate, the comparatively low cost of copper salts and the ease of evaluation of the plated copper. Furthermore, its high cell potential provides a substantial driving energy. The determination of the copper is readily done colorimetrically by dissolving it in 5 per cent ammonium hydroxide solution containing 2 per cent potassium per-sulfate. The blue color of the copper ammonic complex ion so developed may be compared with standards prepared by dissolving known quantities of copper powder in the same reagent. In routine screening it is seldom desirable to determine the amount of copper plated. It is usually more important to determine the concentration of inhibitor required to prevent the plating of copper.

By M. Felsenthal

Engineers are frequently faced with the problem of having to predict oil recovery from a solution gas drive reservoir in the early life of a field. This is often the time when actual laboratory or field-derived gas-oil permeability (kg/k0) data are not yet available. Although excellent collections of k0/k0 data for various types of rocks may be found in the literature,1,2,3 a need exists for a more detailed correlation of k0/k0 data with core properties such as porosity, permeability, clay content or pore-size distribution. It is the purpose of this article to present this type of correlation for sandstone material. The correlation is based on laboratory tests performed on over 300 core samples from 19 reservoirs in the United States and Canada. EXPERIMENTAL Cores were analyzed by routine procedures, using samples of conventional size (7/8-in. diameter, 7/8-in long) cut parallel to the bedding planes. Air permeability was measured at 1.67 atm mean pressure in a Hassler-type apparatus. Porosity was determined with the aid of a Ruska bulk volume meter and a Boyle's Law rock volume meter, An evaporation method' was used to evaluate irreducible water saturations. Core samples were saturated with 25,000 ppm NaCl brine. The brine was reduced to the irreducible saturation, and the remainder of the pore space was filled with kerosene. This was followed by multiphase flow tests using the so-called "dispersed feed" technique.5 n this technique, gas and oil (kerosene and kerosene-saturated air) were dispersed with the aid of a porous Alun-durn feed head, the face of which contained a waffle-shaped system of Lucite-lined grooves. Oil entered the back side of the feed head and emerged at the raised areas of the front face. Gas was introduced directly into the system of grooves at the front face. The gas-oil mixture then entered the test core which was kept in capillary contact with the feed head. Permeabilities were determined from rate and pressure measurements and saturations from weights. All determinations were performed at equilibrium or steady-state conditions. Pore-size distributions were determined on selected samples by a mercury injection technique.6 GENERAL CORRELATIVE TRENDS When examining data from a given reservoir, it was noted that k0/k0 vs saturation curves became generally less steep as specific air permeability increased, a development that has a favorable effect on oil displacement efficiency by solution gas drive. This trend has also been reported by others in the literature.' An effect of porosity on k0/k0 data was also noted. This effect was not generally discernible in a study of relative permeability data for a given reservoir but became apparent when data for sandstone reservoirs of similar lithology but differing average porosity were compared. For instance, in a comparison of argillaceous and/or calcareous sandstones from 11 reservoirs ranging in average porosities from 14 to 28 per cent, a definite trend was noted, indicating that for a given permeability k0/k0 curves became less favorable (i.e., steeper) as porosity increased. A similar trend was observed for a group of comparatively clean sandstones from five reservoirs ranging in average porosities from 15 to 21 per cent. For a given permeability and porosity, comparatively clean sandstones gave more favorable k0/k0 curves than argillaceous and/or calcareous sandstone or chert reservoirs. The laboratory studies also showed that least favorable k0/k0 curves were exhibited by shaly sandstone, conglomerate, and sandstone containing carbon inclusions. Table 1 lists the various source reservoirs classified by lithology types. To facilitate a correlation of k0/k0 data with core characteristcis, it was found expedient to work with

By J. G. Patchett, R. W. Rausch

Fresh waters and the presence of clay in many Rocky Mountain and West Coast sands require special methods of log analysis. Archie's saturation equation requires addition of a shale correction term, and the SP equation must also be modified to account for clays. Suitable equations were developed several years ago, but have not been widely used due to the algebraic complexity. A computer-oriented method has now been developed to overcome this problem. The basic shaly sand equations are rearranged in four different ways to permit solution for various sets of available input data. Essential to application of the method is the correction of observed SP values to those that would be observed if the resistivity of the formation waters were exactly interchangeable with the activity. A graphic method for doing this is given. Where conditions require consideration of the effect of clay in the sands, the method presented has been found to improve the accuracy of water-saturation determinations. INTRODUCTION Log interpretation in many Rocky Mountain and West Coast basins is complicated by rapid vertical and lateral changes in water resistivity. Calculation of formation water resistivity from the SP curve becomes difficult in zones that contain clay, since changes in SP deflection may be due to changes in either clay content or water salinity. In hydrocarbon-producing reservoirs, the problem is further complicated because hydrocarbon saturation also reduces the SP.1 A log interpretation system using computers has been developed to provide a solution to this problem, based on equations proposed by de Witte.2 Four different simultaneous solutions of de Witte's equations have been made. Each solution method uses a different set of input data as independent variables. Thus, a choice of solution method is possible, depending upon the logs run and the availability of other data. Two of the solutions do not require a knowledge of water resistivity. This system is intended to be used primarily in multiple sandstone-shale sequences of low and moderate resistivities where the principal contaminant in the sandstones is clay. However, where sufficient regional data are available, interpretation in single-zone sandstone reservoirs can also be improved by using the method. THEORY AND HISTORY OF SHALY SAND ANALYSIS The log interpretation formula originally proposed by Archie3 in 1941 is applicable only to rock-fluid systems wherein the rock has negligible electrical conductivity. In 1949, Patnode and Wyllie4 showed that if the rock itself can be considered conductive due to the presence of clay, a different calculation approach is necessary. During the following years, this problem was investigated at great length, as was the related problem of the effect of rock conductivity on the SP.5-11 These investigations established functional relationships between SP, resistivity, water saturation and water resistivity for such a formation. Refs. 2 and 12 provide summaries of these studies. Unfortunately, practical use of these relationships required that water resistivity be known independently from the SP. Although log interpretation methods for rock systems containing clay were proposed at that time,' they were not generally accepted for routine use. There are three principal reasons for this. First, in many field situations involving high-salinity water, rock conductivity may be neglected (even if present) without introducing appreciable error. This may be seen by considering the following expression for waier-saturated rock.' 1/R2=1/R1+1/FRn....(1) where 1/R, is conductivity due to clay. As Rw becomes small, I/FRw becomes much greater than 1/R, which may be neglected. Where 1/R, may be neglected, the sandstone is called clean. If the term may not be neglected, the sandstone is termed dirty or shaly. For resistivity purposes, the classification between clean and shaly sands then depends not only upon the conductivity due to shale in the sand, but also upon the resistivity of the associated water (shale is used here to mean surface condition due to disseminated clay). A sand of given conductivity might safely be treated as clean in association with high-salinity water, but would require shaly sand methods if associated with fresher waters. Shaly sand methods are not required in many areas having saline waters; but in Rocky Mountain and West Coast sands having relatively fresh waters (often more than 0.3 ohm-m resistivity at formation conditions), the shaly sand methods are needed. Errors Rw calculations from the SP due to the presence of shale are likewise related to water salinity. In saline water formations drilled with fresh mud, the ratio of mud filtrate resistivity to water resistivity is high, the SP is large and the presence of shale can introduce large errors in water resistivity calculated by the conventional method. When the resistivity ratio is low, the errors are smaller. At zero SP, no error would result from shale. Thus, from the SP viewpoint, a given rock could be shaly if associated with a saline water, and clean in association with a fresh water, which is the opposite of the resistivity-oriented definition above.

By J. P. Timmons, J. H. Moran, G. K. Miller, M. A. Coufleau

A prototype equipment has been designed and built for the digital recording of well logs on magnetic tape at the same time that the regular film recording is made. The format of the digital tape produced is such that it can be used directly at the input of the ZBM 704, 7090 or other models of ZBM computers which accept digital magnetic tape. This apparatus has been used for the experimental field recording of dipmeter tape logs which were subsequently computed by means of an ZBM 704 or 7090. In this paper the equipment and the digital tape are described briefly, and their application to the computer-interpretation of dipmeter data is discussed. A principal element in the interpretation of the dipmeter log is the correlation of the three microresirtivity dipmeter curves to determine the depth displacements between them. Several correlation methods for computer use are considered, with particular attention to their sensitivity to error and their consumption of computer time. The tape data were used to compute information content of the dipmeter microresistivity curves in terms of their frequency spectra. The results show that the sampling rate used in recording the digital information is quite adequate and illustrate a use of the digital tape in evaluating the characteristics of new tools. Some examples of field results are shown. It can be foreseen that, when digital tape recording becomes available for general field use, a whole new realm of possibilities will be opened up for the processing of other well logs through computations, which hitherto were not feasible because they were too laborious and time-con.sunzing. INTRODUCTION The last few years have seen a revolution in the design and production of data-processing equipment. Stored-pro-gram digital computers have progressed from a research curiosity to the basis of a major industry. There are now hundreds of such machines in daily use in the United States. With the acceptance of a technique that was, in fact, already clearly described by John von Neumann in 1945, the last decade has seen great strides in the development'of components, reliability, programming systems and, most spectacularly, in the sheer number of machines built and in use. In 1957 there were enough digital computers available to the oil industry to justify the suggestion that it would be worthwhile to investigate the possibility of using these machines in processing well log data.' The first result of this investigation was the appearance of what may be referred to as the input-output bottleneck. Well logs are customarily recorded on film. To get these data into a machine required then (and still does): a time-consuming semi-automatic reading of the film; conversion of the log data to digital form; and recording these digital data in some medium acceptable for computer input, such as cards, magnetic tape, or punched paper tape. However, the recording, reading, and re-recording could only result in deterioration of the data. Therefore, it was concluded that the fist step should be the development of a new, more direct recording technique supplemental to the film recording, which would provide easy access to the digital computer. There are many solutions to the problem of recording log data in an easily recoverable form. After careful consideration it was decided to adopt the boldest solution which, it was felt, was also the most elegant. It was decided to record well logs directly, in the field, on magnetic tape in such a way that this tape could be used without further modification as an input to the IBM 704 or 7090 computer. To realize practical field recording of magnetic tape logs, it became necessary to develop in a rather small package, an analog-to-digital converter, a tape recorder, and the necessary multiplexing and control circuits to allow the simultaneous recording of a multiplicity of logging signals. The magnetic tape recording was to be made simultaneously with the conventional logging operation in such a way as not to interfere with it. Along with the development of hardware, it was necessary to begin development of interpretation techniques and machine programs that would exploit the power of the digital computer. Here, again, there is a long list of possible applications. After much consideration it was decided to concentrate on the interpretation of the dipmeter log as a first application. It is the object of this paper to describe in some detail the developments sketched in the last three paragraphs.

By L. S. Allen, R. L. Caldwell, W. R. Mills

Borehole models used in the study of nuclear well logging are reviewed and the merit of heterogeneous us homogeneous formation simulation is discussed. .A heterogeneous model for simulating sandstone formations of variable porosity is described. Simulation adequacy was checked by steady-state and pulsed-neutron measurements. INTRODUCTION In the course of developing new methods of nuclear well logging, the problem of testing new logging instruments under controlled conditions always arises. This testing is usually done in wells of known lithology, porosity and fluid saturation, or in laboratory test facilities which simulate the borehole environment. Although the former approach is possibly more satisfying, it is frequently difficult or inconvenient to find wells with enough variation in the cited parameters for a truly comprehensive study, On the other hand. laboratow- test facilities can simulate a wide variety of formation and borehole conditions if care is exercised to maintain adequate simulation under all conditions. Workers concerned with the development of nuclear well logging methods have reported several types of research facilities in the past. Tittle, et a1 used cubical tanks filled with Ottawa silica sand to study the steady-state behavior of thermal and epithemal neutrons in simulated sandstone logging environments.1 In addition to the effects of fresh vs salt water, a wide variety of casing sizes and compositions were investigated. Tittman also used a large tank of Ottawa sand to measure the slowing down length of sio2;2 for making the same measurement in CaCO3, however, an assembly of solid Vermont marble blocks was used. More recently a very useful and carefully planned test pit for nuclear logs was constructed under the auspices of the API.3 This facility consists of three massive limestone sections, each 6-ft thick and more than 5 ft in diameter. The blocks are buried, one on top of the other, in a concrete- lined pit having a total depth of 24 ft. Each section is made of a different limestone to achieve porosity variation, the selection being Carthage (2 per cent), Indiana (19 per cent) and Austin (26 per cent). The blocks are traversed by a 7 7/8-in. diameter borehole and are saturated with fresh water. Great care was taken to insure total saturation of the stone. Homogeneous mixtures and materials have not been used exclusively for simulating earth formations. Tittle, et a1 4 showed that a heterogeneous lattice of water and graphite could be used to simulate Liquid furfural with very little effecton measured thermal and epi thermal neutron distributions. Subsequently, a lattice-type model for simulating limestone earth formations was designed and built by Berry This limestone model was constructed of 7/8- x 11/4- x 36411. marble rods glued together with a plastic resin to form a 36-x 42- x 42in. cube. The cube was honeycombed vertically with 7/8- x 7/8- x 3641-1. holes for porosity and had a 111/2-in. diameter hole in the center to serve as the borehole. Lower porosities were simulated by placing marble rods in the square holes. The finished cube was used with the borehole vertical and set in a tank to hold liquid for filling the holes in the rock matrix. A second type of heterogeneous borehole model has been proposed by Kukharenko, et a1.6= In this concept the model is composed entirely of solid material. The hydrogen equivalent of formation fluids is simulated by making the model of alternating layers of dense rock and celluloid. Additional hydrogen is included by drilling vertical holes in the rock sections and filling them with solid forms of hydrogeneous material. The volume of the holes is equal to the volume of the celluloid layers, so an axial and radial dispersion of the hydrogeneous material is thus obtained. One such model was composed of alternate 50 mm-thick sand layers and 2 mm-thick celluloid sheets. Approximately 200 appropriately spaced plastic tubes were used in each sand section. The plastic tubes were filled with paraffin for additional hydrogen concentration. This arrangement yielded a model with a water-equivalent hydrogen concentration of 10 per cent. Experimental data taken in this model and in a model consisting of the same material but homogeneously dispersed showed that the heterogeneous system adequately

Jan 1, 1966

By C. S. Matthews, H. M. Girner, C. D. Williams, M. J. Edwardson, H. R. Parkison

Quantitative interpretation of electric logs requires knowledge of formation temperature. In this paper, methods are developed for computing changes in formation temperature caused by circulation of mud during drilling operations. The basis of the method is the mathematical solution of the differential equation of heat conduction. The solution of this equation is presented in a series of graphs. These graphs are used to determine formation temperature disturbance at various radii for arbitrary mud circulation histories. Example comparisons with field results show reasonable agreement. It is concluded that, in general, the temperature disturbances caused by circulating mud are small beyond 10 ft from the wellbore but are quite significant near the wellbore. INTRODUCTION Quantitative interpretation of electric logs requires knowledge of the formation temperature in order to establish the resistivity of the formation water with accuracy. To determine the formation temperature, the temperature disturbances produced by circulating drilling mud must be evaluated. -The objective of this investigation was to develop a method for the numerical determination of these temperature disturbances at any distance from the wellbore as a function of time. The first step was to solve the differential equation describing the temperature behavior in the formation during and after mud circulation. This solution was used to calculate a series of curves relating temperature disturbance to shut-in time for various circulating periods. An "exact" method for computing formation temperatures with the use of these temperature-disturbance plots is described, and the results of an application of this method to a well in Montana are presented. Procedures for approximating formation temperatures are also discussed. BASIC EQUATIONS AND ASSUMPTIONS With the assumptions that (1) cylindrical symmetry exists, with the borehole as the axis, (2) heat flow is 1. The formation can be treated as though it is radially infinite and homogeneous in extent, as regards heat flow. 2. The presence of mud cake can be disregarded. 3. The effect of heat generated by bit action is negligible. 4. After mud circulation ceases, the rate of radial heat flow at the wellbore is negligible and is assumed zero for purposes of calculating temperature change with time. These assumptions are believed to be reasonable because of the agreement achieved between calculated and measured borehole temperature in a well, to be discussed later. Further, the agreement at the wellbore indicates that temperatures in the formation were also approximately correct. since it is the formation temperatures that determine wellbore temperature after ceasing circulations SOLUTION OF THE DIFFERENTIAL EQUATION AND ITS APPLICATION The solution of Eq. 2 for the temperature behavior during and after a single period of mud circulation is

By E. A. Breitenbach

A computer program has been developed to afford rapid and complete quantitative log analysis for exploration and production decisions. The computation consists of automatic selection of tops and bottoms of porous intervals from the digitized data, and then point-by-point calculations within each selected interval. Nearly all log types can be analyzed. This paper presents the calculation techniques found to be appticable to machine evaluation and gives examples of their use. INTRODUCTION As the quantitative interpretation of well logs entails repetitive use of charts and equations, it is natural that digital computer programs would be written. A Universal Log Interpretation Computer Program (ULICP) has been developed to afiord rapid and complete analysis for exploration and production decisions. Application of this program enables the log analyst to: (I) apply rapid quantitative log analysis for exploration and production decisions on either a field, a well or on a single zone basis; (2) apply concepts of interpretation requiring detailed numerical analysis; (3) analyze all porous intervals on each log, rather than a few selected zones, and complete the analysis in less time than previously required. Computations are reported for every digitized point in each zone; (4) use empirical techniques applicable to a given area as an integral part of the computation; (5) experiment with the empirical coefficients and exponents in the interpretation equations to find the best possible solution; and (6) automatically plot both the original and computed data to scales analogous to the field prints. The primary utility of a log interpretation program stems from its ability to do an overwhelming amount of work with very little man-power. If a calculation procedure or thought process can be formalized to the extent that step-by-step logic can be written, a computer program can be developed that follows this logic. The major question is one of economics. A feasibility study of the costs for such a program indicates that digital processing is economical primarily as a means of increasing the productivity of the log analyst. In effect, the probability of missing productive intervals in any well, because of a lack of time to do detailed calculations, is greatly reduced. The nucleus of ULICP is programmed to compute large sections of a suite of logs by selecting zones automatically and then performing all pertinent computations on a data point by data point basis within each zone.'.' An entire suite of logs can be processed in this manner with very little manual intervention. Sufficient programming logic is available so that each log analyst can request computations pertinent to his area. These requests are made by simple additions or deletions of information on the input header cards. Hence, the log analyst is always in complete control of the computation process. The evaluation of a suite of logs requires pre-editing, digitization, computation, presentation of results and interpretation. Work by the log analyst is reqilired only in pre-editing and interpretation. Thus, he is allowed more time for comprehensive interpretation, rather than calculation. For continuity, the discussion of ULICP is organized sequentially: pre-editing, digitization, computation and presentation of results. DISCUSSION PRE-EDITING The extent of pre-editing prior to computation is dependent on the format of the original data. For analog prints, it requires inspection, correlation and editing of the logs, plus the entering of required data an special forms. For digitized data such as magnetic tape field recordings, only the special forms are necessary. The process for analog prints will be given here. The inspection and correlation process involves the selection of sections for digitization and the correlation of the traces to a common depth. To decrease the cost of digitization, traces that exhibit large variations in shale formations can be redrawn to a non-zero baseline. The next step is to enter all pertinent data on the input header cards. The header cards presently used for ULICP are presented as Figs. 1 through 6. Cards 1, 2, 3 and 4 (Fig. 1) are defined as the Main Header Cards. They describe the particular well for output identification and give basic information. Cards 5 through 15 (Figs. 2 through 6) are defined as the Block Header Cards. As such, they define the log types and the interpretation parameters for the block of data immediately following. Cards 1 through 10 are required for every computer run. Cards 11 through 14 pertain only to nuclear logs. Card 11 is required if any nuclear log is supplied, and cards 12 through 14 are required only when gamma-ray, density or neutron log data, respectively, are supplied. Card 15 (Fig. 6) must be supplied when cross-plot calculations are required. Either two- or three-component

Jan 1, 1967

Jan 1, 1969

By P. E. Baker

An improved method of logging formation density has been developed in which the formation is bombarded with a collimuted beam of gamma rays. By means of a scintillation detector and pulse height discriminator, the gamma-ray energy band is accepted and recorded which corresponds to deepest penetration into the formation. Laboratory tests on a field tool revealed no borehole diameter effect for smooth holes and no effect of formation chemistry, except insofar as chemistry affects density. In extensive field tests, the density log has exhibited satisfactory agree-ment with core measurements and has correlated accurately with other logs. INTRODUCTION This paper describes an improved method of logging formation density by means of back-scattered gamma rays to penetrate the formation together with energy discrimination to select only that energy (or frequency) range corresponding to deepest penetration into the forma- tion. This method of density logging was developed at the La Habra laboratory of California Research Corp.* Field instruments have been built and tested by McCullough Tool Co. and commercial service has begun in some areas. All of the data discussed here were obtained for the original field tool. The density log can be a valuable adjunct to existing logs because of its use in determining porosity quantitatively. It is useful also in connection with seismic and gravity surveys where formation density information is of interest. PHYSICAL PRINCIPLES The intensity, I, of a beam of gamma rays after traversing a distance, x, in an absorbing medium is given by I = loe -ax where I, is the intensity of the beam entering the medium and a the attenuation constant or macroscopic cross section of the medium for gamma-ray absorption. The macroscopic cross section depends on the number and types of atoms per unit volume and the gamma-ray energy. In brief, it is the sum of the atomic cross sections of all atoms per cubic centimeter. The density of the material is the sum of the masses of all atoms per cubic centimeter. Thus, the density and the macroscopic cross section for gam-ma-ray absorption vary in propor- tion to the number of atoms per cubic centimeter. The density of the material increases, of course, if the atomic masses increase without a change in the number of atoms per cubic centimeter. When the density increases in this latter way, the macroscopic cross section increases also, but disproportionately. This results from the fact that part of the gamma-ray absorption arises from the photoelectric effect and the pair-production effect**, which are proportional to the sixth and to the second power, respectively, of the atomic number of the absorber. The remainder of the absorption results from the Compton effect, and is directly proportional to the atomic number. The gamma rays that undergo Compton scattering are not immediately absorbed. Since these rays are scattered in all directions, a fraction will emerge from the face of the medium at which they entered. If a gamma-ray shield is placed between the source and detector, most gamma rays reaching the detector will be those scattered out of the medium. The intensity will depend on absorption and scattering in the medium, and so indicates the density of the medium. For gamma rays for which Compton scattering prevails, the intensity

Jan 1, 1958

By S. E. Szasz, E. J. Moore, B. F. Whitney

An accurate value of formation water resistivity R, is essential in calculating formation porosity and fluid saturation from electrical well logs. In the cases where R, has not been measured directly, it must be obtained from other data, e.g., the SP curve. This paper deals with another approach: how to calculate R, from the chemical analysis of the formation water. INTRODUCTION It is known that the resistivity of aqueous solutions of pure salts depends on their concentration and on the temperature; the concentrations are given in MPL (mg of solute per liter of solution), or sometimes in ppm (mg of solute per kg of solution): MPL = ppm X specific gravity. Values for different pure salts are available in the literature, but not for solutions of mixtures which are of practical interest. The major component of the dissolved material in almost all formation waters being sodium chloride, it is customary to express the resistivity of formation waters in terms of equivalent sodium chloride concentration, i.e., the concentration of a solution of pure NaCl which has the same resistivity at a given temperature as that of the formation water under consideration. Thus, the problem of calculating R, from the chemical anaylsis can also be stated as how to convert the other constituents of the solute into equivalent NaCl concentration. Salts dissolved in water are at least partly dissociated into ions, and do not conserve their identity. If known amounts of several salts are dissolved in water, the solution does not necessarily contain the same salts in the original proportion, but perhaps some other combination of the ions, along with free ions in solution. This is why the chemical analysis of formation waters is often given in terms of ions, as if all dissolved salts were completely dissociated. Our problem then boils down to how to convert the concentrations of the various ions to equivalent concentrations of Na' and C1-. Dunlap and Hawthorne' have proposed to convert the concentration of all other ions to equivalent Na' and C1-concentrations by means of constant multipliers; e.g., 0.95 for Ca"; 2.0 for Mg"; 0.27 for HCO 3-; 0.5 for SO, -, etc. Their factors were based on measurements made at 68F on 26 formation water samples from the Texas Gulf Coast, ranging in concentration from 1,500 to 75,000 ppm. The Dunlap method is widely used in electric log interpretation, and is often extrapolated beyond its original concentration range. A comparison of R, values calculated by this method and values actually measured on formation water samples has shown large discrepancies, especially at higher concentrations. Therefore, two new methods were developed at Sinclair Oil Corp.'s Tulsa Research Center to calculate equivalent sodium chloride concentration from the chemical analysis of formation water samples. FUNDAMENTAL CONSIDERATIONS The resistivity of a solution, or its reciprocal the conductivity, at a given temperature is determined by the charge, concentration and mobility of the ions actually present. Monovalent ions such as Na' or C1- always carry the same charge. Compounds of polyvalent ions, however. may show incomplete dissociation, e.g., NaSO; + Na' instead of SO,-- + 2Na'. This happens especially in more concentrated solutions. Only very dilute solutions are completely dissociated, as assumed in the chemical analysis report. At higher concentration, the degree of dissociation depends not only on the nature and concentration of the particular salt under consideration but also on the nature and concentrations of the other solutes. Mobility of the ions depends on the viscosity of the solution. It also depends on the degree of hydration of the ions, which in turn is a function of the nature and the charge of the ions and also of the amount of free water available per ion, i.e., the total ionic concentration. The net effect is that the conductivity increases slower than proportional to the concentration, even if a solution contains only one salt such as NaC1, and is different for different salts (Fig. 1). Conductivity can even decline with a further increase in concentration, e.g., if additional salt is little dissociated but ties up some of the free water and/or causes an increase in viscosity. In solutions containing more than one salt, the contribution of one salt to the total conductivity depends not only on the fractional concentration of this same salt, but also on the concentration of all other solutes. A perfect method would give the conductivity or resistivity of a solution as a function of the concentrations of all solutes present. This is so complicated as to be impractical, and a simpler method must be found which is of acceptable accuracy. The Dunlap method, on the other hand. is too simple because it askmes that at any concentration the contrih-

Jan 1, 1967

By J. L. Martin, W. M. Campbell

A new electric logging method, called displacement logging, often gives a direct indication of the presence of mobile hydrocarbons in hydrocarbon-bearing formations. This method is based on the detection of a bank of connate water which is formed between the invading mud filtrate and virgin formation fluids in these cases. The existence of this phenomenon has beer, verified by laboratory studies and by actual field logs taken in known oil sands. INTRODUCTION During the past decade increasing use has been made of conventional electric logs to determine the amount of oil or gas in formations traversed by a borehole. This determination requires knowledge of several factors, such as true resistivity of the formation, formation factor, and water resistivity. Sometimes all of the factors necessary to make a reliable saturation calculation are not available. Also, the computations and corrections which are necessary in such a study often require the services of an experienced log analyst. It has long been the desire of exploration people to have a logging device which will locate hydrocarbon-bearing formations directly without requiring complex interpretation methods. The displacement log, while not completely fulfilling this desire, appears to be a Step in that direction. Basis for the Device The basis for this device stems from a phenomenon which occurs when mud filtrate invades a formation which contains displaceable hydrocarbons. As the mud filtrate invades the formation, most of the oil or gas is pushed out of the invaded zone, leaving residual oil. However, in addition to displacing most of the oil or gas, the connate water is also swept out, since the originally irreducible water is mobile in the presence of a water phase.1,2,3 Tonsequently, this irreducible water piles up as a bank between the virgin oil zone and the invading filtrate. If the mud filtrate is of higher resistivity than the formation water— as is commonly the case—the resulting annular zone, containing the bank of formation water, will be of lower resistivity than either the virgin oil-bearing formation resistivity or the invaded zone resistivity. This resistivity configuration, going from high to low to high again as the distance from the borehole increases, is found only in those zones which contain displaceable hydrocarbons. A device which would reliably detect this low resistivity zone would give a direct indication of mobile hydrocarbons. Illustrations This phenomenon is illustrated in Fig. 1 and 2. Fig. 1 shows a sketch of a zone containing salty connate water plus displaceable hydrocarbons, along with graphs showing fluid content and electrical resistivity vs distance from the borehole, before invasion occurs. Fig. 2 shows this zone, together with fluid content and resistivity graphs. after invasion by a fresh water mud. The resistivity pattern on moving away from the borehole is high: low: high. The absence of the low zone phenomenon in water zones is illustrated in Fig. 3 and 4. Fig. 3 shows a sketch of a zone containing salty formation water and residual oil, along with graphs showing fluid content and electrical resistivity vs distance from the borehole, before invasion occurs. Fig. 4 shows this water zone, together with fluid content and resistivity graphs, after invasion bv a fresh water mud. In this case the resistivity pattern on moving away from the borehole is high: low since there is no change in water saturation at the head ol

Jan 1, 1956

By R. H. Widmyer, G. M. Wood

This investigation was prompted by the absence of a simple, accurate method to predict optimum second stage separation pressure in three stage separation operations. Three stage separation as used here includes two separators plus the stock tank. The first stage pressure is usually fixed by gas sales conditions and the third stage is at atmospheric pressure. Thus, the second stage pressure is the only one that may be varied in-discriminantly during production operations. No variations in temperature were considered in this study, even though it will vary among systems, for a change in temperature has little effect on the optimum pressure. Optimum pressures may be determined from flash calculations, but these are tedious unless a computer is available. An early attempt to develop a simpler method&apos; yielded an equation which gave the optimum pressure only as a function of initial and final pressure. Experience, however, confirms theoretical relationships which show that composition as well as pressure and temperature must be a factor. This indicates that the basic relationship should be an equation of the form, where P, is second stage pressure, P, is first stage pressure, A is a function of P. (stock-tank pressure) and the system composition, and n is a function of the number of stages and the system composition. Fig. 1 shows the basic variation of P2 with P for a fixed system composition. A series of such figures clearly showed the effect of composition. The peak of each curve reflects that second stage pressure at which the stock-tank recovery (gal/MMcf) residue gas is a maximum value. DEVELOPMENT OF NEW CORRELATION Wellstreams having 17 different analyses, ranging from low gravity crude to condensate, were used to determine the effect of composition. Each wellstream was flashed through three stages to determine the optimum second stage pressure for each first stage pressure. Equilibrium vaporization ratios proposed by Latham and Campbell were used throughout. The heptane plus fraction, for simplicity, was assumed to have the properties of octane. The results were subjected to an empirical analysis in an attempt to develop a correlation- that would be (1) simple, (2) accurate, and (3) eliminate the need for any flash vaporization calculations. This analysis yielded two equations, one for wellstreams having a specific gravity greater than one (air = 1.0) and one for those less than one. For crude streams (specific gravFor condensate (specific gravity < 1.0),

By N. L. Muench, J. S. Osba

Research conducted previously has demonstrated that spectral analysis of gamma radiation from the naturally radioactive elements present in earth materials permits detervination of the relative concentrations of these elements. This research suggested that spectral analysis of gamma radiation resulting from neutron bombardment of elements not nor-mally radioactive might serve to identify these non-radioactive elements, and might permit their quantitative determination. To investigate this possibility, laboratory experiments were conducted in which various earth materials were subjected to bombardment by neutrons from a polonium-beryllium source, and in which the spectra of the resultant gamma radiation were analyzed. The geometry of the lab-ortory system was designed to simulate wellbore conditions. The laboratory work demonstrated that some elements not only can be identified with relative ease, but also can be determined quantitatively. This finding indicates that the use of an intense source of more energetic neutrons may lead to the identification and measurement of additional elements of interest by means of induced nuclear reactions. INTRODUCTION In recent years the applications of nuclear physics to the producing branch of the petroleum industry have become increasingly numerous. One significant development, an outgrowth of nuclear science which is of potential importance in well logging, has been the analysis of gamma-ray spectra of naturally radioactive elements in earth strata. The work of Brannon and Osoba1 has demonstrated that presently available equipment and techniques may be used to determine the relative concentration of thorium, potassium, and uranium in underground strata by means of gamma-ray spectral analysis. Success of the research on spectral analysis of naturally radioactive elements suggested that spectral analysis of induced radioactivity might serve to identify, and perhaps measure quantitatively, those elements which normally are not radioactive, but which become radioactive on neutron bombardment. It was to investigate this possibility that the research described in the following sections was undertaken. THEORY OF INDUCED NUCLEAR REACTIONS Of fundamental importance is the fact that an element, when bom- barded with neutrons, emits gamma rays of energies characteristic of that element. Therefore, in theory, an element may be identified by analysis of the energy spectrum of the neutron-induced gamma radiation. If a neutron impinges upon the nucleus of an atom, any one of several reactions may occur. For identification of elements in earth formations the more important are listed below: 1. The neutron may be elastically scattered, as in a collision between two billiard balls. In this case the total kinetic energy is conserved, and no nuclear radiation is emitted. 2. The neutron may be inelast-ically scattered, as in a collision between two automobiles in which case the total kinetic energy is not conserved. The lost kinetic energy reappears in the form of gamma radiation. 3. The neutron may be captured, in which case the neutron becomes a part of the target nucleus, and a gamma ray is emitted. For example, if the target nucleus is H1, the pro-duced nucleus will be H3, and gamma radiation of a particular energy will be emitted. 4. The neutron may produce nuclear fission, or may enter into miscellaneous reactions which result in the emission of

Jan 1, 1958

Jan 1, 1966

By R. H. Winn

In this paper a heterogeneous carbonate reservoir is considered as a succession of layers of formations of radically different porosities, permeabilities, water saturations, and rock matrix types; i.e., the heterogeneity is vertical rather than radial to the wellbore. Presumably each layer is more or less homogeneous. To determine the "net oil in place" in this reservoir it is desirable to know the porosity and water saturation of each layer. In addition, the section must be subdivided into that which will produce oil, that which will produce water, and that which will produce little or nothing. In numerous wells drilled in these reservoirs radioactivity surveys, guard electrode surveys, and caliper surveys have been made. These logs have been compared with core analysis data, when available, and the analysis of the surveys compared with production data. This study, still in progress, is intended to provide improved analytical techniques if such are possible. This paper is a progress report on the work. DETERMINATION OF POROSITY The neutron-gamma log has long been recognized as a measure of hydrogen concentration.1,2,3 In reservoirs that are free of clay, shale, or other hydrogen containing matrices, it has been accepted as a measure of porosity.4,5 Empirical methods to relate relative neu-tron-gamma intensity to porosity are often employed." To provide a more precise determination of porosity from the neutron-gamma log each instrument is calibrated immediately prior to each survey by a method which refers all tools to a hydrogenous cylinder. To insure stability a calibration check is made after each survey. These calibration records are printed as an integral part of the log that is recorded in standard counts per second. A detailed description of the calibration procedure is available in the literature.&apos; The procedure employed insures that each Survey is numerically reproducible within the limits of statistical variation. To relate the standard neutron counts per second to porosity and hole size, records were made in test wells constructed of blocks of limestone. The porosities of the blocks were accurately measured and the blocks completely saturated with fresh water. Fig. 1 presents some of the results obtained in these test wells.&apos; The lines shown in Fig. 1 were obtained by deriving an empirical equation relating the 11 points shown so that the relationship may be projected to larger and smaller hole sizes. A detailed description of the tool is available in the literature.&apos; Field tests indicate that the tool is adjacent to the wall of the hole most of the time when the surveys are made in uncased holes drilled with saline muds. In this circumstance porosity may be calculated from Fig. 1 if the hole size is known. When fresh muds that form thick mud cakes are employed, the centralizing effect of the mud cake must be considered and the porosity determination is less reliable. In cased holes the combined effect of variations in hole diameter and the unknown eccentricity of the casing in the hole may make porosity determinations extremely unreliable. In any event, an accurate knowledge of hole size is essen-

Jan 1, 1958