Neutron Tool Research Articles

A new approach to the interpretation of nuclear borehole logs

Abstract When interpreting nuclear borehole logs it is usually assumed for simplicity that the tool signal and a formation property, e.g. its density, are uniquely related. However, while the tool signal will usually depend primarily on one formation property, it will also be affected by other formation properties. To take this multi-parameter dependence of all the nuclear tools into account, a new approach to nuclear log interpretation has been developed. In this approach four nuclear tools are considered: the gamma density tool, the neutron porosity tool, the pulsed neutron tool and the natural gamma tool. The responses of these four tools depend on formation density (ρ), porosity (φ), neutron absorption (σ a ), and geological type (including shale content). For each of the four tools a physical model has been developed. The signals measured with the four tools and the four models represent together four equations with four unknowns: ρ, φ, σ a and geological type. These four equations may be solved by iteration and a consistent set of formation properties obtained. The interpretation approach has been implemented on a personal computer. Examples of results of such interpretations are presented.

Evaluating Gas Content of Black Warrior Basin Coalbeds From Wireline Log Data

Summary Coalbeds have long been known to contain natural gas in varying amounts. To the mining industry, this is basically a hazard to be controlled and avoided where possible. To the petroleum industry, it is an exploitable resource that holds significant economic potential because of shallow drilling and the promise of long well life. The variability of coal as a gas reservoir raises the need for oil-and gas-well explorers to develop the capability of evaluating coalbed gas content to identify and produce coal gas plays. Current petrophysical models for gas reservoir analysis in traditional clastic or carbonate environments are notably inadequate for evaluation of gas content in coalbeds. However, numerous physical models exist for coals throughout the country, primarily from the efforts of the mining industry. A theoretical model was chosen and used to establish a method for evaluating gas content of coalbeds in the Black Warrior basin of Alabama. Core data in the form of proximate analysis, ultimate analysis, and gas desorption measurements were used, along with wireline data, to build the models. Results of data analysis in wells with coals ranging from low-volatile bituminous to high-volatile B bituminous are compared with core gas desorption measurements to demonstrate the accuracy of the process. The thin coals in the Black Warrior basin require high-resolution logging (especially neutron and density tools) to obtain acceptable results. Wireline computations on a prospective well can be obtained much sooner than core desorption measurements and can provide analyses in zones where samples were not recovered. Implementation of these techniques in conjunction with core measurements can help significantly in determining the economic viability of coalbed-methane prospects.

Application of an MWD Propagation Resistivity and Neutron Tool: ABSTRACT

Application of an MWD Propagation Resistivity and Neutron Tool: ABSTRACT

Compaction Monitoring in the Ekofisk Area Chalk Fields

In late Nov. 1984, the subsidence phenomenon was recognized in the Ekofisk field. To determine the magnitude and areal extent of the formation compaction, a program for measuring compaction with electric logging tools was initiated. Initial time-lapse surveys performed with cased-hole neutron tools indicated that reservoir compaction was occurring, but the accuracy of the determination of compaction rate was low. In addition to the cased-hole neutron survey, radioactive markers and a gamma ray (GR) detection tool were used to determine compaction rate in the reservoir more accurately and to determine whether compaction was occurring in the overburden. A program for implanting radioactive-marker bullets and subsequent monitoring with a four-detector GR tool was implemented. There are currently 13 wells equipped with radioactive markers in the compaction monitoring program. Compaction monitoring accuracy using the four-detector GR tool was found to depend on wellbore geometry, completion design, and radioactive-marker placement. This paper gives the results of the program to date and describes the operational procedures and analysis techniques used for compaction monitoring in the greater Ekofisk area chalk fields.

Statfjord Field: Development Strategy and Reservoir Management

Summary This paper reviews reservoir performance and management of the Statfjord field after 8 years of production. The reasons behind the reservoir development strategies and field experiences are presented. The field comprises three reservoirs produced simultaneously with designated wells for each reservoir: the Upper and Lower Brent and the Statfjord. The two Brent reservoirs are produced with a waterflood, while the Statfjord reservoir is produced with a high-pressure miscible gasflood. The original development plans have been refined on the basis of field performance through an extensive monitoring program and use of reservoir simulation. The induced gamma ray spectra (IGRS) log is used to monitor water movement in the Brent reservoirs, while the compensated neutron tool (CNT) is the main tool used to monitor the gasflood in the Statfjord reservoir. The acquired data have improved the geologic model and the knowledge of fluid movements in all three reservoirs. This resulted in a large and complex reservoir simulation model with more than 20,000 gridblocks. Introduction The Statfjord field, the largest producing oil field in Europe, is located in the prolific northern part of the Viking graben, 125 miles [200 km] northwest of Bergen, Norway, on the U.K./Norwegian boundary (Fig. 1). The field was discovered in March 1974 with the drilling of Well 33/12-1 and was delineated with drilling of 11 additional wells. The field lies mainly in Norwegian Blocks 33/12 and 33/9, but also extends into U.K. Blocks 211/24 and 211/25. The field was unitized in 1976 with Mobil as operator. The equity redetermination of 1979 resulted in 84.1 % of the field being in the Norwegian sector and 15.9 % in the U. K. sector. The operatorship was transferred from Mobil to Statoil in Jan. 1987. The Statfjord field, which is 15 miles [24 km] long and averages 2.5 miles [4 km] in width, is located in a westerly tilted and eroded Jurassic fault block. The main recoverable reserves, about 75 %, are located in the Middle Jurassic Brent group, while the remaining 25 % is in the Lower Jurassic/Upper Triassic Statfjord formation. Both Brent and Statfjord have exceptionally good reservoir characteristics. The estimated ultimate recovery is around 3 × 109 bbl [475 × 106 M3] of oil and 3 × 10 12 Scf [85 × 109 std M3] of gas. Both Brent and Statfjord reservoirs contain highly undersaturated low-sulfur crude oil. Reservoir data and fluid properties are given in Tables 1 and 2. The Statfjord field is being developed with three gravity-based Condeep-type platforms and single-point mooring for offshore oil tanker loading. Production from Platform A commenced in Nov. 1979, while Platform B came on production in Nov. 1982 and Platform C in June 1985. Regular gas sales by pipeline to the U.K. and the European continent began in Oct. 1985. Eighty-nine development wells have been drilled to date to achieve the current average daily oil-production and water-injection rates of 750,000 STB/D [119 000 stock-tank M3 /d] and 1,050,000 B/D [ 167 000 M 3 /d], respectively. As of Dec. 1987, nearly 1.2 × 109 bbl [190 × 10(6) M3] Of oil. or about 40% of the recoverable reserves, has been produced.

Porosity Calibration of Neutron Logs, SACROC Unit (includes associated papers 15799 and 15846 )

Summary A major field study has been initiated at the Scurry AreaCanyon Reef Operators Committee (SACROC) unit to provide a detailedgeologic description of the reservoir. To supply porosity information to the digital database, a new technique has been developed thattransforms neutron deflection logs into accurate neutron porosity logs. The technique uses computer programs to analyze statistically the neutronlog to be rescaled as well as nearby modern porosity logs. An equationis derived that transforms neutron log deflection into porosity. Oncethe porosity response of a particular tool is determined in areas ofgood porosity control, that response can be applied successfully inareas of little or no control. Introduction The SACROC unit reservoir description project (RDP)is a major field study and has as its overall goal thecreation of a digital geologic database of wide scope andrelevance to the operation's current and foreseen needs. The purpose of the RDP is to provide support to theengineering staff at this major CO2 and waterflood siteand to assist the geologists in infill wellsite selection. To accomplish these goals, an immense amount of data mustbe incorporated into the system. This paper describes a major part of the system: thegeophysical well log processing. All gamma ray andneutron logs have been digitized, but only the neutronlogs are undergoing transformation. They are being rescaledin terms of calibrated limestone porosity to provide adense three-dimensional control on the porositydistribution. The SACROC unit covers about 50,000 acres [200 x 10(6)m2], the major part of the giant Kelly-Snyder field(Fig. 1). The reservoir is in the Canyon limestone, a late Pennsylvanian stratigraphic reef or carbonatebuildup(Fig. 2). Although composed for the most partof clean limestone, it also contains shale, shaleylimestone, dolomite, and chert. Having produced over 10(9) bbl [160 x 10(6) m3] of oil, the unit has been converted to tertiary recovery methods(CO2 injection). If reliable porosity logs could beobtained for each of the 1,600 + wells in the unit, fluidmovement would be much more predictable, and floodefficiency could be improved. Throughout the unit is a total of 400+ modern neutronporosity logs(mostly sidewall neutron porosity [SNP]). The remaining 1,200 + wells have been logged with theold-style neutron deflection logs. Core control is limitedto 124 wells, 95 of which have a cored interval greaterthan 30 ft [10 m]. Preliminary tests showed that the old neutron logs couldbe transformed to accurate limestone porosity if sufficientmodern porosity-log control existed in nearby offset wells. Because virtually all of the porosity control exists in thenorthern one-third of the unit, accurate porosity transforms could be developed in that area, but the remainingtwo-thirds of the wells would have to be treateddifferently. We decided to develop carefully the most accuratetransforms possible for well logs in the area of good controland then to apply the appropriate transforms, accordingto the particular neutron tool used, throughout the restof the field. A survey of the log files indicated that 11 differentlogging companies have used at least 70 different neutronlogging tools at SACROC since the field's discovery in1948. Each tool was given an alphanumeric tool code sothat as transform equations were developed and storedalong with other well data, they then could easily berelated to the particular service company and toolconfiguration used. This would eventually permit anygroup of transforms to be called up and compared to others. With nearly 1,000 logs transformed to date, this automaticcross-referencing system has been an obvious advantage. Also, as logs are processed in those areas that lack control, the tool code permits the proper, previouslygenerated transform to be selected and used automatically. Digitization As each well is digitized, additional information to be usedin various phases of the project is recorded with a keypunch. Fig. 3 is a printout of these data for a typical well. After a log is digitized, a playback plot of the log asdigitized is overlaid on the original log for quality-controlpurposes. After the playback plot passes quality control, a second plot is requested with all of the backups shiftedto their correct position and any neutron-zero baseline shifts removed. The result is a continuous trace of neutrondeflection in inches from neutron zero. At this point, the log is ready for transform development. Porosity Transform Development The relationship between neutron log deflection andporosity was presented by Brown and Bowers. JPT P. 468^

Gravel Pack Evaluation

Neal, Marvin R., SPE, Schlumberger Offshore Services Summary Many wells in the Gulf of Mexico are completed with internal gravel pack assemblies to prevent sand production. A reliable method for evaluating the pack is needed to reduce the chances that a work over will be required later. Field logs made with tools chosen from test-well experiments confirm the application of wireline logging techniques to provide a qualitative evaluation of gravel pack quality. pack quality. Introduction Many logging tools have potential for investigating the annular space between screen and casing in an internal gravel pack. Among these are neutron porosity, acoustic velocity, density, and pulsed neutron decay time tools. The choice of tools used for the examples in this paper was based on previous experience and the known factors of depth of investigation and response to metal. The tools chosen werea compensated neutron tool, 3 3/8-in. diameter,a nuclear fluid density meter, 1 11/16-in. diameter, anda dual-spacing gamma ray tool, 1 11/16-in. diameter. Measurements were made in two test wells equipped with standard gravel pack assemblies in 9 5/8-in. and 7-in. casing. Response to various problem gravel pack situations was studied by making successive runs in partially packed sections. partially packed sections. The compensated neutron, nuclear fluid density meter, and dual-spacing gamma ray tools all gave equally good results, and each responded well to changes in the density of the material in the annulus between screen and casing. This paper describes the test wells and the logs that were obtained from various gravel pack situations. Actual field logs that further validate the effectiveness of the technique are shown. Theory Compensated Neutron Tool The compensated neutron tool uses a neutron source and two detectors (near and far). In its conventional usage the count rates from the two detectors are used to compute a ratio from which porosity can be determined. The compensated neutron tool measures the hydrogen index of a formation by emitting fast neutrons into the formation and detecting those that are slowed to the thermal energy level. This is possible because hydrogen has the greatest slowing effect on neutrons. The fluids that are contained in the pore space are rich in hydrogen atoms, whereas the formation rock contains little or no hydrogen. Therefore, the compensated neutron log reflects primarily the fluid-filled part of the formation. Two detectors are used to eliminate part of the effect of the surrounding borehole. The difference in the spacing causes one detector to be more affected by the borehole than the other and allows computation of a ratio of the two that is relatively free of borehole effects. Gravel pack logging utilizes the fact that the detectors are affected differently by the area between the tool and the formation. When the space between the screen and casing is filled with fluid, the tool is looking at a zone that has an apparent 100% porosity. When that fluid is displaced with gravel, the apparent porosity is greatly reduced. Proper calibration and scaling of the near and far count rates cause them to read on top of each other in zones that are 100% packed and to separate in zones of less than 100% pack. The greatest separation is in zones that have no pack at all. Nuclear Fluid Density Meter Tool The nuclear fluid density meter uses a gamma ray source and a single detector. The source emits gamma rays into the borehole and the surrounding area. Some of the gamma rays are scattered back to the detector. The number of gamma rays returning to the detector is an indication of the density of the material through which they traveled. JPT p. 1611

Geophysical Logging in Carbonate Aquifersa

ABSTRACTSome logging methods are inherently superior to others for the analysis of limestone and dolomite aquifers. Three such systems are the density, neutron, and acousticvelocity logs.Relative percentages of limestone and dolomite, average matrix (grain) densities of the rock mixtures, and porosity of the rock mass can be calculated from density, neutron, and acoustic‐velocity logs. Wells drilled before the 1960's generally were logged with an uncalibrated neutron tool. In some instances, the resultant curve can be empirically calibrated.With supporting data from resistivity logs, apparent water resistivity (Rwa) can be estimated. Depending on rock and mud resistivities, the two most useful resistivity curves for water‐quality studies are the deeply‐focused laterolog and the induction log. For older wells, the standard electric log may be used, if the drilling mud is not salt‐saturated.Rwa contour maps derived from geophysical data are useful to outline areas of recharge, direction of probable ground‐water flow, and location and salinity of brine areas. Another useful application of well logs is estimation of ground‐water temperature. These temperatures are obtained from maximum‐reading thermometer data that generally are recorded on each logging run. Maps of porosity can be used to outline potential areas for completing large‐yield wells.

A Neutron Logging Method for Locating the Top of Cement Behind Borehole Casing

Introduction A few methods are known in the oil industry for determining the top of cement in the annulus around a casing. Before the advent of electrologging surveys, inflow and circulation methods provided information on the quality of cementation jobs. Temperature logging is presently the technique commonly used for this purpose. The radioactive tracer survey and sonic log based on the principle of "cycle skipping" also have been used principle of "cycle skipping" also have been used advantageously in selected cases. These surveys indicate only the presence or absence of cement. The cement bond log, however, gives additional information with respect to the bonding of cement to the casing. Grosmangin et al. have given a comprehensive account for analyzing the quality of cementation using the cement bond log. A new technique is proposed in this paper for locating the top of cement behind the casing using a neutron log. This method was used successfully in a North Kadi well in the Cambay basin, India, where temperature survey control was inadequate and the services of a cement bond tool were not available. Discussion The neutron curve responds primarily to the amount of hydrogen present in the borehole fluid and the fluid in the pores of the rocks. The neutron log is also sensitive to the pores of the rocks. The neutron log is also sensitive to the casing, cement sheath, and borehole diameter. However, the borehole effect is reduced considerably in the SNP log. Better results are achieved with the CNL log, where the effects of mud column, mud cake, rugosity, and even casing and cement are reduced considerably . On a neutron curve, the top of the fluid in a well is indicated by a "right-hand shift" in the neutron diagram. This is known as a fluid shift in the neutron log. As the neutron device is continuously pulled up hole, the device records the hydrogen content of the rocks of the wellbore and borehole fluids. Of these two fluids, the borehole fluid is more effective in slowing down the fast-moving neutron. Consequently, the intensity of the secondary gamma ray is reduced. As the tool is pulled farther up, there is an abrupt increase in the secondary gamma ray radiation on leaving the borehole fluid. This gives rise to the fluid shift on the neutron curve. Based on this principle, an anomaly is expected on the neutron log at the principle, an anomaly is expected on the neutron log at the interface of set cement and the overlying fluid zone in a cemented casing. The geometry of the cemented casing comprises a cemented and an overlying cement-free fluid zone sandwiched between the casing and the wellbore. As the neutron device is pulled up in a borehole against the cemented zone, the borehole fluid has little influence on the recording of the log. The effect of the fluid inside the pipe is insignificant on the neutron log because it is the common fluid against both the cemented and cement-free columns in the annulus. As the tool moves up into the fluid column overlying the set-cement zone, the borehole fluid slows down the fast-moving neutrons and reduces the secondary gamma ray intensity, giving rise to a "left-hand shift" on the neutron curve. The neutron log in Fig. 1 was recorded with an uncompensated neutron tool in a North Kadi exploratory well. In this well, the temperature survey did not indicate conclusively the quality of cementation against the pay zone. The cement bond log could not be recorded because the tool was not available. Under these circumstances, a neutron log was planned to check the quality of cementation. In this well, a shift is indicated in the neutron curve recorded after the production-casing cementation at 840 m. This shift can be explained by the increase in the hydrogen ion concentration in the fluid column as compared with that of the zone of set cement. This shift is interpreted to represent the top of cement in the well. To dispel the concern that any lithological change caused this anomaly, an analysis was performed with the SP curve, gamma ray, microlog, and formation density log. This analysis did not show any significant lithological break at this depth. JPT P. 1089

Analysis of Pulsed-Neutron Decay-Time Logs in Acidized Carbonate Formations

AL-SAIF, A.S., ARAMCO ABQAIQ, SAUDI ARABIA COCHRANE, J.E., MEMBER SPE-AIME, ARAMCO, DHAHRAN, SAUDI ARABIA EDMONDSON, H.N., SCHLUMBERGER TECHNICAL SERVICES, PARIS, FRANCE YOUNGBLOOD, W.E., MEMBER SPE-AIME, SCHLUMBERGER OVERSEAS, DHAHRAN, SAUDI ARABIA Abstract The measurement of thermal neutron decay times by means of pulsed neutron tools has become an important reservoir-monitoring technique. In many types of reservoirs, these measurements permit the location of oil remaining behind casing. A requisite condition for the application of this method is knowledge of formation porosity and chloride content. This knowledge usually is derivable from the open-hole logs run before completion of the well. However, when the producing zones are treated with hydrochloric acid, either of these parameters may be changed. This paper presents examples of dual-spacing thermal neutron decay-time logs in Arabia, where prior acidizing bas altered the log response to The prior acidizing bas altered the log response to The point of producing erroneous conclusions unless point of producing erroneous conclusions unless this effect is accounted for. A hypothesis is advanced explaining this phenomenon as the result of either or both the porosity increase created by acidization and the retention of chlorides from the acid by the formation. Although no way has been found to differentiate positively between the two effects, experience indicates that the cumulative effect observed on The decay-time log is permanent during the water-free productive tile of the well. Thus, the recognized production-monitoring technique, known as time-lapse decay-time logging, is still valid and useful providing that The original "reference" decay-time log is run after acidization. This paper investigates various aspects of the problem and details ways in which it has been problem and details ways in which it has been dealt with in practice. Introduction A dramatic acid effect on pulsed neutron decaytime measurements was recognized by the Arabian American Oil Co. (ARAMCO) late in 1973. Before this time, ARAMCO was successful in using periodic decay-time logs to monitor water-saturation changes in nonacidized carbonate reservoirs. During 1973, a number of logs were run in acidized wells in the Arab D reservoir of the Ghawar field for the purpose of detecting sources of water production. Results were confusing at best until a base log recorded in a clean oil producer revealed be acid effect producer revealed be acid effect Extensive inquires were made to shareholder companies, other Arabian Gulf operators, and to Schlumberger. It was found that, although acid effects had been recognized, no correction techniques had been devised. The only guideline given was to disregard water-saturation calculations in acidized formations. Since such calculations were the primary reason for running decay-time logs for monitoring, and most ARAMCO wells were acidized during completion, this guideline apparently left no alternative but to cease decay-time logging in carbonate reservoirs. Since there were no other techniques for water-saturation determination in cased holes, however, A was recognized that a workable solution to this problem had to be found. Early in 1974 a controlled evaluation program was begun to study the acid effect on dual-spacing decay-time measurements. The program considered the following questions. Is be acid effect truly caused by acidization of carbonate reservoirs? Is it a permanent effect, or does it disappear with oil or water production? What is the physical nature of the effect and can it be accounted for in water-saturation calculations? Can the anomalous behavior be used to evaluate the effectiveness of acid treatments? Dual-spacing decay-time logs were obtained in many wells, before and after acid treatments that displayed a variety of characteristics (such as, rates, volumes, concentrations, use of diverting agents, etc.). Also, open-hole porosity and resistivity measurements were obtained before and after treatment to study the effects of acid on other parameters. SPEJ P. 453

The Dual-Spacing Neutron Log-CNL

A new dual-spacing neutron tool developed for porosity determination and gas detection in open or cased boreholes is combinable with various other logging tools. Simultaneous recording saves rig time, and an overlay log presentation facilitates wellsite interpretation. presentation facilitates wellsite interpretation. Introduction A two-detector neutron tool has been developed to determine porosity in open and cased boreholes. The response of the tool is determined by the rate at which the thermal neutron population decreases with distance from the source. This rate of decrease is principally determined by the porosity of the formation. The quantity measured is the ratio of counting rate from the near-spaced detector to that from the far-spaced detector. The surface control panel converts this ratio to a porosity index, which is then displayed on a linear scale. Individual counting rates, count-rate ratio, and porosity index may also be recorded on magnetic tape for subsequent machine computation. The dual-spacing measurement provides compensation for the effects of hole conditions - salinity, temperature, mud type, and mud cake. The residual effects are generally small. The log has therefore been named the compensated neutron log (CNL*). The porosity uncertainty caused by statistical variations is diminished through the use of a high-yield chemical neutron source. A unique feature of the tool is that it may be run in combination with other logging tools such as the formation density, thermal decay time. sonic, and induction-spherically focused logs. Such combinations make possible overlay presentations for wellsite interpretation, and offer the advantages of reduced rig time and positive depth matching. Dual-Spacing Neutron Detection Principle - Fast neutrons from a point source radiate into an infinite homogeneous medium and are slowed down by repeated collisions until they are in thermal equilibrium with the atoms of the medium. The thermal neutrons continue to diffuse, but, on the average, suffer no further energy loss. Eventually they experience absorbing collisions (capture collisions) and disappear from the system. In the process of slowing down, the neutron "cloud" spreads out spatially. The width of this spatial distribution is frequently characterized by a single property, called the slowing-down length. This is a measure of the size of the cloud of neutrons of a particular energy. It is proportional to the mean particular energy. It is proportional to the mean distance that neutrons travel from the source in slowing down to that energy. An alternative theoretical model emphasizes the so-called group-diffusion theory. In this case, one treats all the neutrons in the medium (usually down to some preassigned energy) as if they had the average energy of the group. This approach, also, leads to the definition of a characteristic parameter, the diffusion length, which is a measure of the width of the spatial distribution of the neutrons in the group. P. 1073

The Sidewall Epithermal Neutron Porosity Log

Abstract A sidewall epithermal neutron tool has been developed to substantially reduce environmental effects that have previously complicated neutron log interpretation. Designed for operation in uncased wells, the device provides increased accuracy in both liquid-filled and empty holes. A brief discussion of neutron moderation, diffusion and capture shows that logs using epithermal neutron detection depend on a smaller number of formation-characterizing parameters than those using thermal neutron or capture gamma ray detection. Thus, they come closer to providing an unambiguous determination of hydrogen content. In this new device a directionally sensitive epithermal neutron detection system has been incorporated in a sidewall source-detector skid to minimize borehole effects. The effects of variations in borehole size and shape, mud type, temperature and salinity are greatly reduced. Small residual borehole effects are then computationally accounted for in the surface control panel to provide a borehole-corrected neutron log. The log presents a direct recording of neutron-derived porosity on a linear scale. With corrections for bore hole effects already applied, this direct recording of porosity simplifies log interpretation. Furthermore, comparison with a linear porosity presentation of a formation density log permits sandstones, limestones and dolomites to be readily identified. Thus, in complex or variable lithology, porosity is determined with greater accuracy and reliability that heretofore. Laboratory data and field results demonstrate the improvements in neutron logging afforded by this new sidewall epithermal neutron logging device. Introduction The new Sidewall Neutron Porosity (SNP) logging system, designed for use in uncased wells, provides reliability and accuracy never before achieved with neutron logs. The effects of variations in borehole diameter and shape, fluid salinity, mud weight and temperature parameters that have long complicated neutron log interpretationare suppressed or corrected for by this sidewall epithermal neutron detection system. Furthermore, to simplify interpretation the SNP log presents a direct recording of Computed porosity on a linear scale. The performance improvements achieved by this new system arise primarily from the combination of two important design features. First, though not unique to this new tool, an epithermal neutron detection system is used, Epithermal neutron detection substantially reduces the perturbing influences of the thermal neutron absorption properties of rock matrices and water salinity. Second, the neutron detection system is mounted in a directionally sensitive sidewall skid to greatly minimize borehole effects. The purpose of this paper is to explain briefly the advantages offered by the detection of epithermal neutrons, to describe the SNP equipment and the log, to present calibration and correction data and to give examples of interpretation methods made especially convenient by this new system. ADVANTAGES OF EPITHERMAL NEUTRON DETECTION Epithermal neutron detection, of all the neutron methods in current commercial use, provides the simplest determination of formation hydrogen content. Advantages of epithermal neutron detectors, as compared with thermal neutron and gamma ray detectors, are probably best explained by a brief review of the life of a source-emitted neutron. A fast neutron from the source will eventually be captured by the nucleus of an atom. However, before capture is likely to occur, the fast neutron (energy of 100,000 electron volts (ev) or more) will be slowed down until it is in thermal equilibrium with its surroundings. The neutron is then considered to be a "thermal" neutron (average energy of 0.025 ev at 25C). The overwhelming majority of the slowing down thus occurs in reaching epithermal energies (energies just above thermal 0.5 to several ev). After reaching thermal energy, the neutron will "diffuse" through the formation with, on the average, no further energy change until capture. Upon the capture of the thermal neutron, relatively high energy gamma rays are usually emitted. Hydrogen plays a very important role in the process of slowing down fast neutrons. On the other hand, the absorption effects of water salinity and low concentrations of other strong thermal neutron absorbers relatively unimportant in the moderation process are very important in thermal diffusion, capture and the production of gamma rays. Thus, by detecting only epithermal neutrons with the SNP, spurious effects due to these thermal neutron absorbers are greatly minimized as compared to the effects on tools using thermal neutron or gamma ray detectors. The epithermal neutron detection system affords another important advantage. JPT P. 1351ˆ