Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part I. Fundamentals and Techniques

1971 ◽  
Vol 23 (6) ◽  
Author(s):  
C. Clavier ◽  
W. Hoyle ◽  
D. Meunier
Keyword(s):  
Thermal Neutron ◽  
Decay Time ◽  
Neutron Decay ◽  
Quantitative Interpretation

The Thermal Neutron Decay Time Log

1970 ◽  
Vol 10 (04) ◽  
pp. 365-379 ◽  
Author(s):  
J.S. Wahl ◽  
W.B. Nelligan ◽  
A.H. Frentrop ◽  
C.W. Johnstone ◽  
R.J. Schwartz
Keyword(s):  
Thermal Neutron ◽  
Time Constant ◽  
Decay Time ◽  
High Energy ◽  
Decay Time Constant ◽  
Neutron Decay ◽  
Fast Neutrons ◽  
Neutron Density ◽  
Thermal Neutrons ◽  
Open Hole

Abstract Thermal Neutron Decay Time (TDT) logging tools in 3-3/8 and 1-11/16-in. diameters have been developed for detection and evaluation of water saturation in cased holes. These tools utilize a system of movable and expandable detection time-gates which are automatically adjusted as the log is being run. The two principal detection gates are positioned in time after the neutron burst according to an optimization criterion. An additional gate, delayed until most of the decay has taken place, permits correction for background. This place, permits correction for background. This Scale Factor gating method provides, in each bed, a thermal-decay-time measurement of maximum statistical precision consistent with removal of borehole effects present in the early part of the decay period Increased reliability is afforded by use of digital techniques. Thermal neutron decay time tools employ capture-gamma-ray detection. This choice was based on an extensive series of experiments made to compare gamma-ray detection and direct detection of thermal neutrons. Measurements of thermal neutron decay time constant are affected by local changes in neutron density in the vicinity of the sonde, caused by flow of neutrons by diffusion from one medium to another. The measured decay time constant (T meas) of neutron density at any point may differ, therefore, from the intrinsic decay time constant (T int) produced by absorption alone. The basic physics of neutron diffusion and absorption is reviewed. When the borehole and the formation have different decay time constants and diffusion coefficients, diffusion couples the two regions. Consideration of such effects sheds light on the conditions required for reduction of borehole effects on measured values of the decay time constant. The choice of source-detector spacing is affected. and, for accurate quantitative interpretation, departure curves are required. Departure curves are presented showing the effects of varying cement thickness, casing diameter. and casing fluids Illustrative log examples are shown. Introduction The Thermal Neutron Decay Time (TDT) log provides a determination of the time constant for provides a determination of the time constant for the decay of thermal neutrons in the formation. Hence, it reflects primarily the neutron absorptive properties of the formation. These properties are properties of the formation. These properties are useful in formation evaluation. The most important area of application is in logging cased hole. Because chlorine is by far the strongest thermal neutron absorber of the common earth elements, the TDT log responds largely to the amount of NaCl present in the formation water. As a result, this present in the formation water. As a result, this log resembles the usual open-hole resistivity logs and is easily correlatable with them. When information on lithology and porosity is known or is provided by open-hole logs, a log of neutron provided by open-hole logs, a log of neutron absorption properties permits the solution of a wide variety of problems: saturation determination, oil-water contact location, detection of gas behind casing, etc. Measurements of the thermal neutron decay time constant are made by first irradiating the formation with a pulse of high-energy neutrons from a neutron generator in the sonde, and then, a short time after the neutron source is turned off, determining the rate at which the thermal neutron population decreases. After each neutron burst, the high-energy neutrons are quickly slowed down to thermal velocities by successive collisions with the nuclei of elements in the formation and borehole. The relative number of thermal neutrons remaining in the formation is measured during detection intervals which follow each burst. Between each burst and the beginning of the first detection interval is a delay time which permits the originally fast neutrons to reach thermal permits the originally fast neutrons to reach thermal energy and allows "early" borehole effects to subside. SPEJ p. 365

Accurate Thermal-Neutron Decay Time Measurements Using the Far Detector of the Dual-Spacing TDT A Laboratory Study

1979 ◽  
Vol 19 (01) ◽  
pp. 59-66 ◽  
Author(s):  
William B. Nelligan ◽  
Stephen Antkiw
Keyword(s):  
Thermal Neutron ◽  
Cross Section ◽  
Decay Time ◽  
Neutron Capture ◽  
Water Saturation ◽  
Systematic Errors ◽  
Neutron Decay ◽  
Thermal Neutron Capture ◽  
Capture Cross ◽  
Laboratory Measurements

Abstract To improve the accuracy of saturation-change determinations in reservoirs, thermal-neutron-decay time was measured in the laboratory with a long-spacing TDT (trade mark) device. The far detector of a TDT-K sonde was used in 17.1 and 30.4% porosity sandstone formations for several formation-fluid salinities ranging from 0 to 247,000 ppm NaCl. A 7-in. (17.8-cm) casing cemented in a 10-in. (25.4-cm) borehole was used with and without a 2 7/8-in. (7.3-cm) tubing centered in a gravel pack. Complete decay curves are constructed from measurements made in successive channels of a multichannel analyzer. Values of formation intrinsic decay time calculated from nuclear-capture crosssections are compared with decay times measured with the far detector using the Scale Factor TM gating system. Results show That far-detector measurements are less influenced by diffusion and indicate the usefulness of the large source-detector spacing for determination of changes in water saturation () from logs run at different times in the history of a producing well. Although errors in values calculated producing well. Although errors in values calculated for are reduced by using values measured with the far detector, oar analysis shows that further improvement in accuracy can be obtained by using the correction data derived from the laboratory measurements. Introduction In the TDT time-lapse technique, changes in water saturation, Sw, are determined from the corresponding changes in the measured formation thermal-neutron-capture cross-section. The relationship used to find the saturation change, in an oil-bearing formation is ....................(1) where is the macroscopic thermal-neutron-capture cross-section of the formation at the time of a first measurement, is the cross-section at the time of a subsequent measurement, is the formation porosity, and and are the macroscopic porosity, and and are the macroscopic thermal-neutron-capture cross-sections of formation water and oil, respectively. It is clear from Eq. 1 that it is not necessary to know, the cross-section of the rock matrix. Furthermore, the accuracy of is not affected by systematic errors in the measured values of, if these errors are the same for the two measurements, and . When the errors in and are not equal, however, the effect on the result for from Eq. 1 may be significant. Therefore, for use in secondary- and tertiary-recovery programs, it is of interest to look for ways of improving accuracy available in the present state of the art. One possibility is to use a longer source-detector spacing for the TDT sonde. A longer spacing (632 mm) is available by using the far detector of the TDT-K tool. however, in most cases in practice, it is the near-detector recording that is shown for the curve on the log. This is because the higher counting rate available at this detector (spacing of 343 mm) is needed to obtain an acceptable statistical validity with a single pass in the well. The measurements with the longer spacing are useful when statistical uncertainty is reduced by recording slowly or by conducting several runs and averaging them. To investigate performance with the longer spacing, laboratory measurements determined the changes in systematic error caused by changes in the of the formation fluid when all other parameters were constant. We found that parameters were constant. We found that measurements with the far detector of the TDT-K sonde had smaller systematic errors that were less sensitive to changes in the formation fluid than those from the near detector. SPEJ P. 59

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

1975 ◽  
Vol 15 (06) ◽  
pp. 453-466 ◽  
Author(s):  
A.S. Al-Saif ◽  
J.E. Cochrane ◽  
H.N. Edmondson ◽  
W.E. Youngblood
Keyword(s):  
Saudi Arabia ◽  
Decay Time ◽  
Water Saturation ◽  
Carbonate Reservoirs ◽  
Neutron Decay ◽  
Monitoring Technique ◽  
Acid Effect ◽  
Before And After ◽  
Open Hole ◽  
Pulsed Neutron

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

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