The Thermal Neutron Decay Time Log

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

Stratton Field, Texas Gulf Coast: A Successful Cased-Hole Re-Evaluation of an Old Field to Determine Remaining Reserves and to Increase Production Level

“…A -5. Method B (1) requires known total porosity, or cf>D, (2) accepts the DDPN-Iog values of N J * and F J * as recorded, and (3) adjusts the slope of Eq. A-5 by determining the s value required to fit the cf> D porosity to the DDPNlog value of N J *.…”

Evaluation and Monitoring of Enhanced Recovery Projects in California and Cased-Hole Exploration and Recompletions in West Texas Based on the Continuous Carbon/Oxygen Log

“…JANUARY 1983 Si is the silicon curve for the capture gamma ray energy window of 2.35 to 5.08 MeV. JANUARY 1983 Si is the silicon curve for the capture gamma ray energy window of 2.35 to 5.08 MeV.…”

“…These features JANUARY 1983 The great thickness of Permian sediments is caused by deposition in a subsiding basin. These include, across the central part from east to west, the Eastern shelf, the Central basin platform, the Delaware basin, and the Diablo platform.…”

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

“…Laboratory experiments indicate that the reduction in Tiog is porosity dependent and may exceed 25 percent at low porosities. 4 For practical interpretation the effect of diffusion can often be compensated for by using an apparent capture cross-section for the rock matrix equal to 1.6 times the theoretical value.…”

“…1 -5 In Ref. 4 the special features (capture gamma ray detection, Sliding Gate* detection system) of the thermal neutron decay time log are described.…”