A New Thermal Neutron Decay Logging System- TDTTM-M J .E. Hall, Schlumberger Well Services C.W. Johnstone, SchlumbergerWell Services J.L. Baldwin, * Schlumberger Well Services L.A. Jacobson, Schlumberger Well Services Summary A new thennal-neutron decay-time logging system (TDTTM_M) has been developed to provide greater ac- curacy and increased statistical precision by improve- ment in the manner of detection of the rate of decay of the thennal neutron concentration. Its far-spacing detec- tor can be used to provide a E measurement with a great- 1y reduced diffusion effect as compared with the standard measurement. Introduction Pulsed neutron capture logs provide measurements of the thennal-neutron decay-time constant (7) or its inverse, the macroscopic capture cross section (E). These parameters are useful in differentiating between hydrocarbon- and water-bearing fonnations. Such logs are especially useful in detecting hydrocarbons in cased holes. Thennal neutron capture measurements typically are made by irradiating a fonnation with bursts of fast [i.e., 14-MeV (2.24 pJ)] neutrons and measuring the decay of the thennal neutron density in the fonnation. I This is accomplished by counting the gamma rays emit- ted by fonnation nuclei during discrete time intervals, or gates, following each neutron burst. Today, the widely used pulsed neutron logging system (TDT™ -K) con- sists of a sliding-gate system in which three measure- ment gates are used. 2 The first two gates are timed to detect the fonnation capture'gamma rays, while the third measures the background gamma rays. The last 15 years have seen significant evolution in pulsed neutron capture logging. Improvements have in- creased logging speed and reduced statistical uncertain- ty.2 Laboratory studies have defined tool response and established correction factors. 3 ,4 Dual detector systems are now available that allow estimation of porosity in ad- dition to measurement of E. 2,5 This latter feature per- *Now with Superior Oil Co. 0149-2136/82/0001-9462$$00.25 Copyright 1982 Society of Petroleum Engineers of AI ME JANUARY 1982 mits stand-alone water saturation estimates with pulsed neutron equipment. In spite of this continual improvement, the increasing- ly stringent requirements on through-casing water satura- tion detennination for monitoring and enhanced recovery programs have outpaced these developments. 6 To meet these increased requirements on accuracy and precision, Schlumberger Well Services has developed a new generation of TDT tools-the TDT-M. Through im- provements in neutron output, techniques for measuring and recording decay time, and data reduction, these goals can be met. TDT-M Tool Description and Features The dual-spacing TDT-M tool has a diameter of 43 mm (1 11/16 in.) and a length of 9.9 m (32.5 ft), including telemetry section, casing collar locator, and cable head. The pressure rating is 1150 atm (17,000 psi), and the temperature limit is 177°C (350°F). The static measure point is 2.1 m (83 in.) from the bottom of the tool. The telemetry section includes a gamma ray detector. The TDT-M tool is 15 cm(6 in.) longer than the TDT-K tool when the latter is combined with a gamma-ray tool. The TDT -M tool transmits counting-rate data in digital fonn to the surface from 16 time gates for each detector, in addition to gamma ray and casing collar signals. Tool status infonnation also is transmitted to the surface. A downhole memory is used to store gate counts between telemetry transmissions. The telemetry system is bidirec- tional. Downward telemetry provides commands that are used to tum on the neutron generator and to change the gate timing program, as is discussed later. The surface equipment requires a computer for the detennination of 7 and E from each detector, as well as for the near/far counting rate ratio and other curves that may be recorded. Either a Cyber Service Unit (CSU™) or a microprocessor panel CMP-A is required. The CMP-A ™ also is used with the PLT-A ™ production logging tool string. 199