A New Thermal Neutron Decay Logging System--TDT (TM) -M

Summary A new thermal-neutron decay-time logging system (TDT TM-M) has been developed to provide greater accuracy and increased statistical precision by improvement in the manner of detection of the rate of decay of the thermal neutron concentration. Its far-spacing detector can be used to provide a sigma measurement with a greatly reduced diffusion effect as compared with the standard measurement. Introduction Pulsed neutron capture logs provide measurements of the thermal-neutron decay-time constant (tau) or its inverse, the macroscopic capture cross section (sigma). These parameters are useful in differentiating between hydrocarbon- and water-bearing formations. Such logs are especially useful in detecting hydrocarbons in cased holes. Thermal neutron capture measurements typically are made by irradiating a formation with bursts of fast [i.e., 14-MeV (2.24 pJ)] neutrons and measuring the decay of the thermal neutron density in the formation. This is accomplished by counting the gamma rays emitted by formation nuclei during discrete time intervals, or gates, following each neutron burst. Today, the widely used pulsed neutron logging system (TDT TM-K) consists of a sliding-gate system in which three measurement gates are used. The first two gates are timed to detect the formation 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 increased logging speed and reduced statistical uncertainty. Laboratory studies have defined tool response and established correction factors. Dual detector systems are now available that allow estimation of porosity in addition to measurement of sigma. This latter feature permits stand-alone water saturation estimates with pulsed neutron equipment. In spite of this continual improvement, the increasingly stringent requirements on through-casing water saturation determination for monitoring and enhanced recovery programs have outpaced these developments. To meet these increased requirements on accuracy and precision, Schlumberger Well Services has developed a new generation of TDT tools--the TDT-M. Through improvements 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 deg. C (350 deg. 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 form to the surface from 16 time gates for each detector, in addition to gamma ray and casing collar signals. Tool status information also is transmitted to the surface. A downhole memory is used to store gate counts between telemetry transmissions. The telemetry system is bidirectional. Downward telemetry provides commands that are used to turn on the neutron generator and to change the gate timing program, as is discussed later. The surface equipment requires a computer for the determination of tau and sigma 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 TM) or a microprocessor panel CMP-A is required. The CMP-A TM also is used with the PLT-A TM production logging tool string. JPT P. 199^