Time-Derived Sigma for Pulsed Neutron Capture Logging

Abstract

Summary This paper reviews field applications of the Dresser Atlas Neutron Lifetime Log™ (NLL) service and examines recent technical advances in the instrumentation and data-analysis system. The major improvement discussed is a new method for computing S, the thermal neutron capture cross section of an earth formation. In the new method, the time after the neutron burst is measured for each gamma ray pulse detected by the instrumentation system within a gate of fixed width. This "average pulse time" is uniquely related to the thermal neutron decay rate observed in a borehole environment. The technique discussed is applicable for any condition where the neutron or gamma ray flux is time-dependent. The advantages of this signal-processing method, however, are most apparent for cases of rapid flux change with time, as in an exponential decay. A simple two-gate method of measurement suffers, statistically, from the lower count rate of the second gate. An average-pulse-time measurement over the same range covered by both gates of a two-gate system is less affected by statistical deviations than the ratio normally formed between the two gates. Some important advantages result from this type of signal processing when applied to the NLL service. First, the statistical accuracy of the log is improved; second, the instrumentation required for implementation of the method is not complicated. Also, the average-pulse-time measurement can be made with either digital or analog equipment, and since it involves only simple arithmetic computations, the method is much simpler to implement than either least-squares or correlation methods. Introduction The NLL service1 was introduced in the early 1960's; it was the first measurement through casing that could distinguish and quantitatively evaluate oil and gas in saline formation water environments. More recently, the Dual Detector Neutron Lifetime Log (DNLL) service2 has been used (1) to explore for hydrocarbon resources behind pipe, (2) to monitor the production behavior of wells, (3) to plan workovers properly, (4) to determine residual oil saturation for enhanced-recovery projects, etc. Although the carbon/oxygen3 (C/O) log provides a reliable cased-hole formation evaluation in fresh and brackish formation water environments, the DNLL service is applicable when logging through casing and/or tubing in saline environments. As discussed in detail in the literature, the basic measurement is the thermal neutron absorption (capture) cross section, S, of a subsurface formation. Advanced methods of processing the data recorded by such a pulsed neutron capture logging system are presently under development. These improvements include (1) the average-time technique4 and (2) the sigma-dependent positioning of a single gate exhibiting a fixed gate width of 600 µseconds. The basic theoretical considerations of these improvements are discussed, and supporting field data are presented.