Thermal neutron decay logging using dual detection

A Dual-Spacing Thermal Neutron Decay Time logging tool has been developed which embodies several new features to improve the recording of SIGMA and tau and to provide additional information. Lower statistics and faster logging speeds are achieved by use of the Complete Scale Factor system and an increase in the yield of the pulsed neutron source. The tool has a 1-/sup 11///sub 16/-inch diameter, and has two detectors. It is combinable with a Gamma Ray logging tool. Some of the data it provides are: SIGMA, the neutron capture cross section; tau, the thermal neutron decay time; a ratio curve, derived from the count rates of the two detectors; a count-rate display for comparison of the count rates from the two detectors; and a quality control curve, which is used to validate the SIGMA and ratio recordings. The ratio and SIGMA data can be used to calculate an apparent porosity (phi/sub K/ for the formation and an apparent water salinity (WS/sub a) for the formation water. The count-rate display is designed to facilitate gas identification. Response data are shown for SIGMA and ratio measurements in laboratory sandstone formations of 18 percent and 34 percent porosity with cased boreholes. Borehole sizes weremore » 8-in., 10-in., and 12-in., and casing sizes were 5 /sup 1///sub 2/-in., 7-in., and 9 /sup 5///sub 8/-in. Results of an extensive field-test program are discussed. Methods for using and analyzing the logs provided by this tool, run either in casing or in casing and tubing, are described. Log examples are shown.« less