METHOD FOR DETERMINING THERMAL NEUTRON CAPTURE CROSS-SECTION OF EARTH FORMATIONS USING MEASUREMENTS FROM MULTIPLE CAPTURE GAMMA RAY DETECTORS

摘要

A method for determining thermal neutron capture cross-section of an earthformationpenetrated by a wellbore from counts of capture gamma rays detected at spacedapartlocations from a source irradiating the formations with bursts of high energyneutrons. Anapparent capture cross-section is determined from counts detected at one ofthe spaced apartlocations. The apparent capture cross-section is adjusted by a correctionfactor calculatedfrom counts detected at each of the spaced apart locations. The correctionfactor iscalculated from an empirical relationship of known values of formation capturecross-section,wellbore diameter and wellbore capture cross section, with respect to countsdetected at eachof the spaced apart locations. Coefficients of the empirical relationship aredetermined bysimulating counts which would be detected in response to the known values offormationcapture cross-section, wellbore diameter and capture cross section. Thecoefficients areadjusted until differences between capture cross-section calculated by therelationship fromthe simulated counts and the known values of formation capture cross-sectionare minimized.In the preferred embodiment, coefficients are calculated for each one of atleast two timeregions into which the counts between successive bursts are segregated.