Pulsed neutron well logging method for determining multiple formation parameters

摘要

A method for well logging includes emitting a plurality of bursts of high energy neutrons into a wellbore and formations surrounding the wellbore. During and for a selected duration after at least one of the plurality of bursts, gamma rays are detected at at least one location spaced apart from the emitting and characterizing an energy of the detected gamma rays. After the last burst, gamma rays are detected and energy spectrum and rates of detection with respect to time thereof are determined. The foregoing is repeated for a selected number of times. After the selected number of times background gamma rays are measured. At least one of the numbers of detected gamma rays during a selected time interval and an energy spectrum of the detected gamma rays during the selected time interval is used to determine selected formation properties.

主权项

1. A method for well logging, comprising:
(a) emitting a plurality of bursts of high energy neutrons having a first selected duration into a wellbore and formations surrounding the wellbore; (b) during at least one of the plurality of bursts and for a second selected duration after the at least one of the plurality of bursts, detecting gamma rays at at least one location spaced apart from a position of the emitting and characterizing an energy of the detected gamma rays, and using numbers of the gamma rays detected after the at least one burst to correct numbers of gamma rays detected during the at least one burst for capture gamma ray effects; (c) after a last one of the plurality of neutron bursts, detecting gamma rays and detection rates thereof with respect to time at the at least one location and characterizing an energy thereof for at least part of a third selected duration; (d) repeating (a), (b) and (c) for a selected number of times; (e) after the selected number of times measuring gamma rays in a background time interval to provide correction for other measurements of gamma rays by removing background gamma radiation; and (f) using at least one of the numbers of detected gamma rays during a selected time interval, the rate of detection with respect to time and an energy spectrum of the detected gamma rays during the selected time interval to determine a neutron porosity, thermal neutron capture cross section, inelastic gamma ray spectral elemental concentration and capture gamma ray spectral elemental concentration by performing (a) through (e) only once for any selected depth interval along the wellbore.