CORRECTING EFFECTS OF BOREHOLE CAPTURE CROSS SECTION AND NEUTRON DIFFUSION IN THERMAL NEUTRON DECAY BOREHOLE LOGGING

摘要

An on-line environmentally-corrected measurement of the thermal neutron decay time ( tau ) of an earth formation traversed by a borehole is provided in a two-detector, pulsed neutron logging tool, by measuring tau at each detector and combining the two tau measurements in accordance with a previously established empirical relationship of the general form: tau = tau F+A( tau F+ tau NB)+C, where tau F and tau N are the tau measurements at the far-spaced and near-spaced detectors, respectively, A is a correction coefficient for borehole capture cross section effects, B is a correction coefficient for neutron diffusion effects, and C is a constant related to parameters of the logging tool. Preferred numerical value of A, B and C are disclosed, and a relationship for more accurately approximating the A term to specific borehole conditions.