K. J. Dunn, D. S. Guo, and T. A. Zalan
Chevron Oil Field Research Company, La Habra, California
ABSTRACT
We used Polyachenko’s analytical black borehole model and Monte Carlo simulation to study the gas/steam saturation effect on pulsed neutron capture (PNC) logs. Results of the Polyachenko model are substantiated by Monte Carlo computation simulating PNC tool response. Many empirical observations of PNC count rates for the near and far detectors are consistent with predictions of the model. Based on the present study, we find that:
(1) For single lithology formations saturated with relatively fresh water or oil, the near and far detector count rates can be properly scaled so that they show little or no separation on a log.
(2) As gas saturation increases from zero to low values, both near and far counts increase, with far counts increasing at a slightly higher rate. For high gas saturations, far counts increase much faster, whereas near counts decrease slightly.
(3) For limestones, both near and far counts are much less than those in sandstones and dolomites of similar porosities.
(4) As formation water salinity increases, the separation of near and far count rate profiles in gas zones decreases and may disappear for highly saline formation fluid.
A field example is shown to be consistent with results of our modeling study. Actual behavior of near and far count rates is dictated by factors specific to the particular PNC tool and may show variations between tools. Nevertheless, our present study gives for the first time a sound physical basis for many empirical observations of PNC logs and provides useful guidelines for log interpretation.