The accuracy of pulsed neutron capture logs for residual oil saturation determinations.

Abstract

Pulsed neutron capture logs have been used to determine residual oil saturations for many years. A previous study found that at low values of residual oil saturation (ROS) conventional pulsed neutron logging techniques did not have the accuracy necessary for enhanced oil recovery decision making requirements. Special log-inject-log techniques were developed in order to reduce the uncertainty in values of ROS measured with pulsed neutron capture logs. The expected accuracy of these log-inject-log techniques has been reported to be within (+OR-)5 saturation percent.

A study of the uncertainty associated with ROS values determined with pulsed neutron capture logs was made using Monte Carlo simulation techniques. Field data was obtained from tests reported in the literature. The total uncertainty associated with saturations determined by both conventional and log-inject-log procedures involving pulsed neutron capture logs was found to be 3 to 4 times higher than previously published. This increase in uncertainty was due only to the parameters required in the interpretive equations. Additional uncertainty introduced by the log-inject-log process itself was not modeled. This fact makes the estimates in uncertainty presented here optimistic.