A NEW ALGORITHM FOR CORRECTING NEUTRON DECAY LOGS FOR BOREHOLE AND DIFFUSION EFFECTS

摘要

Pulsed neutron capture (PNC) logs have been in routine use for decades for the measurement of formation capture cross section. The measurement is robust, accurate, and statistically precise. However, borehole and diffusion effects on the measurement are difficult to characterize and use since they require accurate knowledge of not only borehole size but also borehole capture cross section.rnThis paper presents an innovative approach to correct pulsed neutron logs for borehole and diffusion effects without the knowledge of borehole capture cross section or water salinity. This is accomplished through the utilization of all the information available in the time decay spectra of both the near and far detectors. The correction to the apparent formation capture cross section of one of the detectors is described as a mathematical model of the relative counts in different gates of the time decay spectra of both detectors. The coefficients of the model are obtained from a nonlinear least-squares fit of the model to data from different borehole and formation conditions. The model was optimized using over 5000 data points generated from accurate Monte Carlo simulations covering a very large range of down-hole conditions. The accuracy of the new algorithm when tested on modeling data was found to be on the order of 0.5-1 cu over the entire range of formation and borehole capture cross sections.