Numerical simulation study of fast neutron cross section logging using D-T neutron source with long source distance detector

[Background]: Fast neutron cross section (FNXS) logging plays a crucial role in evaluating tight gas reservoirs and monitoring carbon dioxide geological sequestration. The accuracy and detection characteristics of FNXS logging are primarily influenced by the source-to-detector distance of the long detector. Therefore, investigating the effects of source-to-detector distance selection on FNXS logging is essential. [Purpose]: This study aims to optimize the source-to-detector distance for the long detector in FNXS logging and analyze the impact of different source-to-detector distances on its detection characteristics. [Methods]: Firstly, utilizing the actual parameters of a multi-detector pulsed neutron logging tool, a Monte Carlo numerical simulation method is employed to analyze the gamma response of gas layers at various source-to-detector distances. Subsequently, based on FNXS measurement sensitivity and error analysis, the optimal range for the long detector’s source-to-detector distance is determined. Finally, a three-detector pulsed neutron tool model, based on Schlumberger’s Pulsar tool, is constructed to analyze the detection depth and vertical resolution at source-to-detector distances of 30 cm, 60 cm, and 90 cm. [Results]: Results indicate that the optimal range for the long detector’s source-to-detector distance in FNXS logging is between 85 cm and 115 cm. As the source-to-detector distance increases, the detection depth increases, while the vertical resolution decreases. At a source-to-detector distance of 90 cm, the detection depth and vertical resolution are 22 cm and 76.2 cm, respectively. Moreover, the increased detection depth of the long detector makes it suitable for measurements in cased holes, but unsuitable for thin-layer measurements. [Conclusions]: This study provides theoretical support and key parameter references for enhancing FNXS measurement accuracy and optimizing instrument design.