A differential-ratio neutron logging method for high-precision monitoring of hydrogen storage in depleted gas reservoirs

Depleted gas reservoirs present promising opportunities for underground hydrogen storage (UHS), but accurate measurement of hydrogen saturation ( S H 2 ) is crucial for leak detection, containment integrity assessment, and operational safety. The traditional thermal neutron capture cross section is greatly influenced by neutron transport effects in gas-bearing formations which decreases measurement accuracy. This study proposes a dynamic monitoring method for estimating S H 2 based on the differential ratio ( DR ) obtained from triple-detector pulsed neutron–neutron logging (TPNN). A comprehensive formation–borehole–instrument model was developed using Monte Carlo simulations to evaluate the effects of various factors, including temperature, pressure, lithology, shale content, baseline fluid composition, and borehole conditions, on the DR response. From these simulations, quantitative correction algorithms were derived based on calibration charts. The results indicated that DR was more sensitive to hydrogen–methane substitution than traditional logging parameters and maintained strong discrimination across varying porosity and saturation levels. Validation in a synthetic well demonstrated that, after correcting for bound water, S H 2 values derived from DR closely matched reference measurements, with typical errors of ±5%. These findings confirm that, when properly constrained and corrected for key geological and engineering factors, the DR method provides accurate and reliable real-time monitoring of S H 2 in complex UHS environments. This method provides a strong technical foundation for safe operations, thorough risk assessment, and reliable leak detection in UHS. • Differential-ratio method enhances sensitivity to hydrogen–methane substitution in neutron logging. • Correction framework adjusts for temperature, pressure, lithology, shale, and borehole effects. • Monte Carlo simulations evaluate key factors influencing hydrogen saturation measurement. • Bound water model improves hydrogen saturation accuracy in complex pore fluid systems. • Method attains ±5 % accuracy in synthetic wells, enabling safe underground hydrogen storage.