A Molecular Perspective on Hydrogen Behavior in Moist Depleted Shale: Implications for Geological Storage

Abstract Hydrogen, as a promising low-carbon energy carrier, plays a pivotal role in the global energy transition, gaining increasing momentum worldwide. However, its inherently low volumetric density presents a significant challenge for large-scale storage. Underground hydrogen storage (UHS) in shale has emerged as an effective long-term solution, attracting widespread interest. The presence of residual methane and moisture, however, complicates gas behavior under nanoscale confinement, making it challenging to characterize occurrence mechanisms. In this study, a realistic kerogen-based amorphous matrix is developed, and Grand Canonical Monte Carlo (GCMC) simulations are employed to elucidate the spatial distribution and dominant interactions of gas and water under diverse ambient conditions a range of molar compositions and moisture content, using a three-site model to capture hydrogen's unique electrostatic characteristics. Our findings indicate that moisture content predominantly governs methane adsorption in a kerogen matrix, reducing capacity by up to 36% at 5 wt% moisture and a 0.25 methane molar fraction, while hydrogen adsorption, primarily influenced by molar fraction, exhibits minor enhancements. Elevated methane molar fractions, coupled with higher moisture levels, significantly enhance selectivity, potentially reaching 1.0 with a 4.5-fold increase, though methane's dominance persists. Meanwhile, due to hydrogen's low compressibility, even minimal injections cause notable pore-pressure increases, with a 13 MPa rise observed at a 0.25 molar fraction. Storing large hydrogen volumes under moist conditions with a high molar fraction boosts formation energy and production efficiency but elevates pressure by 42~58% above typical shale formation initial pressures at a 0.50 molar fraction, risking geomechanical instability. The results contribute to a deeper understanding of gas dynamics under confinement, with implications for optimizing the storage and production processes during UHS.