Assessment of hydrogen geo-storage capacity in depleted shale formations: Multiphysics storage mechanisms and the impact of residual gas in place

Despite the growing interest in hydrogen as a potential source for supplying the increasing global demand for energy, the design, construction, and maintenance of hydrogen storage and transport infrastructure poses major challenges. Unlike natural gas, compressing hydrogen in storage units requires relatively high pressure which could be unfeasible and, hence, large storage compartments are needed to ensure a continuous supply. Depleted hydrocarbon reservoirs present naturally occurring storage compartments that come with existing infrastructure to allow for injection, production, and monitoring of hydrogen. In this study, depleted organic-rich shale formations were investigated for their suitability for the geo-storage of hydrogen. The approach adopted followed a realistic scenario of hydrogen injection in the presence of residual amounts of natural gas (represented by methane) to capture hydrogen/natural gas interactions with the surrounding rock formations at some degree of confinement as well as the effect of that on the storage mechanisms. The investigation was carried out at the molecular scale using molecular simulation considering some of the organic and inorganic constituents of shales. The results showed that, despite the degree of confinement, hydrogen was taken up mainly by different nanopores as free compressed phase, while methane was predominantly adsorbed. These results were used to derive a reservoir-scale volumetric model to determine the total hydrogen storage capacity in depleted shale formations. The findings reported in this study provides some critical insights for the assessment of subsurface reservoirs for geo-storage projects.