Modeling the diffusivity of hydrogen and the associated cushion gas in depleted hydrocarbon reservoir caprocks

To enable a large-scale hydrogen (H-2) economy, the availability of geographically widespread geo-storage options is crucial. Such geo-storage targets include saline water aquifers, salt caverns, or depleted hydrocarbon reservoirs. The geologic settings, availability of pre-existing surface facilities and downhole well connections make depleted hydrocarbon reservoirs very attractive geo-storage targets. It is, however, important to ensure that the reservoir caprock is capable of containing hydrogen effectively. While leaks through existing or man-made fractures or faults can lead to a rapid loss of hydrogen, a more insidious problem is the diffusive transport of hydrogen through the shales that form the caprock. This study investigates the transport of H-2, CH4, and CO2 through the tight nanoporous media that comprise the caprock system. We consider both binary (H-2 and CH4) and ternary (H-2, CO2, and CH4) gas mixtures that are relevant for hydrogen geo-storage when considering cushion gas requirements. Our modeling considerations also include different surface chemistries, including organic and clay minerals as well as brine salinity conditions. We observe that the diffusivity of hydrogen in the gas phase to be of the order of 10(-7)m(2)/s and only marginally faster than the diffusive transport of methane and carbon-dioxide through the same pores. Considering that reservoir caprocks have prevented the migration of methane and other hydrocarbons for millions of years, our results are promising and point to the effective containment of hydrogen in depleted oil and gas reservoirs. However, the biotic and abiotic geochemistry of hydrogen can pose concerns and therefore the results presented in this study dictate the need for additional work that includes the reactive transport of hydrogen.