Gas transport in shales with applications to geological storage of H₂ and CO₂

Geological storage of hydrogen (H₂) and carbon dioxide (CO₂) in depleted hydrocarbon reservoirs leverages shale caprock as a physical trap for subsurface storage. In addition, shale formations are suggested as suitable geo-storage sites due to their large capacity. Accordingly, this paper investigates H₂ and CO₂ mass transport through shale reservoirs as target caprocks or formations for leakage or geologic storage, respectively. We utilize a species-based model that incorporates different gas storage and transport mechanisms. The model considers the gases to exist in both free and sorbed phases and thus incorporates the adsorption potential of the gases in modeling their transport. By considering varying multicomponent systems where CH₄ exists initially in shale nanopores, we simulate and analyze H₂ and CO₂ injection processes. Simulation results are obtained for each species and subsequently compared to reveal how the free- and sorbed-phase concentration changes during the injection processes. The impact of pore size on diffusion rate in the long term is analyzed. The availability of free gas and the preferential storage sites are also highlighted by analyzing the fractional pore-volume fraction of each phase in shale formation. For fluid-rock systems with lower adsorption affinity, their potential for mass transport through the nanopores is higher than for those with high adsorption affinity. Fluid-rock systems with higher adsorption affinity show a significant contribution of the sorbed phase to the total flux. Overall, this study sheds light on gas transport and storage in shales and the effectiveness of shale caprocks in containing geologically stored H₂ and CO₂.