Petrophysical Impacts of CO₂ as Cushion Gas in Subsurface Hydrogen Storage

Depleted oil and gas reservoirs are promising sites for large-scale hydrogen storage due to their low operational costs, high containment security, and effective recovery. Using CO2 as a cushion gas helps maintain reservoir pressure, reduce costs, and support hydrogen production. However, injecting a H2–CO2 mixture can affect rock pore properties, and there is limited knowledge of this impact. This study investigates the effects of CO2 injection on the petrophysical properties of Indiana Limestone (L) and Bandera Gray Sandstone (B). Six core plugs were treated with a H2–CO2 mixture for 90 days at 75 °C and 1400 psi. Results indicated an increase in gas porosity by 7.59% for Bandera Gray Sandstone and 5.98% for Indiana Limestone, with corresponding increases in NMR porosity (9.5% for B and 6.6% for L). Gas permeability also rose by 13.54% for Bandera Gray Sandstone and 9.62% for Indiana Limestone. These findings suggest that CO2 injection can lead to pore expansion and increased permeability, potentially enhancing hydrogen storage capabilities. However, significant changes in the pore structure are unlikely during short storage cycles (1–3 months). Long-term effects, such as fracture development and further pore expansion, warrant an additional investigation. Overall, the study demonstrates the stability of sandstone and limestone rocks in the presence of CO2 as a cushion gas under the tested conditions for short-term hydrogen storage.