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.