Pycnometric evaluation of shale caprocks during underground hydrogen storage in depleted reservoirs

Feasibility of underground hydrogen storage (UHS) in depleted reservoirs relies on the sealing integrity of caprocks. This study examines matrix porosity and permeability changes in Marcellus, Eagle Ford, Wolfcamp, and Mancos shale caprocks exposed to pressurized hydrogen (6.9 MPa) over 10, 45, and 90 days. Marcellus and Eagle Ford shales exhibited significant increases in porosity and permeability due to cyclic hydrogen exposure, inducing micro-fracturing and pore enlargement along grain boundaries. In contrast, Wolfcamp and Mancos shales showed minimal changes, indicating greater structural resistance. These findings suggest that pressurized hydrogen injection-depletion cycles can compromise caprock integrity in susceptible shales, increasing risks of hydrogen leakage. Meanwhile, structurally resilient shales are better suited for UHS applications, as they minimize diffusive loss of the stored hydrogen. This study underscores the importance of comprehensive reservoir characterization to evaluate caprock performance, guide risk assessments, and optimize UHS operations, ensuring safe and economically viable large-scale UHS operations.