An Experimental Investigation of Geomechanical Effect on Depleted Oil and Gas Reservoir Storage Rock During Underground Hydrogen Storage: A Case Study in North Dakota's Red River Formation

Abstract This study examines the geomechanical effects of depleted oil and gas reservoir rocks during underground hydrogen storage (UHS), with a specific focus on the Red River Formation in North Dakota. As the transition to renewable energy sources accelerates, hydrogen storage in geological formations presents a promising solution for large-scale energy storage. However, numerous challenges and barriers exist that prevent this technology from becoming a widely available decarbonization solution. The UHS process comprises multiple cycles of injection and withdrawal. These cycles may elevate the risk of fracturing or seal integrity failure in certain reservoirs. Furthermore, it is crucial to assess the potential alteration of rocks caused by hydrogen exposure to understand its impact on the mechanical properties of the reservoir rock and caprock. This study investigates the effect of geomechanical change in the mechanical and petrophysical properties of reservoir rock and caprock rocks (Carbonate + Anhydrite), crucial factors for securing subsurface hydrogen containment. Core samples from three wells in the Red River Formation's Zone B were subjected to high-pressure, high-temperature (HPHT) conditions simulating underground storage environments, including 30-day hydrogen exposure at 2000 psi and 140°C. Advanced analytical techniques including Nuclear Magnetic Resonance (NMR) porosity measurements, pulse-decay permeability analysis, and ultrasonic velocity measurements were employed to assess geomechanical changes before and after hydrogen exposure. The results revealed distinct lithological control on hydrogen-rock interactions with favorable implications for storage integrity. Carbonate reservoir samples demonstrated systematic porosity increases ranging from 9.7% to 23.1%, with corresponding permeability enhancements of 12.7% to 25.8%. Despite these petrophysical changes, mechanical properties showed improvement, with dynamic Young's modulus increasing by 1.7% to 11.4% and compressional wave velocities enhancing by 8.6% to 30.1%. Poisson's ratio increased by 3.6% to 13.7%, indicating altered deformation characteristics that remain within acceptable operational ranges. Anhydrite caprock samples exhibited exceptional stability and enhanced sealing capacity under hydrogen exposure. Porosity changes were negligible (±2%), while permeability remained ultra-low with minimal variations (±4.5%). Remarkably, caprock mechanical properties showed significant strengthening, with Young's modulus increasing by 10.4% to 18.3% and compressional wave velocities improving by up to 22.1%. These enhancements indicate improved structural integrity and resistance to deformation under operational pressures. The systematic mechanical property improvements in both reservoir and caprock formations create a geomechanically favorable environment for hydrogen storage operations. These findings advance the understanding of geomechanical processes in UHS and provide crucial insights supporting the safe and efficient implementation of hydrogen storage in depleted carbonate reservoirs with anhydrite seals, particularly in similar geological settings throughout the Williston Basin.