An Experimental Study on the Caprock Integrity of Reservoirs to Assess the Repurposing Depleted Bakken Formation Oil and Gas Fields for Underground Hydrogen Storage

Abstract The escalating greenhouse gas emissions have compelled global economies to implement climate change mitigation strategies. Geological hydrogen storage in depleted oil and gas reservoirs emerges as a groundbreaking solution, offering a dual benefit of repurposing existing geological structures while advancing sustainable energy storage, potentially facilitating the transition to a low-carbon economy. However, the potential for hydrogen leakage over extended storage periods is a significant concern. To assess the risk of leakage, it is essential to understand the interactions between hydrogen, brine, and the reservoir caprock integrity. In this study, we collected core samples from three depleted oil and gas reservoirs in the Bakken Formation (W17351, W21884, and W24881), from three distinct fields: Antelope, Alger, and Ranch Coulee. The Upper Bakken formation serves as a seal for potential underground hydrogen storage in the Middle Bakken reservoir. The core samples were subjected to hydrogen and brine exposure under high-pressure, high-temperature (HPHT) conditions in an autoclave reactor for 1 and 5 days to simulate reservoir conditions and assess the impact of hydrogen-brine-caprock interactions on the reservoir's integrity. We analyzed the samples' porosity, permeability, and mechanical properties before and after long-term exposure to hydrogen-brine using Nuclear Magnetic Resonance (NMR), permeability measurements, and ultrasonic measurements, X-ray diffraction (XRD) and scanning electron microscopy (SEM) to assess changes in their properties. The results showed a consistent increase in permeability across all samples, with the magnitude of increase varying based on exposure duration. NMR measurements indicated a substantial, time-dependent increase in porosity for all samples. Mechanical properties, such as Young's modulus and Poisson's ratio, decreased after exposure to hydrogen-brine, suggesting increased susceptibility to deformation and reduced ability to withstand stresses. SEM analysis revealed the development of fracture pores, interparticle pores, and dissolution-induced pores, as well as changes in elemental composition. XRD analysis showed changes in the relative abundances of minerals, with a decrease in clay and quartz content and an increase in K-feldspar content. These findings have significant implications for the integrity and sealing capacity of the Upper Bakken formation when considering its suitability for underground hydrogen storage. The observed changes in permeability, porosity, mechanical properties, microstructure, and mineralogy raise concerns about the potential for hydrogen leakage and the long-term stability of the reservoir seal.