Chemo-Mechanical Investigation of Caprock Integrity and Leakage Risks in Underground Hydrogen Storage: a Focus on Shale Discontinuities

ABSTRACT: Hydrogen is widely recognized as a critical component in future energy, driving the exploration of industrial-scale hydrogen storage. Porous geological formations have emerged as a promising option for temporary storage, complementing traditional sites like salt domes. However, uncertainties persist regarding the sealing efficiency of caprocks that overlie porous sedimentary formations identified for underground hydrogen storage. To address these concerns, we conducted a core-flooding experiment under subsurface pressure conditions to evaluate how brine-hydrogen injection affects the sealing efficiency of a Mancos shale core containing permeable bedding planes. The saturation fluid was equilibrated with the rock mineralogy to ensure that any observed geochemical reactions were limited to hydrogen-mineral interactions. Using a triaxial direct-shear (TDS) system, we measured the effective permeability of both brine and hydrogen at various brine saturation levels in the intact core. This was followed by controlled shear fracturing along the bedding planes and subsequent multiphase-flow characterization. After inducing shearing along the bedding planes, we measured the effective permeability of the fractured core and collected effluent samples every 10-12 hours, coinciding with each permeability measurement. For the intact core, results indicated that brine had significantly higher effective permeability than its absolute brine permeability when hydrogen was introduced into the flow system - a trend also observed in the fractured core. Additionally, shear activation along the bedding planes did not significantly increase the effective permeability of hydrogen and brine compared to the intact core. These findings raise questions about whether the existing relative permeability concept is applicable to brine-hydrogen two-phase flow in shales. They also highlight the need for theoretical modifications to better capture the unique behavior of brine-hydrogen two-phase flow in these formations.