Evaluating Carbonate Reservoir Rocks for Underground Hydrogen Storage: A Comprehensive Laboratory Approach

Abstract Underground Hydrogen Storage (UHS) in porous media represents a promising solution for long-term, large-scale energy storage. Despite its potential, UHS development faces challenges such as low storage efficiencies and potential geochemical reactions between hydrogen and reservoir rocks. This study investigates the behavior and interactions of carbonate rocks, specifically limestones, under reservoir conditions for UHS, focusing on both geochemical and geomechanical changes induced by hydrogen exposure. Static reactivity tests were conducted on various limestone samples at a constant pressure of 500 psi, exposed to different temperature conditions (25°C and 50°C) and brine over 30 days. Both dry and saturated states were tested to simulate diverse reservoir conditions. Parameters such as porosity, permeability, and Uniaxial Compressive Strength (UCS) were measured before and after exposure. Results showed that porosity remained stable across all conditions, suggesting hydrogen exposure does not significantly alter pore structure. However, permeability exhibited mixed responses, with increases observed in two out of three limestone types, indicating hydrogen exposure could enhance fluid flow in certain carbonate rocks. All samples exposed to elevated temperatures and saturation conditions demonstrated a reduction in UCS, indicating a weakening of mechanical integrity, which could impact structural stability and safety of UHS operations. The extent of weakening varied among limestone types, suggesting mineralogical composition plays a crucial role in rock response to hydrogen exposure. These findings provide critical insights into the geochemical and geomechanical behavior of carbonate rocks under hydrogen exposure. While UHS in carbonate reservoirs appears feasible, the significant reduction in UCS highlights potential risks to mechanical stability. Understanding these changes is essential for assessing the feasibility and safety of UHS projects in carbonate reservoirs. Future research should focus on long-term exposure tests and the development of mitigation strategies to address the mechanical weakening of reservoir rocks.