Minireview and Perspectives of Gas–Rock Interactions in Shale Reservoirs for Symbiotic Storage of H _<b>2</b> and CO _<b>2</b>

Shale formations are increasingly considered for the symbiotic storage of CO2 and H2, where permanent carbon sequestration is combined with flexible energy storage. This review focuses on gas–rock interactions in shale and examines how geochemical reactions, pore structure evolution, and mechanical property changes influence containment and storage performance in a symbiotic gas-storage scenario. A synthesis of recent laboratory and modeling studies shows that CO2 strongly interacts with carbonate, feldspar, and clay minerals, leading to dissolution, secondary mineral precipitation, and swelling. Adsorption capacity under reservoir pressure conditions was observed to typically range from 0.5 to 3.5 mmol g–1 and porosity may increase from 1.4% to 3.5% after prolonged exposure to CO2. Moreover, it also leads to reductions in strength and stiffness from 30 to 45%. On the other hand, hydrogen exhibits comparatively lower adsorption capacity, generally ranging less than 0.05 mmol g–1. It interacts weakly with shale through chemical pathways and yet affects rock integrity by promoting microcracking, softening, and transport through pore networks, especially in organic-rich zones. When both gases are injected, their effects are interdependent: CO2 modifies mineralogy and sorption sites, which alters H2 diffusion and migration pathways. Comparative analysis further indicates that CO2-related changes dominate over long time scales, while H2 introduces additional challenges under cyclic injection withdrawal, raising questions about fatigue and seal reliability. Despite significant progress, critical knowledge gaps remain, such as long-duration CO2–H2 exposure tests under reservoir pressure–temperature conditions, the role of microbial activity, and the upscaling of laboratory results to field operations. Future research should focus on integrated experimental and modeling approaches that capture these coupled processes and provide a basis for evaluating shale formations as a safe dual-purpose storage medium.