Geo-Mechanical Assessment of Creep Deformation Phenomena: Relevant for CO2 and H2 Storage

Abstract Underground hydrogen storage (UHS) is emerging as a critical technology for large-scale, long-duration energy storage in the transition to low-carbon energy systems. By utilizing geological formations such as depleted reservoirs, deep saline aquifers, and salt caverns, UHS offers the potential for seasonal balancing of renewable energy generation and demand. However, repeated injection and withdrawal cycles can induce complex hydro- mechanical responses in the host formation, influencing both storage performance and long-term containment integrity. In this study, a fully coupled two-dimensional poromechanical model was developed to investigate the geomechanical stability of a homogeneous deep saline aquifer subjected to cyclic hydrogen injection and withdrawal. The formulation integrates a pressure equation, well representation, and Biot’s linear poroelasticity with an explicit creep deformation component, enabling the simulation of pore pressure, displacement, stresses, volumetric strain, and deformation-driven porosity and permeability evolution. Single-cycle simulations revealed strongly localized pressure build-up and volumetric strain near the injector, accompanied by reductions in effective stress and the formation of an annular zone of elevated deviatoric stresses. Property changes were confined to the near-well region, with permeability enhancement driven by elastic dilation. Two-cycle analyses demonstrated cumulative effects, with residual porosity and permeability increases persisting after production phases, particularly near the injector where directional permeability rose by up to 27%. Creep inclusion highlighted a ratcheting mechanism, whereby volumetric strain and property changes accumulate over successive cycles even under constant loading conditions. These findings underscore the significance of incorporating time- dependent deformation processes into the geomechanical assessment of UHS systems, as cyclic operations can induce progressive, spatially localized changes that influence both injectivity and containment performance.