Short- and Long-Term Responses of Reservoir Rock Induced by CO2 Injection

CO2 injection for geologic carbon storage is commonly conducted into sedimentary reservoir rock and influences the stress state and pore pressure resulting in coupled hydro-mechanical processes. Consideration of the presence of aqueous fluids in reservoir formations is crucial for describing their mechanical response, both short- and long-term. As CO2 is injected into the reservoir, the chemical reaction of the acidic mixture of CO2 and water with rock minerals may alter the poromechanical and hydraulic responses that need to be appropriately characterized. We conduct laboratory experiments aimed at accurate measurements of the stress-dependent poroviscoelastic rock properties to describe short- and long-term deformations. The chemical effects of CO2 treatment on the poroviscoelastic and hydraulic properties are explored by duplicating each experiment before and after CO2 treatment, while Berea sandstone, Apulian limestone, and Indiana limestone are selected as representative reservoir materials. CO2 treatment increases the bulk compressibility of the sandstone and limestones by 15-20%. The undrained response is characterized through measurements of Skempton's B coefficient, which decreases for Berea sandstone, but increases for the limestones. For calcite-rich rock, the porosity and solid compressibility increase by 4-6% and 20%, respectively, while these parameters remain unchanged for silica-rich rock. As the time-dependent behavior is measured indirectly by monitoring the pore pressure buildup under an undrained condition, the CO2 treatment appears to significantly facilitate the viscous response by 50-60% for all materials. Additionally, CO2 injection causes an increase in permeability but does not change the porosity-permeability exponents for the tested rock. Porosimetry and microimaging analyses reveal that for the limestones the main effect is caused by the dissolution of calcite, but for the sandstone the main explanation for the observed results is stress corrosion cracking. This study outlines CO2 injection influence on the poroviscoelastic response of reservoir rock and emphasizes the importance of properly characterizing the time-dependent behavior for geologic storage projects.