Geomechanical Alteration of Carbonate Rocks in CO ₂ -Saturated Brine: In-Situ and Ex-Situ Analysis

The injection of CO2-saturated brine can significantly alter the mechanical properties of carbonate rocks through geochemical reactions and wormhole formation. However, previous studies primarily relied on ex-situ measurements to evaluate these changes, which may not accurately represent the subsurface conditions. This study presents a comparative analysis of the mechanical properties of Indiana limestone samples before and after CO2-saturated brine treatment, utilizing both in-situ and ex-situ techniques. Ex-situ measurements, including hardness and dynamic Young’s Modulus (YM) derived from impulse hammer and acoustic measurements, showed reductions ranging from 19 to 36% and 9–13% respectively, indicating rock fabric deterioration. However, in-situ static YM measurements, conducted at elevated temperature (60 °C), confining pressure (13.4 MPa), and pore pressure (10.3 MPa) using a core flooding system with an in-situ scanner and triaxial setup, revealed no significant change in YM after wormhole creation. This discrepancy is attributed to the presence of incompressible CO2-saturated brine filling the dissolved rock at high pore pressure, potentially masking the mechanical degradation observed ex-situ. Furthermore, fast iterative digital volume correlation (FDIVC) analysis of in-situ CT images demonstrated that volumetric and deviatoric deformation decreased with decreasing effective stress, suggesting that rock failure or subsidence might not occur during CO2 injection of CO2 into water aquifers, especially as the pore pressure increases. This study’s main contribution lies in demonstrating that ex-situ measurements can overestimate CO2-induced weakening of carbonate rocks, while in-situ tests provide a more realistic assessment of mechanical stability under storage conditions. These findings emphasize the critical need for in-situ geomechanical characterization when evaluating the integrity and safety of CO2 sequestration reservoirs.