Geomechanical Alteration of Carbonate Rocks in CO₂-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.