Impact of Sequential Injection of Dry scCO₂ and CO₂–Saturated Brine on the Petrophysical and Mechanical Properties of Carbonate Formations

Global warming is a pressing issue that necessitates the use of saline aquifers as a potential solution for CO₂ sequestration. Saline aquifers, particularly those in carbonate formations, offer a promising option due to their widespread prevalence and significant storage capacity. This paper investigates the effects of injecting dry scCO₂ and CO₂-saturated brine on the petrophysical and mechanical properties of carbonate rocks, and assesses the viability and effectiveness of CO₂ storage in saline aquifers. The study evaluated porosity (ranging from 20.18% to 23.76%), permeability (from 26.14 mD to 427.21 mD), and the mechanical properties of rock samples under varying confining pressures (14 to 28 MPa). The injection process was simulated in in situ CO₂ storage conditions by introducing supercritical CO₂ and CO₂-saturated brine at 3500 psi and 60 °C. Advanced analytical techniques, including nuclear magnetic resonance (NMR) imaging and microcomputed tomography (micro-CT), were employed to capture the evolution of the rock’s structural and flow properties during the injection sequences. The results show that sequential injection of dry scCO₂ and CO₂-saturated brine altered the pore structure, permeability, and mechanical response of the tested carbonate cores. Dry scCO₂ injection caused moderate changes in NMR-derived porosity and permeability, whereas CO₂-saturated brine injection produced stronger pore-structure alteration, including wormhole formation in some samples. Porosity increased by up to 11%, and permeability increased by 45% in one core and approached a twofold increase through wormhole breakthrough in another. These changes indicate improved local pore volume and injectivity under the tested conditions. However, the injections also reduced dynamic Young’s modulus and increased Poisson’s ratio, indicating mechanical weakening of the carbonate framework. Overall, the results demonstrate that CO₂–brine–carbonate interactions should be evaluated by considering storage capacity, injectivity, and storage security together. Although dissolution-induced pore enlargement and wormhole formation may improve local flow capacity, they may also create preferential flow pathways and reduce mechanical integrity. Therefore, the impact of CO₂ injection in carbonate saline aquifers should be interpreted as a balance between improved reservoir performance and potential storage-security risks.