Characterization and Performance Evaluation of Biochar‐Modified Wellbore Cement Under Geological CO 2 Storage Conditions

ABSTRACT The integrity of wellbore cement needs to be maintained to ensure the effectiveness and stability of geological CO 2 storage (GCS). However, the chemical instability of wellbore cement in CO 2 ‐rich environments poses a serious risk of CO 2 leakage. This vulnerability arises from CO 2 ‐induced alteration of cement hydration products, leading to detrimental physicochemical changes in the cement matrix. This study examined the ability of biochar‐modified wellbore cement to resist CO 2 attack. The cement samples were initially cured in a 1 wt% NaCl solution for 14 days under the conditions of 17 MPa and 62°C, mimicking deep downhole conditions. Afterward, the samples were completely immersed in an autoclave containing CO 2 ‐saturated brine solution, which was maintained under the same pressure and temperature conditions. To evaluate the feasibility of using biochar to mitigate CO 2 alteration, microstructural and mineral composition changes in wellbore cement samples were characterized both before and after exposure to CO 2 using various techniques, including XRF, micro‐CT, and SEM‐QEMSCAN. The characterization results indicate that adding biochar enhanced CO 2 alteration resistance of the cement matrix. With the addition of 2% biochar, the carbonation depth was reduced to 556.1 µm, compared to 594.7 µm in the reference sample after CO 2 exposure. In addition, BC‐2 sample showed a smaller reduction in compressive strength (16.25% decrease after 28 days of CO 2 exposure), indicating less degradation when exposed to CO 2 compared to the RF sample (40.49% decrease after 28 days of CO 2 exposure). In short, biochar exhibits moderate resistance to CO 2 alteration and shows potential for application in wellbore cement systems to inhibit CO 2 penetration.