Characterization and Performance Evaluation of Biochar‐Modified Wellbore Cement Under Geological CO2 Storage Conditions

The integrity of wellbore cement needs to be maintained to ensure the effectiveness and stability of geological CO2 storage (GCS). However, the chemical instability of wellbore cement in CO2‐rich environments poses a serious risk of CO2 leakage. This vulnerability arises from CO2‐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 CO2 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 CO2‐saturated brine solution, which was maintained under the same pressure and temperature conditions. To evaluate the feasibility of using biochar to mitigate CO2 alteration, microstructural and mineral composition changes in wellbore cement samples were characterized both before and after exposure to CO2 using various techniques, including XRF, micro‐CT, and SEM‐QEMSCAN. The characterization results indicate that adding biochar enhanced CO2 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 CO2 exposure. In addition, BC‐2 sample showed a smaller reduction in compressive strength (16.25% decrease after 28 days of CO2 exposure), indicating less degradation when exposed to CO2 compared to the RF sample (40.49% decrease after 28 days of CO2 exposure). In short, biochar exhibits moderate resistance to CO2 alteration and shows potential for application in wellbore cement systems to inhibit CO2 penetration.