Supercritical CO2-Modified Biochar as a Novel Additive for Enhancing CO2 Corrosion Resistance in Wellbore Cement: Characterization and Mechanisms

In the wellbore system of a CO₂ geological storage site, Portland cement is not adequately resistant to CO₂ corrosion. Therefore, it is essential to incorporate suitable additives that can hinder CO₂ corrosion in order to guarantee wellbore integrity. This study introduces supercritical CO₂-modified biochar (SCBC) as a wellbore cement additive, enabling wellbore cement to be resistant to CO₂ attack. Prior to exposure to CO₂, the samples were initially cured in 1 wt % of NaCl solution for 14 days under the conditions of 17 MPa and 62 °C, mimicking typical GCS conditions. Afterward, the samples were exposed to CO₂-saturated brine for 14 days, with the same pressure and temperature as the curing conditions. The performance of BC (BC with no supercritical CO₂ treatment) and SCBC samples to resist CO₂ attack and their influence on the hydration and strength of the wellbore cement were investigated and compared both with each other and with the control cement sample without BC (RF). The results indicate that SCBC demonstrates greater effectiveness in mitigating CO₂ corrosion (with a 30.97% inhibition efficiency of carbonation) when incorporated into wellbore cement, compared with the RF. This represents a 24.48% increase in the inhibition efficiency of carbonation compared to BC with no ScCO₂ modification. The compressive strength of the SCBC increased from 23.66 to 29.97 MPa, representing a 26.66% increase following 14 days of CO₂ exposure. In contrast, the compressive strength of RF decreased by 28.66%, while the compressive strength of BC with no ScCO₂ modification declined by 24.69% under the same conditions. The characterization results of the SCBC revealed two primary reinforcement mechanisms: (1) promoting the growth of calcite induced by carbonate formation within the cement matrix and (2) preventing CO₂ infiltration due to the preloaded CO₂ within the pores of the BC, along with its water-holding capacity, which aids in internal curing within the cement matrix.