A prediction model for carbonation depth of cement sheath of carbon capture utilization and storage (CCUS) wells

The cement sheath of CCUS well is vulnerable to carbonization corrosion upon protracted exposure to a CO₂-rich setting, which reduces the strength of the cement sheath and increases the porosity, eventually leading to CO₂ leakage. Predicting the carbonation depth and regularity of the cement sheath of CO₂ injection wells allows an estimation of the service life , to ensure safe operation of CO₂ injection wells. However, most of the current prediction models for CO₂ corrosion depth are still semi-empirical models, which are fitted to experimental data but are not universally applicable. This may be resolved by our CO₂ corrosion depth prediction model supported by the law of mass conservation, diffusion convection equation, and calcium precipitation rate. The influence of seven factors on the corrosion depth was analyzed and ranked. The rise in corrosion time, temperature, chloride ion content, CO₂ partial pressure, water-cement ratio, and water saturation increases corrosion depth and CO₂ content, in addition to porosity and permeability, while increasingly corrosion-resistant material causes the opposite effect. The cement sheath begins to be seriously corroded by CO₂ partial pressure exceeding 10 MPa, chloride ion content over 0.20 mol/L, or temperature higher than 70 °C. Water saturation significantly affects corrosion, and the CO₂ corrosion depth at 0.8 is 10.16 times that at 0.6. The CO₂ content at the distance of 0.2 m–0.93 m from the corroded end surface basically does not change after 7 years of corrosion. Water-cement ratio increased to 0.48 provides conditions for a large amount of CO₂ accumulation in the cement sheath. The addition of corrosion-resistant materials can reduce the initial porosity and permeability of cement sheath. The seven factors is ranked in descending order of influence as water saturation, corrosion-resistant material, water-cement ratio, CO₂ partial pressure, corrosion time, chloride ion content, and temperature.