Enhancing Cement Integrity in CO2 Sequestration Wells - Advanced Strategies and Implications for Environmental Regulatory Standards

Abstract Geological carbon sequestration requires injecting CO2 into the deep formation through wellbores with an injection rate as high as ~1 Mt/year. This process can lead to substantial temperature drops near the wellbore, potentially causing the cement debonding from the casing or formation, resulting in severe leakage. In this paper, we first analyzed the transient wellbore temperature and pressure profile across the well's depth using both an analytical model and one dynamic multiphase flow simulator, with cross validation. We then adapted these results for the subsequent well integrity analysis using a fully coupled thermoporoelastic model with transient solutions. Our analysis shows that the cooling effect depends on injection rate, surface CO2 temperature, and reservoir pressure. For a simulated Class VI well, a combination of high injection rate, low surface CO2 temperature, and depleted reservoir could result in the wellbore temperature that close to the bottom dropping by 60 ℃. Similar significant cooling scenarios have been observed at the Aquistore field located in Canada, the Alwyn field, and the Goldeneye field in the U.K. The maximum allowable cooling temperature for a typical cement formulation with 10 GPa elastic modulus, and 0.15 Poisson's ratio is 40 ℃. Therefore, this cooling effect could compromise wellbore integrity and cause well leakage through cement-casing debonding. Multiple practical strategies have been found to enhance cement integrity during CO2 injection, including modifying cement formulation to be more ductile and resilient, enhancing the initial state of stress in the cement using a pre-stressing method or expansive additive, and adding an insulated coating layer to the tubing. Pre-heating CO2 at the surface is effective but can be expensive and impractical. Using a protective annulus fluid with a lower thermal conductivity and a new tubing material with low thermal conductivity have been found to be ineffective. Overall, our comprehensive analysis enables us to assess the long-term impacts of CO2 injection on well integrity and promote sustainable and effective geological carbon sequestrations with proper environmental protection protocols.