Computational Study of Surface Signature of Anthropogenic CO 2 at a Potential Carbon Sequestration Site,San Juan Basin

Abstract

Abstract

Injection of anthropogenic carbon dioxide into geologic formations is a technology that can be deployed in the relatively short term in order to avoid potential harm to the environment caused by excess CO₂ in the atmosphere. Success of sequestering CO₂ in underground reservoirs is strongly dependent on the prevention of leakage back into the atmosphere and the ability to mitigate should significant leakage occur. Both detection of leakage and reliable risk mitigation plans require a robust monitoring system. The space and time span of CO₂ sequestration projects is large, which results in trade-offs between cost and robustness of monitoring. In order to make a cost-effective decision without compromising monitoring effectiveness, knowledge of CO₂ transport in the vadose zone and seepage mechanisms into the atmosphere is essential. This study focuses on the simulations of hypothetical CO₂ leakage into the ∼100 m thick vadose zone at an actual site in the San Juan Basin, the United States. Hypothetical leaks were assumed to occur through abandoned wellbores whose integrity had been compromised below the water table. Results show that, at the leak rates analyzed, CO₂ did not express itself at the wellhead for extended periods due to the extremely thick vadose zone. It was also seen that even after decades of simulated leakage, point measurements of CO₂ flux into the atmosphere may not reach levels distinguishable from the background. The regional seepage pattern, however, is both measurable and distinguishable. This finding can be used for designing cost-effective and robust near-surface monitoring networks and algorithms.