USING SOIL CO2 CONCENTRATION GRADIENTS TO IDENTIFY CCS RESERVOIR LEAKAGE

Among the primary concerns associated with carbon capture and sequestration (CCS) is reservoir leakage, resulting in atmospheric CO₂ release. This necessitates reliable monitoring strategies at injection sites to verify successful trapping and storage, and thereby ensure the effectiveness of the operation. Traditional surface-based leakage monitoring strategies rely on discrete measurements of soil CO₂ flux utilizing chamber-based methods. Here, we develop a concentration-based monitoring technique by directly measuring the shallow soil CO₂ concentration gradient during a small-scale injection experiment in DeKalb, IL. The experimental setup included 8 pairs of sample ports (5- and 9-inches deep) connected to a 16-port manifold feeding a Los Gatos Research Ultraportable Greenhouse Gas Analyzer. Using this setup, we measured the background soil CO₂ concentration gradient continuously at 30-minute intervals for approximately two weeks. After obtaining the background data, we simulated a leaking CCS reservoir by injecting a small quantity of food grade CO₂ into a shallow piezometer while continuing CO₂ concentration monitoring for two additional weeks. By comparing the experimental data against discrete measurements of soil CO₂ flux obtained by periodic sampling with a closed-chamber, we demonstrate that the concentration-based technique is an effective alternative to traditional surface flux monitoring.