Soil gas monitoring at the Illinois Basin – Decatur Project carbon sequestration site

Abstract An extensive monitoring, verification, and accounting (MVA) program was carried out from 2011 to 2016 as part of the Illinois Basin – Decatur Project (IBDP), a large-scale carbon capture and storage (CCS) demonstration project in Decatur, Illinois, USA. As part of the monitoring program, a near-surface monitoring network for soil gas was used to characterize the natural variability of CO 2 and identify anomalies that could be associated with CO 2 leakage in the vadose zone. In the fall of 2011, 21 permanent soil gas sampling nests were installed on-site and three nests were installed off-site as reference locations. Soil gas was collected from three depth intervals (15–30, 47–61, 107–122 cm) at each nest. Fixed gas (CO 2 , N 2 , O 2 , and light hydrocarbon) concentrations, stable carbon isotopic composition (δ 13 C), and radiocarbon ( 14 C) content of CO 2 in soil gas were monitored during the pre-injection, injection, and post-injection phases from 2011 to 2016. Both CO 2 concentrations and carbon isotopic compositions showed large spatial and temporal variations; however, the relationships among CO 2 , N 2 , and O 2 concentrations in soil gas indicated that these variations should be attributed to natural processes and not CO 2 leakage. Soil CO 2 concentrations responded to temperature and precipitation variations and increased with increasing soil depth. While 14 C cannot be used to determine the occurrence of CO 2 leakage at IBDP site due to the indistinctive signature between injected CO 2 and soil gas CO 2 , experiences at IBDP validated that δ 13 C can serve as an effective natural tracer at this site. Results from this project have value for future industrial CCS projects that plan to use soil gas monitoring techniques for site characterization and leakage detection.