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

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 CO₂ and identify anomalies that could be associated with CO₂ 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 (CO₂, N₂, O₂, and light hydrocarbon) concentrations, stable carbon isotopic composition (δ¹³C), and radiocarbon (¹⁴C) content of CO₂ in soil gas were monitored during the pre-injection, injection, and post-injection phases from 2011 to 2016. Both CO₂ concentrations and carbon isotopic compositions showed large spatial and temporal variations; however, the relationships among CO₂, N₂, and O₂ concentrations in soil gas indicated that these variations should be attributed to natural processes and not CO₂ leakage. Soil CO₂ concentrations responded to temperature and precipitation variations and increased with increasing soil depth. While ¹⁴C cannot be used to determine the occurrence of CO₂ leakage at IBDP site due to the indistinctive signature between injected CO₂ and soil gas CO₂, experiences at IBDP validated that δ¹³C 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.