7-Years of Soil Gas Sampling for Assurance Monitoring at an Onshore CCS Demonstration Site in Victoria, Australia

Carbon capture and storage (CCS) projects aim to capture carbon dioxide (CO2) from hard-to-abate industries and the existing energy industry, for permanent storage in suitable geological formations. CCS technologies offer promising solutions to achieve net-zero emissions and ease the transition towards clean energy. Environmental monitoring is employed to demonstrate minimal influences on the near-surface environment by the CCS operation and provides assurance to the regulators and local communities. Soil gas monitoring is one of the techniques available for monitoring onshore sequestration sites. The Otway International Test Centre (OITC), Australia’s first demonstration site for deep-well CO2 injection, has demonstrated over 95,000 tonnes of CO2 sequestration into a depleted gas reservoir and a deep saline aquifer. Due to the unique regulatory framework under which the site is permitted, a requirement of the site’s EPA license for the injection of CO2 into the subsurface is soil gas monitoring, which commenced prior to the first injection in 2008. At the OITC, soil gas samples were collected at more than 100 sites across a study area of 3.8 km2 from a 1-meter depth, with soil gas baseline values established in 2007 and 2008. The samples were analysed for major gas concentrations (i.e., oxygen (O2), nitrogen (N2), CO2 and methane (CH4)), 13C analysis on CO2, and selected samples were tested for tracer, including sulphur hexafluoride (SF6), Krypton (Kr) and Xeon (Xe). Based on available data, the injection operations at the OITC have no undesirable impacts on the near-surface environment and, when managed appropriately, CCS operations can be conducted with a high level of confidence. The 17-year soil gas data from the OITC shows high year-to-year variabilities in soil gas CO2 concentrations, posing a major challenge to ensure robust soil gas baseline monitoring. For this reason, the use of soil gas monitoring for regulatory processes is not supported, however, as a general site check and as a means to garner community confidence, it has proven to be useful. To address the challenge of this naturally occurring variability, a multi-step verification process has been implemented to enhance confidence in identifying or ruling out anomalies. This process incorporates tracer analysis, baseline analysis, and adapted analysis methodologies demonstrated in research papers, such as the process-based analysis. Furthermore, research was conducted to review the evolution of soil gas science in the CCS industry and to optimise the monitoring strategies with data collected from the OITC as a case study.  Valuable lessons highlighted the efficacy of risk-based monitoring adjacent to identified storage formations. For example, monitoring near relatively high-risk legacy wells with compromised well integrity or for highly faulted regions with great geological uncertainty. Risk-based monitoring that includes several locations with higher temporal resolution is supported for future large-scale CCS sites.