CO2 Injection in Faulted Formations: Insights from the Otway Shallow Fault Project

Abstract The risk of CO2 leakage through faults is a fundamental component of Geological Carbon Storage (GCS) project design and risk management. Current practice is to avoid faults entirely - a cautious and understandable response to limited understanding of CO2 migration through fault zones. Although fault-related fluid flow has long been studied in the hydrocarbon industry, few experiments have directly injected fluids into faults under controlled conditions. The Otway Shallow Fault Project, at the Otway International Test Centre (OITC), did just that - targeting a shallow fault characterised using boreholes and high-resolution seismic data. In April 2024, a first-of-its-kind controlled injection experiment was conducted. Sixteen tonnes of gaseous CO2 were injected adjacent to Brumbys Fault and closely monitored, providing a unique opportunity to study CO2 migration in faulted formations. The project involved extensive pre-injection characterisation, including detailed formation evaluation and reservoir modelling. A preparatory water pump test helped assess downhole conditions and refine models using non-compressible fluid behaviour. The experiment was designed to remain open-ended, simulating a range of scenarios to minimise uncertainty. During CO2 injection, a suite of advanced monitoring technologies, including reverse Vertical Seismic Profiling (VSP), Cross-hole tomography, downhole fibre optics (measuring acoustics, temperature, and strain), and downhole pressure/temperature gauges, enabled high-resolution tracking of plume movement and fault zone response. Preliminary results showed very rapid CO2 migration, at a speed that exceeded expectations, and demonstrated high sensitivity of fibre optic sensors to minor strain variations. The experiment also revealed previously undetected structural features that influenced fluid flow. Data from downhole gauges and surface CO2 flux measurements informed final reservoir model updates and are being used to calibrate simulations against real-world observations. Although full analysis is ongoing, initial findings offer crucial insights into the behaviour of CO2 in faulted geological settings. This work provides a valuable calibration point for improving fault leakage risk assessments and enhancing the predictive capabilities of simulation models. Ultimately, the study underscores the complexities of structural features within a fault zone and how this can influence CO2 migration in storage areas. The findings offer essential guidance for future industrial-scale GCS projects facing similar geological challenges.