Tracking a Subsurface CO2 Plume with Time-Lapse Pressure Tomography in the Otway Stage 3 Field Project

We present results from a field trial of pressure tomography in the Otway Stage 3 project, Victoria, Australia. Pressure tomography involves the inversion of cross-well pressure test data to form a coarse-grained map of subsurface petrophysical properties, e.g. diffusivity-porosity-thickness. When performed in a time-lapse manner throughout CO2 injection, the gas saturation can also be tracked. Six wells, drilled at depth ≈1500 m in the Paaratte formation, are used to track the migration of 15 kt of injected CO2 with a series of cross-well pressure tests in three time-lapse surveys. We develop a Bayesian inversion scheme to infer the CO2 saturation in time, using two complimentary methods; a flow-agnostic adjoint method, and a topography driven gravity-current method. We present the methodology, post-processed data and maximum aposteriori probability (MAP) estimates of the plume migration, comparing to observations from seismic monitoring using time-lapse offset VSP and 4D-VSP. Results are consistent with seismic monitoring, with key features invertible from the time-lapse surveys: the centre of mass of the plume, the merging with an in-situ legacy gas plume, and the plume migration towards the monitoring array extremes. We demonstrate that pressure tomography is a viable, unobtrusive, long-term monitoring technique for carbon storage projects.