Tracking CO2 plumes via time-lapse pressure tomography

Downhole methods for monitoring applications in CCS projects are increasingly attractive because of their low surface footprint and ease of stakeholder engagement. When several wells are available, inversions from multiple water injections tests, also known as pressure tomography (PT), is a viable method of tracking gas saturation in the reservoir interval. This technique can be employed in a time-lapse setting with surface requirements comprising only pumps, water facilities, and instrumentation.

We have developed a parallelised, adjoint-based PT inversion framework using a single-phase effective-media flow model in the presence of gas. This is demonstrated on experimental data from the 2020–2021 Otway stage 3 project, where 15kT of CO2-rich gas was injected into the Paaratte formation at ∼1500m depth, in 3 stages of 5kT each, with monitoring via PT and seismic methods. The inversion method is applied on baseline data for inference of spatial diffusivity and porosity fields, then on 3 monitor PT surveys for the inference of the evolving gas saturation field. These inversions have successfully imaged the migrating gas plume, and remobilisation of the legacy gas plume in the SE direction updip of the injection. Comparisons with simultaneous migrated seismic differences on surface-vibrator to downhole-fibre transects are encouraging and persuasive.