Time-Lapse Pressure Tomography for Gas Plume Monitoring

Some monitoring contexts place significant constraints on surface access for field operations: reducing surface footprint is a common priority for CO2 projects, for example. Less intrusive downhole methods may be preferred. Of possible downhole techniques, pressure monitoring has the advantage of accessing the saturation/permeability/porosity distribution most directly, as opposed to inferences from elastic impedances (seismic) or resistivity (electrical). A suite of controlled well tests observed at multiple wells can be used to monitor plume location via an inversion framework we call “pressure tomography”. Preprocessing the data to remove environmental signals such as barometric pressure and earth tides is crucial. Computational methods for inverting the processed data based on both nonlinear adjoint theory and low-dimensional object-based models have been developed. The information-destructive nature of the forward physics, where gas strongly buffers the pressure, can be partially compensated for by use of Bayesian priors that promote strongly contiguous plume reconstructions. Prototype inversion studies based on the Stage 3 injection program of the CO2CRC Otway project indicate that plume imaging is workable for well arrays that adequately illuminate the plume. Conversely, large uncertainties appear if parts of the plume are remote from or poorly illumined by the testing array.