Time-lapse pressure tomography for characterizing CO₂ plume evolution in a deep saline aquifer

A time-lapse pressure tomography inversion approach is applied to characterize the CO₂ plume development in a virtual deep saline aquifer. Deep CO₂ injection leads to flow properties of the mixed-phase, which vary depending on the CO₂ saturation. Analogous to the crossed ray paths of a seismic tomographic experiment, pressure tomography creates streamline patterns by injecting brine prior to CO₂ injection or by injecting small amounts of CO₂ into the two-phase (brine and CO₂) system at different depths. In a first step, the introduced pressure responses at observation locations are utilized for a computationally rapid and efficient eikonal equation based inversion to reconstruct the heterogeneity of the subsurface with diffusivity (D) tomograms. Information about the plume shape can be derived by comparing D-tomograms of the aquifer at different times. In a second step, the aquifer is subdivided into two zones of constant values of hydraulic conductivity (K) and specific storage (S_s) through a clustering approach. For the CO₂ plume, mixed-phase K and S_s values are estimated by minimizing the difference between calculated and "true" pressure responses using a single-phase flow simulator to reduce the computing complexity. Finally, the estimated flow property is converted to gas saturation by a single-phase proxy, which represents an integrated value of the plume. This novel approach is tested first with a doublet well configuration, and it reveals a great potential of pressure tomography based concepts for characterizing and monitoring deep aquifers, as well as the evolution of a CO₂ plume. Still, field-testing will be required for better assessing the applicability of this approach.