The effects of heterogeneity in Carbon Capture and Storage (CCS) in saline aquifers have been investigated extensively and are known to have important bearings on the storage capacity of the aquifer. In CCS projects, the time-lapse seismic survey has been proposed as a valuable tool for monitoring of CO2 movement. However, the potential of the time-lapse seismic data for heterogeneity characterization and geologic model updating has not been fully explored. One of the biggest challenges in the quantitative use of time-lapse seismic data during CCS is the complex movement of the CO2 influenced by compositional effects, geochemical reactions, phase changes and gravity segregation.
In this paper, we first introduce compositional streamlines to understand and visualize the flow and transport of CO2 in the presence of mineral precipitation/dissolution, residual trapping and buoyancy effects. To start with, individual component fluxes are generated by a finite difference fully implicit compositional simulator incorporating all the relevant physics of CO2 sequestration. The fluxes are then utilized in novel streamline tracing algorithms to generate phase and component streamlines depicting the movement and the trapping of CO2 in the aquifer. Next, we utilize the compositional streamlines to determine the sensitivity of the time-lapse seismic attributes specifically, interpreted saturation differences, to changes in reservoir properties such as permeability and porosity. The sensitivities are then used in an inverse modeling algorithm to calibrate the geologic model to time-lapse seismic data. The outcome is an improved description of permeability heterogeneity that is consistent with the 4-D seismic response and improved predictions of the CO2 storage capacity.
We have investigated the benefits of time-lapse seismic data integration in improving the performance assessment of CO2 sequestration using examples involving CO2 injection under realistic conditions. The first example examines the value of the 4-D seismic data integration in the estimation of storage capacity. The second example systematically studies the impact of viscous to gravity ratio on the performance of time-lapse seismic monitoring and heterogeneity characterization during CCS.