Integration of Continuous Active-Source Seismic Monitoring and Flow Modeling for CO2 Sequestration: The Frio II Brine Pilot

We present preliminary results from the integrated analysis of continuous active-source seismic monitoring (CASSM) data acquired as part of the Frio II brine pilot CO2 injection experiment. Our approach is to link retrospective 3D multiphase flow modeling (TOUGH2) to a complete suite of synthetic geophysical datasets via an accurate model of the seismic response induced by CO2 emplacement. Through detailed comparison of the Frio II field observations and our synthetic seismic dataset we were able to sequentially refine our flow model, thus gaining insight into the CO2 storage performance characteristics of the Frio site and, potentially, carbon sequestration on a larger scale. In the Frio II crosswell CASSM experiment, the seismic source was located near the top of the formation in the injection well and a string of receivers was located along the observation well, sited updip of the injection well. During the first two days of CO2 injection, seismic velocity along various crosswell raypaths decreased as the CO2 plume migrated. A 3D numerical model of the multiphase, multicomponent fluid flow was developed based on a layered permeability distribution extrapolated from wireline logs and core analyses. This model generated a time-series of simulated saturation distributions which were subsequently transformed into models of time-varying geophysical properties. The petrophysical model used for this transformation couples White's patchy fluid saturation model with NIST's CO2 equation of state and the brine property equations developed by Batzle & Wang. Synthetic seismic travel-time datasets for the true CASSM geometry were calculated using a non-linear eikonal solver, while complete waveforms were generated using a high-order acoustic time-domain finite- difference simulation system. Comparison of calculated and observed seismic travel times constrained the simulated plume evolution, and thereby improved retrospective estimates of the permeability model. In particular, the pattern of arrival of CO2 along various seismic ray paths within the reservoir suggests strongly localized flow of CO2 along preferential paths. Such localized flow may limit capillary trapping, and reduce the rates of dissolution in brine and reaction with rock minerals.