The movement of sequestrated CO2 revealed by seismic attenuation spatial and temporal changes in Frio-II site, USA

Abstract:

Continuous active source seismic measurements (CASSM) were collected in the crosswell geometry during scCO₂ injection at the Frio-II brine pilot (Liberty, TX). Previous studies (Daley et.al. 2007, 2011) have demonstrated that spatial-temporal changes in the picked first arrival time after CO₂ injection constrain the movement of the CO₂ plume in the storage interval. To improve the quantitative constraints on plume saturation using this dataset, we investigate spatial-temporal changes in the seismic attenuation of the first arrivals. The attenuation changes over the injection period (~60 h) are estimated by the amount of the centroid frequency shift computed by the local time-frequency analysis. Our observations include: at receivers above the packer seismic attenuation does not change in a physical trend; at receivers below the packer attenuation sharply increases as the amount of CO₂ plume increase at the first few hours and peaks at specific points varying with distributed receivers, which are consistent with observations from time delays of first arrivals. Then, attenuation decreases over the injection time with increased amount of CO2 plume. This bell-shaped attenuation response as a function of time in the experiment is consistent with White’s patchy saturation model which predicts an attenuation peak at intermediate CO₂ saturations. Our analysis suggests that spatial-temporal attenuation change is an indicator of the movement/saturation of CO2 plume at high saturations, a system state for which seismic measurements are typically only weakly sensitive to.