CO2 saturation and thickness predictions in the Tubåen Fm., Snøhvit field, from analytical solution and time-lapse seismic data

Abstract

CO2 migration in a saline aquifer is governed by viscous, capillary and gravitational fluid forces at an early stage of injection, where the dominant flow regime is site specific and controls the fluid migration in the pore space. This study combines the CO2 saturation inverted from time-lapse seismic methods with an analytical expression to define the CO2 flow regime, saturation distribution and layer thickness in the Tubåen Fm. following CO2 injection. Quantitative estimates of the CO2 saturation from time-lapse seismic amplitude versus offset (AVO) and spectral decomposition are compared to a viscous dominated analytical expression of CO2 injection into a saline aquifer. The spatial extent of the CO2 plume obtained from time-lapse spectral decomposition and inverted from time-lapse AVO analysis display good agreement with the analytical expression. The CO2 is limited to an area close to the injection well, with an elongated shape in the channel direction. Comparison between the time-lapse seismic and analytical expression shows that the fluid flow is dominated by viscous forces. CO2 saturation within the plume is constant and close to the residual brine saturation. The influence of gravity is ignorable on the reservoir CO2 flow. CO2 fills the entire sandstone unit up to approximately 50m away from the injection before the CO2 layer thickness is reduced to a thin wedge that propagates below the overlying shale unit. Reduction in CO2 saturation away from the injection well is a reduction in effective CO2 saturation relative to the thickness of the horizon. The maximum radius of the CO2 layer from the analytic expression is 750m, of which 400m is above the time-lapse noise level. Time-lapse seismic analysis reveals the CO2 layer radius is 405m in the direction of the local fluvial channel and 273m in the perpendicular direction.