Large-scale subsurface injection of CO2 has the potential to reduce emissions of atmospheric CO2 and improve oil recovery. Studying the effects of injected CO2 on the elastic properties of the saturated reservoir rock can help to improve long-term monitoring effectiveness and accuracy at locations undergoing CO2 injection. We used two vintages of existing 3D surface seismic data and well logs to probabilistically invert for the CO2 saturation and porosity at the Cranfield reservoir using a double-difference approach. The first step of this work was to calibrate the rock-physics model to the well-log data. Next, the baseline and time-lapse seismic data sets were inverted for acoustic impedance Ip using a high-resolution basis pursuit inversion technique. The reservoir porosity was derived statistically from the rock-physics model based on the Ip estimates inverted from the baseline survey. The porosity estimates were used in the double-difference routine as the fixed initial model from which CO2 saturation was then estimated from the time-lapse Ip data. Porosity was assumed to remain constant between survey vintages; therefore, the changes between the baseline and time-lapse Ip data may be inverted for CO2 saturation from the injection activities using the calibrated rock-physics model. Comparisons of inverted and measured porosity from well logs indicated quite accurate results. Estimates of CO2 saturation found less accuracy than the porosity estimates.

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