Impacts of relative permeability formulation on forecasts of CO2 phase behavior, phase distribution, and trapping mechanisms in a geologic carbon storage reservoir

A critical aspect in the risk assessment of geologic carbon storage, a carbon-emissions reduction method under extensive review and testing, is effective multiphase CO2 flow and transport simulation. Relative permeability is a flow parameter particularly critical for accurate forecasting of the multiphase behavior of CO2 in the subsurface. The relative permeability relationship assumed and especially the residual saturation of the gas phase greatly impacts predicted CO2 trapping mechanisms and long-term plume migration behavior. A primary goal of this study was to evaluate the impact of the selection of relative permeability formulations on the efficacy of regional-scale CO2 sequestration models. To accomplish this, we selected the San Rafael Swell area of East-central Utah as a case study to evaluate the impact of relative permeability formulations on CO2 plume movement and behavior. A 2D vertical cross section model was built to simulate injection of CO2 into a brine aquifer for 30 years followed by 970 years of post-injection monitoring. We evaluated five different relative permeability relationships to quantify their relative impacts on forecasted flow results of the model, with all other parameters maintained uniform and constant. Because of the interdependence between phase saturation and relative permeability in numerical simulations we expected to see some variation in phase behavior, phase distribution, and CO2 trapping mechanisms. An almost 60% difference in residually trapped gas was observed across the different relative permeability relationships. Large variations of up to 40% in the saturation of both dissolved phase and mobile supercritical phase CO2 were also observed. The pressure buildup and decay observed was not significantly affected by the relative permeability formulation until after injection pressures have dissipated. Results of this analysis suggest that CO2 plume movement and behavior are significantly dependent on the specific relative permeability formulation assigned, including the assumed residual saturation values of CO2 and brine. More specifically, different relative permeability relationships translate to significant differences in CO2 plume behavior and corresponding trapping mechanisms. (C) 2016 Society of Chemical Industry and John Wiley & Sons, Ltd