Gas migration and residual trapping in bimodal heterogeneous media during geological storage of CO2

Abstract

This study investigates the effect of heterogeneity on CO2–brine two-phase flow behavior and capillary trapping at the field scale. A model based on macroscopic invasion percolation is developed to simulate CO2 migration and trapping in strongly heterogeneous systems with bimodal permeability distributions. Stochastic simulations are performed on heterogeneous permeability fields generated by considering both transition probability based lithofacies distribution and multi-Gaussian random fields, representative of characteristics often encountered in relevant sedimentary basins. The heterogeneity cases of different lithofacies proportions are designed such that they share the same ensemble mean of upscaled permeability over the simulation domain. We find that the CO2 spreading and trapping is strongly influenced by the heterogeneity. The ensemble means of sweep efficiency and domain-average CO2 saturation after primary drainage show a slight decrease with decreasing sandstone proportion p1 from 1.0 to 0.6 but a much steeper decline when p1 further decreases to 0.4. The upscaled two-phase flow properties such as the relative permeabilities are also influenced by the lithofacies proportion. Residual or capillary trapping and its uncertainty is shown to be dependent on the heterogeneity and the choice of trapping models. Our findings have broad implications within the context of CO2 sequestration, as the gas spreading and entrapment are the fundamental processes leading to long-term storage security.