Digital Rock Core Simulation of CO2 Storage, Showing the Impact of Heterogeneity on Fluid Flow and Residual Trapping

Abstract Residual trapping is the mechanism by which capillary forces immobilize Carbon dioxide (CO2) at the core scale. This mechanism depends on the interplay of capillary heterogeneity and hysteresis to reliably trap CO2 during imbibition. However, while the importance of sub-core scale capillary pressure heterogeneity on residual trapping is established, it has not been extensively incorporated into CO2-brine numerical simulations. In this study, digital rock core numerical simulation approach was used to implement residual trapping and hysteresis in the context of CO2 storage. Steady-state CO2-brine drainage followed by imbibition was simulated on a 3D model of a Bunter sandstone rock core. Drainage was conducted at a range of capillary numbers to investigate the role of capillary heterogeneity on CO2 distribution and trapping. Furthermore, the heterogeneity orientation was changed to examine how this would impact the system's multiphase flow properties and trapping efficiency. The simulation results showed that CO2 distribution and trapping is sensitive to capillary heterogeneity under capillary limited conditions that is, when capillary forces dominate over viscous forces. The impact of heterogeneity is reduced at high rates corresponding to viscous dominated conditions. And most importantly, significant amount of the injected CO2 was residually trapped by capillary heterogeneity. Depending on the drainage injection rate and core orientation, between 57% and 71% of the initial CO2 was left behind after brine injection. The residual CO2 saturation generally increased with the initial saturation. Higher sub-core scale trapping efficiencies were recorded when the core orientation was changed to approximate a vertical core. This is understood to be a result of local accumulation of CO2 at core boundary locations where a low capillary entry pressure pore is set against a high capillary entry pressure pore. In all, this study underscores the influence of sub-core scale capillary heterogeneity on CO2 distribution and the need to implement hysteresis for proper characterization of CO2 trapping in numerical simulations.