Impact of Heterogeneity and Miscibility on scCO2 Drainage Flow Patterns and Implications for Experimental Interpretation

In this study, we investigated the coupling effect of sub-core-scale heterogeneity and miscibility on the development of flow pattern under drainage in porous media. We performed experiments with two Bentheimer sandstone cores (usually considered homogeneous) both with mild heterogeneity. For both cores, two drainage experiments were performed: supercritical ${\text{CO}}_2$ (${\text{scCO}}_2$) was first injected to displace ${\text{CO}}_2$-saturated brine; then the core was re-saturated with clean brine (containing no dissolved ${\text{CO}}_2$) and ${\text{scCO}}_2$ was injected to displace and dissolve within the unsaturated brine. In total, four drainage experiments were performed, and the in situ flow pattern during the development of flow pathways was tracked continuously for each experiment with quasi-dynamic X-ray micro-computed tomographic imaging, which enables visualization of the three-dimensional, dynamic invasion pattern of the invading fluid. For the core with more minor heterogeneity, miscibility did not have significant impact on the resultant flow pattern, and the flow pathways were mostly dictated by core geometry. However, in the core with slightly higher core heterogeneity, we observed significantly different flow pathways between experiments conducted under immiscible and partially miscible conditions. We conclude that the impact of miscibility on the development of the flow pattern is not as significant as small levels of sub-core-scale heterogeneity. The flow pattern in the mildly heterogeneous cores appeared to be highly sensitive to small changes, such as the changes resulting from miscibility during the initial stage of primary drainage.