Viscous-dependent fingering dynamics of gas invading into multi-fluids

To realize the transition of our society to a low-carbon future with innovative subsurface energy solutions, understanding the dynamic behavior of gas invading multi-fluid systems in underground pore space is critical. In this work, a joint approach of flow imaging and digital image processing is employed to investigate the fingering dynamics of gas invading multi-fluids in porous media. We examined various gas (G) invasion scenarios of a high-viscosity defending liquid (HL), low-viscosity defending liquid (LL), and their co-existing multi-fluid system, focusing on the viscosity effect. Quantification of phase saturation shows that the displacement efficiency follows the order of G$\to$(L$\to$L) $>$ L$\to$L $>$ G$\to$L, regardless of the varieties in injection flow rate in the viscous-dominated flow regime. In other words, the enhancement in displacement efficiency and potential energy savings are achieved by solely introducing a third phase without the cost of the higher pumping power. When gas invades the HL and LL multi-liquid system, the fingering pattern in G$\to$(HL$\to$LL) and G$\to$(LL$\to$HL) significantly differs and highly depends on the sequential occupation of HL and LL in the pore spaces. The previously unobserved yarn-liked gas pattern in G$\to$(LL$\to$HL) is suspected as the main reason for the fast gas displacement. Through Local dynamics analysis, we identified that the preferential invasion into interconnected LL channels and the inhibitory effect of scattered HL on bypass invasion are the primary mechanisms behind the formation of yarn-liked fingers. We classified two distinct categories of ganglia mobilization and connection in G$\to$(LL$\to$HL), i.e. “catch up to connect” and “expand to connect”. Finally, the topological connectivity of the gas finger in G$\to$(LL$\to$HL) is evaluated using Euler number. Euler number shows an ascending trajectory before breakthrough, followed by a rapid descent and stabilization at steady state. This signifies that disconnected ganglia emerge before breakthrough and subsequently expand and reconnect. Our new findings are of great importance for subsurface extraction/storage strategy innovation through enriching multi-fluids injection scenarios.