Impact of dead-end pores on the residual mass in miscible viscous displacement: A pore-scale perspective

Viscous fingering instability occurs in porous media when a low viscosity fluid displaces a higher viscosity fluid. Classical Darcy-scale studies have well-established that the underlying dynamics adversely affects miscible displacement operations. However, our current understanding is limited to oversimplified media, where dead-end pores marked by stagnant flow have been largely overlooked. To accurately capture geological porous structures where such structures are commonly observed, a pore-scale evaluation is required. Following this motivation, using numerical simulations in a prototype medium, the interplay between dead-end pores and miscible viscous fingering is investigated, focusing on the quantitative analysis of macroscopic front length, total residual mass, and cleanup efficiency. The temporal evolution of residual mass that depicts heavy asymptotic tailing is characterized by three distinct regimes at initial, intermediate, and asymptotic times, which is distinct from the Darcy-case. In addition to the role of viscous fingering in a displacement process, dead-end pores are shown to further hinder it by amounting to a surprisingly larger amount of residual fluid accumulation and lower cleanup efficiencies compared to an equivalent Darcy-case. The increased efficiency reduction is directly related to the local mass transport across the dead-end pores that act as hot spots for fluid entrapment, corroborated by comparison with two synthetic granular media and by using a conceptual model. To this end, this study unravels the combined impact of dead-end pores and viscous fingering on displacement processes, providing crucial insights towards the improvement of technologies such as groundwater remediation, CO 2 geological sequestration, underground hydrogen storage, etc. • Pore-scale simulations unravel the interplay between dead-end pores and viscous fingering. • Dead-end pores act as local hot spots, reducing the cleaning efficiency substantially. • Efficiency reduction is directly related to pore-scale mass transport across dead-end pores. • This holistic impact of dead-end pores is explained using a conceptual model.