Effects of Capillary and Viscous Forces on Two-Phase Fluid Displacement in the Microfluidic Model

Abstract

Immiscible fluid−fluid displacement in porous media is an important phenomenon that impacts the field of underground energy and environmental engineering, such as enhanced oil/gas recovery, geological CO 2 sequestration, and transport and remediation of groundwater pollutants. In enhanced oil recovery, the occurrence of the fingering instability of the two-phase flow interface causes a significant reduction in the oil recovery factor for waterflooding. In geological CO 2 sequestration, the efficiency of CO 2 capillary/residual trapping is limited by the noncompact displacement pattern, presenting only part of brine displaced by scCO 2 . Although extensive studies have investigated viscous and capillary fingering in porous media, how viscous and capillary forces affect the crossover is not well understood. Quantifying and characterizing the transition of immiscible fluid−fluid displacement patterns are therefore important. In this article, comprehensive investigation of capillary number and viscosity ratio effects on displacement patterns and efficiency was established by a series of displacement microfluidic experiments in a homogeneous pore network. Three pairs of nonwetting−wetting immiscible fluids with viscosity ratios M ranging from 1/1000 to 1000 were used to perform the displacement experiments. The drainage experiments of water displacing silicone oil were conducted under five flow rate conditions with capillary number log 10 Ca ranging from −7.19 to −3.45, while the imbibition experiments of silicone oil displacing water displayed at four flow rates with log 10 Ca varying from −7.25 to −2.52. The relationship between displacement efficiency S inv and fractal dimension D f as a function of M and Ca was established to quantitatively determine the transition of displacement patterns (capillary fingering, viscous fingering, stable displacement, and ordered dendritic). The results indicated that D f increased with S inv under unfavorable conditions (M < 1) and decreased as S inv for M > 1, and lower invading fluid saturations were observed in the crossover zone between capillary fingering and viscous fingering for M = 1/20 and 1/200. Based on the pore-scale analysis of pore-filling events and microscopic observation, the theoretical model of the transitions for invasion modes were proposed to determine the critical capillary corresponding to viscosity ratio and contact angle. This study extends the classic log 10 M−log 10 Ca phase diagram and quantifies the dynamic effects of capillary force and viscous force on the immiscible fluid−fluid displacement process.