Influence of porous media heterogeneity on water-oil immiscible displacement and mechanism analysis

Water is usually injected into the reservoirs during the development of oil and gas resources to improve oil recovery due to its effectiveness and economic feasibility. Predicting the remaining oil distributions, fluid flow pathways, and oil recovery require a solid understanding of the water-oil immiscible displacement process at the pore scale. Previous researchers have divided immiscible fluids displacement patterns into three types, namely capillary fingering, stable displacement, and viscous fingering. These patterns are strongly influenced by two dimensionless numbers, fluid viscosity ratio (M) and capillary number (Ca). Then, some phase diagrams based on viscosity ratio and capillary number can be established to describe the parameter region for different displacement patterns. However, several recent studies have shown that the heterogeneity of porous media has a significant effect on immiscible displacement. At the same time, heterogeneity is also a major characteristic of oil reservoirs. In this study, the Navier-Stokes (N-S) equation is employed to simulate the water-oil immiscible displacement in two micro models, one with a homogeneous porous medium and another with a heterogeneous one. The dynamic change of the two-phase interface is tracked by the phase-field method. The porous medium was initially saturated with oil as the wetting fluid and water were injected. A series of displacement experiments were performed for six different capillary numbers, ranging from logCa=-3.48 to -2.63, with a fixed oil-water viscosity ratio of 30. The effects of capillary number and heterogeneity on the flow patterns were analyzed, and the displacement efficiency and breakthrough time of injected water were also investigated. The unstable displacement caused by capillary fingering occurred at low capillary numbers, and the viscous force gradually dominated the displacement process with the increase of injection rate. Nevertheless, the flow pattern did not change from capillary fingering to stable displacement but directly to viscous fingering. The latter was associated with the high oil-water viscosity ratio in our experiments. Under the same displacement conditions, the fluid dynamics at moderate capillary numbers for homogeneous and heterogeneous models were different. More specifically, heterogeneous porous media enhanced the displacement instability and promoted immiscible drainage to viscous fingering. The latter resulted in reducing the critical capillary number of viscous fingering. The primary reason was that the heterogeneity resulted in a decrease in the sweep efficiency and an increase in the flow rate of injected water in pore throats. In order to quantify the impact of heterogeneity on displacement efficiency, we also compared the oil recovery of different porous media when the injected water broke through at the outlet. The oil recovery generally increased with the capillary number prior to the flow pattern changing to viscous fingering. However, there was a slight drop in oil recovery when logCa increased to -2.78 and -2.93 for homogeneous and heterogeneous models, respectively. The drop was related to the transition from capillary to viscous fingering in porous media. At the same time, the oil recovery in the homogeneous model was higher. The difference in flow patterns resulted in an early breakthrough for the heterogeneous model. And the formation of viscous fingering would reduce the difference in water breakthrough time. This study revealed the influence of porous media heterogeneity on immiscible displacement and provided the theoretical background for a fundamental understanding of the water-oil flow in oil reservoirs. © 2022, Science China Press. All right reserved.