dc.description.abstract | When one fluid displaces another in a disordered porous medium, the displacement can be unstable that leads to the reduction of displacement efficience. Viscous fingering is an unstable phenomenon that occurs when a less viscous fluid displaces a more viscous one, depending on different viscosities, surface tension, and wettability between the two fluids. In order to confirm the feasibility of a new method for enhancing displacement effects, this study used the commercial software ANSYS Fluent to simulate the two-phase flow in a quasi two-dimensional porous media. The flow was considered to be laminar and incompressible, and the two-dimensional transient Navier-Stokes equation with additional source terms of Darcy resistance and continuous surface force to describe the viscous force induced by the boundary and capillary forces between the two fluids. The fluid volume (VOF) algorithm was used to track the interface of the two-phase flow.
The physical domain is a thin porous medium sandwitched between two solid flate plates. Mirco cylindrical obstacles were used to produce two porous media with different capillary pressures. The upstream region was filled with big obstacles, and the downstream region was filled with small obstacles. The displacement efficiency was improved by increasing finger movement resistance at the heterogeneous interface. Five different radius ratios (1:1, 1:0.7, 1;0.5, 1:0.45, 1:0.4) of big obstacles region to small obstacles region were considered. The five situations represented five kinds of capillary pressure differences.
The porous medium was initially filled with water. Next, the water exited porous medium from outlet and the air entered porous medium from inlet. The simulation model was verified by experimental comparisons and grid convergence tests. The simulation results showed that the displacement effect was getting better when the radius ratios were from 1: 1 to 1: 0.5. However, the displacement efficiency started getting worse from 1:0.5 to 1:0.4. The areas of air were mainly distributed in the big obstacle regions near the heterogeneous pore-scale boundary, and had a fan-shaped distribution pattern. There were two main reasons why the displacement efficiency could not be improved for pore-size ratio getting smaller: First, if the air fully filled the heterogeneous pore-scale boundary, the water in the big region cannot be drained; second, the viscous pressure drop from the fingertip to heterogeneous pore-scale boundary is larger than the capillary pressure difference, so the capillary resistance was not sufficient anymore to resist the fingers. | en_US |