Characterization of two-phase displacement mechanisms in porous media by capillary and viscous forces estimation using the lattice Boltzmann simulations

Abstract

This paper proposes a method for characterization of the two-phase displacement mechanisms (viscous fingering, capillary fingering, stable front, and crossover) in porous media. The approach is based on separate estimation of the capillary and viscous forces and subsequent calculation of the ratio between them (designated as W). The value W determines the dominance of capillary force and is essentially the inverse capillary number. The advantage of this approach consists in taking into account the scale of the samples and predicting the flow mechanisms using parameters measured in laboratory conditions. The balance between viscous and capillary forces is controlled by surface tension and fluid viscosities. For flow simulation, the lattice Boltzmann equations and the color-gradient model are applied. The values of W equal to 0.5 and 2, which define the boundary of the crossover zone and separate it from the flow mechanisms, were determined and the proposed method was successfully validated on three two-dimensional samples of porous media with different permeability coefficients. It revealed a directly proportional effect of permeability on W. As a result, the surface tensions, which determine the boundaries of viscous fingering, capillary fingering, stable front, are shifted towards smaller values with increasing permeability. Also, the effect of the permeability and displacement mechanisms on the displacement efficiency before, after and at quasi-steady regime was investigated. The tendency to efficiency decrease with increasing surface tension and decrease in viscosity ratio was revealed for quasi-steady regime. The absence of the mobile wetting fluid after breakthrough is detected for crossover and capillary fingers. No-displacement regime after breakthrough and, consequently, effectiveness drop to minimal values in quasi-steady regime occurs at lower surface tensions with increasing permeability.