Pore-scale study of drainage processes in porous media with various structural heterogeneity

Multiphase flow in porous media is widely encountered in many scientific and engineering fields. In this work, Voronoi diagram is used to reconstruct porous structures with low permeability and high heterogeneity, and the color-gradient lattice Boltzmann method is employed to simulate two-phase displacement with high viscosity ratio. Three benchmarks are utilized to validate the model, which are the Young-Laplace test, the contact angle test and the relative permeability in a 2D channel. Effects of capillary number, structural heterogeneity and wettability are analyzed. The results show that a higher capillary number leads to viscous fingering with a higher recovery rate. The connectivity rate is used to quantify the continuous wide pores, showing that structure heterogeneity will increase the probability of continuous wide pores, which promotes the formation of gas channeling, thus reducing the recovery rate. However, local structure modifications, providing that the overall homogeneity is retained, would not induce significant differences in recovery rates and displacement patterns. Complex pore structures will also cause capillary snap off, leading to more dead-end fingerings and more complicated two-phase dynamic behaviors. Meantime, a more wetting invading phase would induce a stop-wait-overlap phenomenon, which would increase the recovery rate and render the displacement more compact.