Wettability effect on the invasion patterns during immiscible displacement in heterogeneous porous media under dynamic conditions: A numerical study

Abstract

This paper investigates the wettability effect on the characteristics of two-phase immiscible flows in porous media at different capillary numbers. This issue presents both applied results useful to engineering disciplines and explores the fundamentals of displacement mechanisms, especially in the mode of compact displacement. The main findings of this study are based on numerical flow simulations in a digital model of heterogeneous porous structure, for which the lattice Boltzmann equations are used in combination with the multi-relaxation time and color-gradient models. This work is one of the few which studies the wettability effect in porous structures with a random arrangement of particles. The influence of the contact angle and the capillary number is presented in the form of images of the fluid distribution at various contact angles and capillary numbers, which were associated with the phase-diagrams for displacement efficiency and fractal dimension, as well as fluid-fluid interfacial length and finger width. It was found that increasing capillary number suppresses the wettability effect on displacement characteristics. This article first identifies four crossover zones between capillary and viscous fingering and compact displacement. It was shown that the critical contact angle, which determines the appearance of compact displacement, increases with the capillary number. Particular attention is paid to the displacement mechanisms for compact displacement. Analysis of the pressure signature at various contact angles and capillary numbers clarifies the role of touches and overlaps that stabilize interfacial front during compact displacement and determines the real driving force for this invasion pattern. This article is the first to establish a relationship between pore-filling events and fluctuations in the pressure signature during compact displacement. It was found that an increase in the pressure signature is characterized by a uniform displacement of all menisci along with the entire fluid-fluid interface, while invasion with a sharp decrease in pressure is determined by the burst-like displacement of a concave meniscus in narrowly localized areas of the pore space.