Study on non-Newtonian fluid displacement patterns based on the pore network model

An unsteady pore network model was developed to simulate three distinct displacement patterns-capillary fingering, viscous fingering, and crossover in immiscible displacement of non-Newtonian fluids. The model considered both fluid compressibility and shear rheology. To simulate the water flooding process using the pore network model, water was injected into the pore network that was initially saturated with a non-Newtonian fluid at various capillary numbers and viscosity ratios. The simulation results showed that three different displacement patterns were obtained by the increase of injection capillary number. To explain the formation of these patterns, a competitive mechanism between capillary and viscous forces was used, while a characteristic front flow rate was used to distinguish the boundaries between patterns. Capillary fingering was characterized by fluctuating inlet pressure and disordered growth of fingers that inhibited front velocity, resulting in a small characteristic front flow rate. In contrast, viscous fingering was characterized by inhibition of fluid flow towards the inlet and lateral growth of fingers. The inlet pressure decreases significantly with increasing intrusion fluid saturation. In the crossover zone, the invading fluid tended to occupy a small number of paths consisting of larger radii and breakthrough towards the outlet with a faster characteristic flow rate, resulting in thinner fingers and lower intrusion efficiency. In this study, simulations were conducted using a Newtonian fluid as the displaced phase. We observed that shear rheology attenuates the effect of the viscous force of the displaced phase, resulting in a wider crossover zone. This study improves the understanding of how capillary and viscous forces control displacement patterns and has practical significance for improving oil recovery and CO2 geological storage efficiency.