基于孔隙网络模型的非牛顿流体驱替模式研究

本文提出了一种考虑了流体可压缩性与非牛顿流体剪切流变性的非稳态孔隙网络模型，用以研究非混相驱替中的毛管指进、黏性指进和两者之间的过渡区这三种不同的驱替模式.通过在孔隙网络模型中饱和非牛顿流体，研究不同注入毛管数和不同黏度比情况下的注水驱替模式.结果表明，非稳态孔隙网络模型可以模拟再现三种不同的流体驱替模式，并提出特征前缘流量用于区分三种驱替模式.毛管指进的入口压力的相对波动较大且手指的无序生长会抑制前缘的移动速度，因此特征前缘流量变化较小.黏性指进会抑制指进横向生长和向入口方向的流动，入口压力在突破时快速降低.在过渡区，驱替流体倾向于占据由更大半径孔喉组成的少量路径，并以更大的特征流量向出口处快速突破，导致更细的手指与较低的驱替效率.本文还对比了被驱替相为牛顿流体时的情况.结果表明，剪切流变性会减弱被驱替相黏性阻力的影响，从而使得过渡区的范围更宽.本研究有助于更好地研究复杂流体间黏性力与毛管力对驱替模式的影响，对于提高油气采收率和二氧化碳地质封存效率具有现实意义.

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.