Flow Visualization of Viscous Fingering in Heterogeneous Porous Media

Abstract

Abstract Flow visualization experiments and numerical simulations were performed to investigate the combined effects of viscous fingering and permeability heterogeneity in four different two-dimensional glass bead packs. In each model, unstable displacements were performed at three different flowrates and mobility ratios. The experiments were also simulated numerically, using a particle tracking simulator. In the homogeneous model, fingers grew through the mechanisms of spreading, splitting, shielding and coalescence. Fingering patterns were sensitive to mobility ratio but not to flowrate. Analysis of pressure distributions in and around fingers showed that viscous crossflow drives the finger growth mechanisms. In the heterogeneous models flow was largely determined by the patterns of heterogeneity. In all experiments, flow followed high permeability paths seen in the M=1 displacements, with viscous fingering effects overlain, despite the fact that permeability contrasts were mild (four-to-one). Simulations yielded finger patterns remarkably similar to those seen in the experiments. Thus, the simulator used represents with reasonable accuracy the physics of finger growth in heterogeneous porous media. physics of finger growth in heterogeneous porous media Introduction A complete description of the miscible flooding process would include phase behavior, viscous instability, and permeability heterogeneity, as well as crossflow driven by permeability heterogeneity, as well as crossflow driven by capillary, viscous, and gravity forces. Given the range and complexity of the interactions of those displacement mechanisms, experimental investigation of a subset seems warranted. In this paper, we focus on the combined effects of two key factors, viscous fingering and permeability heterogeneity. Viscous fingering during flow in a porous medium is the result of displacing a more viscous fluid by a less viscous one. Instead of a stable, straight displacement front, an uneven, fingered front occurs. These viscous fingers propagate rapidly, causing early breakthrough and poor propagate rapidly, causing early breakthrough and poor sweep efficiency. The severity of the fingering is determined by the mobility ratio of the fluids. Permeability heterogeneities also can cause nonuniform flow. Consider, for example, a porous medium with uniform permeability except for a high permeability layer from injector permeability except for a high permeability layer from injector to producer. Even if the fluids have the same viscosity, there will be preferential flow in the high permeability streak, and the heterogeneity will cause early breakthrough and reduced sweep efficiency. If the injected fluid has lower viscosity than the fluid initially in place, preferential flow in the high permeability streak will be enhanced, and fingers may form in both high and low permeability zones. P. 211