Effect of wettability on immiscible viscous fingering: Part I. Mechanisms

Most immiscible viscous-fingering experiments have been conducted using Hele-Shaw cells or linear displacements with high permeabilities. In linear displacement experiments, the effect of varying velocity profiles on viscous fingering is not tested. Only a limited number of studies have been conducted in five-spot displacement patterns. These studies have not provided any comparative insight into fingering length scales during high-IFT imbibition and drainage in water-wet and oil-wet porous media, respectively. To eliminate the limitations of previous studies, high-IFT, adverse-viscosity-ratio, water–oil displacements are conducted in a quarter-five-spot pattern with a relatively intermediate permeability under water-wet and oil-wet conditions, respectively. Statistically representative fingering patterns are captured by devising experimental techniques and image analysis algorithms to correlate statistical information including the evolution of the fingering length scales with time in water-wet and oil-wet micromodels. Mechanistic regimes of viscous fingering in imbibition and drainage are proposed. The patterns of fingering suggest a profound effect of wetting properties on the complex interplay between several pore-scale mechanisms during high-IFT, adverse-mobility-ratio imbibition and drainage, including the interfacial energy effects, capillary invasion, transverse flow, viscous crossflow, bypassing, film flow, and tip splitting, shielding, and spreading.