Recent Improvements in Micromodel Experimentation and Pore-scale Simulation of Multiphase Systems

Recent efforts in the Environmental Molecular Sciences Laboratory at PNNL have resulted in improved experimental methods to fabricate silicon micromodels with different wettability and improved numerical methods to predict multiphase fluid displacement at the pore scale. Wettability is a key parameter influencing capillary pressures, permeabilities, fingering mechanisms, and saturations in multiphase flow processes within porous media. Glass-covered silicon micromodels provide precise structures in which pore-scale displacement processes can be visualized. The wettability of silicon and glass surfaces can be modified by silanization. However, similar treatments of glass and silica surfaces using the same silane do not necessarily yield the same wettability as determined by the oil-water contact angle. Surface wettability modifications and cleaning pretreatments were investigated to determine conditions that yield oil-wet surfaces on glass with similar wettability to silica surfaces treated with the same silane. Both air-water and oil-water contact angles were determined. Displacement experiments completed in these micromodels have shown unstable and stable displacement patterns, related to capillary and viscosity ratios of the fluids. A series of high-resolution numerical experiments were conducted using the Pair-Wise Force Smoothed Particle Hydrodynamics (PF-SPH) multiphase flow model. The novel model was used to simulate experiments that showed viscous fingering, capillary fingering, and stable displacement of immiscible fluids for a wide range of capillary numbers and viscosity ratios. It was demonstrated that the steady state saturation profiles and the boundaries of viscous fingering, capillary fingering, and stable displacement regions compare favorably with micromodel experimental results. For displacing fluid with low viscosity, we observed that the displacement pattern changes from viscous fingering to stable displacement with increasing injection rate. When a high viscosity fluid is injected, transition behavior from capillary fingering to stable displacement occurred as the flow rate was increased. These observations also agree with the results of the micromodel laboratory experiments. The results confirm that the PF-SPH method can be used for predictive modeling of multiphase flow under a wide range of conditions.