A Microfluidic Investigation of the Synergistic Effect of Nanoparticles and Surfactants in Macro-Emulsion-Based Enhanced Oil Recovery

Summary Injecting oil-in-water (O/W) emulsions stabilized with nanoparticles (NPs) or surfactants is a promising option for enhanced oil recovery (EOR) in harsh-condition reservoirs. Stability and rheology of the flowing emulsion in porous media are key factors for the effectiveness of the EOR method. The objective of this study is to use microfluidics to (1) quantitatively evaluate the synergistic effect of surfactants and NPs on emulsion dynamic stability and how NPs affect the emulsion properties, and to (2) investigate how emulsion properties affect the sweep performance in emulsion flooding. A microfluidic device with well-defined channel geometry of a high-permeability pathway and multiple parallel low-permeability pathways was created to represent a fracture/matrix dual-permeability system. Measurement of droplet coalescence frequency during flow is used to quantify the dynamic stability of emulsions. An NP aqueous suspension (2 wt%) shows excellent ability to stabilize the macro-emulsion when mixed with a trace amount of surfactant (0.05 wt%), revealing a synergistic effect between NPs and surfactant. For a stable emulsion, when a pore throat is present in the high-permeability pathway, it was observed that flowing emulsion droplets compress each other and then block the high-permeability pathway at a throat structure, which forces the wetting phase into low-permeability pathways. Droplet size shows little correlation with this blocking effect. Water content was observed to be much higher in the low-permeability pathways than in the high-permeability pathways, indicating different emulsion texture and viscosity in channels of different sizes. Consequently, the assumption of bulk emulsion viscosity in the porous medium is not applicable in the description and modeling of the emulsion-flooding process. Flow of emulsions stabilized by an NP/surfactant mixture shows droplet packing in high-permeability regions that is denser than those stabilized by surfactant only, at high-permeability regions, which is attributed to the enhanced interaction between droplets caused by NPs in the thin liquid film between neighboring oil/water (O/W) interfaces. This effect is shown to enhance the performance of emulsion-blockage effect for sweep-efficiency improvement, showing the advantage of NPs as an emulsion stabilizer during an emulsion-based EOR process.