Influence of dispersed water in water in oil emulsions on oil recovery and pressure response in porous media

Oil recovery from reservoirs containing emulsified crude is often hindered by elevated oil-phase viscosity, pore-throat blockage, and excessive pressure drops, all of which limit displacement efficiency. This study provides a systematic experimental assessment of water-in-oil (W/O) emulsion flow in porous media, with a focus on the coupled effects of dispersed water content, pressure dynamics, and temperature on oil recovery. Stable emulsions were prepared from crude oil sourced from a southwest Iranian reservoir using optimized salinity conditions (6000 ppm NaCl+10,000 ppm Na₂SO₄) to ensure long-term droplet stability. Controlled flooding tests were conducted in 2-3 D sand packs with dispersed water fractions of 0%, 10%, 20%, and 40%, while continuously monitoring pressure evolution before and after breakthrough. The results reveal a strong, non-linear relationship between dispersed water content and flow behavior. At low water fraction (10%), emulsions exhibited flow characteristics nearly identical to non-emulsified oil, maintaining low pressure fluctuations (~ 0.2 psi) and achieving the highest recovery at ambient temperature. At moderate fraction (20%), sweep efficiency improved but viscosity-induced resistance increased; elevated temperatures (up to 90 °C) mitigated this effect, reducing fluctuations and boosting recovery to 42.4%. At high fraction (40%), irreducible water saturation approached zero, but severe pore-throat blockage produced prolonged high-pressure peaks, reduced displacement efficiency, and slower post-injection pressure decay. Post-test analysis indicated that entrapped water droplets acted as micro-elastic elements, sustaining residual pressures (~ 8 psi after 1 h) and delaying full pressure stabilization. These findings demonstrate that while moderate water fractions combined with thermal optimization can enhance sweep coverage without compromising mobility, excessive dispersed water significantly degrades displacement performance. The study offers new mechanistic insight into W/O emulsion transport in high-permeability reservoirs and provides practical guidance for designing emulsion-flooding operations to balance recovery gains with operational stability.