Pore-Scale Investigation of Emulsion Formation and Its Impact on Oil Displacement Efficiency in Micromodels

Abstract

After conventional water flooding, most portion of oil remains trapped in porous media due to poor mobility ratio, capillary forces and reservoir heterogeneity. To address this issue, chemical enhanced oil recovery (EOR), particularly surfactant flooding, has become commonly used to improve oil displacement efficiency by IFT reduction and wettability alteration. However, mechanisms of in-situ emulsification at pore scales and their impact on oil recovery under varying reservoir conditions require further research. This study investigated pore-scale mechanisms of emulsification and their impact on oil displacement efficiency using micromodel experiments. Three tests were conducted to evaluate the effects of oil viscosity, salinity, and heterogeneity: (i) low-viscosity oil in a heterogeneous micromodel with Caspian Sea water, (ii) high-viscosity oil under the same conditions, and (iii) high-viscosity oil in a complex heterogeneous micromodel using NaCl brine (10,000 ppm). The pattern of the micromodel was monitored in real-time using a digital camera to observe oil displacement and in-situ emulsification mechanism. In Test 1, which used low-viscosity oil with Caspian Sea water, showed the highest recovery factors among all three tests: secondary waterflooding achieved a recovery factor of 0.345, secondary surfactant flooding reached 0.366, and tertiary surfactant flooding yielded 0.528. Despite these recovery results, in-situ emulsification was limited. In contrast, Test 2 with high-viscosity oil showed significantly more and stable O/W droplets. In Test 3, NaCl brine produced unstable droplets that coalesced rapidly rather than contributing to flow diversion, while the complex heterogeneous micromodel promoted localized droplet generation at pore constrictions. Overall, the results demonstrate that oil viscosity, water composition, and micromodel heterogeneity produce in-situ emulsions and improve displacement efficiency. These findings highlight the importance of considering such injection strategies and reservoir specific conditions when designing surfactant flooding for mature reservoirs with high water-cut.