Visualizing in-situ emulsification in porous media during surfactant flooding: A microfluidic study

Hypothesis: It is well recognized that crude oil could be emulsified during surfactant flooding process, and recently such an in-situ emulsification was found practically helpful for enhanced oil recovery (EOR). However, no direct proofs are reported yet to unravel how emulsion is formed in porous media and how important to increasing oil recovery factor due to unavailability to visualize the emulsification process, thus it is desirable to verify visibly the formation of emulsion in porous media and the contribution of emulsification to EOR process. Experiments: Two types of microfluidic chips with heterogeneous and homogeneous pore geometries respectively were employed to simulate the underground oil reservoir environment. Sodium dodecylbenzenesulfonate (SDBS) was selected as a model surfactant, and its aqueous solution was injected into the paraffin oil-saturated microchip to mimic the displacing process. A series of tests were conducted by varying SDBS concentration, electrolyte content, injection rate, and pore-scale snapshots were captured for qualitative and quantitative analysis of in-situ emulsification during the surfactant flooding. Findings: Both oil-in-water (O/W) and water-in-oil (W/O) emulsions are formed in microchips during the surfactant flooding. Increasing SDBS concentration, migration distance, injection rate, or addition of electrolyte tends to form smaller O/W particles through snapping action at pore throat, and vice versa. Smaller size endows oil with a better mobility to go through the pore throat, and up to 24% extra emulsion can be achieved through emulsification entrainment; bigger droplets can block the dominant paths, thus improving sweep efficiency and increasing oil recovery factor up to 30% compared to water flooding. Furthermore, W/O emulsification was found to be a time-dependent process influenced by SDBS concentration, and oil was recovered by diffusing surfactant solution into oil phase and replacing the oil occupied space in porous media. (c) 2020 Elsevier Inc. All rights reserved.