Impact of water salinity differential on a crude oil droplet constrained in a capillary: Pore-scale mechanisms

Low-salinity water flooding can be effectively used for enhanced oil recovery. Given the complex physical and chemical processes involved, several controlling mechanisms have been proposed to describe oil re-mobilization in the presence of water solution with low salinity. Osmosis and water-in-oil emulsification are among these mechanisms. However, our current knowledge about these processes is limited and their associated time scales are not well understood. In this study, we have used 11 capillary tubes with an inner diameter of 800 µm to inject a sequence of low-salinity water, crude oil, and high-salinity water phases and to observe the evolution of the system. The monitoring was done for a period of 40 days. We used two setups, a CMOS camera and a confocal laser scanning microscopy, to capture dynamics of the oil droplet re-mobilization as well as the 2D/3D water–oil interfaces. Additionally, microscopic pore pressures were directly measured at both low and high-salinity water phases containing the oil droplet using two fiber-optic sensors. We observed that in the water-wet capillaries the oil droplet moved a distance of about 524 µm. The contact angles at both low and high-salinity water interfaces with crude oil gradually decreased by 34.32° and 18.23°, respectively, during the first 15 days. We found that the pressure difference between high/low-salinity water phases reached a plateau with a maximum value of 1.65 kPa during a period of 24 days. Further, based on these changes and their time scales, we propose a hypothesis about emulsification and water diffusion through the oil phase.