Microfluidics study on immiscible displacement mechanisms of oil by hybrid low salinity water and polymer in fractured porous media.

Application of hybrid low salinity water and polymer flooding (LSPF) as an enhanced oil recovery (EOR) method takes advantage from the wettability alteration by low salinity water and enhancement of mobility ratio by addition of polymer. Recently it is shown that polymer affects the possible interactions in oil/brine/rock system e.g., interfacial tension, contact angle, pH and zeta potentials. However, in the case of highly heterogeneous systems such as fractured porous media, the present understanding of the effect of LSPF on pore scale displacing mechanisms and enhanced oil recovery is very limited compared to the low salinity water. With this aim, in the present study, a series of injection scenarios were performed in fractured micromodels. Different experiments were conducted to illustrate the effects of brine composition, addition of HPAM to brine solution and injection scenario (either secondary or tertiary) on pore scale displacement mechanisms, sweep efficiency and oil recovery. Sea water (SW) and twice concentrated SW (2xcSW) and 10 times diluted SW (10xdSW) as well as 1000ppm HPAM (based on the previously study on the optimum oil/brine and rock/brine interactions) were considered. The results showed that in the case of secondary injection and absence of HPAM, the difference between the recovery factors of different brines is minimal and none of them are capable of producing the oil from the matrix. In the secondary injection scenario, combination of 2xcSW and polymer didn't improve the oil recovery considerably, but the success of the hybrid process in the cases with lower salinity (i.e., SW and 10xdSW) was remarkable. For tertiary injection, adding HPAM to the formerly injected brine (which was injected in the secondary model) improved the EOR considerably. The amount of improvement was significantly larger in the case of brines with lower salinities. For all the investigated hybrid cases, the ultimate oil recovery after tertiary injection scenario was larger than secondary counterparts. The results are discussed based on the possible interactions in the oil/brine/rock system, rheological properties and visualized displacing mechanisms in the case of each investigated injectant and injection scenario.