Microfluidics experimental investigation of the mechanisms of enhanced oil recovery by low salinity water flooding in fractured porous media

Spontaneous imbibition of water from fracture into the matrix is considered as one of the most important recovery mechanisms in the fractured porous media. However, water cannot spontaneously imbibe into the oil-wet rocks and as a result oil won't be produced, unless the capillary pressure barrier between fracture conduits and matrix is overcome. Wettability alteration is known as the main affecting mechanism for low salinity water flooding (LSWF), however, its effectiveness in fractured porous media has been less investigated, especially in the case of possible pore scale displacement mechanisms. In this study, the effectiveness of LSWF (diluted seawater) on oil recovery is compared to the formation water (FW) and seawater (SW) in four different fractured models. Each of these fractured models is representing a different sub scale of the NFR. In addition to the effect of fracture geometry, the effect of wetting conditions was studied. Contact angles, Interfacial Tension (IFT) and zeta potentials are measured to have more insight on the Crude-oil/Brine/Rock system under investigation. The results of micromodel experiments show a positive effect of LSWF in both water-wet and oil-wet states. Sessile drop and pendant drop tests show that contact angle alteration and IFT reduction cannot be the dominant mechanisms. Visual investigations on dynamic displacement tests, suggest that the formation of micro-emulsions and elastic oil/water interface are the main mechanisms of recovery in the case of LS and FW injections, respectively. Elastic FW/oil interface causes discontinuous water phase in the fracture, which in turn reduces the mobility of the FW in the fracture and facilitates the diversion of the flow from fracture towards the matrix. This mechanism results in better sweep efficiency for the case of FW compared to the case of SW. Nevertheless, the dominant formation of microemulsion in the case of LSW is more effective in the diversion of the flow from fracture into the matrix (compared to the effect of interface elasticity in the case of FW). This mechanism along with slightly better imbibition (less oil-wet state) in the case of LSWF enhances its recovery performance compared to the FW in both water-wet and oil-wet systems.