Interfacial tension induced-transport in shale: A pore-scale study

While unconventional reservoirs have quickly become a main contributor to oil and gas production, the transport mechanism in shale is not well understood. Although water does not imbibe into oil-wetted rock matrix, it has been observed in laboratory measurements that it can. The amount of water imbibed into the rock matrix increases when the salinity of the injected water decreases. Wettability alteration is often considered as the main reason for this phenomenon. One of the physical mechanisms for the imbibition of low salinity water into an oil-wetted rock matrix is the variation of the interfacial tension and the contact angle. The change of salinity initiates the change of interfacial tension driving the movement of the interface imbibing water into oil-wetted pores. The imbibition of the water into the rock matrix is a complicated process requiring a comprehensive understanding of the mass transport at pore scale. Hence, pore scale modeling was used in this research study for understanding the physics of transport at pore scale caused by interfacial tension variation or interfacial tension-induced transport. First, a simplified pore-scale model is presented to simulate the imbibition of water into an oil-wetted pore. Next, a numerical solution and procedure are outlined to simulate the transient mass transport in the pore as the result of interfacial tension variation due to salinity. The results indicate that the interfacial tension-induced transport is a very important mechanism for water imbibition into oil-wetted pores. How fast the water imbibes into the pore depends on the salinity gradient and the type of solute used. When salinity at the inlet of the pore is low, more solute diffuses out of the pore resulting in higher cumulative recovery from the pore. Reducing the interfacial tension between oil and water forces oil out of the pores and improve the oil recovery. Hence, low salinity water is recommended for low salinity water injection enhanced oil recovery.