Viscous crossflow as the mechanism for polymer enhanced oil recovery in viscous oils: Evidence of crossflow between bypassed oil and viscous fingers

Until recently, screening criteria typically stated that the upper limit of oil viscosity was ∼150 mPa.s for successful polymer flood implementation. This was predicated on the understanding that polymer flooding operated via a mechanism of “mobility control” whereby the polymer reduced the mobility ratio between the water and viscous oil, ideally to near unity, thus giving a much more “stable” water-oil displacement process. In recent years, there has been a significant number of both laboratory and field observations of effective polymer flooding of heavy oil with viscosities, μ o >>150 mPa.s. In these cases, the mobility ratio with polymer flooding is still significantly >>1, yet much higher recovery efficiencies have been observed compared with the waterflood. Sorbie and Skauge (2019) proposed that this greatly improved and unexpected performance was due to a form of viscous crossflow taking place during polymer displacement of the viscous oil. Similar to inter-layer crossflow, the injection of the viscosified water phase into a pre-fingered water displacement results in crossflow at the trailing front of water out of the viscous finger and crossflow of bypassed oil into the viscous finger region at the leading front. This paper demonstrates the viscous crossflow mechanism using evidence from a series of laboratory and numerical experiments for heavy oils in the range of 100 to 2,000 mPa.s under tertiary polymer flooding. First, a number of recent visualised examples of viscous crossflow are presented, then a series of miscible simulations performed to demonstrate viscous crossflow the absence of relative permeability, capillary pressure etc. before a series of immiscible calculations. It is also shown via modification of the wettability of the porous media that capillarity can easily supress viscous fingers at the laboratory scale, and in such cases viscous crossflow cannot take place, since there are no immiscible viscous fingers to crossflow into, and polymer flooding operates through the mechanism of classical “mobility control”.