Evidence of Viscous Crossflow during Polymer Enhanced Oil Recovery

Summary Since the early 1960s polymer flooding has been shown to be an effective enhanced oil recovery (EOR) method across hundreds of applications. Until recently, screening criteria typically stated that the upper limit of oil viscosity was ∼150 mPa.s for successful implementation. This was predicated on the understanding that polymer flooding operated via a mechanism of mobility control whereby the addition of water-soluble polymer to the injection stream reduced the mobility ratio between the water and oil to near unity, giving a stabledisplacement. In more recent years, there has been a significant number of laboratory and field observations of effective polymer flooding of heavy oil with viscosities >> 150 mPa.s. In these cases, the mobility ratio with polymer flooding is still significantly >>1, yet high recovery efficiencies are observed. Sorbie and Skauge (2019) suggested that this was due to a form of viscous crossflow taking place during polymer injection. Similar to viscous crossflow between layers, the injection of the viscosified water phase results in crossflow at the trailing front of water out of the viscous finger and crossflow of bypassed oil into the viscous finger at the leading front. This work demonstrates the viscous crossflow mechanism via a series of laboratory and numerical experiments. First, a visualised example of viscous crossflow is presented, then a series of miscible simulations performed to demonstrate viscous crossflow the absence of relative permeability, capillary pressure etc. along with a series of immiscible calculations. The immiscible simulations directly match literature experiments of viscous crossflow and show a high degree of portability between experiments of the same adverse viscosity ratio. In each case, viscous crossflow is evidenced and literature results justified under these assumptions.