Water flooding has been applied for more than seventy years in both conventional and unconventional heavy oil reservoirs. Although it is generally accepted that the mechanisms of water flooding in heavy oil systems are totally different than that of light oil reservoirs, there is not a systematic study to specifically investigate water flooding in heavy oil systems. This article presents the findings of core flooding experiments in water-wet systems and gives some insights on the interplay between capillary and viscous forces in imbibition displacement processes.
Seven different oils of various viscosities, ranging from 1 to 15,000 mPa.s at 25 °C, were used in nineteen core flooding experiments where injection velocity was changed from 0.7 to 24.3 ft/D (2.5 × 10−6 m/s to 86.0 × 10−6 m/s). An in-line densitometer was used to precisely determine breakthrough time. Capillary forces and instability analysis were used to quantify the balance between viscous and capillary forces.
On physical grounds, the capillary number, which is the relative magnitude of viscous and capillary forces during a displacement, should be the first-order influence on residual oil saturation. However, Abrams (Abrams, 1975) showed that accounting for the viscosity ratio improves the correlation to residual oil saturation for oil viscosity below 37 mPa.s. Our observations extend the range of oil viscosity to 15,000 mPa.s and when combined with 178 datasets from the literature indicate that viscosity ratio has much more influence than capillary number on residual oil saturation.
Standard models such as Buckley-Leverett theory predict that oil recovery at water breakthrough depends only on phase mobilities. However, our observations indicate that flow velocity also influences breakthrough oil recovery. At oil to water viscosity ratios smaller than 20, breakthrough oil recovery monotonically increases with increasing injection velocity. For intermediate viscosity ratios (20 < μo < 160), breakthrough oil recovery increases with decreasing injection velocity. At higher values of viscosity ratios, breakthrough oil recovery is almost independent of injection velocity. In these cases, late time oil recovery remarkably increases with decreasing injection velocity. This effect is more pronounced in more viscous oil systems suggesting the importance of imbibition in these systems. Our observations prove that water flooding, if applied at the most optimized mode that is a strong function of oil viscosity, can still be a very efficient EOR technique.