This paper aims at evaluating the interactions between gravity segregation and viscous fingering during water injection into viscous oils, in order to estimate the recovery factor accurately, e.g. before assessment of tertiary EOR mechanisms. Based on simulations of homogeneous and heterogeneous reservoir sector models with horizontal wells, we wish to identify the dimensionless groups affecting the water breakthrough time and the oil recovery, and to quantify the impact of viscous instabilities in 3D domains versus 2D vertical cross-sections.
High-resolution numerical simulations, based on very fine grids and a high-order spatial discretization scheme implemented in our parallel in-house research reservoir simulator, were required to properly capture the complex flow patterns of these two-phase immiscible displacements. We analyze the competition between viscous and gravity effects using different values of injection flow rate, oil viscosity, density difference between oil and water and domain aspect ratio. This analysis is carried out in 2D and 3D, for simple permeability distributions with different correlation lengths, by inspection of production data and saturation maps.
For the investigated range of parameters, capillary effects are negligible and we identify three flow regimes before water breakthrough. When gravity dominates, viscous fingering is strongly attenuated by the rapid formation of a gravity tongue but a ‘ridge instability’ phenomenon may occur. In contrast, when viscous forces are preponderant, the gravity tongue is very weak or non-existent, and the flow pattern in vertical cross-sections is often characterized by one dominant viscous finger. In the transition regime, the gravity tongue and the viscous fingers coexist and may reach the producer quasi-simultaneously, leading to an optimum injection velocity in terms of breakthrough sweep efficiency. We then examine the dependency of water breakthrough and oil recovery upon the end-point mobility ratio and the viscous-to-gravity ratio . We also compare the early-time and late-time impacts of permeability heterogeneity and transverse viscous fingering.
In the context of viscous oil recovery by water injection, high-resolution simulations are required to represent the interplay between gravity segregation, viscous fingering and permeability heterogeneity. In the present study, under low capillary effects, it is quite remarkable that the post-breakthrough recovery is well predicted by 2D simulations and is mostly controlled by two dimensionless groups. This may be useful to create screening models for quick-look estimation of oil recovery on different sector models representative of an oil field.