Quantifying Viscous Cross-flow and its Impact on Tertiary Polymer Flooding in Heterogeneous Reservoirs

Tertiary polymer flooding is believed to be an effective strategy for recovering the remaining oil from mature oil reservoirs. It improves displacement efficiency in homogeneous reservoirs by increasing the viscosity of injected fluid. It should also improve conformance by reducing the tendency of the injected fluid to channel through more permeable layers and channels. Studies in the 1990 have shown that it may also improve macroscopic sweep in heterogeneous reservoirs through viscous cross-flow between zones of contrasting permeability. Nonetheless many oil companies are cautious about deploying this very expensive technology in such reservoirs. Uncertainty in the geological model means that engineers and manager perceive that there is a greater risk that the recovery improvement may be less than expected. In this paper, the relative importance of reservoir heterogeneity, viscosity ratio, and gravity on tertiary polymer flooding performance are investigated. This is accomplished by numerical simulation. We first investigated the impact of permeability contrast, aspect ratio and gravity in a simple two layered model. The study was then extended to more realistic models of geological heterogeneity taken from the SPE 10 Model 2 (which is a synthetic model of a Brent sequence). The reservoir heterogeneity in these models was quantified using an index derived from maps of the vorticity of single phase flow in these models. This vorticity heterogeneity index has been demonstrated to provide a good measure of the impact of heterogeneity of recovery and breakthrough in a number of previous publications. We compared the relative contribution to recovery from improved displacement efficiency, viscous cross flow and gravity in all cases. The results show a non-trivial relationship between the incremental recovery of polymer flooding versus permeability contrast and the size of the layers for different oil-water mobilites. We find that viscous cross-flow can result in significant incremental recovery in the layered model. In the more realistic heterogeneous models we find that incremental oil recovery from tertiary polymer injection is higher in the more heterogeneous systems and that this incremental oil recovery is a result of viscous cross-flow. Thus geological heterogeneity may not always have an adverse impact on improved oil recovery.