The mechanisms of enhanced oil recovery by polymer flooding in stratified systems were investigated using reservoir simulation at a range of model resolutions. The objectives of this work were to understand the effects of grid refinement on the simulated flow behavior and, in light of this, to examine the validity of some seminal coarse-scale reservoir simulation studies reported in the literature.
Simplified one- and two-dimensional models, in which aqueous phase viscosity varied as a function of polymer concentration, were used to simulate the behavior of polymer floods and waterfloods in homogeneous and layered systems. Advanced visualization techniques enabled a detailed analysis of localized crossflow between layers. This provided a clearer understanding of flow behavior than was available in previously reported studies, in which localized flow patterns were inferred from the aggregate system responses obtained from reservoir simulation and laboratory coreflood experiments.
In common with the literature, results indicate that crossflow between layers is of central importance to the polymer flood recovery mechanism in stratified systems where the layers are in vertical communication. In addition to improving the fractional flow properties of the flood, the polymer crossflow enhanced the vertical volumetric sweep efficiency.
A one-dimensional polymer flood model demonstrated the mobilization of formation water, which was initially at irreducible saturation, to produce a water bank with the saturation profile of a shock front ahead of the injected polymer solution. Simulation results were analyzed in the context of Buckley-Leverett displacement theory. The variation in polymer concentration between injector and producer wells produced a fractional flow curve which, at intermediate concentrations, deviated from the characteristic "S-shaped" curve of a waterflood, and instead exhibited a linear profile of progressively increasing viscosity, consistent with the disperse polymer flood front that was observed behind the formation water shock front.
The high-resolution studies revealed additional features of the recovery that were not observed at the coarse scale, including a secondary oil bank, formed by oil crossflow ahead of the polymer slug, and fingering of chase water into the rear of the slug. The variation of grid resolution was observed to produce substantial changes in the distribution of incremental oil production between layers, but to have a more limited impact on cumulative production. In contrast to previous findings, the crossflow behavior of the polymer flood model was found to be highly sensitive to vertical, as well as horizontal, grid refinement. This demonstrates the importance of grid resolution to predictions of oil in place and to the forecast distribution of production between intervals in the well.