In this paper we investigate the contribution of capillary and viscous cross-flow to oil recovery during secondary polymer flooding. Cross-flow can be an important mechanism in oil displacement processes in vertically communicating stratified reservoirs. Using polymers will change the balance of these contributions. Previous numerical investigations have shown that the amount of viscous cross-flow is controlled by the layer permeability contrast and a dimensionless number that characterises the combined effects of water, polymer and oil viscosities. The highest viscous cross-flow values were observed during favourable mobility ratio floods in reservoirs with a layer permeability ratio close to 3.
The purpose of the laboratory study was to validate previous numerical studies of cross-flow performed using commercial reservoir simulators. A series of experiments were performed in glass beadpack using analogue fluids comprising water, glycerol solution (to represent the polymer) and paraffin oil. All porous medium and fluid properties (including relative permeabilities and capillary pressure curves) needed for the numerical simulations were determined independently of the displacement experiments. Two beadpacks were constructed of two layers of different permeabilities parallel to the principal flow direction. In one of the packs a barrier was placed between the two layers to prevent cross-flow. Comparing the recoveries from these enabled us to quantify the contribution of cross-flow to oil recovery. The mobility ratios examined in the experiments ranged from very unfavourable to very favourable. The layer permeability ratio was approximately 2.5.
Good agreement was obtained between experiments and simulations, without the need for history matching, demonstrating that the simulation correctly captures the physics of crossflow. The incremental oil recoveries attributable to cross-flow and mobility control both fell within the error margins of the experimentally calculated values. The experiments showed that capillary cross-flow dominated over viscous cross-flow on laboratory length scales. Having validated the simulator, we then used it to show that wettability (with and without capillary pressure) can modify the impact of cross-flow on oil recovery.