Enhanced oil recovery mechanisms of polymer flooding in a heterogeneous oil reservoir

Taking reservoir rocks and fluids of the Daqing, Dagang and Changqing oilfields as research objects, the EOR mechanisms and technical approach of polymer flooding were discussed. By comparing the displacement performances of ordinary polymer, glycerol, polymer in “sheet-net” structure and heterogeneous weak gel at the same viscosity and concentration, the relationship between the viscosity of polymer displacement agents and displacement performance was demonstrated, and the method of improving polymer flooding effect was worked out. The main mechanism of polymer flooding to increase oil recovery is the swept volume expansion of water injection due to polymer retention in porous media. The viscosity of polymer agents has no positive correlation with polymer flooding effect. Although polymer of “sheet-net” structure has strong capacity in increasing viscosity, it has poor compatibility with pore throat structure of reservoir rock, low injectivity and low shear resistance. Heterogeneous weak gel system has higher adsorption and capture capacity in porous media, which is easy to retain in porous media, and can effectively establish seepage resistance in high permeability layers (zones). Compared with polymer solutions with the same viscosity or concentration, it has stronger ability to expand swept volume. Long term injection of polymer flooding agents will inevitably lead to fluid entry profile reversal, and thus worsening of polymer flooding effect. Alternate injection of high retention and low or non-retention displacement agents can further improve the displacement effect of polymer flooding agents.