Theoretical Study on Oil Bank for Chemical Enhanced Oil Recovery

Summary Daqing Oilfield has carried out a large scale chemical flooding industrial application for more than 20 years, and the annual oil production for chemical flooding has been more than 10 million tons since 2002. All the chemical flooding projects performance exhibited very good oil bank forming processing mechanism, so far the main method to be used for revealing that whether the oil could be enriched and aggregated to form the oil bank in chemical flooding process is to observe the water cut of produced fluid, lacking of theoretical demonstration. This paper has conducted a series of theoretical studies on the oil bank forming process for chemical enhanced oil recovery. Firstly, an oil production well surveillance method has been carried out, demonstrating that the oil saturation in the reservoir between injector and producer increased gradually with injection of chemical displacement fluid. Secondly, a set of oil recovery mechanism experiments have been conducted to obtain the relative permeability curves and capillary desaturation data for the chemical viscoelasticity fluid, providing the basic quantization relationship to characterize the process mechanism for chemical enhanced oil recovery. Finaly, based on the fractional flow theory, a kinetic mathematical model was established to reveal oil bank forming mechanism for chemical flooding process, demonstrating that, for chemical enhanced oil recovery, the oil could be enriched and aggregated gradually in displacement fluid front to form the oil bank eventually. According to the flowing properties of chemical displacement fluid in the porous media, several influence factors on the oil enrichment and aggregation have been investigated by using the kinetic mathematical model on oil bank forming, mainly including the polymer concentration gradient in the front, the relationship of polymer viscosity and concentration, the polymer solution elasticity, the permeability reduction factor causing by polymer, the interfacial tension, and the wettibility alteration. The research results show that the chemical flooding with higher viscoelasticity, higher chemical concentration, lower interfacial tension has a big potential to increase oil recovery, providing very useful theoretical method to optimize the injection parameters for chemical flooding scenario design.