Study On Change of Reservoir Physical Properties and Seepage Characteristics After Polymer Flooding

In Daqing Oilfield, the industrial application of polymer flooding began in 1995. So far, the amount of original oil in place produced by polymer flooding is 11.4×108 t, the oil recovery factor after polymer flooding is 57.6%, and the reservoirs developed by polymer flooding has the remaining reserves of 4.83×108 t, which need to be further developed by means of other methods. Therefore, it is necessary to study the change of reservoir physical properties and seepage characteristics after polymer flooding, so as to provide basis for the following EOR method development.

Daqing Oilfield has abundant sealed coring well data before and after polymer flooding, well logging data, field development data of polymer flooding, and EOR field test data after polymer flooding. By using these precious first-hand data comprehensively, this paper analyzed the change of reservoir properties after polymer flooding, and carried out microfluidic visual experiment, physical simulation experiment and seepage theoretical study to figure out the seepage characteristics of reservoirs after polymer flooding.

The research results show that the reservoir properties change greatly after polymer flooding, for long-term flushing of polymer increases the permeability significantly. In addition, compared with low-permeability reservoirs, the polymer retention and shale content of high-permeability reservoirs are much lower and the permeability increase amplitude is larger, which further intensifies the heterogeneity degrees of reservoirs after polymer flooding. After polymer flooding, the distribution of remaining oil is more scattered, the remaining oil saturation gets lower further, only just 40.9%, which is 11.9% lower than that after water flooding. The retention of polymer in reservoirs has significant influence on permeability, compared with the reservoirs after water flooding, the retention of polymer has greater influence on middle- and low-permeability reservoirs and the water relative permeability decreases more. After polymer flooding, permeability, relative permeability and oil saturation change significantly, the seepage resistance of high-permeability reservoirs decreases greatly, and the seepage resistance of middle- and low-permeability reservoirs increases greatly, so that water preferential flow channels formed, the water preferential flow channels thickness percentage is only 15.9% and the percentage of remaining geological reserves is only as small as 4.6%, but the water injection percentage is high up to 60%.

These research results are of great guiding significance to the further selection of EOR method and the development of oil displacement agent after polymer flooding, and they have been successfully applied to the EOR field test after polymer flooding.