The Micro-flow Mechanism Study On Polymer Flooding In Second Type B Reservoirs

The second type B reservoirs had become the main polymer injection and exploitation object in Daqing Oilfield,and its geological reserves reach 689 million tons.Compared with the second type A reservoirs,the second type B reservoirs have more serious heterogeneity,lower permeability and worse physical properties.The development effect of polymer flooding in the second type B reservoirs which has been put into production is not ideal.In order to find out the deep reasons for the uneven production of second type B reservoirs,a lot of experimental and theoretical studies have been carried out.However,there are few researches on the micro-flow mechanism of polymer flooding in second type B reservoirs.Therefore,based on the geological characteristics and physical property parameters of the second type reservoirs,a micro pore model was established to study the micro-flow law of polymer flooding in the second type B reservoirs,the influence of pore structure,fluid properties and rock wettability on the microscopic oil displacement efficiency were revealed.Firstly,a two-phase flow mathematical model of viscoelastic polymer solution was established.The logarithmic conformation method was used to transform the flow model.The VOF free interface tracking method was used to track the two-phase interface in real time during the displacement process,and the finite volume method was used to discretize it,a numerical method is developed to describe the viscoelastic polymer-oil two-phase flow.In this paper,the flow mechanism of polymer solution in different pore channels of the second type reservoir is described in detail,and the influence of different reservoir characteristics,pore structure,fluid properties and rock wettability on the flow mechanism is analyzed respectively from the distribution characteristics of saturation,pressure and velocity in the flow field.The results show that for the ideal model,the recovery of the second type A reservoir is about 2.1% higher than that of the second type B reservoir.For the real pore model,the micro-displacement efficiency decreases with the deterioration of the connectivity,separation and pore throat radius of the formation channels The micro-displacement efficiency of the second type B_P2 model is 15.9% higher than that of the second type B_G1model,and the micro oil displacement efficiency of the second type A_S3 model is 10.9%higher than that of the second type B_P2 model.For the parallel channel model,the final micro-displacement efficiency of B_P2 reservoir is 55.2%,which is 27.2% higher than that of B_G1 reservoir.For the ideal model of the second type B reservoir and the model of complex porous media,increasing the relaxation time of polymer solution leads to the increase of elasticity,micro-displacement efficiency gradually increases,but when the high elastic to a certain value,the micro-displacement efficiency began to decline,shows that the elasticity of polymer solution in a certain range increase the micro-displacement efficiency play a bigger role.With the increase of viscosity,the micro-displacement efficiency increases.When the polymer solution with viscosity ratio of 3:1 is injected,the micro-displacement efficiency of the polymer solution with viscosity ratio of 2:1 and 1.5:1 is 1.56% and 4.96% higher than that of the polymer solution with viscosity ratio of 3:1 and 1.5:1,respectively.In addition,when the wetting angle 60 ° is water-wet,residual oil in the porous medium model mainly includes the blind side residual oil and residual oil and pore throat clusters of residual oil,and wetting angle is 120° oil-wet under the condition of residual oil saturation is greater than the state of wettability to water-wet,the oil membrane residual oil,the two large differences in the microscopic oil displacement efficiency,water-wet is approximately 15% higher than oil-wet final recovery.The research results provide an important theoretical support for the development of polymer flooding in the second type B reservoirs of Daqing oilfield and have important guiding significance for improving oil recovery of the oilfield.