The Micro-Flow Mechanism Study On Viscoelastic Polymer Flooding Considering The Wettability

Chemical flooding is the main technology to enhance oil recovery in China.Polymer flooding has entered the stage of industrial application,and has been successfully applied in sandstone reservoirs,complex fault block reservoirs,high temperature and high salt reservoirs.Most of the oil fields in China are continental deposits with serious reservoir heterogeneity.The geological reserves developed by water flooding account for more than 85% of the developed geological reserves.The annual output of crude oil from 2000 to 2016 exceeds 10 million tons.Compared with water flooding,144 million tons more oil was produced,and the average oil recovery rate was increased by 13%,providing support for stable and high oil production.At present,there have been some researches on the influence of single factor of viscoelasticity of polymer solution and wettability of rock wall on the microscopic flow mechanism of polymer flooding,but there are few researches considering the synergistic effect of polymer solution elasticity and wettability on the flow mechanism.Therefore,on the basis of the established porous medium model,the influences of the elasticity,wettability,rock particle shape and gravity on the flow mechanism of polymer solution were studied in this paper,which provides an important theoretical basis for further enhancing the oil recovery by polymer flooding in actual reservoirs with different wetting characteristics.In this paper,a two-phase microscopic flow model of viscoelastic polymer solution and crude oil was established.The free interface capture method of VOF(Volume of Fluid)was used to trace the two-phase interface in the displacement process in real time.The finite volume method was used to discretization,and the logarithmic conformation method was used to transform the constitutive equation and momentum equation.The dynamic contact Angle model,which was used to estimate the velocity value of the contact line by the velocity value of the control unit near the contact line,takes into account the micro-wetting characteristics of the rock wall and forms a numerical calculation method for viscoelastic polymer solution-crude oil two-phase flow considering the dynamic wetting effect.The results show that for the dead end tunnel model,the oil displacement efficiency was the highest under water wet condition,it was87.12%,followed by the neutral wet condition,it was 85.21%,and the oil displacement efficiency under oil wet condition was the lowest,it was 84.51%.The oil displacement efficiency increases with the increase of elasticity in both wet wall and wet wall conditions.When Weissenberg number We=0.05,the microscopic oil displacement efficiency was the highest.When We increases from 0.01 to 0.05,the oil displacement efficiency increases by 1.19%under wet water conditions,but only increases by 0.4% under oil wet conditions.It shows that the elasticity of polymer solution plays a greater role under the condition of water and humidity.The flow mechanism of polymer solution in different porous media was analyzed and studied.Based on the characteristics of saturation and pressure distribution,the influences of wettability and rock particle shape on flow characteristics were studied.For the double layer heterogeneous ideal model with three different rock particle shapes,the round particle model with high roundness had the highest displacement efficiency 3.34% and 11.48% higher than hexagonal and rhomboid models,respectively;Based on the circular particle model,the influence of wettability was studied.The oil displacement efficiency under wet and water conditions was higher than that under wet and water conditions.The closer the contact angle was to 90°,the higher the oil displacement efficiency was.For the two-layer heterogeneous real model,the polymer solution was superior to the high permeability layer.The displacement scheme with gravity was affected by the crossflow caused by gravity and viscous force,and the microdisplacement efficiency was 9.87% lower than that of the non-gravity displacement scheme.The different locations of polymer flooding front lead to uniform distribution of oil saturation in each layer.Considering the wettability,vertical capillary force crossflow will be formed.The oil displacement efficiency under water wet condition was 3.9% higher than that under oil wet condition.The research results can not only expand and enrich the microscopic flow mechanism of viscoelastic polymer solution,but also provide reference and guidance for polymer flooding design in mine field.