| This paper was supported by China Petroleum and Chemical Industry Federation(Grant No.2015-01-03).With the development of economic society and the rising demand for oil resources,the exploitation of viscous oil,bitumen and oil shale is an important challenge for the oil industry while the recoverable reserves of conventional crude is decreasing constantly.However,because of the requirement of heat,steam or volatile solvents,the conventional thermal recovery method become uneconomical in certain cases especially when the reservoir is close to the permafrost,relatively thin,or off-shore with no steam generation facility.Polymer flooding,the representative of chemical EOR method,is imperative to improve recovery for viscous oils.Chemical flooding can enhance sweep efficiency and production rate.The mechanism of polmer flooding conventional oil has been studied deeply,so it has been applicable in industry.However,the mechanism of polymer displacement viscous oil,especially the difference between displacement conventional crude oil and viscous oil still need studying deeply on the pore scale flow.The goal of this paper is to develop the micro flow mechanism of viscoelastic polymer displacement viscous oil basis on the Bohai oilfied.The study results are promising and provide a theoretical foundation for the viscous oil field polymer flooding,moreover,the study can establish and develop theory of viscoelastic polymer displacement oil.This paper is set in Suizhong 36-1 polymer flooding in Bohai oilfield,the viscostic and elastic characteristics of AP-P4 hydrophobic associative polymer with different concentration are measured based on steady-state shear and small-amplitude oscillatory shear mode,the relaxation time is measured.By matching the experimental rheology data,the Oldroyd B can describe the rheology of hydrophobic associative polymer.The Power law constitutive equation of the rheological properties of the viscous oil in Suizhong 36-1 block were obtained by rheological testing.Basis on the OpenFOAM platform,the mathematical model of viscoelastic fluid and power law fluid two-phase flow based on the VOF interface tracking method was established.So we develop solver of Viscoelastic and power law fluid basis on the two solvers of polymer solution single-phase unsteady flow ViscoelasticFluidFoam and two-phase steady flow InterFoam.The theoretical simulation method of viscoelastic polymer solution and viscous oil two phase flow is developed.We establish the physical model of static oil droplet in deadendand dynamic oil droplet in circular tube.The microscopic displacement mechanism on static and dynamic oil droplet is simulated by Polyflow,remeshing technique was used to for the moving interface.Elastic effect on the conventional oil droplet and viscous oil charateristics was compared.The results show that although the viscous oil droplet's deformation caused by elasticity of the polymer solution is less than that of the conventional oil,the higher the elasticity of the polymer solution is,the greater stress and velocity of the viscous oil droplets,the more obvious of the oil droplets' deformation,the flow field are dominated by elasticity,reflecting the more conducive to the oil droplets stripping.The complex porous media pore model is established using the SnappyHex Mesh tool of OpenFOAM platform,the model is consistent with the microscopic etching glass model.The unsteady flow law of different elastic polymer solutions in this model is studied by using Viscoelastic FluidFoam solver.The results indicated that the flow pressure difference is related to the injection time and the elasticity of the polymer solution.When the injection time is doubled under the constant injection rate,the flow pressure difference can be increased 37.2%,the elastic effect becomes more and more obvious.When the elasticity of the polymer solution is increased under the constant injection rate,the velocity contour area increases,which causes the additional pressure drop to be larger,which is more conducive to the improvement of the sweep efficiency.The flow characteristics of polymer displacement viscous oil in contranction and deadend was simulated using the solver.The results show that the polymer elastic effect begins to appear when the displacement front reaches the position where the pore shape changes.With the elasticity increasing,the normal stress acting on the two-phase interface increases,and the velocity of the two-phase flow increases.At the same time,the pressure difference increase until polymer breakthough occurred at the outlet,which contributes to the reduction of viscous oil saturation and the improvement of displacement efficiency.Under the condition of simulation calculation,compared with the pure viscous polymer flooding at the same viscosity,viscoelastic polymer flooding,whose relaxation time is 0.50 s,can increase the displacement efficiency of the viscous oil by 3.02% and 1.36% in contraction and deadend model respectively.The polymer displacement viscous oil experiment is developed to study the mechanism of elastic effect in porous media.The micro flow characteristics in deadends model and microscopic model displacement experiment show that the elasticity of the polymer solution in deadends make the two-phase interface becoming asymmetric U-shaped distribution,which reflects the "entangling" and "drag" effect on viscous oil.It also can enhance the kinetic energy of viscous oil droplets in deadends.This phenomenon is obvious as the concentration of polymer increase.The elastic contribution to recovery is larger when injectrion polymer in low sweep efficiency with water flooding.The recovery of elastic contribution is 3.13%,4.33% and 5.18% respecitively when water sweep efficiency is 60%,40% and 20% injecting polymer.One polymer slug and suceesive slugs displacement viscous oil in core experiment show that hydrophobic associating polymer can further reduce the oil saturation than purely viscous xanthan with same viscosity.Compared with waterflood,the viscosity contribution to viscous oil is 7.81%,the elastic contribution to recovery is 7.54%.As De of polymer solution increase,the elascity increase which reduce the oil saturation further,while the concentration is above critical association concentration.The elastic contrition to viscous oil recovery enhance 5.06% when the concentration increase 3 times on creitial associtation concentration.The research of this paper can provide the significant knowledge base of viscous oil.The result will enhance scientificalness and accuracy of viscous oil simulation and engineer design.Moreover,this study will enrich the microscopic mechanism of viscoelastic polymer flow in porous media. |
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