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In order to analyze the stress and deformation of different permeability of residual oil film in the complex pore, which are affected by the viscous-elasticity of the fluid, the hydrodynamic displacement mechanism is explored from the stand-point of hydrodynamics, that is, the residual oil film displaced by alternating injection of different concentrations of the polymer solution, viscous-elastic fluid flow equation is established in the complex pore by choosing continuity equation, motion equation and the upper convected Maxwell constitutive equation. The flow field is computed by using the method of numerical analysis. Not only the stress and deformation of residual oil film on the different permeability of micro pores, but also the analysis of the flooding mechanism of alternating injection of different concentrations of the polymer solution is got. The results show that the larger the viscous-elasticity of polymer solution is, the bigger the normal deviatoric stress acting on the residual oil film is; the distribution of normal deviatoric stress has the abrupt change. The stronger the viscous-elasticity of the polymer solution is, the bigger the horizontal stress difference acting on the residual oil film is and the more obvious the deformation is; the high-concentration polymer solution is suitable for high-permeability micro pores. Low-concentration polymer solution is suitable for medium and low-permeability micro pores. Alternating injection of polymer solution can improve Volumetric Sweep Efficiency and increase the deformation of residual oil film, which is conducive to enhancing oil recovery.

After water flooding, polymer flooding has a favourable effect on flooding effects, which largely enhance the oil recovery [

In order to analyze the force and deformation of the residual oil film in different permeability micro pores, Two-dimensional parallel channel model is shown in _{1}:H_{2}:H_{3} as 1:2:3.

In

Polymer solution with complicated rheological proper

belongs to viscous-elastic fluid. Based on experiment research, the upper convected Maxwell constitutive equation is suitable to describe the viscous-elastic polymer solution. Continuity equation, motion equation and constitutive equation [

Continuity equation:

where: is velocity vector.

Momentum equation:

where: F is mass force; ρ is density; P is stress tensor.

where: I is unit tensor; –p is static pressure; T is deviatoric stress tensor.

Constitutive equation:

where: λ is relaxation time, which is the characterization of fluid elasticity; η is shearing viscosity; A_{1} is one order Rivlin-Ericksen tensor.

Weissenberger number is a dimensionless quantity which is used to describe elasticity of fluid. The bigger its value is, the stronger the elasticity of the fluid is. Calculation formula of Weissenberger number is:

where: We is Weissenberger number; v is characteristic velocity; l is characteristic length.

In order to analyze the force acting on the oil film from fluid of different viscous-elastic flowing through different permeability micro pores, the stress field is calculated when viscous-elastic fluid flows residual oil film under the different conditions that We is 0.1, 0.2, 0.3 respectively. In addition, normal deviatoric stress τ_{nn} and the difference of horizontal stress p_{nh} on the residual oil film surface are calculated.

As shown in

permeability micro pore, the normal deviatoric stress of high-permeability micro pore is 6 times than that of it with the same oil film.

When the polymer solution of the We = 0.3 is injected, the normal deviatoric stress acting on residual oil film differ greatly from low and medium-permeability micro pores from high-permeability micro pores. High viscouselasticity of the high-concentration of polymer significantly increase the force and deformation of the residual oil film, but high concentration makes polymer molecules overlap and molecular clew size large, which makes them difficultly enter the medium and low-permeability layer, especially in low-permeability layer.

Therefore, the high concentration of high molecular weight polymers mainly enriched in the high-permeability layer, generating large normal deviatoric stress to the residual oil film in high-permeability layer. This shows that the force and deformation of residual oil film in the micro pore are affected by viscous-elasticity of driving agent. High-concentration of polymer is available for high-permeability micro pore.

When the polymer solution of the We = 0.3 is injected, the normal deviatoric stress acting on residual oil film is less the same between low and medium-permeability micro pore and high-permeability micro pore. This is due to the low-concentration polymer solution can be relatively easy to enter the low-permeability layer. Compared to the high-concentration polymer solution, the total amount of the low-concentration polymer solution entered is large. Due to the increase in the amount of the polymer entered low and medium-permeability micro pore, the degree of deformation increases. So the low-concentration polymer is suitable for low-permeability micro pore.

Both the microscopic visualization flooding result [

As can be seen from

have little difference when low concentration of polymer solution is injected.

Through the analysis of the force and deformation of residual oil film, we find that alternating injection different viscous-elastic driving fluid can enhance oil recovery. General injection is taken, force and deformation of the residual oil is very small in low and medium-permeability micro pore, displacing effect is not good, the degree of use is low; force and deformation of the residual oil is large in high-permeability micro pore. Therefore, alternating injection takes advantage of principle that different driving agent flooding will make different force and deformation of the residual oil, which can improve excessive absorption of fluid in high-permeability micro pore and insufficient absorption of fluid in low and me-

dium-permeability micro pore, make more low-concentration polymer into low and medium-permeability micro pore, Each permeability layer takes what he needs; fluidity is controlled well, the residual oil of force and deformation is more, and the swept volume is expanded, at the same time, it will play a better role in flooding.

• The normal deviatoric stress of viscous-elastic polymer solution acting on oil film increases with the increasing of We. The distribution of normal deviatoric stress occurs corresponding to mutations, which are more conducive to the deformation of the oil film.

• The horizontal stress difference of driving fluid acting on oil film increases with the elasticity of driving fluid increased.

• For high-permeability micro pore, high-concentration polymer solution is injected which can acquire the largest normal deviatoric stress and the most obvious deformation. For low and medium-permeability micro pore, the normal deviatoric stress of injecting lowconcentration polymer solution is more or less the same with that in the high-permeability micro pore.

• Alternating injection different viscous-elasticity polymer solution can enhance oil recovery, which is consistent with the results of laboratory experiments.