The Experimental Research On The Mechanism Of The Increase In Displacement Efficiency In Viscoelastic Polymer Flooding

About the researches in polymer flooding mechanism, the most accepted mechanism is: for certain fluid-reservoir system, the displacement efficiency is determined by the viscous pressure gradient of the driving fluid and the restrain force of the residual oil, the former is proportional to the viscosity and velocity of the fluid, the latter is proportional to the interfacial tension, the ratio of them is defined as capillary number. According to this theory, the interfacial tension between polymer solution and oil is equal to that of water, and the macro pressure gradient is equal either, then the capillary numbers of polymer flooding and water flooding are near, the displacement efficiency of them should be equal, this is not agree with the results of polymer flooding lab tests and field tests. Then the macro forces can not explain the increase in displacement efficiency in polymer flooding, the increase in displacement efficiency of polymer flooding can only be caused by micro forces which do not increase the macro pressure gradient. In this paper, the rheological properties of different kinds of polymer solutions are tested, the viscoelasticity of different polymer solutions are compared systemically, methods in characterizing the viscosity and elasticity of polymer solutions are found; the analyzes of forces on residual oil show that the elasticity of driving fluid can change the micro flow lines in porous media, then increase the micro force which acting on the protruding part of residual oil blob and mobilize the protruding part; through flooding experiments on artificial cores, the influences of micro force on different types of residual oil, and the influences of capillary number and viscoelasticity on displacement efficiency are verified.Three kinds of polymers are studied in this research, they are polyacrylamide, comb polymer and associative polymer. Through dynamic mechanics test and steady state shear test, the viscoelasticity of different polymer solutions are measured, and the viscoelastic features of different polymer solutions are studied through comparison of the test results. The results show that, the viscosity and elasticity of polyacrylamide solutions increase with molecular weight and concentration both in dynamic mechanics test and steady state shear test, the viscosity is more sensitive to concentration while the elasticity is more sensitive to molecular weight; compared with polyacrylamide with equal molecular weight, comb polymer solution has higher viscosity but lower elasticity, when comparing different kinds of polymers, the results from dynamic mechanics test are not as precise as the results from steady state shear test, the slope of first normal stress difference vs. shear rate can characterize the elasticity which influences the displacement efficiency of different polymer solutions; when the concentration is low, the viscoelasticity of associative polymer solution is the same as that of polyacrylamide solution, with the increase of concentration, the viscosity of associative polymer solution starts to increase sharply, the concentration when this increase occurs is the critical associative concentration (CAC), the test results show that, when the concentration is higher than CAC, the viscosity of associative polymer solution can be greater than polyacrylamide with larger molecular weight, the association has no contribute to elasticity, which is still mainly determined by the molecular weight of polymer.The micro states and distributions of residual oil in different porous media are studied in this research, and the stress distribution of residual oil blobs under steady state are analyzed, the results show that, under the driving forces, there will be a protruding part along the flow direction which provide enough capillary force to counter the driving forces, the changes in micro velocity will influence the micro force acting on the protruding part of residual oil blob. The normal stress effects of viscoelastic fluids is stronger than that of Newtonian fluid, which make the change in flow lines stronger too; otherwise, the velocity profile of viscoelastic fluid in porous media is more uniform, the velocity of fluid near the residual oil blobs is greater, so the micro force of viscoelastic fluid is much more bigger than that of Newtonian fluid, and the higher the elasticity, the higher the micro force. The micro force is the main reason which makes the displacement efficiency of viscoelastic fluid flooding greater.Through polymer flooding experimtnes on both oil wet and water wet heterogeneous artificial cores, the influences of viscoelasticity and capillary number on displacement efficiency and residual oil saturation are studied. The pressure gradient of core is used in calculating the capillary number, which avoids the influences of viscosity of driving fluid and permeability of porous media. For polyacrylamide, comb polymer and associative polymer flooding, the increases in capillary number and elasticity of solutions are all effective in increasing the displacement efficiency and decreasing the residual oil saturation. The comparison of flooding results with different polymer solutions which have specific viscoelastic features show that, the changes in displacement efficiency and the changes in elasticity of polymer solutions match regularly. The elasticity of comb polymer and associative polymer is lower than that of polyacrilymide solution, and the displacement efficiency is lower, the viscosities of polymer solutions do not influence the displacement efficiency. Rationally increase the elasticity of the driving fluid can achieve the same effect of ultra low interfacial tension. The field test results of high concentration polymer flooding show that, the increase in recovery ratio is more than 20%OOIP, which is nearly two times than conventional polymer flooding, and the front of polymer solution is more uniform, the sweep efficiency is greater.In conclusion, the elasticity of polymer solution can increase the displacement efficiency in porous media and maintain the macro pressure gradient constant. The mechanism of viscoelastic fluid increases the displacement efficiency is: the elastic fearures of the fluid can change the flow line in pores, then increase the micro force on the protruding part of the residual oil blob and make it mobile. This mechanism can explain the phenomena of polymer flooding in micro and macro scale, it is helpful in designing, synthesizing and screening the chemical agents, in designing and optimizing the plans, and in developing the micro fluid mechanics of chemical flooding,...