In this paper, the visco-elastic characteristics of polyacrylamide solution have been studied experimentally. The mechanisms of the effect of polymer solution with visco-elastic characteristic on each type of residual oil after water flooding are analyzed and the mechanisms of polymer solution with visco-elastic characteristic increasing microscopic oil displacement efficiency are studied. By the analysis of microscopic experiments of percolating flow, corresponding relationships between the characteristic parameter describing the visco-elastic behavior of polymer solution and displacement efficiency of residual oil in "dead ends" are given. It is shown that the larger the visco-elastic behavior of polymer solution, the higher the displacement efficiency of residual oil in "dead ends".

A phenomenon that is not seen during water flooding is observed during polyacrylamide solution flooding in the microscopic experiment of percolating flow, that is, residual oil can be pulled into "oil threads" by the polyacrylamide solution. So a new type of oil flow channel, i.e. "oil thread" channel, can be formed and residual oil flows downstream through the "oil thread" channel. The probability of forming a steady "oil thread" flow channel in polymer flooding is also analyzed theoretically and proved.

The research indicates that the mechanism of polymer solution microscopically increasing oil recovery is due to the visco-elastic characteristic of the polymer solution, the sweeping force acting on the residual oil for the visco-elastic polymer solution is larger than that of water. The residual oil is not pushed out by the polymer solution but pulled out by the polymer solution. It is also found that every type of residual oil after water flooding can be decreased by visco-elastic polymer solution, and the larger the visco-elastic property, the stronger the capability of the polymer solution to "sweep out" the residual oil.

In brief, the results indicate that polymer flooding can not only macroscopically increase the sweep efficiency but also microscopically increase the oil displacement efficiency and a new type of flow channel, "oil thread", can also be formed.Besides new understanding on the mechanism of the flow of fluid through porous media and the displacement of residual oil, the above results should be beneficial to designing new EOR methods to further increase the recovery of oil fields.

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