The viscoelastic behavior of HPAM solutions and its effects on oil recovery were studied by means of core flowing tests and mathematical modelling. The mechanism of improving displacement efficiency due to viscoelastic effects was explained. This paper first presents a method to determine the first critical velocity (or critical viscoelastic velocity), Vce, at which the straining flow occurs. The experimental results indicated that the Vce increases with increasing permeability of porous media. Vce less than 1 m/d was observed.

There is a second critical velocity (or critical transit velocity), Vcr, at which the rheological behavior of HPAM solution transits from pseudoplastic to dilatant behavior. Vcr can be also determined through experiments. Vcr ranges from 1.74 to 3.30 m/d and also increases with increasing permeability of cores in our experiments. Within the range from Vce to Vcr, elongational flow occurs although the HPAM solution still exhibits pseudoplastic behavior. Beyond Vcr, the strain viscosity increases rapidly with velocity and straining flow governs the flow behavior. The results from core flowing tests showed that the displacement efficiency evidently increases with the velocity beyond Vce and reaches a maximum when the velocity approaches Vcr. The model index B (renamed Han’s index in this paper) describing the viscoelasticity of HPAM in porous media [1] has been proved to be the relaxation time for HPAM molecules[2]. The effect of property parameters of both porous media and polymer solution on Han’s index, H, was investigated by core flowing tests with artificial cores. The property parameters include injection rate, salinity, hydrolysation degree of HPAM and core permeability. A comprehensive Han’s index, He, was obtained as a function of these parameters by using multi-variable regression. Polymer flow tests in cores showed that the relative increment of displacement efficiency varies with He as a normal distribution function, i. e., there exists a optimum He which leads to a maximum of the oil recovery.

To investigate the influence of the rheological behaviour of HPAM on the prediction of on recovery in polymer flooding, one dimensional numerical simulation was conducted.

It is believed that viscoelastic behaviour will inevitably occur during HPAM solution flushed through oil formation and it plays an important role in oil recovery. Viscoelastic property of polymers should be considered as a screening criterion and it is necessary to take into account the viscoelastic effect in polymer flood numerical simulation.

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