Recently, there are increasing interest on polymer flooding due to its high enhanced oil recovery (10%- 20% OOIP in field tests in Daqing) and much lower cost compared to surfactant-polymer (SP) flooding and alkali-surfactant-polymer (ASP) flooding. How to select polymer molecular weight (Mw) and concentration to improve mobility ratio and avoid blocking in less permeability strata is important but difficult issue. Maximum allowable polymer Mw and concentration for given permeabilities by using different techniques is studied. Maximum allowable polymer Mw and concentration for a given permeability in polymer flooding varies according to different methods. Even in Daqing Oilfield, where the largest polymer flooding industrial application in the world has been carried out since 1996, and the annual oil production from polymer flooding is more than 10 million tons, no agreement on the maximum allowable polymer Mw and concentration exists. Different criteria of resistance factors (Fr) and residual resistance factor (Frr) used in China are reviewed and compared. These criteria are generally based core flow tests with less or no consideration on heterogeneity and oil saturation effect. Recent studies shows that Frr is misleading and incorrect because of insufficient injection volumes. Too high viscosity and molecular may block low/less permeability layer. Polymer molecular is seen as a sphere and the average diameter ratio to average pore-throat should be higher than 5 or 10 to avoid blocking. Though the pore- throat of a core can be got from mercury intrusion method, the average diameter of hydrolyzed polymer (HPAM) is subjected to change with different method, not to say the scattered range of Mw. Fr and Frr was frequently used in determining whether a certain Mw or concentration polymer is blocked in a certain permeability. However, the critical values varies. Another theory connects the polymer Mw and concentration with allowable flow rate in field test.

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