ABSTRACT The molecular weight of polymer is important to the efficiency of polymer flooding. Polymer solutions with higher molecular weight have better absorption, better capitation, higher resistance and residual resistance coefficients. All these imply higher recovery factor. But for reserviors of middle and low permeability, higher injected molecular weight means more unswept pore volume, lower controlling degree and recovery factor. Therefore, both higher injected molecular weight and controlling degree must be fully considered, so as to obtain a higher recovery factor in a given block. On the basis of lab experiments and numerical simulations, two matching charts are formed. One is about the relationship between molecular weight and layers and the other is about controlling degree on recovery rate. They are used to optimize the molecular weight for polymer flooding. This new method is applied in the project for polymer flooding in secondary layers of Daqing oilfield and an enhanced recovery factor of more than 8% is obtained. INTRODUCTION In polymer flooding, the solution viscosity is enhanced and water phase permeability is reduced. Sweeping volume is enlarged to increase recovery factor. Polymer molecular weight is the main factor in polymer flooding. Higher molecular weight means lower water phase permeability and higher recovery rate. However, in the industrialization of polymer flooding in Daqing Oilfield, it is indicated molecular weight should be optimized in accordance with permeability. Ultrahigh molecular weight needs high injection pressure and causes blocks in the layers, which leads to poor development. To the question, a new method is put forward to optimize the molecular weight.

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