For many highly watered oilfields at late stages with reservoir pressure greater than the saturation pressure the measured gas-oil ratio is often considerably exceeds its origin value. During the interaction of oil and water in the reservoir the partial transition of light components occurs from oil to water and their carrying out to the surface dissolved in passing extracted water is done. Herewith the properties of the reservoir oil because of the change of component composition become different with time from its properties at the beginning of the field development. In the paper the estimation of influence of change of reservoir oil viscosity, which is caused by contacting with injected water, on the recovery factor is provided. On the base of Backley - Leverett equation the parametric analysis is provided of influence of type of water injection on the oil recovery factor for different parameters of reservoir and phase permeability. It is shown that the increase of oil viscosity caused by change of it component composition and physical and chemical properties during the contact with injected water leads to decrease of oil recovery factor down to 2% from the proposed value, when the variation of oil viscosity is neglected. By the example of one of the oilfields form Western Siberia, which is operated at late stage, the estimation of influence of change of reservoir oil viscosity on the recovery factor and accumulated oil production during remaining period of time is done. It is obtained that under conditions of change of reservoir oil viscosity on reaching the water cut up to 95% the proposed oil recovery factor will decrease by 6% and the proposed accumulated oil production will decrease by 10%. The obtained results indicate about importance of taking into account the change of oil viscosity during contact with water when planning the basic indicators of oil recovery.
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