In production fluid snubbing systems of oil production wells equipped by electrical submersible pumps (ESP), non-associated gas usually leads to degradation of flow and pressure performance of the pump. Depending on the amount of non-associated gas in the production fluid flowing through the pump, ESP performance may vary from a slight deterioration to a complete blockage of the liquid phase due to the formation of gas slugs in the inter-blade channels of the pump impellers. The ratio of the volume flow rate of gas bypassing the ESP to the total volume flow rate of non-associated gas before the pump intake is defined as the natural separation factor, and its correct prediction is an important part of designing and optimizing any mechanized method of production fluid lifting. Methods proposed by P.D. Lyapkov, Serrano and Marquez (empirical and mechanistic ones) for calculating natural gas separation ratio in the borehole annular space when the pump takes the gas-liquid mixture above the perforated section of the production string were verified. Mechanistic Marquez method and empirical Serrano method showed calculation accuracy that is acceptable for solving engineering problems. P.D. Lyapkov method and empirical Marquez method showed a significant overestimation of the calculated data over the experimental results. The analytical method for calculating ratio of natural separation of gas was developed for the case of production fluid pumping from the level below the perforation section of the production string. The method involves assumptions (that have been confirmed by numerical experiment) that in the well perforation zone, the reduced fluid and gas velocities and static pressure gradient change linearly along the longitudinal coordinate. Comparison of calculated data obtained by Marquez mechanistic method and the developed analytical method under similar operating conditions showed that lowering the pump below the interval of well perforation provides a more than twofold increase in the natural gas separation ratio.
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