Mechanistic Modeling of Electrical Submersible Pump ESP Boosting Pressure Under Gassy Flow Conditions and Experimental Validation

Abstract

As an artificial lift method for high-flow-rate oil production, electrical submersible pumps’ (ESP) performance surfers from gas entrainment, a frequently encountered phenomenon in ESPs. When it occurs, ESPs can experience moderate or severe head degradation accompanied with production rate reduction, gas locking and flow instabilities. For the design and operation of an ESP-based production system, the accurate model is needed to predict ESP boosting pressure under gassy flow conditions. In this paper, a simplified mechanistic model is proposed to model gas-liquid flow inside a rotating ESP. The model not only maps flow patterns in ESPs but also captures the multiphase flow characteristics in terms of in-situ gas void fraction, boosting pressure, bubble size, etc. The experimental facility for testing ESP gas-liquid performance comprises of a 3″ stainless steel fully closed liquid flow loop and ½″ semi-open gas flow loop. A radial-type ESP with 14 stages, assembled in series, was horizontally mounted on the testing rig. Pressure ports were drilled at each stage to measure stage-by-stage pressure increment. The mixture of gas and liquid is separated in a horizontal separator, where excessive gas was vented and the liquid continues circulation. Experimental data were acquired with two types of tests (mapping tests and surging tests) to completely evaluate the pump behaviors at different operational conditions. The water/gas flow rates, ESP rotational speeds, intake pressure etc. were controlled in the experiments. The new model starts form from Euler equations, and introduces a best-match flowrate at which the flow direction at ESP impeller outlet matches the designed flow direction. The mismatch of velocity triangle in a rotating impeller results from the varying liquid flow rates. Losses due to flow direction change, friction, and leakage etc., were incorporated in the model. Based on the force balance on a stable gas bubble in a centrifugal flow field, the in-situ gas void fraction inside a rotating ESP impeller can be estimated, from which the gas-liquid mixture density is calculated. The predicted ESP boosting pressures match the corresponding experimental measurements with acceptable accuracy.