Abstract

In oilfield applications, an Electrical Submersible Pump (ESP) comprised of centrifugal pump, seal, and motor is placed inside a well to provide energy to lift reservoir fluids from the formation to the surface when the reservoir’s natural lift energy is insufficient. The use of an ESP as an Electrical Submersible Power Recovery Turbine (ESPRT) is the focus of this paper. As fluid is injected from the well surface to the pump discharge, through the pump stages, and out the pump intake, the ESPRT shaft rotates in reverse (counterclockwise from an uphole vantage point), compared to an ESP shaft. The moving fluid in the stages transmits energy to the impellers, which turns the motor shaft; the induction motor thereby becomes a generator. We simulated and optimized the performance of a conventional ESP stage using Computational Fluid Dynamics (CFD) and single-phase water.

Before any laboratory or field testing, we simulated ESPRT performance at various speeds and flow rates to predict the performance of the turbine. The CFD prediction of the turbine follows the performance in the third quadrant of a typical four-quadrant pump curve. During the pre-field test CFD study, we did not attempt to predict the stage thrust. Later field trial results, however, showed the impellers operated in up thrust due to fluid flow through the turbine stages producing an upward reaction force on the impellers. We used the field test results to improve the CFD model and, afterwards, the stage design in order to reduce up thrust. The turbine, and pump upon which it is based, are designed to operate in down thrust.

We investigated the effects of changing the diameter of the impeller balance holes and impeller-to-diffuser clearance at the impeller’s skirt and balance ring. Through several CFD iterations, we identified the optimum balance hole and clearance sizes; decreasing the size of the former feature and increasing the latter feature moved the impeller into the down thrust regime. Neither change reduced the stage head and corresponding power output.

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