Gas entrainment is frequently encountered in electrical submersible pump (ESP) as an artificial lift method for oil production. When this occurs, ESP suffers from moderate to serve performance degradation depending on inlet gas volumetric fraction (GVF). The resulted pressure surging may cause vibrations and short run-life of ESPs. For better design of ESP system, a mechanistic model is needed to accurately predict its performance with gas-liquid flow. Similar to modeling multiphase pipe flow, the flow pattern identification and classification inside a rotating ESP is thus of great importance. In this paper, we propose a new mechanistic model to map flow patterns in ESP operated under gassy flow conditions. The model is validated by comparing to experimental results with good agreement.

The experimental facility for testing ESP two-phase performance was designed and constructed. The main flow loop comprises a 3" stainless steel liquid flow loop and V" gas flow loop. A radial-type ESP with 14 stages assembled in series is horizontally mounted on a testing bench. Pressure ports were drilled at each stage to measure stage-by-stage pump pressure increment. The mixture of gas and liquid is separated in a horizontal separator, where excessive air is vented and liquid continues circulation. Experimental data are acquired with two types of tests (mapping and surging tests) to completely evaluate the pump behaviors at different operational conditions. The water/gas flow rates, ESP rotational speeds, intake pressure and surfactant concentrations are controlled in the experiments.

For two-phase flow, ESP pressure increment suffers from more severe degradation as gas flow rate increases. With the performance curves obtained in surging or mapping tests, ESP flow patterns including dispersed bubble flow, bubbly flow, intermittent flow and segregated flow can be identified. The pattern transition boundaries are mapped. Starting from the free body diagram on a stable single bubble, the transition boundaries of dispersed bubble flow to bubbly flow and bubbly flow to intermittent flow are formulated. Based on the combined momentum equation, the transition criterion of intermittent flow to segregated flow is derived. The flow pattern map calculated from the new mechanistic model agrees well with that detected from ESP performance curves.

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