An Improved Model for Predicting Separation Efficiency of a Rotary Gas Separator in ESP Systems

Abstract

Abstract An improved model is presented capable of predicting the separation efficiency of a rotary gas separator in ESP systems. The model incorporates a new two-phase flow inducer model capable of calculating the inducer head. The inducer head generated by a rotary gas separator has been identified as a key parameter that distinguishes between a separator's high and low efficiency regions. This information was previously determined empirically, but now can be calculated. The new model predicts more accurately the liquid rate at which a rotary gas separator should be installed. A comparison of the model's predictions with water-air and hydrocarbon-air experimental data indicates that the improved model performs better than earlier models. Introduction In designing ESP systems for gassy oil wells, a rotary gas separator (RGS) is still one of the most commonly used gas handling devices capable of minimizing the amount of free gas going into the ESP pump section (Fig. 1). The basic components of an RGS consist of intake ports, an inducer section, a centrifuge chamber as well as cross-over and outlet ports (Fig. 2). After entering the RGS through the intake ports, the gas and liquid phases are pressurized by the inducer section, and then separated in the centrifuge chamber by centrifugal force. Due to centrifugation, the liquid, as the heavier component, is pushed toward the outer wall while the gas, as the lighter component, accumulates at the center. The crossover re-directs the liquid phase back to the center thereby allowing it to continue its path toward the ESP pump section. Meanwhile, the gas is re-directed toward the outer wall and is expelled back into to the annulus through the outlet ports. An ESP system design for gassy oil wells should address two important questions:how much free gas can an RGS separate, andat what maximum liquid rate, if any, should the RGS be installed? The importance of answering these questions is two fold. First, the amount of remaining free gas exiting the RSG corresponds to the amount of free gas that the ESP must ultimately handle, which in turn affects the pump's performance. Second, ESP system mechanical integrity must be guaranteed. Since one of the main issues related to ESP operation is vibration, this sets an additional guideline of whether or not to install an RGS. In an attempt to answer the above questions, Alhanati1 developed a mechanistic model to predict the separation efficiency of an RGS. Based on his model, the separation process in an RGS system occurs in two distinct flow domains; i.e., within the tubing-casing annulus and within the centrifuge chamber inside the RGS (Fig. 3). The separation process will take place, as long as the inducer can provide enough head (?P+) to compensate for the pressure drop across the outlet ports (?P-). A typical separation efficiency curve, as predicted by the Alhanati1 model, shows three distinct efficiency regions (high, transition and low) as a function of the liquid flow rate (Fig. 4). The high efficiency region reflects the combined separation efficiency occurring within both the tubing-casing annulus and the centrifuge chamber. The transition region corresponds to the flow rates where the inducer is overloaded, and is no longer able to provide enough head to compensate for the pressure drop across the outlet ports. Under these conditions, some of the gas separated within the centrifuge chamber will not be expelled back into the casing. The low efficiency region corresponds to a situation where none of the free gas entering the RSG is expelled back into the casing. Under these conditions, the separation efficiency of the RGS is reduced to the natural separation efficiency occurring within the tubing-casing annulus. The Alhanati1 model has been verified by experimental data gathered by Sambangi2 and him for water-air systems, and by Lackner3 for hydrocarbon-air systems.