Application of a Bubble Tracking Technique for Estimating Downhole Natural Separation Efficiency
Authors: Liu B. , Prado M. 
Published: March 2010

Abstract
Liquid velocity and pump intake pressure are known to have strong effects on the separation efficiency of gas and liquid which may impact the flow efficiency of downhole pumps. This paper describes a new model to evaluate the effectiveness of this separation. The model correctly predicts the decline in separation efficiency with higher liquid flowrates, which is shown to be in agreement with presented experimental data. A calibration procedure is conducted which shows a descending trend of bubble size when the pump intake pressure is increased. The new model shows good agreement with the experimental data in the bubble flow regime, since it handles the two-phase natural separation problem by combining the single-phase flow field solution with a bubble tracking method. It is the first CFD-based simplified model which incorporates the multiple effects that govern the natural gas-liquid separation process. If a correlation for bubble size is properly obtained, the model will cover the omprehensive effects of operational and geometric conditions and provide an accurate prediction of gas-liquid separation efficiency over the full spectrum of normal bottomhole pump operating conditions.
Finally, the single-phase flow field is solved in a 2D Cartesian coordinate system. The acceleration of the liquid phase, a nature of fluid behaviour in a cylindrical system, is not considered. This is a necessary improvement in the model to properly calculate the slip velocity in the radial direction.

Introduction
In pumping wells, the presence of free gas can cause several operational problems. The gas ingested by the pump can lead to poor pump performance and low overall efficiency of the entire pumping system. For any pumping lift system, estimating natural separation efficiency is a very important task whether the well is equipped with a bottomhole gas separator or not. A sketch of the natural separation process is shown in Figure 1. The movement of the free gas in the annulus space in front of the pump intake is the result of gravitational, inertial, and interfacial effects. The gas phase being lighter than the liquid tends to continue flowing vertically due to buoyancy effects. On the other hand, the gas phase is also being dragged due to interfacial forces and pressure gradient effects toward the pump intake by the liquid phase. As a consequence, part of the gas stream will eventually follow the liquid hase entering the pump and the rest of the gas will continue its vertical movement. The fraction of gas that is vented through the annulus is known as natural separation efficiency.
FIGURE 1: Sketch of the natural separation process. Available in Full Paper.
Lea(1) was the first investigator to conduct test programs to evaluate the effects of free gas on the performance of submersible pumps; his results included a graph of natural separation efficiency as a function of liquid flow rate and gas fraction at the pump intake. Alhanati(2) derived the first mechanistic model to estimate the natural separation efficiency for vertical wells.