Summary
Electrical submersible pump (ESP) installations are commonly used in the oil industry to aid fluid flow from the reservoir to the surface. As with any pump, the presence of free gas at the pump intake can adversely affect the operation of an ESP. One method of reducing the amount of free gas the pump has to process is to install a rotary gas separator.
In this work, the effect of viscosity on the separator's performance is investigated. New experimental data were gathered that covered a broad range of operational conditions in terms of pressures, liquid flow rates, gas-liquid ratios (GLRs), and rotational speeds. The experiments were conducted on a field-scale experimental facility with a commercially available separator. The working fluids were water, two mineral oils, and air. An existing mechanistic model (based on physical principles) predicting the bottomhole gas-separation efficiency in ESP installations was then evaluated with the data. Based on this investigation, improvements were implemented in the model to better capture the influence of viscosity on the downhole gas-separation process.
The results of the study indicate that there are two regions of separation efficiency with a pronounced transition between them: one region in which the rotary gas separator is very effective (separation efficiencies between 80 and 100%), and the other in which it is not effective at all (separation efficiencies between 30 and 55%). The transition location depends on the fluid physical properties, operational conditions, and geometry of the separator. The mechanistic model can predict this behavior and agrees well with the data that are obtained during this investigation. Fluid viscosity in the range of investigation (1 to 50 cp at 100°F) is found to have only little influence on gas-separation efficiency. This may indicate that the effects of turbulence at high rotational speeds dominate the behavior of flow inside the separator.
Introduction
When the pressure in an oil well is insufficient to push the liquid to the surface or to sustain the flow at an adequate rate, an artificial lift system must aid the natural flow. Such a method supplements the natural driving force of the reservoir and increases the production rate by reducing the flowing bottomhole pressure. From the available artificial lift methods, the oil industry commonly chooses electrical submersible pumps (ESP). A typical ESP installation is depicted in Fig. 11; operational details are described elsewhere.1–3
The desire for an extended period of maximum production without creating unfavorable operating conditions governs the design of an ESP installation. To avoid low efficiency and operational problems, one parameter that needs to be monitored is the presence of free gas at the pump intake. As with any pump, an ESP is only able to handle a certain percentage of free gas. Ref. 4 established a critical value of 10% free gas by volume that an ESP can handle without a problem. Beyond this value, action has to be taken to avoid shutdowns and improve ESP performance. Several options are available to improve ESP performance in gassy oil wells.5 One is to install a gas separation device ahead of the ESP, preferably a rotary gas separator (see Fig. 2).
The question remains of how effective the rotary gas separator is for different operational conditions. The literature gives an extensive performance overview for these separators under field conditions. 6–9 Some work has been done to study them in the laboratory4,10–13; however, no quantitative modeling effort whatsoever has been reported, except for one study.14 Most authors indicate that fluid physical properties influence the performance of the separator,4,6,15,16 but this issue has never been examined. In summary, until recently, bottomhole gas separation was poorly understood because no available and reliable quantitative model for gas-separation efficiency existed.
Ref. 14 for the first time presented a mathematical model, based on fundamental physical principles, to predict bottom-hole gas separation efficiency. A relationship was developed that predicts the efficiency of the separator as a function of the amount of produced gas and liquid, operational conditions, and downhole geometry. Experimental data taken on a field-scale test facility, with air and water as working fluids, matched the theoretical findings. The result of the study indicates that there are two distinct regions of separation efficiency divided by a sharp transition (see Fig. 3). In one region, the separation efficiencies are between 80 and 100%, while in the other, they are between 30 and 55%. The location of the transition region depends on fluid physical properties, operational conditions, and geometry. The only limitations to the applicability of the model cited in this study were the well inclination angle and liquid viscosity. The final result is a practical design tool in determining rotary gas separator efficiencies that are applicable to the majority of field conditions.
In light of the previous study and the available information, the goal of this study was to extend and improve the existing model, focusing on viscosity and turbulence effects. Specifically, the goals for this work include:Modification of an existing experimental facility to gather new data with different fluids.Obtain a full set of data using a fluid with a viscosity of at least one order of magnitude greater than water.Investigate the turbulent two-phase phenomena associated with the separation process.Study the effect of liquid viscosity on the separation process.Implement the experimental and theoretical findings into the model.
The Downhole Gas-Separation Process
To properly study the bottomhole gas separation process both experimentally and theoretically, one must understand the mechanisms involved. First, some of the produced gas will always be separated because of annulus ventilation. This phenomenon, commonly called annulus or natural gas separation, is caused by gravity. The remaining free gas and the liquid are sucked into the rotary gas separator (see Fig. 2). Upon entering the equipment, the fluid mixture is pressurized in the inducer section and separated inside the separation chamber by centrifugal forces and density differences. The centrifugal forces push denser liquid to the outside, while the less dense gas accumulates near the center. Finally, the crossover section feeds the pump with the heavier fluid and expels the lighter fluid back into the annulus.
