Effect of Viscosity on Downhole Gas Separation in a Rotary Gas Separator

Abstract

Abstract Electrical submersible pump (ESP) installations are commonly used in the oil industry to aid the flow of fluids 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 that the pump has to process is to install a rotary gas separator. In this work, the effect of viscosity on the performance of the separator is investigated. New experimental data were gathered, covering a broad range of operational conditions in terms of pressures, liquid flow rates, gas-liquid ratios and rotational speeds. The experiments were conducted on a field scale experimental facility using a commercially available separator. The working fluids were water, two mineral oils and air. An existing mechanistic model (based on physical principles) that predicts 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 that have 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 location of transition 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 cp to 50 cp at 100 ºF), is found to have only little influence on gas separation efficiency. This may indicate that the effects of turbulence, at high rotational speed, dominate the behavior of the flow inside the separator.