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

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.