Abstract
A downhole separator is an equipment placed before an artificial lift pump, which increases the pump run time by separating the fluids and only sending the liquid to the pump. Understanding the fluid dynamics inside this apparatus entails an engineering challenge necessary to evaluate its efficiency. Computational Fluid Dynamics (CFD) simulations are applied in this study to characterize the performance of a centrifugal static downhole separator. This separator is suggested to be used upstream of the sucker rod pumps, performing best under 700 BPD. Transient multiphase simulations were run using ANSYS Fluent 2023 in a fluid domain using a mesh with 152,446 elements. The fluid domain simulates a static centrifugal separator. The mesh was selected after a grid independence study. The turbulence is modeled using the Realizable k-epsilon with an enhanced wall treatment model for the swirling motion inside the separator. The multiphase flow is modeled using a Euler-Euler approach with the Eulerian model and a particle size of 0.001 m. Different boundary conditions are used for the simulations with fixed-velocity inlets and fixed-pressure outlets.
The simulations were conducted with an inlet gas void fraction of 0.45 and varying outlet pressures. Cases were simulated with both water and oil as the liquid phase. Eddies or vortexes were formed in certain corners of the separator, prompting re-design opportunities to minimize turbulence inside the separator. Additionally, eddies were observed in the separator's top outlet as the fluids were expelled at high velocities from the helical section to the annulus. The gas-liquid flow was studied inside the helical paths through pressure, volume fraction, and velocity contours. In most cases, an average of 90% of the liquid was expelled through the bottom outlet, while 80% of the gas left the fluid domain through the top outlet, which displays good efficiency. The importance of optimizing the casing-tubing pressure difference was verified for the correct convergence of the cases and ideal separation. For example, with the optimum casing-tubing pressure difference control at a GVF of 0.45, 98.3% of the liquid left the domain through the bottom into the tubing, while 85% of the gas left through the top and to the casing.
Due to the conventionally high simulation times, CFD simulations of downhole separators are limited in the open literature. However, due to CFD's improved computation and modeling capabilities, it has recently gained popularity. This study’s simulations can provide invaluable insights into liquid-gas flow in downhole separators and help to optimize their designs.
