Oil and gas producing rates have increased rapidly with the development of shale oil and fracturing technology. Besides, advances in horizontal wells have increased the slugging issue, especially in complex geometry wells. Therefore, Artificial lift systems, especially rod-pumps and electrical submersible pumps, always suffer from associated gas and require an economical way to avoid problems like gas lock, gas pound, gas interference and slugging. Among all kinds of the downhole separator, and the gravitational separator are the most economical devices, which can handle severe slugging problems.

The rule of thumb liquid maximum downward velocity for the gravitational separator is 0.6 in/s [13, 14]. However, the criterion needs to be improved by considering pressure, temperature and fluid properties. This article first uses CFD simulations to validate the critical liquid velocity and then obtains pressure field, velocity profile, gas distribution and sensitivity factors under complicated field situations. The results could be used to develop an empirical or even a mechanistic efficiency prediction model in the future.

In this paper, 2-D simulation is first utilized to study the critical separation velocity and effective parameters. Comparing with Stokes’ law, the simulation shows density, and surface tension have a strong effect of critical separation velocity, while viscosity has lower influence. Then extended 2-D simulations are conducted on different inner tube to annulus connection geometry, which shows a strong effect on separation efficiency. Later on, 3-D CFD simulations are generated based on a newly designed separator by TUALP and an existing design from the Don-Nan separator. Simulations are used to validate 2-D conclusion and illustrate the improvement of the new design.

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