Field Tests of a Decentralized Downhole Gas Separator
Authors: McCoy J.N. , Podio A.L. , Woods Mike , Guillory C. 
Published: December 2007

A decentralized downhole gas separator has been designed and successfully field tested in numerous beam-pumped wells which were subject to severe gas interference. This paper presents a detailed description of the new gas separator which improves the performance of beam pumped wells. The decentralized gas separator places the gas separator housing against the casing wall by use of a bow spring on one side of the separator. The gas separator has ports on the opposite side of the spring, which are positioned against the wall where the liquid concentration is greater than in the larger section of the annulus where the flow is predominantly gas. A significant number of wells suffer from gas interference. If the pump is positioned above the formation, liquid separation from gas is very difficult especially at high annular gas flow rates. Significant increases in oil production are possible with use of the decentralized gas separator when gas interference hinders efficient pump operation.

Examples of dynamometer cards and acoustic liquid level tests are given prior and after wells were equipped with the new decentralized gas separator. More than 50 wells are analyzed. The extremely high success ratio of the decentralized gas separator indicates a very good probability of improving oil production when gas interference is occurring in the downhole pump.

Introduction

Since gas interference remains one of the main sources of inefficiency of pumping systems an improved downhole gas separator was designed to maximize the liquid flow into the pump and minimize the amount of gas by using a decentralized geometry and careful consideration of flow areas and hydrodynamic flow patterns. The new separator has been tested extensively in the field and to date at least 200 units have been built and have been installed in wells ranging from Canada to South America. This paper summarizes the results of some of these field tests.

Separator Design

Visual studies of multi-phase flow in clear tubes have shown that large gas bubbles normally exist in gaseous liquid columns, such as gaseous liquid columns in the casing annulus of an oil well, where the liquid remains in the annulus but gas flows through the liquid and is produced at the surface. When the tubing is concentric with the casing, the gas distribution will be uniform throughout the annular area. When the tubing contacts one side of the production casing, so as to form an eccentric annulus, gas will flow preferentially up the larger side of the annulus. In this case the liquid concentration will be significantly higher in the narrow portion of the flow stream where the two tubes are nearly touching than in the annulus of a concentric casing-tubing arrangement carrying the same gas and liquid flow. For the eccentric geometry a continuous circulation of fluid takes place with liquid being entrained from the narrow side into the high gas concentration and high fluid velocity wide side, then carried upwards some distance and then as the gas slips through, the liquid is shed back to the narrow side. The liquid near the narrow side of the annulus then moves downwards under its own hydrostatic and eventually is re-entrained into the wide side of the annulus. (See Figure 1).

A similar condition will occur in a pumping well if the gas separator is pushed against the casing wall using a decentralizer. The decentralizing bow-spring forces the gas separator against the casing wall so that an enlarged area exists on one side of the gas separator allowing gas flow upward. The fluid ports (A and B) are located diametrically opposed to the decentralizer placing them in the region of the wellbore which has the highest concentration of liquid. Thus the fluid entering into the separator's quieting chamber will have a much lower concentration of gas than if the separator were concentric with the casing.

Due to the pumping action, fluid is removed from the lower portion of the gas separator through the dip tube. Fluid (both liquid and gas) will flow into the gas separator annulus through the ports that are positioned near the casing wall. P. 257