Downhole Gas Separators-A Laboratory and Field Study
Authors: McCoy J.M. , Patterson J. , Podio A.L. 
Published: March 2010

Downhole gas separators are often the most inefficient part of a sucker rod pump system. This paper presents laboratory data on the performance of five different gas separator designs. Only continuous flow was studied. Field data is presented on one of the designs. The field data indicates that success or failure of the gas separator is dependent upon the fluids and wellbore pressures, as well as the mechanical design of the gas separator. Successful and unsuccessful examples of gas separator performance in the field are shown along with field fluid data properties. While the study is not complete, this is the first of hopefully several papers that will present the results of this investigation.

Introduction

Patterson and Leonard(1) studied different downhole gas sep-aration designs for coalbed methane operations in Wyoming. In these designs, the inlet to the gas separators were smaller than normally used and, along with some baffles (thought to allow gas to vent from inside the gas separator), obtained good gas separation in the field installation. It was these installations which prompted the laboratory study of the gas separator geometry to understand if the ‘rules-of-thumb’ used by the industry for gas separator design were valid.

The objectives of this paper are to give a clearer insight into the mechanisms of gas interference in pumping wells and to present the results of recent laboratory and field studies on the flow characteristics and performance of some downhole gas separators.

In these applications, the separation of gas from liquid is achieved through gravity separation without the introduction of other mechanisms (centrifugal forces, nozzles, etc.). Thus, the difference in density between the gas and liquid is the main driving force for separation. This implies that forces that oppose the effect of gravity, such as viscous drag caused by high fluid velocity and turbulence, will be detrimental to the separation process. Thus, high velocity of liquid or gas should be avoided, if possible.

The Pumping Wellbore as a Gas-Liquid Separator

The preferred pumping installation for maximum pump efficiency requires installing the pump intake below the lowest point of fluid entry into the wellbore and requires having an open casing-tubing annulus from the bottom to the wellhead.

Gas and liquid enter the wellbore through the perforations and liquid flows to the bottom of the well under the action of gravity.

The lighter gas bubbles rise through the liquid, forming a gaseous liquid column from the bottom of the perforated interval to the fluid level. Then, gas flows through the casing-tubing annulus to the wellhead where it exits to the flow line. Practically 100% liquid accumulates at the bottom of the well and enters the pump intake to be discharged by the pump into the tubing.

In a large number of wells, it is not possible to install the pump intake below the lowermost fluid entry point in the wellbore. Typically, this is caused by an insufficient rathole, the presence of liners or gravel packs, concerns regarding sanding up and sticking the pump or just the operator's preference.