This paper provides insights on the design of downhole gas separators based on the laboratory study of various separator designs. A downhole gas separator, also known as a gas anchor, may be installed below the pump to separate free gas from the produced liquid. The free gas-produced downhole is usually separated through the casing-tubing annulus (the casing-tubing annulus acts as a natural gravity separator) while the liquid is produced through the tubing. However, inefficient gas anchor designs are widespread and an acceptable guide for their optimum design does not currently exist.Laboratory testing of downhole gas separators has been ongoing since January 2005 at The University of Texas at Austin Petroleum Production Engineering Facility (UTAPPEF) using an instrumented full-scale model of a wellbore and separator constructed with clear acrylic pipe to visualize the fluid mechanics of the separation process. An air and water mixture is injected through the well's perforations. The air and water flow rate measurements are used to measure and define a performance plot of each separator design. The separator designs tested differed in entry-port configuration, size of dip tube, and the relative position of the separator-fluid entry ports with respect to the well's perforations. Based on the results of the tests, a new separator design that includes the effect of centrifugal forces to separate the gas and liquid phases was developed.The results show that, for the conditions in the laboratory, 100% separation was achieved whenever the entry ports were located 1-to 2-feet (ft) below the bottom-most casing perforation thereby dispelling the predominant industry-held opinion that more distance is required betweent the separator-fluid entry ports and the bottom-most casing perforation. Similarly, laboratory results equally show that an optimum dip tube length of 5.5 ft is sufficient for the optimal separation of free gas from the produced liquid by the separator. This clearly runs contrary to the accepted industry practice, as many industry models employed for the estimation of dip-tube length were found to over-estimate the dip-tube length, and this subsequently results in the undesirable increased pressure drop across the separator. Lastly, the entry port geometry does not appear to have a significant impact on the separator performance as long as sufficient flow area is present. The efficiency of all gravity-driven separators was limited by the liquid velocity inside the separator annulus. When the liquid velocity inside the separator averaged approximately 6 in. per second or less, an almostto-complete gas separation was achieved. On the other hand, the centrifugal separator had a liquid capacity 70% greater than any of the gravity-driven, static-downhole gas separators.

The objective of this paper is to provide insight on the design of downhole gas separators based on laboratory studies of various designs. A downhole gas separator, also known as a gas anchor, may be installed below the pump to separate free gas from the produced liquid. The gas that is separated downhole is produced through the casing-tubing annulus and the liquid is produced through the tubing. Unfortunately inefficient gas anchors are common and an acceptable guide for their optimum design does not exist. Laboratory testing of downhole gas separators has been ongoing since January 2005 at the University of Texas at Austin using an instrumented full scale model of a wellbore and separator constructed with clear acrylic pipe to visualize the fluid mechanics of the separation process. An air and water mixture is injected through the well's perforations. The air and water flow rate measurements are used to measure and define a performance plot of each separator design. Both continuous and intermittent flow conditions were applied in the tests. The separator designs that were tested differ in entry port configuration, size of dip tube and relative position of the separator with respect to the well's perforations. Based on the results of the tests, a new separator design that includes the effect of centrifugal forces to separate the gas and liquid phases was developed. The results show that, for the conditions in the laboratory, 100% separation was achieved whenever the entry ports were located 1 to 2 feet below the bottom-most casing perforation. The entry port geometry does not appear to have a significant impact on the separator efficiency as long as sufficient flow area is present. The efficiency of all gravity driven separators was limited by the liquid velocity inside the separator annulus. When the liquid velocity inside the separator averaged approximately 6 in/s or less, an almost to complete gas separation was achieved. On the other hand, the centrifugal separator had a liquid capacity 70% greater than any of the gravity driven static downhole gas separators. Introduction Effectively separating free gas from liquid at the bottom of the well optimizes the performance of any pumping system. This insures that only liquid enters the pump and all the gas flows up the casing-tubing annulus. The most effective gas separator is the casing annulus, since its large volume provides the best opportunity for gas-liquid gravity separation to occur. When the pump intake is set below the perforations all the gas will be produced through the casing. Nevertheless, when the pump intake cannot be set below the perforations, a downhole gas separator (also known as a gas anchor) is installed below the pump. Gas anchors are usually built by operators with material that is readily available and with a simple construction, hence the name 'Poor-boy'. Many gas anchors constructed this way are inefficient, and unfortunately, there is no reliable methodology for building an effective gas anchor. This paper provides a better understanding of the variables that affect the performance of downhole gas separators based on the analysis of laboratory results. Different gas anchor designs were tested in a full scale laboratory well with a water and air mixture injected into its perforations. The injection rates ranged from approximately 150 BPD to 750 BPD for water and from 15 to 120 MMCFD for air. In addition, the flow rate of air that entered through the separator was measured. Using these water and air flow rate measurements, we obtained performance curves of each separator design. All the separators tested were six feet long. Three separator designs used gravity forces to separate the gas from the liquid and had a 5.5 ft long dip tube. The fourth design was a static (no rotating parts) centrifugal separator with a wire reinforced PVC hose as a spiraled dip tube inside the separator. The separator designs were tested for continuous and intermittent flow to simulate the effect of artificial lift selection (electrosubmersible and progressing cavity pumping versus sucker rod pumping). We studied the effect of variations on entry port configuration, dip tube diameter, and entry port position relative to the casing perforations.

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