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.

This paper attempts to establish the effects, if any, of wellbore inclination on the performance of downhole gas separators. Different downhole gas separator designs have been extensively studied in the laboratory and far reaching conclusions have been drawn from observations made in the laboratory for vertical wellbores. However, it is pertinent to note that most wellbores are not truly vertical and a survey which shows the deviation of the wellbore from a vertical reference point is often present in most wellbore configurations. Hence, this paper compares the performance of different downhole gas separator designs in a vertical wellbore set up with that of the same downhole gas separators operating under similar conditions but installed in an inclined wellbore set up. Technology has made it possible to drill horizontal wells which are fast becoming ubiquitous in most oilfields. With gravity forces dominating separation in static downhole gas separators; it is certainly logical to study the effect of wellbore inclination on the separation efficiency of downhole gas separators as transport properties of multi-phase fluids are affected by gravity. With the above mentioned arguments, it becomes imperative to study downhole gas separators under conditions which best simulate oilfield operations. The approach utilizes an instrumented full scale model of an inclined well-bore and separator constructed with clear acrylic pipe. The base case consists of the entire wellbore and separator set up installed vertically and subsequent experiments were performed with the wellbore inclination angle at 45o. An air and water mixture is injected through the well's perforations. The air and water flow rate measurements define a performance plot of each separator design with respect to wellbore inclination. Continuous flow conditions were applied in all tests Results obtained from laboratory annular bubble rise experiments show that in an inclined set up, the bubbles tend to move along the walls of tubing where the effect of drag is maximum. Results from the annular bubble rise experiments in an inclined wellbore equally show that the upwards motion of the smaller bubbles tend to be considerably reduced and sometimes halted by drag while on the contrary, the larger bubble (Taylor type bubbles) have higher mobility along the walls of the tubing. Also, results from the downhole gas separator performance tests conducted in a wellbore inclined at 45o indicate that at similar operating conditions, a gravity driven downhole gas separator installed in a wellbore inclined at 45o would perform better than the same gravity driven downhole gas separator in a vertical wellbore. While at similar operating conditions, a centrifugal force driven downhole gas separator installed in a vertical wellbore would perform better than the same centrifugal force driven downhole gas separator in a wellbore inclined at 45o. Introduction Laboratory testing of downhole Gas Separators (also known as gas anchors) have been ongoing at the University of Texas at Austin Petroleum Production Engineering Facility (UTAPPEF) since January, 2005. Numerous tests have been conducted on a full scale laboratory vertical well equipped with different downhole gas separator designs with a view of understanding the various parameters that influence the separation performance of downhole gas separators. Bohorquez R, Ananaba V, Alabi O et al (2009) have presented far reaching conclusions regarding factors that influence downhole gas separator performance. However, it is instructive to note that results presented by Bohorquez R, Ananaba V, Alabi O et al (2009) were conducted only on a truly vertical laboratory well. Furthermore, since it is common knowledge that no wellbore is truly vertical; the best laboratory simulation of actual field operations utilizing downhole gas separators is that laboratory experimental model that introduces a deviated wellbore setup.