Summary Pumping systems reduce their efficiency and lifetime when working with free gas at the pump inlet. Downhole gas separators are usually installed upstream of the pump in oil production wells to avoid free gas, but they cannot handle high void fraction. Studies on the so-calledinverted-shroud gravitational separator indicate that it is possible to achieve gas separation efficiencies (GSEs) higher than 97%, even in the presence of very high void fraction. Gas separation inside this kind of separator works in two stages. The most critical stage is related to gas entrainment caused by a plunging free-surface flow in an annular channel formed by the production tubing and the separator itself, where strong kinetic energy dissipation takes place. Studies regarding this phenomenon in inverted-shroud separators are scanty. In this study, we present a semianalytical model supported by an empirical turbulent energy-dissipation correlation. The proposed correlation is based on dimensional analysis. New experimental data of the statistical distributions of bubble diameters during the gas entrainment process are presented. We measured bubbles diameters using a high-speed video camera and an edge detection algorithm. Data of both bubble diameters and GSE were used to adjust and validate the proposed model. The model establishes a transition boundary that separates a region of total gas separation (TGS) from a region of partial gas separation (PGS). In other words, it provides the operating envelope of the separator. The results can be used as a starting point for the design of inverted-shroud separators for field applications.
Summary Gas-liquid separation is a typical process in many applications. For instance, gas separation is critical for the proper operation of electrical submergible pumps in the oil and gas industry as the pumps’ performance and lifetime are severely reduced when working with high gas/oil ratios. Gas-liquid separators are installed in oil production wells to reduce the void fraction at the pump inlet. The inverted-shroud gravitational separator stands out due to its efficiencies higher than 97%. This separator performs the gas separation process in two stages. The first is a segregation process related to inversion from liquid to gas continuous flow, whereas the second stage is related to gas entrainment associated with kinetic-energy dissipation process. The latter is more complex to model in the vertical than in the inclined separator’s position. Previous studies revealed that the liquid flow rate and separator’s inclination are relevant parameters for the gas separation efficiency (GSE). However, studies regarding the effect of the liquid viscosity on GSE are scanty. We evaluate the influence of the liquid viscosity and separator’s inclination on the GSE of an inverted-shroud separator (IS-separator) with water-air and oil-air mixtures. Efficiency maps for each inclination and empirical correlations to predict the GSE in the vertical inclination are proposed. New experimental data collected for several liquid flow rates and separator inclinations are offered in this study as a starting point to develop universal GSE maps. Different gas separation phenomena are observed depending on the flow pattern at the inner annular channel (IAC) of the separator and its inclination. The experiments conducted with the water-air mixture indicated turbulent flow, while the oil-air mixture revealed laminar flow for both inclined and vertical positions. The results suggest that the greater the liquid viscosity, the higher the GSE. The efficiency maps indicate that it is possible to reach total gas separation (TGS) for many experimental conditions. In practice, our approach proposes an alternative technology where the variables that influence the production well’s dynamic fluid level are actively controlled. Therefore, the IS-separator can operate under operational field conditions similar to those tested on a laboratory scale. This fact makes the IS-separator a promising tool for industrial applications.
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