Experimental Analysis of Swirl Tubes as Downhole Desander Device

Abstract

Abstract This work analyses the impact of the viscosity increase on the separation efficiency of swirl tubes based on experimental data. The data is reduced using dimensionless analysis. A functional relationship between the separation efficiency and the dimensionless groups is obtained. It discloses how the separation efficiency depends on the liquid viscosity, flow rate, solid size, and other geometrical parameters of the separator.It helps to push the design of these desander devices to new scenarios with heavy crudes or to oil-water mixtures with low water cuts prone to evolve to oil-water emulsions. Introduction Swirl tubes (1) are often used in oil production as a downhole desander device. Avoiding sand to reach the pumping system and other pipeline accessories they reduce the well workover and servicing activities. The swirl tubes are usually installed in vertical or near vertical well bores above the oil formation; figure 1 depicts the application scenario. The device consists of two concentric pipes which overlap each other forming a chicane. Within the annular space between the pipes there are stationaries vanes to impart a swirl component on the downward liquid stream. At the chicane the liquid stream is diverted upward toward the production line while the solids proceed downstream on the external pipe to the solids reservoir. These separation devices are best fit for continuous operation rather than to slug flow. They handle liquid flow rates from low to moderate range, 20 m[3]/d to 600 m[3]/d.The gas content is typically limited to 2% in volume at operation condition. Eventually higher gas contents can be handled with increase in size of the equipment. With reduced size and no moving parts these devices are often employed in mature fields with high water cut, typically 95% to 97%. Applications with heavy crudes or oil-water mixtures with lower water cuts are not successfully reported. The drop on the separation efficiency or even its complete failure is attributed to the increase of the liquid viscosity due to the formation of oil-water emulsion. The objective of this work is to experimentally study the impact of the liquid viscosity increase on the separation efficiency of a swirl tube under various liquid flow rates. Complementary, it is also investigated the impact on the separation efficiency due to the changes on the solids size, and other geometrical parameters. This paper is structured in three sections: the description of the experimental apparatus, the dimensionless analysis and the experimental results. It then follows the conclusions. Experimental set-up The experimental set-up is a closed loop where the liquid and the solid streams flows. It has a liquid and solids reservoirs, a pump, a solids feeder, the separator itself and a solids catcher. Figure 2 displays a photograph of the set-up. The liquid and the solids reservoirs are open to the atmosphere and have capacities of 1000 liters and 30 liters respectively. The liquid and the solids are simultaneously feed to the suction line of a progressive cavity pump located below the liquid reservoir. The liquid flow control is done by the pump speed driven by a frequency inverter. It allows continuous liquid flow rates within the range of 34 m[3]/d to 170 m[3]/d. Dry solids are feed through a single pitched helix driven by a variable speed motor. The solid stream attains volumetric concentrations within 0.1% to 1%. The solids, feed above the liquid reservoir, fall by gravity straight to the suction line at the bottom of the liquid reservoir. The solids and the liquid mix to each other along the suction line before reaching the pump inlet. The solid-liquid mixture is transported by a 37 mm (1 ½ inch) diameter line from the pump discharge to the separator inlet. The separator, installed at the vertical, receives the liquid with suspended solids by its top end. It has an external casing of transparent Plexiglas and internals of carbon steel. At the bottom end is the separated solids reservoir. The liquid and eventually non-separated solid exits the separator by a side line also at the top end. The return line discharges the liquid with suspended non-separated solids into a scream located above the liquid reservoir. The eventually non-separated solids are catch by the scream while the liquid returns to the reservoir.