The present invention relates to a method and apparatus for gas separation from liquid at downhole of an oil and gas well, using an eccentric pipe-in-pipe configuration. Compared to previous downhole gas separators, the present invention has a simpler structure, less resistance to flow, better velocity field for gas-liquid separation and larger liquid retention volume in order to absorb long slugs of liquid. For wells with sand or solids production, an embodiment with a bottom ball valve allows sand to sink into the bottom hole at shut-in and stop the short circuit flow at normal operation.
Pumps, such as sucker rod pumps, progressive cavity pumps, and electrical submersible pumps (ESP) are used to maintain or increase oil production from subterranean reservoirs. For example, an electrical submersible pump includes a downhole pump and an electric motor to power the pump. The pumps force liquid from the reservoir through the well to the surface. Gases inevitably co-exist with oil during production. They are present in a downhole reservoir either as free gas or escape from liquid solution when pressure becomes lower. Gas involvement in the produced fluids can significantly reduce pump boosting pressure and efficiency. One rule of thumb is that an electrical submersible pump will not tolerate greater than 10 percent gas. When gas fraction reaches the critical value, gas lock condition will occur in an ESP and the pump does not provide any pressure increase. A solution to this gas degradation problem is to separate gas from liquid before the fluids enter the pump. The separated gas can be bypassed and produced through the casing-tubing annulus, or recombined with tubing flow through a gas lift valve at a higher location.
There are a number of existing downhole gas separator designs. In one example, Don-Nan Pump & Supply developed a concentric pipe-in-pipe gas separator which diverts the gas away from entering the pump intake. Through a ported coupling, gas-liquid flow is first directed into the annulus between the two tubes and exits from the top slots on the outer tube. Then, gas flows upward and liquid flows downward in the annulus between the separator and the well casing. At the bottom of the separator, liquid enters the inner tube of the separator through a port and flows toward the pump intake, free of gas. A drawback of this design is the restriction of the small ports to the flow.
For ESP applications, Brown, Wilson and James (U.S. Patent Publication No. 2009/0065202) proposed to use an ESP shroud for gas separation. A potential problem of this method is the entrainment of gas into the ESP shroud by liquid. Due to the small gap between the ESP shroud and the well casing, the local fluid velocity is relatively high. Gas may be dispersed in liquid as small bubbles. These small gas bubbles can be entrained by liquid into the ESP shroud at relatively high flow rate.
Centrifugal separators have also been proposed and used to condition the ESP intake flow. However, this kind of separator consumes additional power and increases pump failure probability. Most of the previous downhole gas separators cannot handle low frequency slugging due to insufficient volume of liquid and counter-current flow of gas and liquid.
It is known that cross-sectional area downhole is at a premium. It is desirable to develop a downhole gas separator that will maximize volume of through-put while accommodating various fractions of gas.
In some wells utilizing pumps, there are alternate periods where the pump is on and liquid is drawn to the surface by force of the pump and where the pump is off and sand and solids settle to the bottom of the well.
It is accordingly also desirable to develop a downhole gas separator that will accommodate sand or solids while efficiently separating gas from liquids.
The present invention uses an eccentric pipe-in-pipe configuration for downhole gas separation. In one preferred embodiment, a gas-liquid mixture flows from the well into an opening in an inner tube from the bottom. The mixture exits at the top from the inner tube across and through an outer tube wall. Then, separation occurs in the annulus between the outer tube and well casing with gas rising upward and liquid flowing downward.
At the bottom of the annulus, liquid enters a conduit having a crescent shaped cross-section formed between the two tubes through openings in the outer tube. Substantially free of gas, the liquid flows upward in the crescent shaped conduit.
A top of the gas separator apparatus is connected to a pump inlet, such as an ESP motor shroud. The single-phase liquid from the separator flows through an annulus between the ESP motor and the shroud. Efficient heat transfer between the ESP motor and the flowing liquid helps maintain the ESP motor temperature and prolong the pump run life.
The cross sectional area of the inner tube is slightly larger than the crescent shaped area between the inner and outer tubes considering the separated gas flow rate. A sufficiently large cross sectional annulus area between the separator apparatus and the well casing is important since it determines the largest bubble the liquid can entrain at a given flow rate. When sufficient separator apparatus length is used, the annulus volume between the separator apparatus and the well casing can eliminate the gas and liquid fluctuations due to hydrodynamic or low frequency long slugs (e.g. from a horizontal well). The liquid flow rate through the ESP needs to be controlled either by ESP rotation speed or a control valve based on the liquid level in the annulus monitored with liquid level sensors or pressure transmitters.
For wells with sand production, an alternate preferred embodiment of the gas separator of the present invention with a different flow path is used. The gas-liquid mixture from the bottom hole flows into a crescent shaped conduit between an outer tube and an inner tube of the apparatus from the bottom hole. It exits at the top from the crescent shaped conduit across and through openings on the outer tube wall. Then, separation occurs in the annulus between the separator and well casing, with gas rising upward and liquid flowing downward.
At or near the bottom of the annulus, liquid enters the inner tube through an opening across the outer tube. Substantially free of gas, liquid flows upward into the pump intake above.
A ball valve is installed at the bottom of the inner tube. During normal operation, the ball valve is closed by the suction force of the pump with the ball in an upper position. At shut-in condition, a ball in a ball valve cage will fall by gravity to its lower position and the valve will open. This allows sand particles to sink into the bottom hole through the opening. When production resumes, the ball valve will automatically close.
The embodiments discussed herein are merely illustrative of specific manners in which to make and use the invention and are not to be interpreted as limiting the scope of the instant invention.
The gas separator apparatus 10 is suspended from equipment installed downhole. In the present case, the apparatus 10 is suspended from a shroud of an ESP pump or motor 14 by a connection 20. It will be appreciated that the separator apparatus 10 may be suspended from other equipment within the spirit and scope of the invention.
The apparatus 10 includes an inner cylindrical tube 30 having an outer diameter smaller than an inner diameter of an outer tube 26. The outer tube 26 is generally concentric with the axis 44 of the well and, in particular, with the casing of the well. The inner tube 30 is eccentric from the outer tube and is aligned or set against an inner wall of the outer tube 26.
A gas-liquid mixture from a reservoir flows to the bottom hole 42. Then, the gas-liquid mixture flows into the inner cylindrical tube 30 of the separator apparatus 10 as shown by arrows 60. The gas-liquid mixture then moves upward and exits through the inner tube and outer tube at the outlet 24. In the annulus formed by the well casing 12 and the outer tube 26, gas and liquid separate by gravity. Gas flows upward by floatation as shown by arrows 22 and liquid flows downward by gravity as shown by arrows 28. The cross sectional area of the annulus between the well casing 12 and separator apparatus outer tube 26 should be sufficiently large so that the liquid downward flow velocity will be low and only very small gas bubbles can be entrained by the liquid.
At the bottom of the gas separator apparatus 10, the annulus between the well casing 12 and the outer tube 26 is plugged by a packer 38. The packer 38 creates a fluid tight seal and centers the apparatus 10.
Liquid flows from the annulus into an area having a crescent shaped cross-section formed by the separator outer tube 26 and the separator inner tube 30 through a plurality of openings 36 in the separator outer tube 26 near the bottom of the apparatus 10. The crescent shaped cross-section may be best seen in
Liquid flows upward in the crescent shaped conduit and continues into a shroud 16 for an ESP motor 14 through a connection 20. The liquid continues upward toward the surface as shown by arrows 18. A further benefit of the present invention may be seen. Heat transfer between the ESP motor and the flowing liquid helps maintain the ESP motor temperature.
The cross-sectional area of the inner tube 30 is slightly larger than the cross-sectional crescent shaped area between the inner and outer tubes considering the separated gas flow rate. In addition, a sufficiently large cross-sectional annulus area between the separator apparatus and well casing is important.
The separator apparatus 70 is suspended from equipment installed downhole. In the present embodiment, the apparatus is suspended from a shroud 16 of an ESP motor 14 through a connection 20. It will be appreciated that the separator apparatus 70 may be suspended from other equipment within the spirit and scope of the invention.
The gas separator apparatus 70 includes an outer tube 72 having an inner diameter. The outer tube 72 is generally concentric with the center line axis 80 of the well. An inner tube 74 having an outer diameter smaller than an inner diameter of the outer tube 72 is within the outer tube 72 and eccentric therefrom. The inner tube 74 is set against or aligned against an inner wall of the outer tube 72.
The gas-liquid mixture flows from the bottom hole 42 into a crescent shaped area formed by the separator outer tube 72 and the separator inner tube 74, and moves upward and exits at an outlet or outlets 76 passing through the outer tube 72. In the annulus formed by the well casing 12 and the gas separator outer tube 72, gas and liquid separate by gravity. Gas flows upward by floatation as shown by arrows 22 and liquid flows downward by gravity as shown by arrows 28.
The cross sectional area of the annulus between the well casing 12 and separator outer tube 72 is established sufficiently large so that the liquid downward flow velocity will be low and only very small gas bubbles can be entrained by the liquid. At the bottom of the separator apparatus 70, the annulus is plugged by a packer 38. The packer 38 creates a fluid tight seal and centers the apparatus 10.
Liquid flows from the annulus into the inner tube 74 of the gas separator apparatus through an opening or openings 78 through the outer tube 72 and the inner tube 74 near the bottom.
A ball valve has a ball 46 within a cage 48 located between the inner tube 74 and the well. The ball 46 is moveable vertically a small distance within the cage 48. During operation of the pump during oil production, the ball 46 is drawn upward in the cage 48 by force of the pump and motor. The bottom of the inner tube 74 is blocked by the ball 46 during normal operation of the pump so that fluid in the bottom hole 42 is prevented from passing into the inner tube.
Liquid flows upward inside the inner tube 74 and continues into a shroud 16 or inlet of an ESP or other type of pump through the tubing 74. During shut-in condition when the pump and/or motor is off, the ball 46 will fall by gravity to the bottom of the cage 48 and leave an opening between the ball 46 and its cage seat. Sand or solid particles 50 can sink through the opening into the bottom hole, as shown in
The cross-sectional area of the inner tube 74 is slightly larger than the cross-sectional crescent shaped area between the inner and outer tubes. The embodiment of the gas separator apparatus 70 shown in
While the invention has been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the invention's construction and the arrangement of its components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification.
Whereas, the present invention has been described in relation to the drawings attached hereto, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the spirit and scope of this invention.
This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 62/250,335, filed Nov. 3, 2015, which is herein incorporated in its entirety by reference.
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Entry |
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Jyothi Swaroop Samayamantula, An Innovative Design for Downhole Gas Separation, taken from http://www.don-nan.net/papers/gas_separation.pdf on Oct. 16, 2015. |
Number | Date | Country | |
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62250335 | Nov 2015 | US |