Patent Publication Number: US-11028682-B1

Title: Eccentric pipe-in-pipe downhole gas separator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     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. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Related Art 
     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 &amp; 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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view of a preferred embodiment according to the present invention of an eccentric pipe-in-pipe gas separator apparatus located in a well downhole and connected to an electrical submersible pump (ESP); 
         FIG. 2  illustrates a bottom view of the eccentric pipe-in-pipe gas separator apparatus shown in  FIG. 1 ; 
         FIG. 3  illustrates a partial sectional, side view of the eccentric pipe-in-pipe gas separator apparatus shown in  FIG. 1  showing bottom openings on an outer tube; 
         FIG. 4  illustrates a cross-sectional view of an alternate preferred embodiment of the present invention, showing an eccentric pipe-in-pipe gas separator apparatus with a bottom ball valve to avoid sand or solids accumulation during shut-in; 
         FIG. 5  illustrates the eccentric pipe-in-pipe gas separator apparatus shown in  FIG. 4  with sand particles settling through the opening of the bottom ball valve during shut-in condition; and 
         FIG. 6  illustrates a partial sectional, side view of the eccentric pipe-in-pipe gas separator apparatus with the bottom ball valve shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
       FIG. 1  illustrates a simplified diagrammatic cross-sectional view of one preferred embodiment of a gas separator apparatus  10  in accordance with the present design. A cylindrical well casing  12  extends downhole from the surface (not shown) and terminates in an open end. The eccentric pipe-in-pipe gas separator apparatus  10  is installed vertically or installed with an inclination angle (not shown) close to the bottom hole  42  of a subterranean well. Dashed line  44  illustrates the center line of the well. 
     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  FIG. 2  taken along section line  2 - 2  of  FIG. 1 .  FIG. 2  illustrates a bottom view of the eccentric pipe-in-pipe gas separator apparatus  10  shown in  FIG. 1 . The crescent shaped conduit at the bottom is blocked by a sealing plate  40 . Accordingly, liquid in the crescent shaped conduit cannot return to the bottom hole. 
     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. 
       FIG. 3  illustrates a partial sectional, side view of the gas separator apparatus  10  showing the plurality of bottom openings  36  through the outer tube  26 . The bottom openings  36  permit liquid to pass from the annulus into the crescent shaped conduit. 
     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. 
       FIG. 4  illustrates a simplified diagrammatic view of another preferred embodiment of an apparatus  70  and method for downhole gas separation in accordance with the present invention for wells with sand or other solids production. A cylindrical well casing  12  extends downhole from the surface (not shown) and terminates in an open end. The separator apparatus  70  is installed vertically or installed with an inclination angle (not shown). A gas-liquid mixture from a reservoir flows to the bottom hole  42  of a subterranean well. Dashed line  80  illustrates the center line or axis of the well. 
     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  FIG. 5 . 
       FIG. 6  illustrates a partial sectional side view of the gas separator apparatus  70  with implementation of the bottom ball valve feature. The apparatus  70  is rotated 90° from views in  FIGS. 4 and 5 . The opening  78  through the outer tube  72  and inner tube is visible. The ball  46 , the cage  48  and the inner tube  74  are shown in dashed lines. 
     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  FIGS. 4 through 6  avoids sand or solids accumulation during shut-in condition. 
     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&#39;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.