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BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to gas extraction in oilfield applications and, in particular, to an improved system, method and apparatus for a gas extraction device for down hole oilfield applications. 
     2. Description of the Related Art 
     The separation of gasses and liquids carried out in a well bore is common. The separation of gasses and liquids at the seabed as part of a subsea oilfield exploitation is becoming increasingly common. Separating the gas and using a high head centrifugal pump to pump the liquids vastly improves the project economics (e.g., asset net present value and recovery factor). The separation of the gas from the liquid also results in improved flow assurance. Moreover, pumping fluids that contain excessive amounts of gas can cause gas lock in a pump or can cause a pump to overheat and fail prematurely. 
     Currently, in a well bore, the accepted method of controlling the gas-liquid interface level is to manually control the amount of fluid produced by artificial lift, such as a down hole electric submersible pump (ESP). Generally, the ESP is installed and the production rate is set. If the pump encounters a gas lock condition, it is shut down to allow the well to recover, restarted and a new lower production rate is manually set. This is continued until the ESP is operating in a continuous and stable manner. Conversely, if the pump does not gas lock when the ESP is first installed and is operating in a stable manner, the production rate is manually increased in steps until a gas lock condition occurs. After recovery, the production rate is then reduced to the point of the last stable operation. The object is to produce the maximum fluid available from the well with the pumping equipment. 
     In such ESP applications, the liquid travels through production tubing to the surface. Excess gas gathers at the top of the well within the casing and is typically vented at the well head to a separate gathering system. Alternatively, the gas is connected into the liquid production line down stream of the wellhead. In some cases, however, production would benefit from gas entering the liquid production stream down hole within the well casing. 
     SUMMARY OF THE INVENTION 
     Embodiments of a system, method, and apparatus for a gas extraction device for down hole oilfield applications are disclosed. The invention utilizes the flow of the liquid, created by an artificial lift device, to produce conditions by which the gas is drawn into tubing. A venturi creates a low pressure area through a constricted section of the tubing. The pressure within the throat of the venturi drops lower than the pressure in the casing. The device may be axially adjusted to allow communication ports to a lower pressure area of the casing annulus. The lower pressure in the venturi draws the gas into the liquid stream and into the production tubing above the device. The gas is then transferred through the well head within the liquid stream. 
     The venturi may be located near the pump exit; however it may not be able to generate a pressure that is sufficiently low enough to entrain the gas. As the fluid is withdrawn from the well, the fluid level drops. This lowering of the gas-liquid interface in the well increases the gas pressure in the upper regions of the casing. The venturi may be located high enough in the well so that the pressure balances allow the venturi to evacuate the gas from the casing annulus. As the gas enters the flowing liquid in the tubing it lightens the liquid and begins to add buoyancy to the vertical flow forces. This reduces the burden on the pump and increases its performance. The venturi may be located low enough, however, to take advantage of the lift assist from the gas and high enough to allow the pressure balance in the venturi to create the evacuation of the gas from the casing. The invention may be used in sub-surface well applications, and may be especially useful in subsea applications where a second line to remove the gas from the bottom hole pumping system or well is not an option. 
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the present invention are attained and can be understood in more detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings. However, the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic diagram of one embodiment of an electrical submersible pump (ESP) assembly for a production system and is constructed in accordance with the invention; 
         FIGS. 2 and 3  are schematic diagrams of an embodiment of a gas extracting device for the ESP assembly of  FIG. 1 , illustrating lower positions of an axially movable sleeve, and are constructed in accordance with the invention; 
         FIGS. 4 and 5  are schematic diagrams of the gas extracting device of  FIGS. 2 and 3 , illustrating upper positions of the axially movable sleeve, and are constructed in accordance with the invention; 
         FIG. 6  is a side view of one embodiment of the axially movable sleeve for the gas extracting device of  FIGS. 2-5  and is constructed in accordance with the invention; 
         FIG. 7  is a schematic diagram of another embodiment of the gas extracting device of  FIGS. 2-5 , illustrating an upper position for the axially movable sleeve, and is constructed in accordance with the invention; 
         FIG. 8  is a schematic diagrams of the gas extracting device of  FIG. 7 , illustrating another, slightly lower position for the axially movable sleeve, and is constructed in accordance with the invention; and 
         FIG. 9  is a schematic diagram of still another embodiment of the gas extracting device of  FIGS. 2-5  and is constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-9 , embodiments of a system, method and apparatus for a gas extraction device for down hole oilfield applications are disclosed. The system may include an electrical submersible pump (ESP) assembly having, e.g., a centrifugal pump, a sucker rod pump, a hydraulic pump, or any kind of pump as well as an ESP. The ESP pumps a gassy fluid in a well or production vessel with the intake flow to the pump routed in such a way that the gas substantially separates from the oil and is not drawn into the pump. Systems are provided to remove the gas to gas processing facilities located at the surface. 
     In a basic embodiment ( FIG. 1 ), the invention comprises a system for controlling a pump  11  in a well or other type of gas-oil separation and production environment, such as a production vessel  23  (e.g., a caisson, canned pump assembly, booster pump assembly, etc.). The production vessel  23  is the sealed vessel that contains the oil and gas to be pumped to the surface. The system uses the pump  11  to retrieve fluid  15  from the production vessel  23 . 
     One or more instruments or sensors  17  are located adjacent the pump  11  to obtain physical property measurements of the fluid  15 . Physical properties such as density, capacitance, etc., that are influenced by the presence of a gas are suitable for these applications. For example, the rotational speed of a turbine flow meter is directly proportional to the gas content in fluid. Although the sensor  17  is shown located at the fluid discharge area (e.g., after gas separation) of the pump, it also may be located at a fluid intake area relative to the pump, or at other positions along the assembly. In addition, the sensor  17  may comprise a plurality of sensors located at different positions along the fluid flow path relative to the assembly. 
     In one embodiment, density measurements may be used as an indicator of the relative proportion of gas  19  in the fluid  15 . A controller  21  coupled to the sensor  17  controls the components of the system. The production values of the system may be modified responsive to the desired properties of the fluids in order to maintain a desired constant or set point level of gas within the production vessel  23 . The desired level of gas within the vessel may be selected based on many criteria and depends on the application. For example, in one embodiment, the set point level may be established at or near the pump intake to provide the maximum gas volume and the maximum gas liquid separation prior to producing the fluid to the surface. 
     As shown in the embodiment of  FIG. 1 , the invention is employed in an oil and gas production system comprising a plurality of wells  31  for producing oil and gas. The production vessel  23  may be provided with an inlet pipe  32  for fluid communication with the plurality of wells  31 . The production vessel  23  contains a volume of oil  15  and a volume of gas  19  produced by the plurality of wells. The production vessel  23  may be provided with a gas port  33  for releasing the gas  19 . 
     The ESP assembly  35  is installed in the production vessel  23  for pumping oil  15  out of the production vessel  23 . The lower end of the ESP assembly  35  is submerged beneath an interface  41  between the volumes of oil  15  and gas  19 . In the embodiment shown, the ESP assembly  35  comprises a motor  43 , a seal section  45  and the pump  11 , and may include a gas separator. The sensor  17  measures a property (e.g., density) of the fluid processed by the ESP assembly  35 . The controller  21  controls the flow rate and other variables of pump  11  in response to the sensor  17 . 
     As described herein, the flow rate of the pump  11  may be modified responsive to the fluid density measurements to maintain a desired level  41  of gas within the production vessel  23 . The fluid density indicates a relative proportion of gas in the oil. The sensor  17  may be located at the fluid discharge or fluid intake areas relative to the pump. In alternate embodiments, the sensor  17  may comprise multiple sensors located at different positions along a fluid flow path relative to the ESP assembly  35 . Such sensors may sense or measure more than one property of the fluid. The automated flow rate control of the pump may be manipulated by, e.g., modifying the speed of the pump. Alternatively, a choke (e.g., discharge choke valve) may be provided in the fluid flow path downstream from the pump to regulate the flow rate of fluid through the pump. 
     Referring now to  FIGS. 2-6 , the fluid production system also comprises a gas extraction device  51  (or “jet pump”) that is located above the ESP assembly  35 . The gas extraction device  51  is adjacent or may be mounted directly to the ESP assembly  35 . The gas extraction device has a body  53  with an axis  55  and an orifice  57  on a lower end for receiving fluids  59  from the ESP assembly  35 . The body  53  also has a radial port  61  formed therethrough for allowing gases to enter an axial passage  63  that extends completely through the body  53 . 
     A sleeve  65  is coaxially mounted within the axial passage  63  of the body  53  for selective axial movement within the body. The sleeve  65  has an axial aperture  67  extending completely therethrough, and a radial aperture  69  in communication with the axial aperture  67 . The sleeve  65  has a lower position (e.g., the lowest position is shown in  FIG. 2 ) wherein fluids flow through the axial aperture  67  and the radial aperture  69  into the axial aperture  67 . The sleeve  65  seals against the radial port  61  in the body  53  in the lower positions. An intermediate lower position is shown in  FIG. 3 . 
       FIGS. 4 and 5  depict upper positions (e.g., the uppermost position is shown in  FIG. 6 ) for the sleeve  65 . In the upper positions, the fluids  59  flow only through the axial aperture  57 . The radial aperture  69  in the sleeve  65  at least partially aligns (see, e.g.,  FIG. 4 ) with the radial port  61  in the body  53  to draw gases into the fluids flowing through the axial aperture  67 . 
     In other embodiments, the sleeve  65  may further comprise a radial opening  71  in communication with the axial aperture  67  in the sleeve for permitting additional fluids to flow into the sleeve when the sleeve is in the lower positions ( FIGS. 2 and 3 ). The radial opening  71  is sealed between the sleeve  65  and the body  53  by seal  81  when the sleeve  65  is in the upper positions, as shown in  FIGS. 4 and 5 .  FIG. 6  illustrates that the radial opening  71  may be provided with an oval shape.  FIG. 6  also shows that the radial aperture  69  may be provided as an elongated slot formed in the sleeve  65  and is located below the radial opening  71 . 
     The axial aperture  67  in the sleeve  65  may comprise a venturi having a throat  73  ( FIG. 2 ) located between opposed divergent channels  75 . The radial aperture  69  in the sleeve  65  intersects the throat  73  as shown. In addition, the radial opening  71  formed in the sleeve  65  and in communication with the axial aperture  67  for permitting additional fluids to flow into the sleeve when the sleeve is in the lower position, may be located above an upper one of the opposed divergent channels  75 . 
     In the embodiments shown, the upper and lower ends  77  ( FIG. 6 ) of the sleeve  65  are flared. The body  53  has the axial passage  63  through which the sleeve  65  slidingly extends. The axial passage  63  is tapered at both axial ends  79  and includes seals  81  for sealing against the flares  77  on the upper and lower ends of the sleeve  65 . Engagement between the flares  77  and the tapers  79  limits axial travel of the sleeve  65  in both the uppermost ( FIG. 5 ) and lowermost ( FIG. 2 ) positions. As shown in  FIG. 3 , the radial port  61  in the body  53  intersects the axial passage  63  between the tapered axial ends  79 . 
     Referring now to  FIGS. 7-9 , the sliding sleeve  65  may be operated in a number of ways for the gas extracting device  51 .  FIGS. 7 and 8  illustrate how a hydraulic pump  83  compresses an opposing spring  85  to move the sliding sleeve  65  and allow gas to enter the jet pump.  FIG. 9  shows the system operating only hydraulically to operate the sleeve  65 . 
     Accordingly, the invention may further comprise a hydraulic chamber  87  ( FIGS. 7 and 8 ) to be formed between the body  53  and the sleeve  65  in an annular space. The hydraulic chamber  87  is sealed at a lower end by seals  89  on an inner wall of the body  53  against the sleeve  65 , and on an upper end by seals  89  on a flange  91  of the sleeve  65  that engage the inner wall of the body  53 . The offset spring  85  extends between an upper surface of the flange  91  and a shoulder  93  formed in the body  53 . The hydraulic pump  83  communicates hydraulic fluid to the hydraulic chamber  87  through a conduit  95  extending between the hydraulic chamber  87  and pump  83 . Control means  97  selectively pressurizes the hydraulic chamber  87  such that the hydraulic pump  83  forces the sleeve  65  to the upper position ( FIG. 7 ) and compress the offset spring  85  and, when pressure is reduced, the offset spring  85  biases the sleeve  65  to the lower positions (e.g.,  FIG. 8 ). 
     Referring again to  FIG. 9 , the system may comprise a pair of hydraulic chambers  101  (e.g., upper and lower chambers) that are formed between the body  53  and the sleeve  65  in the annular space. The upper and lower chambers  101  are separated by the flange  91  of the sleeve  65  that engages an inner wall of the body  53  with a seal  89 . The upper and lower ends of the pair of hydraulic chambers  101  are sealed by seals  89  on the inner wall of the body  53  against outer surfaces of the sleeve  65 . The hydraulic pump  83  communicates hydraulic fluid to the pair of hydraulic chambers  101  through separate conduits  95  extending between the pair of hydraulic chambers  101  and the hydraulic pump  83 . The control means  97  selectively pressurizes the pair of hydraulic chambers  101  to selectively force the sleeve  65  between the upper and lower positions as described herein. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.

Summary:
A gas extraction device for down hole oilfield applications utilizes the flow of the liquid, created by an artificial lift device, to produce conditions by which the gas is drawn into tubing. A venturi creates a low pressure area through a constricted section of the tubing. The pressure within the throat of the venturi drops the pressure in the casing. The device is axially adjusted to allow communication ports to a lower pressure area of the casing annulus. The lower pressure in the venturi draws the gas into the liquid stream and into the production tubing above the device. The gas is then transferred through the well head within the liquid stream.