Abstract:
The pump can be utilized in gassy oil wells to prevent gas slugs from locking the electrical submersible pump. A shroud assembly is provided with a bottom that can be fixed to the top of a seal section connected to the top of a motor. Additional lengths of shroud can be added as the shroud assembly is lowered into the well. The electrical submersible pump can then lowered into the shroud and supported from a production tubing string. A hanger can then be attached to the production tubing string to carry the weight of the shroud assembly, motor, and seal section.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to installation of electrical submersible pumps (ESPs), and in particular the installation of ESP equipment inside an inverted shroud. 
     BACKGROUND OF THE INVENTION 
     A typical subsea installation can use an Electric Submersible Pump (ESP) within an inverted shroud. An ESP unit consists of a motor section, a seal section, and a pump section having an inlet and a discharge connected to production tubing and is used to provide artificial lift to liquid from a formation. 
     An inverted shroud can be used in combination with an ESP for use in gassy wells to divert the gas past the entrance of the ESP to reduce the possibility of gas locking. The shroud is a cylindrical steel tube that encompasses the ESP and is sized to allow clearance for fluid to pass both inside past the ESP and outside between the well casing and the shroud. 
     In gassy oil wells, gas and liquid enter the casing from the formation then both travel up the casing past the ESP unit to the top of the shroud. Due to gravity, the liquid can fall back down inside the shroud, which has an open top, and into the entrance of the pump. Gas slugs, however, effectively continue moving past the ESP. This reduces the chances for the ESP to experience gas locking due to gas slugs. 
     The assembly and installation of an inverted shroud with an ESP is very time consuming and difficult because the shroud, the pump, and lengths of production tubing must be assembled in unison as it is lowered into the hole. Parts for the assembly must be manufactured to strict tolerances in order to allow for proper assembly. Further, the diameter of the shroud limits the size of the motor that can be used for the ESP, which in turn affects the capability of the ESP to produce artificial lift. 
     A need exists for a technique that addresses the limitations and shortcomings described above. In particular a need exists for a technique to allow for an inverted shroud to be installed with an ESP in a timely manner and in a manner that does not limit the size of the motor that can be used. The following technique may solve these problems. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present technique, a shroud assembly is provided with a bottom that can be fixed to the top of a seal section connected to the top of a motor. Additional lengths of shroud can be added as the shroud assembly is lowered into the well. This allows for a relatively less time consuming and less difficult assembly process as the shroud can be assembled independently from the electrical submersible pump (ESP) and the production tubing, which in the past have been assembled in unison with the shroud. Further, assembly of the shroud in this manner makes the motor size independent from the inner diameter of the shroud because the motor is not located within the shroud. 
     In the illustrated embodiment, a motor is located at the base of an assembly with a seal section through which the motor shaft passes. A power cable descends from the surface and runs along between the casing and the shroud to serve the motor. The shaft protrudes into a special section of shroud about a foot in length that is bolted onto the seal section. The pump is connected to the protruding shaft and can have multiple stages. The pump can also have a pump positioner or guide at the base to aid in positioning the pump. Additional sections of shroud extend upwards from the special section of shroud and house the ESP within. The shroud sections can be sections of pipe connected end to end and can extend up to 300 feet or more above the ESP. Inlet holes are located approximately at the top end of the shroud to allow formation liquid to enter the shroud and fall down to the entrance of the ESP. 
     The discharge of the ESP located inside the shroud connects to production tubing that extends past the top of the shroud and to the surface. A shroud hanger located at the top of the shroud supports the weight of shroud assembly comprising the shroud, motor, and seal section, and transfers the weight to the production tubing via the hanger. 
     During installation of the shroud assembly and ESP, a clamp at the wellhead holds the assembled components, and a lifting clamp lifts the next component over the wellhead to be assembled. For example, the clamp at the wellhead initially holds the assembled seal section to support the seal section and the motor connected below. The special shroud section, about a foot in length and housing a protruding shaft spline from the motor, is lifted with a second clamp and placed over the seal section located at the wellhead. The special shroud section can then be bolted onto the seal section. Once the special section of the shroud is bolted onto the seal section, the clamp holding the seal section can be released and then replaced by the clamp used to lift and hold the special shroud section so that it sits on the wellhead. This alternating use of the lifting clamp and the clamp at the wellhead is used to add additional sections of shroud. 
     Once the shroud sections are assembled, the ESP can be lifted and lowered down inside the shroud until it engages the shaft spline of the motor protruding into the special shroud section. At this point the top of the shroud is still supported by a clamp at the wellhead. Once the ESP is positioned within the shroud, a section of production tubing is lifted with a clamp and lowered down inside the shroud to connect with the discharge end of the ESP. As with the shroud sections, additional production tubing sections are lifted and connected end to end by releasing the clamp holding the assembled production tubing at the wellhead and replacing it with the clamp holding the last added section of production tubing. A hanger is then installed at the top of the shroud at the point where the length of production tubing is sufficient to extend to or above the top of the shroud. The hanger engages the production tubing to thereby transfer the weight of the shroud assembly to the production tubing, allowing the clamp holding the shroud assembly to be released. The production tubing along with the shroud assembly and the ESP within are then lowered to the desired depth in the well for operation, with additional sections of production tubing added to extend the production tubing up to the wellhead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a seal section and a motor section clamped to the wellhead, in accordance with the invention. 
         FIG. 2  shows a shroud section with a pump positioner attached to the seal section, in accordance with the invention. 
         FIG. 3  shows the top of the completed shroud clamped at the wellhead and the motor and seal attached to the bottom, in preparation to receive a pump, in accordance with the invention. 
         FIG. 4  shows the pump lowered by production tubing into the shroud and mated with the pump positioner, the shroud being hung off the production tubing in accordance with the invention. 
         FIG. 5  shows the pump, seal section, motor, and shroud assembly lowered by production tubing to the desired location in the well, in accordance with the invention. 
         FIG. 6  shows an additional embodiment of the assembly varying in the type of hanger used to support the shroud off of the production tubing in accordance with the invention. 
         FIG. 7  shows another additional embodiment of the assembly varying in the type of hanger used to support the shroud off of the production tubing in accordance with the invention. 
         FIG. 8  shows a sectional top view of the shroud offset from the center of the well to provide clearance for a power cable guard, in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 through 5 , an embodiment of the installation of a shroud  24  with a pump  26  is illustrated. Pump  26  is a rotary pump such as a centrifugal pump or progressing cavity pump. Referring initially to  FIG. 1 , a motor  14  connected to the lower end of a seal section  16  is shown suspended inside a well casing  12 . A power cable  17  is connected to the motor  14  and runs up to the surface of the well. A clamp  18  supports the assembled motor  14  and seal section at the wellhead  10  by holding the seal section  16 . Clamp  18  can be slips or a spider type of supporting system. Clamp  18  may be located on a rig floor of a workover rig. 
     A second clamp (not shown), of the workover rig, typically a pipe elevator, can then lift the next component to be assembled as shown in  FIG. 2 . For example, in this embodiment a special shroud section  20  is lifted with the second clamp (not shown) and can be bolted to the top of the seal section  16  held by the clamp  18  at the wellhead  10 . The clamp  18  at the wellhead  10  is released and replaced by the lifting clamp, thereby moving the assembled components downward into the well. The special shroud section  20  can be approximately a foot in length and houses a spline shaft  22  to mate with and align the pump  26  ( FIG. 4 ). The special shroud section also has an anti-rotational slot or key (not shown) to prevent the pump  26  from rotating. 
     As shown in  FIG. 3 , the shroud  24  can be comprised of sections of pipe, such as casing, connected end to end. The sections of shroud  24  can be lifted by the lifting clamp (not shown) and connected to the previous section of shroud  24  supported at the wellhead  10  by the clamp  18 . The clamp  18  at the wellhead can then be released and replaced by the lifting clamp in the same manner described for the special shroud section  20  above. This procedure of replacing the clamp  18  at the wellhead  10  with the lifting clamp is repeated until the desired shroud length is reached. The uppermost section of shroud  24  has an intake, such as inlet holes  30  in the side wall near the top. The lower end of shroud  24  is closed. 
     Referring to  FIG. 4 , once the shroud  24  sections are assembled, the pump  26  can be lifted and lowered down inside the shroud until it engages a spline shaft  22  and also engages the anti-rotation slot or key (not shown). At this point the top of the shroud  24  is still supported by clamp  18  at the wellhead  10 . Once the pump  26  is positioned within the shroud, a section of production tubing  28  is lifted with a clamp (not shown) and lowered down inside the shroud  24  to connect with the discharge end of the pump  26 . As with the shroud  24  sections, additional production tubing  28  sections are lifted and connected end to end by releasing the clamp  18  holding the assembled production tubing  28  at the wellhead  10  and replacing it with the clamp holding the last added section of production tubing  28 . The tubing inside shroud  24  may be considered to be a lower production tubing string  28 . Shroud  24  remains suspended at wellhead  10  during this process. 
     A hanger  32  is then installed at the top of the shroud  24  at the point where the length of lower production tubing  28  is sufficient to extend to or above the section of shroud  24  having inlet holes  30 . The inlet holes  30  allow formation liquid to enter the shroud  24  and flow down to the entrance of the pump  26  during operation. The hanger  32  engages the upper production tubing  29  to thereby transfer the weight of the shroud  24 , motor  14 , and seal section  16 , to the upper production tubing  29  via the hanger  32 . Once the hanger  32  is installed, the clamp  18  holding the shroud  24  can be released. The lower production tubing  28 , pump  26 , along with the shroud assembly comprising the shroud  24 , motor  14 , and seal section  16 , are then lowered to the desired depth in the well for operation, as shown in  FIG. 5 , with additional sections of upper production tubing  29  added to extend the production tubing up to the wellhead. 
     Hanger  32  has external threads that engage internal threads formed in the upper section of shroud  24 . Hanger  32  has internal upper and lower threads for securing upper tubing string  29  and lower tubing string  28 . 
     In other embodiments illustrated in  FIGS. 6 and 7 , different types of hangers can be utilized. The hangers  34 ,  36  shown are also used to hang the shroud assembly from the production tubing  28 .  FIG. 6  shows a hanger  34  having a lower slip with a lower tapered bowl. The lower tapered bowl has external threads that engage internal threads formed in the upper section of shroud  24 . To prevent upward movement of the production tubing due to thermal growth, the hanger  34  additionally comprises an upper slip with an upper tapered bowl. A set of internal threads on the upper tapered bowl engages external threads on the lower tapered bowl. 
       FIG. 7  shows a hanger  36  having a lower slip with a lower tapered bowl. The lower tapered bowl has external threads that engage internal threads formed in the upper section of shroud  24 . A retainer secures the slip to prevent upward movement of the slip. 
       FIG. 8  shows a sectional top view of the shroud  24  offset from the center of the well to provide clearance for a power cable guard  40  attached to the exterior of the shroud  24 . The electrical power cable  17  is routed inside the guard  40  to protect it from damage. The guard  40  can comprise a continuous channel or can be comprised of a plurality of spaced apart channels. 
     In an additional embodiment (not shown), the power cable  17  can run inside the shroud  24 . The power cable  17  could stab into an electrical connector assembled as part of the special shroud section  20  at the base of the pump  26 . 
     Assembling the shroud assembly comprising the shroud  24 , motor  14 , and seal section  16  prior to the installation of the pump  26  and production tubing  28  can reduce installation time and difficulty by eliminating the need for strict tolerances required when the shroud assembly, ESP, and production tubing are assembled in unison. Further, the size of the motor is not limited by the shroud diameter because the motor is installed prior to and outside the shroud, allowing for a larger motor size. In the example shown in the figures, the outer diameter of motor  14  is greater than the inner diameter of shroud  24 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. These embodiments are not intended to limit the scope of the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. For example, a rotary gas separator could be located in shroud below pump as part of the pump assembly. If so, however, a gas outlet diverter would be connected between a exterior port of the shroud and the cross over of the gas separator.