Patent Publication Number: US-8985226-B2

Title: Electric submersible pump, tubing and method for borehole production

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage entry of international application number PCT/GB2010/050133, having international filing date of Jan. 28, 2010, which was published in English, and which claims priority to Great Britain patent application no. GB 0901542.1, filed Jan. 30, 2009 and to Great Britain patent application no. GB 0920431.4, filed Nov. 23, 2009. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Disclosure 
     This invention relates to systems for the production of well fluids, including for example oil and gas, from boreholes, and to production tubing and electric submersible pump assemblies for deployment in boreholes. 
     2. Description of the Related Art 
     An electric submersible pump assembly (hereafter referred to as an ESP) is deployed in oil wells and other boreholes to transport fluid to the surface, and comprises a pump, i.e. an impeller or other element that acts on the well fluid, coupled to an electric motor that drives it. (It will be understood by those skilled in the art that “a pump” and “an electric motor” include a stack of pumps or a stack of electric motors acting together so as to increase the power of the ESP.) 
     Production tubing may be either sectional, jointed tubing or continuous, coiled tubing, which is lowered down the borehole to provide a conduit through which the well fluid may be pumped to the surface. With the production tubing in place in the borehole, the ESP may then be lowered down the production tube on a flexible tether to a deployed position, typically proximate its lower end, and then sealed to the internal wall of the tubing by a packer so that the outlet of the pump is in fluid communication with the upper portion of the tube, which is used to conduct the well fluid to the surface. Conveniently, the flexible tether may incorporate an electric cable for supplying power to the motor. Alternatively, the tether may comprise a coiled tube, which may be used to conduct the well fluid to the surface, in which case the ESP may simply be suspended in the production tubing without a seal. 
     An arrangement of this general type is disclosed for example in US 2007/0289747 A1. 
     The motor of the ESP generates heat in service, and depending on the power of the pump, may require cooling to ensure the insulation and lubricants of the motor do not break down through excessive heat and damage the motor. 
     At low power, the static, ambient well fluid may be used to dissipate heat from the motor. However, as the power of the motor (or the temperature of the ambient fluid) increases, the static well fluid is no longer capable of cooling the motor and alternative methods have to be used. One known solution involves placing a shroud around the motor and passing fluid through this shroud. This cools the motor more than the ambient well fluid alone would, but at the expense of more components, greater cost and increased diameter of the pump assembly. 
     Alternatively, the motor may be cooled by allowing the well fluid passing through the pump to flow over the surface of the motor within the production tube. 
     In order to provide a conduit between the outer wall of the motor and the inner surface of the production tube, sufficient to carry the full flow of the well fluid passing through the pump so that the well fluid may cool the motor, the motor must necessarily be of substantially smaller diameter than the inner diameter of the production tube. This in turn disadvantageously limits the power of the motor and hence the output of the ESP. 
     Rather than reducing the diameter of the motor, the diameter of the production tube may be increased, which however substantially increases its cost. Moreover, the larger diameter of the production tube reduces the velocity of the well fluid, which in turn reduces its capacity to carry particulates from the well, leading to a buildup of sand and other debris which can clog the pump and the wellbore. 
     In practice, it is found in that, even where the motor is cooled by the well fluid passing over its surface within the production tube, overheating may still occur. 
     BRIEF SUMMARY 
     The object of the present invention is to provide an improved method and apparatus for pumping well fluid from a borehole, which in particular addresses the above mentioned problems. 
     According to the various aspects of the present invention, there are provided a system, a method, an electric submersible pump assembly and a production tube as defined in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various illustrative embodiments of the invention will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the following drawings, in which: 
         FIG. 1  shows a longitudinal sectional view of a first production tube deployed within a well casing; 
         FIG. 2  shows a longitudinal sectional view of the first production tube and casing with a side view of a first electric submersible pump; 
         FIG. 3  shows a diagrammatic cross-sectional view of the first production tubing and electric submersible pump; 
         FIG. 4  shows a longitudinal sectional view of a second production tube deployed in a well casing; 
         FIG. 5  shows a longitudinal sectional view of the second production tube and casing of  FIG. 4  with a side view of a second electric submersible pump; 
         FIG. 6  shows a cross-sectional view of the production tubing and electric submersible pump of  FIG. 5 ; 
         FIG. 7  shows a longitudinal sectional view of a third production tube with a side view of a third electric submersible pump; 
         FIG. 8  shows a longitudinal sectional view of a fourth production tube and a fourth electric submersible pump; and 
         FIGS. 9-12  show a fifth electric submersible pump and a fifth production tube, wherein: 
         FIG. 9A  is a longitudinal section through the production tube; 
         FIG. 9B  is a longitudinal section through the ESP; 
         FIG. 10  is a longitudinal section through the tube and ESP in the deployed position; 
         FIG. 11  is a schematic plan view showing the tube and ESP in the deployed position; 
         FIG. 12A  is a longitudinal section through the upper end portion of the ESP; and 
         FIG. 12B  corresponds to  FIG. 12A  showing the upper end portion of the ESP after separation of the tether at the shear connection. 
     
    
    
     Corresponding reference numerals indicate the same parts in each of the figures. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , production tube  20  is installed in well casing  10 . A seal  22  is located at the lower end of the production tube  20 . The seal  22  has a landing seat  23  which incorporates a throughbore. The production tubing  20  has a region  25  of increased diameter, and a plurality of inwardly projecting protuberances  24  are spaced apart around its inner surface. The protuberances acts as stabilising elements or centralisers, and are formed as dimples which extend inwardly into the tube to substantially the same diameter as the internal diameter of its upper portion. Advantageously, the protuberances  24  formed as rounded dimples provide minimal resistance to fluid flowing through the conduit defined between the pump assembly and the tube, while their rounded contours avoid snagging the pump assembly during deployment. 
     Referring to  FIGS. 2 and 3 , an ESP  21  is made up of a number of motor modules  26  connected together, arranged above a number of pump modules  28  arranged in series which are driven by the motor modules  26 . 
     The ESP  21  is lowered on coiled tubing  32 , which also carries a power supply cable. The lowermost pump terminates with an inlet tube  30 . When the ESP  21  reaches the bottom of the production tube, the pump inlet tube  30  engages with the landing seat  23  of the seal  22 . A pump outlet  27  is located between the pump modules  28  and the motor module  26 . 
     In this position, the motor modules  26  of the ESP  21  are spaced from the inner surface of the production tube  20  by the centralisers  24 , whose points describe a diameter slightly larger than the outer diameter of the electric submersible pump. In operation, the pump modules  28  urge fluid from beneath the seal  22 , through the pump inlet  23 , and the fluid passes out through the pump outlet  27 , and flows through the annulus  36  between the inner surface of the production tube  20  and the outer surface of the motor modules  26 . 
     The region  25  of increased diameter of the production tube allows for a greater rate of flow of fluid. 
     Moreover, the applicant has hypothesised that if the ESP is unsupported along its upper part, it may tilt in the production tube so that one side of the motors rest on the inner surface of the tube. It is believed that when this happens, the reduced fluid flow around the side of the motors resting on the production tube leads to non-uniform cooling of the motor casing. This in turn is believed to result in very slight deformation of the casing, which due to the very small clearance between the rotor and the stator, causes rubbing of the rotor, which explains the problem of overheating and damage to the motor which has been observed in prior art ESPs. 
     The applicant has found in practice that by arranging stabilising elements so as to centralise the motor in the production tube, the overheating problem previously observed is avoided, which is believed to be due to the uniform flow thus achieved around the circumference of the motors and the consequent uniform cooling of the motor casing, so that any thermal expansion is also uniform and does not result in deformation of the casing. 
     Modular motors stacked in series allow a long motor having a small outer diameter to be easily built up so that a large amount of power can be generated for a limited diameter; likewise, modular pumps in series allows the electric submersible pump to develop a large pressure differential between the pump inlet and pump outlet. However, the principles of the invention can equally be applied to ESPs having a single motor and single pump. 
     Referring to  FIG. 4 , the ESP may be supplied with power by a cable  31  which is strapped to the outside of the production tube  20  by cable clamps  55  (visible only in  FIG. 6 ) distributed along the length of the production tube  20  as required. The cable  31  terminates in an electrical connection block  33  which is located beneath an opening  35  in the production tube  20 . As in the previous example, the production tube  20  has a region  25  of increased diameter, the inner surface of which features inwardly pointing centralisers  24 . The region  25  also features inlet ports  37  around the production tube&#39;s circumference. 
     Referring to  FIGS. 5 and 6 , an electric submersible pump comprises a number of pump modules  44  located above a number of motor modules  41 . As for the previous example, the pumps and the motors are connected in series, although it will be seen that in this embodiment, the pumps are situated above the motors. The lowermost pump includes a pump inlet  43 , and a pump outlet  34  is situated above a engagable seal  46 . 
     The electric submersible pump is lowered down the production tube  20  on a wireline  48  to the correct position. As the electric submersible pump nears its this position, a retractable electrical connector  39  extends from the electric submersible pump to project through the opening  35  and engage with the electrical connection block  33 . The electric connector  39  and the electrical connection block  33  may mate using a known mechanism such as that described in UK patent GB2403490. As for the previous embodiment, the motor modules  41  are held in the centre of the increased diameter region  25  by the centralisers. As can be seen in  FIG. 6 , the centralisers may be formed from separate pieces that are fixed in or upon the wall of the production tube  20 . In this cross sectional view, which shows a section through a motor module  44  (comprising a stator  51  and rotor  53 ), it can be seen how the centralisers  24  hold the motor module centrally so that there is a equal area around the entire circumference of the motor housing  58  for the pumped fluid to flow up through the inlets  37  and over the motor module to cool the motor module. 
     Once the electrical connector  39  has engaged with the connection block  33  and the electric submersible pump is supplied with power, the motor modules drive the pump modules  44  such that well fluid is drawn through the inlet ports  37  (and also around the bottom of the electric submersible pump, which is not sealed), over the outside of the motor modules  41 , through the pump inlet  43  and pump modules  44  and then out through the pump outlet  34  and up through the production tube. 
     Referring to  FIG. 7 , rather than the production tube  20  having centralisers formed from dimples, centralising means could be carried on the electric submersible pump itself. When the electric submersible pump is in position, centraliser blades  45  are activated to move from a retracted position inside the body of the electric submersible pump  21  around the motor modules  41  to an extended position where the blades  45  engage with the inner surface of the production tubing  20  in a region  25  of increased diameter. As for the centralisers located on the production tube  20  in the previous embodiments, the centraliser blades  45  secure the electric submersible pump  21 , and the motor modules  41  in particular, in a central position in the production tube  20 . 
     Since the power connection is supplied via a cable  31  attached to the production tube  20 , the wireline  48  may be disconnected from the top of the electric submersible pump  21  and retrieved at the top of the borehole. 
     It will be seen that the principles of spacing the motor from the side of the tube in which the electric pump is disposed can be easily adapted to different downhole systems. Referring to  FIG. 8 , an electric submersible pump comprises a brushless DC motor  64  which drives an impeller type pump  66 . The electric submersible pump is lowered down on a power cable  69  so that the pump inlet  68  lands on a production tube shoe  72 . In this embodiment, a region having a larger inner diameter is formed from a uniform piece of tubing  76 , into which two other lengths of tubing  74 ,  78  (having outer diameters equal to the inner diameter of the tubing  76 ) have been inserted. Centralisers  24  formed or attached on the tubing  76  abut the motor housing  63  to ensure that the motor is spaced from the wall of the tube  76  and pumped well fluid can flow through the pump outlets  71  into the annulus  65  around the entire circumference of the motor  64  to cool it effectively. A valve could be included in the shoe  72  if desired. The centralisers may take any form, provided that a sufficient, preferably annular flowpath is left around the motor. The centralisers could for example be formed from vertical ribs instead of discrete, rounded dimples. 
     Referring to  FIGS. 9-12 , a fifth production tube  100  comprises an enlarged diameter portion  101 , which may be a rigid tube that is jointed to the upper portion  102  above it and the lower portion  103  below it or alternatively may be formed by expanding a continuous coiled tube. A polished bore receptacle (PBR)  110  is sealingly engaged in the lower portion  103  of the tube, and includes a torque anchor which prevents it from rotating in the tube. 
     A fifth electric submersible pump assembly  120  comprises a motor  121  arranged above a pump  122  (i.e. it is a so-called “inverted ESP”), the pump having an inlet  123  and an outlet  124 . The motor is supplied via an electric cable  130 , which functions as a tether  131  for lowering the pump assembly down the production tube into the deployed position illustrated in  FIG. 10 . The cable comprises three conductors  132 , each having a steel core  133  and a copper cladding  134  that carries most of the current, and an outer insulating jacket  135 . The cable terminates in a block  136  which is attached to the upper end portion  137  of the ESP by means of a shear connection, comprising a plurality of dowels  138  which shear to release the block  136  from the upper end portion  137  when sufficient tensile stress is exerted on the tether. This ensures that the tether will detach before it breaks. If detachment occurs, for example, due to the ESP becoming jammed in the tube, then a retrieval tool can be lowered down the tube on a heavier wireline and engaged with an engagement profile  139  on the upper end portion  137 , so that the wireline can then be used to haul the ESP to the surface. 
     In use, the ESP is introduced into the upper end of the production tube  100  and lowered on the tether down the upper portion  102  of the tube. The ESP is provided with a stinger  150  at its lower end which engages in the polished bore receptacle (PBR)  110  so as to locate the ESP in its deployed position with the motor positioned within the enlarged diameter portion of the tube. The stinger includes a seal  151 , which seals the ESP to the production tube between the inlet and the outlet of the pump so that the outlet is in fluid communication with the upper portion  102  of the production tube. The stinger also includes a torque anchor which prevents the ESP from rotating relative to the PBR and hence relative to the tube. The ESP may then be operated to draw well fluid through the pump and expel it via the production tube  100  to the surface. 
     The upper end portion  137  may be provided with a plurality of fixed stabilising elements comprising fins  140  (shown in  FIG. 10 ) which are spaced apart around the pump assembly and extend radially outwardly between the pump assembly and the tube, and which engage the inner surface of the upper portion  102  of the tube so as to space the motor from the enlarged diameter portion of the production tube to define a conduit  111  therebetween having a cross-sectional area sufficient for the passage of the well fluid passing through the pump. By positioning them on the upper end portion  137 , which has a smaller diameter (i.e. a smaller maximum transverse dimension) than the internal diameter  102 ′ of the upper portion  102  of the tube, the stabilising elements can be permanently fixed to the ESP without preventing it from being deployed down the tube from the surface, and serve to space the outer surface  121 ′ of the casing of the motor  121  from the enlarged portion of the tube while allowing the well fluid to flow around the ESP and between the fins  140  as it travels up the tube to the surface. Advantageously, there is no point contact by any part of the tube against the casing of the motor  121 , so localised damage due to vibration of the motor against the casing is avoided. In alternative embodiments, fixed stabilising elements may be positioned on the lower end or another reduced diameter portion of the ESP. 
     Alternatively or additionally, the ESP may be provided with a plurality of stabilising elements  141  (shown in  FIG. 11 ) which are retractable and extendable (e.g. by hydraulic or electromagnetic or other suitable actuation means) from the ESP so that, once the ESP has reached the deployed position ( FIGS. 10 and 11 ), they are extended radially outwardly beyond the outer diameter of the motor casing and beyond the inner diameter of the upper portion  102  of the production tube through which the pump assembly is deployed, so as to engage the inner surface  101 ′ of the enlarged portion  101  of the tube as shown. The elements  141  are spaced around the outer circumference of the ESP proximate the motor and are retracted to allow the ESP to be withdrawn from the tube. Again, the retractable elements  141  space the motor from the production tube while ensuring that the casing of the motor does not make point contact against the tube, which avoids damage to the ESP due to vibration of the motor in service. 
     Both the fixed elements  140  and the retractable elements  141  allow the outer diameter of the motor to be only slightly less than the inner diameter of the upper portion of the production tube through which it is deployed, it being understood that the enlarged diameter portion of the production tube may conveniently be shorter than the length of the ESP. Thus, the major part of the production tube can be no wider than the ESP, so that the flow velocity is advantageously higher than it would be in a larger diameter tube, allowing effective clearance of debris to the surface, while the motor is effectively cooled by the well fluid pumped through the conduit  111 . Moreover, the cooling flow is achieved without reducing the diameter and hence the power output of the motor. Since the enlarged diameter portion can be relatively short, the annulus between the production tubing and the well casing is also advantageously substantially unobstructed. 
     Preferably, the stabilising elements  140  and/or  141  are adapted to locate the motor substantially coaxially in the production tube as shown, so that the conduit  111  defines an annulus as shown between the motor and the tube. As previously mentioned, this is particularly advantageous in that it is found to overcome the problem observed in prior art systems of overheating of the motor in service, which is believed to be due to the fact that, in prior art arrangements, the end or ends of the ESP extending beyond the seal (or, where no seal is present, the whole of the ESP) may lie against the wall of the production tube, which can cause the casing of the motor to expand unevenly due to the reduced fluid flow and hence the reduced rate of cooling in the region where it touches the tube. Of course, the problem is substantially reduced by arranging the motor within the enlarged portion of the tube, even if no stabilising elements are used, so that the well fluid flows freely around the whole circumference of the motor casing. 
     In summary, an electric submersible pump assembly (ESP) is deployed in a production tube in a borehole such that the motor of the ESP is spaced from the inner wall of the production tube, defining a conduit through which the pumped well fluid can flow to cool the motor. The production tube may have an enlarged diameter portion within which the motor is positioned. Alternatively or additionally, the ESP and/or the production tube may be provided with stabilising spacers which extend between the ESP and the tube to preferably centralise the ESP in the tube and support it against vibrational movement, the spacers preferably defining an annular conduit between the motor casing and the production tube. 
     Rather than using a PBR, the stinger might alternatively be arranged to engage directly in the lower portion  103  of the production tube. Alternatively, the ESP might be provided with a packer which expands to engage the upper portion  102  or the enlarged diameter portion  101  of the tube. In alternative embodiments, the lower portion  103  of the production tube might be a larger or smaller diameter than the upper portion  102 , and might be engaged by a stinger or a packer on the ESP; alternatively, the tube might not be provided with a lower portion  103 . 
     The production tube can be any tube that a pump may be deployed in after lowering the tube into a borehole. The tether could comprise a continuous coiled tube, which may be hollow or may be filled with the insulated electric cable. Where the pumped fluid is conducted to the surface in the same tube that the electric submersible pump is deployed in, there must be a seal between the pump inlet and pump outlet; no seal is necessary however when a separate outlet tube, e.g. hollow coiled tubing functioning as the tether, is used to transport the fluid to the surface. 
     The centralisers could also be disposed down the borehole as a separate device to engage with production tubing, and engaged with the electric submersible pump when the pump reaches its deployed position. 
     The enlarged section could also be achieved by mechanically expanding the tubing in the well by deployed an expanding tool down the tubing either on wireline or coiled tubing to create the required larger diameter where the motors will be positioned. 
     In alternative, less preferred embodiments, stabilising elements or protuberances may be provided between the ESP and production tube in a production tube of constant diameter; alternatively, the production tube may be provided with an enlarged diameter portion, and the ESP may be deployed with the motor arranged in the large diameter portion, without the use of protuberances or stabilising elements.