Abstract:
A tubular assembly  10  is disclosed for passing fluids within a hydrocarbon recovery well. The tubular assembly  10  has an insulated connection  12  between insulated tubing segments  14, 16 . Tubing segments  14, 16  are connected via a threaded connection, and the resulting joint  10  is insulated by an external insulating sleeve  18 . The insulating sleeve  18  may have unitary construction, such that the sleeve  18  may only be installed or removed by first breaking apart the joint  10 . Alternatively, the sleeve  18  may comprise a plurality of arcuate partial-sleeves, which may be assembled about or removed from the completed joint  10  without first breaking apart the joint  10.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to insulated pipe strings for use in hydrocarbon recovery wells. More particularly, this invention relates to a tubular assembly having an improved insulated joint between insulated tubular segments.  
         BACKGROUND OF THE INVENTION  
         [0002]    Hydrocarbon recovery wells for producing oil and gas involve using long tubing strings to convey the hydrocarbons from the downhole reservoir to the surface. In many instances it is desirable to maintain temperature and minimize heat loss from substances flowing through the string. In more conventional oil recovery operations, the oil may already be highly flowable within the reservoir. Nevertheless, because the viscosity of oil increases as it cools, minimizing heat loss helps the oil maintain flowability, making it easier and less costly to produce. In some recovery operations, however, oil may be very viscous within the reservoir. It may then be necessary to heat the oil downhole in order to produce it in economically viable quantities. Minimizing heat loss in the string is therefore more critical. In these situations, hot steam is typically passed downhole through the tubing string to release the thickened or trapped oil so it becomes flowable. Insulated tubing strings minimize heat loss from the steam and oil.  
           [0003]    An older method of insulating a tubular assembly involved applying insulation to the outside of a tubing string, such as described in U.S. Pat. No. 3,763,935. The insulation extended from the earth&#39;s surface down to the bottom of the permafrost zone, in a continuous cylindrical form. This method of insulation had several disadvantages, however. Applying thermal insulation in this manner was expensive and time consuming. The insulation was also quite fragile under typical drilling conditions.  
           [0004]    A newer category of insulated tubing strings involves stringing together double-walled insulated tubing segments. Generally, each insulated tubing segment has an outer tube disposed about an inner tube and defining an annular space therebetween. The annular space is sometimes filled with insulating material. Alternatively, a vacuum may be established in the annulus to insulate the tubing. Heat transfer is therefore minimized between the inner wall, which may be exposed to hot oil and steam, and the outer wall, which may be exposed to the cooler interior of the well bore or to atmosphere. U.S. Pat. No. 4,512,721, for example, discloses insulated tubing having a vacuum annulus filled with a “getter material” for absorbing gases that can migrate into the annulus at high temperatures. U.S. Pat. No. 3,680,631 discloses insulated tubing combining the use of vacuum and solid thermal insulation, for passing warm fluids through a permafrost zone.  
           [0005]    Despite their increased durability and ease of assembly, the use of insulated tubing segments has inherent disadvantages. A major problem with joining insulated tubing segments is that excessive heat loss may occur at the joint between segments. This is because an insulated segment is not insulated at its ends where the outer tube is joined with the inner tube to seal the annulus between the inner and outer tube. Heat may therefore be conducted away from the interior of the tubing along a conductive flow path at each joint, at a much higher rate than through the insulated portion of the tube. This results in greater heat loss and reduced efficiency.  
           [0006]    A number of solutions have been proposed to minimize heat loss at the joint between insulated tubing segments. U.S. Pat. No. 4,518,175 discloses an insulated tubular assembly having insulation underneath an external coupler at the tubing joint. A frustoconical member supports and positions the inner pipe relative to the outer pipe of each tubing segment. The frustoconical member extends diagonally upward from an end of the inner tube to an adjacent end of the outer tube. The insulation at the joint is layered between the external coupler and the frustoconical member, and extends over a portion of the insulated tubing annulus. The overlapping annulus insulation and joint insulation essentially provides continuous insulation along the tubular assembly, minimizing heat loss at the joint.  
           [0007]    U.S. Pat. No. 4,415,184 discloses an insulated tubular assembly having insulation between an external coupling and the insulated tubing segments at the tubing joint. A fluid-tight thrust ring seals and joins adjacent ends of the inner and outer tubes of each tubing segment. Insulation is sandwiched between the thrust rings and the external coupler that couples two insulated tubing segments. The insulation may be cast in place during manufacture of the tubing segments, or inserted during installation of the tubular assembly. An additional ring of insulation must be inserted at the joint between the ends of the tubing segments to provide continuous insulation along the joint.  
           [0008]    Yet another way of insulating the joint internal to a coupler is provided by U.S. Pat. No. 4,693,313. An external coupler joins two tubing segments, leaving a substantial gap between the joined ends. The gap created between the coupled segments is insulated by means of a coupling insulator, which typically includes suitable insulation material. A tubular shield is positioned inside the coupling insulator to shield the joint insulation material from fluids passing through the tubular assembly.  
           [0009]    A different approach to insulating the joint is provided by U.S. Pat. No. 4,538,834. The tubing segments are joined with only a slight gap or space left between them at the joint. Condensate of the fluid flowing through the assembly is trapped in the space, which may form a thermal barrier between the fluid flowing through the assembly and the ambient environment.  
           [0010]    The above insulating tubing joints and methods have the drawback that the insulation at the joint is fitted internal to the coupler. Applying insulation during the manufacture of each tubing segment in this way can be complicated and expensive. For example, the insulation must be installed so the coupler will later fit around it when joining two insulated tubing segments. If the coupler does not fit properly during installation, it may not be correctable in the field during installation, while away from the manufacturing facility. If the insulation is instead applied in the field during installation, this can be a complicated or time-consuming step.  
           [0011]    Another complication with the above tubing joints is that it may be difficult or impossible to repair or replace the insulation once in the field. The joint will most likely have to be disassembled to access the insulation. Whether the insulation needs replacement may be difficult or impossible to discern, because it is hidden within the joint. Especially in a long tubing string, a great deal of effort is required to break apart each joint whose insulation needs inspection or repair. A further disadvantage of having insulation on the inside of a coupler of a joint is that the flow through the tubular assembly may be disrupted. For example, the tubular assembly of U.S. Pat. No. 4,693,313 requires installing a shield to prevent flow within the inner tube from impinging the joint insulation. Even with this extra step, the shield might impede the flow or cause turbulent flow.  
           [0012]    The disadvantages of the prior art are overcome by the present invention. An insulated tubular assembly is provided having an improved insulated joint that is easier and less expensive to manufacture, install, repair, and replace.  
         SUMMARY OF THE INVENTION  
         [0013]    An insulated tubular assembly is disclosed for passing fluids within a hydrocarbon recovery well. The tubular assembly is formed by stringing together double-walled tubing segments. The tubular assembly has an improved insulated joint between tubing segments, making the tubular assembly easier and less expensive to manufacture, install, repair, and replace.  
           [0014]    In a preferred embodiment, the assembly includes a plurality of consecutively joined insulated tubing segments. Each tubing segment includes an inner tube, an outer tube, and two opposing ends. An inner tube defines an inner passage for conveying fluids. An outer tube is concentrically disposed about the inner tube, and an annulus is defined between the inner and outer tubes. An annular bridge at each end connects the inner tube to the outer tube. Together, the annular bridges seal the annulus between the inner and outer tube, such as for sustaining a vacuum and/or containing an insulating material. The inner tube has an extension extending outwardly from at least one of the two ends. The insulated tubing segments are joined by a threaded connection, which connects an extension of a first tubing segment with an extension of a second tubing segment to form a joint. An insulating sleeve surrounds the joint.  
           [0015]    In another preferred embodiment, a method is provided for insulating a tubular assembly by stringing together a plurality of insulated tubing segments. Each tubing segment includes two opposing ends, an inner tube defining an inner passage adapted for conveying fluids, an outer tube concentrically disposed about and defining an annulus with the inner tube, and an annular bridge at each opposing end. Each annular bridge connects the inner tube to the outer tube, together sealing the annulus from atmosphere, for sustaining a vacuum and containing an insulating material therein. The inner tube has an extension extending outwardly from at least one of the two ends. The method further involves threadably connecting an extension of a first tubing segment with an extension of a second tubing segment to form a joint. Finally, an insulating sleeve is positioned around the joint to insulate the joint.  
           [0016]    It is a feature of this invention that the threaded connection may comprise a female thread on the extension of the first tubing segment, and a male thread on the extension of the second tubing segment. The male and female threads maybe threadably engaged to form an integral joint between the first and second tubing segments.  
           [0017]    Alternatively, the threaded connection may comprise a tubular coupling having opposing first and second threaded box ends, and a central passageway for passing fluid therethrough. There is also a threaded pin end on the extension of each of the first and second tubing segments. The first and second tubing segments are joined by threading the pin end of the first tubing segment with the first box end of the tubular coupling, and threading the pin end of the second tubing segment with the second box end of the tubular coupling.  
           [0018]    Another feature of this invention is that the insulator sleeve may have an outer diameter substantially equal to or less than an outer diameter of the insulated tubing segments, with an advantage being that the sleeve does not contact the well bore as the tubular assembly is moved within the well bore. Alternatively, the outer diameter of the insulator sleeve may be greater than the outer diameter of the tubing segments, with an advantage that the sleeve may act as a bumper to protect the tubing segment from contacting the well bore as the tubular assembly is moved within the well bore. Yet another feature is the sleeve may be long enough to extend across the entire exposed portion of the tube joint. One related advantage is that insulation of the tubular assembly is continuous along the otherwise exposed portion of the tube joint between the insulated tubing segments. Another related advantage is the transition from tubing segment, across the sleeve/joint, and to the next tubing segment, may be substantially smooth. Alternatively, the sleeve may be longer, to completely cover and extend beyond the entire exposed portion of the tube joint, substantially overlapping with the annulus of each adjoining insulated tubing segment. Another feature is the sleeve may comprise an inner sleeve for thermal insulation, and an outer shell made of a rigid material such as a metal or a plastic. A related advantage of this rigid outer shell is that it may increase the durability of the sleeve, which is especially advantageous when the sleeve is also used as a bumper. A further feature is the sleeve may have a unitary tubular body, such that the sleeve may be positioned at the joint only when the joint is broken apart. An advantage is that a unitary sleeve may have maximum durability. Alternatively, the sleeve may be formed by a plurality of arcuate partial sleeves for interconnection about the tube joint to form a full sleeve. An advantage of this alternative is the sleeve may be positioned about or removed from the joint without breaking the joint. These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 illustrates an embodiment of the tubular assembly, in which the joint comprises a male thread on one insulated tubular segment coupled with a female thread on another tubular segment. An insulating sleeve surrounds the joint.  
         [0020]    [0020]FIG. 2 illustrates an alternate embodiment of the tubular assembly, in which the joint comprises a threaded coupling joining threaded ends of two tubular segments. FIG. 2A illustrates an alternate embodiment of the tubular assembly, in which the insulating sleeve overlaps the insulated tubing segments and a cage surrounds the insulated joint.  
         [0021]    [0021]FIG. 3 illustrates an alternate joint in which the outer tubes have threaded extensions for joining insulated tubing segments.  
         [0022]    [0022]FIG. 4 shows a cross section of the tubular assembly joint, such as A-A or B-B in FIG. 1 or  2 . The insulating sleeve shown has a unitary tubular construction, such that the sleeve cannot be installed or removed from the joint unless the joint is first broken apart.  
         [0023]    [0023]FIG. 5 shows a cross section of the tubular assembly joint, such as A-A or B-B in FIG. 1 or  2 . The alternative insulating sleeve shown has a plurality (in this case, two) arcuate partial sleeves for interconnection about the tube joint to form a full sleeve. This sleeve may be installed or removed from the tubular assembly without breaking apart the joint.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 1 shows an embodiment of an insulated tubular assembly  10 , having a connection  12  between two insulated tubing segments  14 ,  16 . An insulating sleeve  18  surrounds the connection  12 . Each tubing segment  14 ,  16  has a double-walled insulated construction, which includes an inner tube  20 ,  21 , an outer tube  22 ,  23  concentrically disposed about the inner tube  20 ,  21  and an annulus  24 ,  25  between the inner tube  20 ,  21  and the outer tube  22 ,  23 . Each end of each tubing segment  14 ,  16  has an annular bridge  26 ,  27 , which connects the inner tube  20 ,  21  to the outer tube  22 ,  23 . The annular bridge  26 ,  27  may be a fillet weld joining an upset portion  20   a ,  21   a , formed on tubes  20 ,  21  respectively, to outer tubes  22 ,  23 , to seal the annulus from atmosphere. The annulus  24 ,  25  may thereby sustain a vacuum and/or contain an insulating material, to insulate each tubing segment  14 ,  16 .  
         [0025]    The inner tube  20 ,  21  of tubing segments  14 ,  16  each have an extension  30 ,  31  extending outwardly from at least one end of each tubing segment  14 ,  16 . For example, the extensions  30 ,  31  may be portions of the inner tubes  20 ,  21  that extend beyond the bridges  26 ,  27 , respectively. A threaded connection connects the extension  30  with the extension  31 . In FIG. 1, this threaded connection includes a male thread or “pin”  32  on extension  30 , and a female thread or “box”  33  on extension  31 . Thus, the tubing segments  14 ,  16  may be threadably joined to form an integral connection  12 .  
         [0026]    A conductive flow path occurs where the inner tube  20 ,  21  meets the outer tube  22 ,  23  of each tubing segment  14 ,  16 . Thus, despite the insulating properties of tubing segments  14 ,  16  about the annulus  24 ,  25 , excessive heat transfer may be occur at the connection  12 . To minimize heat loss at the connection  12 , the insulating sleeve  18  is provided about the connection  12 .  
         [0027]    [0027]FIG. 2 illustrates another embodiment of the tubular assembly  110  having a connection  112 . Similar to tubing segments  14 ,  16  of tubular assembly  10 , tubing segments  114 ,  116  have inner tubes  120 ,  121 , outer tubes  122 ,  123 , and annuli  124 ,  125  therebetween for sustaining a vacuum and/or contain an insulating material. Annular bridges in the form of frustoconical thrust rings  126 ,  127  connect, by welding, the inner tubes  120 ,  121  with respective outer tubes  122 ,  123  to close off the annulus to atmosphere. Tubing segments  114 ,  116  have extensions  130 ,  131  on at least one end of each tubing segment  114 ,  116 . The extensions  130 ,  131  may be portions of the inner tubes  120 ,  121  that extend beyond the bridges  126 ,  127 .  
         [0028]    The connection  112  of tubular assembly  110  comprises a tubular coupling  135  for receiving extensions  130 ,  131  of tubing segments  114 ,  116 . Tubular coupling  135  has threaded box ends  137  and  138 , which mate with respective pin threads  132  and  133 , to join tubing segments  114 ,  116 .  
         [0029]    Like the tubular assembly  10  in FIG. 1, the tubular assembly  110  in FIG. 2 includes an insulating sleeve  118  to insulate the connection  112 . The sleeve  118  may include annular flanges  142 ,  143  projecting radially inwardly to substantially fill gaps  144 ,  145  between the coupling  135  and bridges  126 ,  127 . The sleeve  118  surrounds the connection  112  to insulate the connection  112 .  
         [0030]    The sleeve of a tubular assembly preferably extends across an entire connection such that the insulation of the tubular assembly may be substantially continuous along the tubular assembly, i.e., from one insulated tubing segment, across a connection, to a next tubing segment. To increase reliability and effectiveness of the insulation, the insulation may extend beyond the connection to substantially overlap the annulus within the insulated tubing segments. An outer diameter of the sleeve may be less than or substantially equal to that of adjacent tubing segments, such that the sleeve is protected from damage as the tubular assembly is moved within the well. Alternatively, the outer diameter of the sleeve may be greater than that of adjacent tubing segments, to radially space the tubing segments from the well bore, thereby acting as a bumper to protect the tubular assembly.  
         [0031]    [0031]FIG. 2A illustrates an alternate way of insulating a connection, such as connection  112  of FIG. 2, to help maximize insulation and ensure the entire connection  112  is insulated. An insulating sleeve  218  extends across the entire connection  112 , and the ends  204 ,  206  of the insulating sleeve  218  overlap the annuli of insulated tubing segments  114 ,  116 , to minimize any heat loss that might otherwise occur through the connection  112 .  
         [0032]    [0032]FIG. 2A also illustrates the use of a cage  220  surrounding an insulating sleeve  218  and the connection  112 . The cage  220  is clamped to insulated tubing segments  114 ,  116  with clamps  208 ,  212 . Hinges  210 ,  214  allow the cage  220  to be clamped around the “made up” connection  112  without breaking out the connection  112 . The insulating sleeve  218  may be split lengthwise to fit around the connection  112  without breaking out the connection  112 . The cage  220  may have additional uses, such as to route hydraulic lines or signal wires for use with downhole tools. The cage  220  helps protect the insulating sleeve  218  and any lines, wires, etc. routed through the cage  220 . The cage  220  is sized to fit downhole in the wellbore.  
         [0033]    [0033]FIG. 3 illustrates an alternate connection  312  between insulated tubing segments  314 ,  316 , in which extensions  330 ,  331  extend from outer tubes  322 ,  323  rather than from inner tubes  320 ,  321 . The extensions  330 ,  331  may simply be portions of the outer tubes  322 ,  323  which extend beyond bridges  326 ,  327 . A tubular coupling  335  has threaded box ends  337 ,  338  which mate with pin threads  332 ,  333 , respectively, to join insulated tubing segments  314 ,  316 . An insulating sleeve  318  surrounds the connection  312 .  
         [0034]    [0034]FIG. 4 is a cross-sectional view depicting an embodiment of an insulating sleeve such as sleeve  18  or  118  having unitary tubular construction. Unitary tubular construction means that at least a portion of the sleeve  18 ,  118  has a continuous tubular shape. Thus, to be completely installed or removed from the tubular assembly  10 ,  110 , the connections  12 ,  112  must first be broken apart. Preferably, the entire sleeve  18 ,  118  has a rigid or unitary “one-piece” construction. Alternatively, this unitary sleeve may comprise a plurality of separate pieces, with at least one piece having a continuous tubular portion. For example, the sleeve  18 ,  118  may still have a separate inner core  51  and outer shell  50 . The inner sleeve  51  and outer shell  50  may be separated from each other by axially sliding the inner sleeve  51  with respect to the outer shell  50 . The primary advantages of the unitary construction is that the sleeve may be stronger and have fewer pieces, if any, to assemble.  
         [0035]    [0035]FIG. 5 depicts an alternate embodiment of an insulating sleeve comprising a plurality of arcuate partial-sleeves for interconnection about the tube connection to form a full sleeve. In this figure, the plurality of arcuate partial-sleeves shown are two arcuate half-sleeves  60 ,  61  surrounded by an outer shell  60 . The outer shell  60  may be a metal band tightened about the half-sleeves  61 ,  62  via a tensioner  65 , as shown. Alternatively, the outer shell  60  may comprise a metal clamp for clamping together the half-sleeves  61 ,  62 . The primary advantage of this sleeve embodiment is the half sleeves  61 ,  62  may be positioned about or removed from a completed connection  12 ,  112 , without breaking apart the connection  12 ,  112 .  
         [0036]    In some embodiments, the sleeve may have a substantially homogeneous construction, such that the entire sleeve is made from a single insulating material or group of insulating materials. More likely, however, the sleeve will comprise an insulating inner sleeve for thermal insulation, such as may be represented by sleeve  51 , and a tubular outer shell, such as may be represented by the outer shell  50  surrounding the inner sleeve  51 .  
         [0037]    The insulating material from which the sleeve is made can comprise a variety of materials that are naturally insulating in that they exhibit minimal to no heat conduction. Thus, various polymeric materials may be employed, either alone or in a mixture with fillers. Non-limiting examples of such polymeric materials include biaxially oriented polytetrafluoroethylene, polyurethane, etc. Additionally, the insulating material of the sleeve can be made from expanded exfoliated graphite, as well as from various expanded inorganics such as silicate materials, including vermiculite, etc. When inorganic and mineral materials such as silicates, expanded graphite, etc. are used, they can include reinforcing fillers such as fiberglass, carbon fibers, etc., as well as binders, which can be incorporated and provide the insulating material with structural integrity. A preferred insulating material can comprise an inorganic material such as an expanded silicate, together with a binder, the binder being any one of numerous polymeric materials, both thermoplastic and thermosetting in nature. When the insulating sleeve is comprised of composite materials such as expanded silicates and binders, it will generally include a tubular outer shell as mentioned above, metallic in nature, and preferably, aluminum, stainless steel, etc.  
         [0038]    The outer shell  50  and inner core  51  may be separable, or they may instead be bonded together at an interface  52 . The harder, tougher outer shell  50  may be especially important to protect the sleeve when the sleeve  18 ,  118  is used as a bumper as described above.  
         [0039]    A string may be assembled as long as necessary from a plurality of insulated tubing segments, such as to reach from the earth&#39;s surface to an underground reservoir. Each insulated tubing segment may have an extension at each of two opposing ends, such that a connection may be formed at each end of the tubing segment. An insulating sleeve may be installed at each connection, such that the tubular assembly is continuously insulated along its length, without interruption at each connection.  
         [0040]    It will be understood by those skilled in the art that the embodiment shown and described is exemplary and various other modifications may be made in the practice of the invention. Accordingly, the scope of the invention should be understood to include such modifications which are within the spirit of the invention.