The subject disclosure generally relates to the field of elongated structures such as coiled tubing and coiled tubing applications in hydrocarbon wells. More particularly, the subject disclosure relates to increasing the reach of coiled tubing by delaying the onset of buckling.

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/530797 filed Sep. 2, 2011, and U.S. Provisional Patent Application Ser. No. 61/530800 filed Sep. 2, 2011, both of which are hereby incorporated by reference herein in their entireties. 
     
    
     FIELD 
       [0002]    The subject disclosure generally relates to methods and apparatus for moving a rod or pipe through a cylinder. Some embodiments relate to the field of coiled tubing and coiled tubing applications in hydrocarbon wells. The subject disclosure also relates to increasing the reach of coiled tubing by delaying the onset of buckling, although it is not limited thereto. 
       BACKGROUND 
       [0003]    Coiled tubing refers to metal piping, used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells, which comes spooled on a large reel. Coiled tubing operations typically involve at least three primary components. The coiled tubing itself is disposed on a reel and must, therefore, be dispensed onto and off of the reel during an operation. The tubing extends from the reel to an injector. The injector moves the tubing into and out of the wellbore. Between the injector and the reel is a tubing guide or gooseneck. The gooseneck is typically attached or affixed to the injector and guides and supports the coiled tubing from the reel into the injector. Typically, the tubing guide is attached to the injector at the point where the tubing enters. As the tubing wraps and unwraps on the reel, it moves from one side of the reel to the other (side to side). 
         [0004]    Residual bend exists in every coiled tubing string. During storage and transportation, a coiled-tubing string is plastically deformed (bent) as it is spooled on a reel. During operations, the tubing is unspooled (bent) from the reel and bent on the gooseneck before entering into the injector and the wellbore. Residual bending is one of the technical challenges for coiled tubing operations and originates from the spool of the coiled tubing on the reel. Although the reel is manufactured in a diameter as large as possible to decrease the residual bending incurred on the coiled tubing, the maximum diameter of many reels is limited to several meters due to storage and transportation restrictions. 
         [0005]    Coiled tubing is susceptible to a condition known as helical buckling of the tubing which leads to lockup. Residual bending of the coiled tubing increases the susceptibility of the coiled tubing to helical buckling and lockup. As the coiled tubing goes through the injector head, it passes through a straightener; but, the tubing retains some residual bending strain. That strain gives the tubing a helical form when deployed in a wellbore and can cause it to wind axially along the wall of the wellbore like a long, stretched spring. Ultimately, when a long length of coiled tubing is deployed in the well bore, frictional forces from the wellbore wall rubbing on the coiled tubing cause the tubing to bind and lock up, thereby stopping its progression. Lock up limits any further progression as the coiled tubing cannot be pushed further by a force applied at the surface. (Lubinski, A., Althouse, W. S., and Logan, J. L., “Helical Buckling of Tubing Sealed in Packers,” SPE 178, 1962). Such lock up limit the use of coiled tubing as a conveyance member for logging tools in highly deviated, horizontal, or up-hill sections of wellbores. 
         [0006]    There are many methods available to extend the reach of coiled tubing. Some of these methods include tractors, tapered coiled tubing strings, alternate materials e.g., composite coiled tubing, vibrator technologies, straighteners, friction reducers, and injecting a light fluid inside the coiled tubing. These methods are aimed at delaying the onset of buckling which as described above leads to lock-up of the coiled tubing string. 
       SUMMARY 
       [0007]    This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
         [0008]    In some embodiments, the subject disclosure relates to methods of delaying the onset of buckling in an elongated structure having an outer surface traversing a tubular path having an inner surface. The method comprises adapting at least one of the outer surface of the elongated structure and the inner surface of the tubular path to increase a coefficient of friction between the outer surface of the elongated structure and the inner surface of the tubular path in a first direction, while maintaining or decreasing a coefficient of friction between the outer surface of the elongated structure and the inner surface of the tubular path in a second direction; and inserting said elongated structure into the tubular path. 
         [0009]    In some embodiments, the subject disclosure relates to an apparatus wound about a reel and for use in a tubular path. The apparatus comprises a hollow pipe wound about the reel to form a coiled tubing, said pipe when unwound from the reel having a length of at least 1000 feet, an outer diameter of between 0.75 inches and 5.0 inches, and adapted to have at least one of (i) an anisotropic bending stiffness, and (ii) an outer surface adapted to increase a coefficient of friction between the outer surface and an inner surface of the tubular path in a first direction while maintaining or decreasing a coefficient of friction between the outer surface of and the inner surface of the tubular path in a second direction. 
         [0010]    Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the subject disclosure. 
           [0012]      FIG. 1  is a graph of axial load as a function of measured depth for a coiled tubing; 
           [0013]      FIG. 2  is a graph of axial load as function of measured depth for a coiled tubing which is approaching a locked up state; 
           [0014]      FIG. 3  illustrates a coiled tubing with a patterned surface; 
           [0015]      FIG. 4  illustrates a modified inner surface of a casing; 
           [0016]      FIGS. 5A and 5B  illustrate cross-sections of a coiled tubing having an anisotropic cross-section; 
           [0017]      FIG. 6  illustrates one topology for creating anistropic stiffness in a coiled tubing; 
           [0018]      FIG. 7  illustrates another topology for creating anistropic stiffness in a coiled tubing; 
           [0019]      FIGS. 8A and 8B  illustrate cross-sections of a strip of varied cross-sectional diameter that may be manufactured into a coiled tubing having anisotropic bending stiffness; 
           [0020]      FIGS. 8C and 8D  depict a strip that is respectively wound and welded to form a tube; and 
           [0021]      FIG. 8E  depicts a partially cut-away tube formed resulting from the winding and welding of  FIGS. 8C and 8D . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. 
         [0023]    In conventional coiled tubing operations, the tubing is stored as a continuous length of pipe wound on a spool. Depending upon the pipe diameter (typically between 0.75 inches and 5 inches in outer diameter) and the spool size, the coiled tubing can range from at least one thousand feet long to 15,000 feet long or even greater length. The pipe or tube is straightened prior to being translated along the borehole or wellbore (the two being used interchangeably herein) either via gravity or via an injector pushing from a surface. Regardless, the end of the coiled tubing being translated into the borehole is load-free. For an extended reach horizontal wellbore, an axial compressive load will build up along the length of the coiled tubing due to frictional interactions between the coiled tubing and the borehole wall. 
         [0024]    A typical example of axial load for a pipe as a function of measured depth is plotted in  FIG. 1 . The wellbore in which the pipe having the load depicted in  FIG. 1  has a 4000 foot vertical section, a 600 foot transition section (bend) which angles away from the vertical (at a rate of about 15° per 100 feet), and then a horizontal section that remains generally horizontal until the end of the wellbore. As seen in  FIG. 1 , for an example tube which is extended about 7600 feet into the wellbore, the first 3200 feet of the tube is in tension (load greater than zero), and the remainder is in compression (load less than zero). 
         [0025]    If the horizontal section of the wellbore is sufficiently long, the axial compressive load on the tube will be large enough to cause the tubing to buckle. A first buckling mode is referred to as “sinusoidal buckling.” In this mode, the tubing snakes along the bottom of the borehole with curvature in alternating senses. This is considered to be a fairly benign buckling mode, in that neither the internal stresses nor the frictional loads increase significantly. As the axial compressive load continues to increase, the coiled tubing will buckle in a second buckling mode referred to as “helical buckling ” This mode involves the tubing spiraling or wrapping along the borehole (wellbore) wall. For a typical cylindrical pipe, this helical buckling occurs at a predictable axial load and wavelength. Once the tubing begins to buckle helically, the normal force exerted by the borehole wall on the tubing increases very quickly and this buckling may have quite severe consequences. In particular, helical buckling causes a proportional increase in frictional loading, which in turn creates an increase in axial compressive load. Once helical buckling has initiated, the axial compressive load increases very quickly to a level such that the tubing can no longer be pushed into the hole. This condition is termed “lock-up.” 
         [0026]      FIG. 2  depicts a plot of axial load as a function of measured depth for an example coiled tubing which is at or almost at a locked up state in the wellbore described before with respect to  FIG. 1  (4000 feet vertical section, followed by a transition section of 600 feet, followed by a horizontal section). In  FIG. 2 , it is seen that the tubing extends over 9000 feet into the wellbore and it can be deduced from the slope of the curve that the tubing at the transition from the 600 foot transition section to the horizontal section is buckling. As seen in  FIG. 2 , almost the entire length of tubing is under compression. 
         [0027]    In certain embodiments, the onset of buckling of a tubing can be delayed by providing the tubing with certain frictional attributes. In one aspect it is desirable to have reduced friction in the axial direction to facilitate insertion of the coiled tubing into the wellbore. In another aspect, it is desirable to have increased friction in the transverse direction, in order to resist the lateral deformation necessary for the coiled tubing to buckle. Thus, embodiments of the subject disclosure relate to providing the coiled tubing and/or a casing in a wellbore with a modified surface(s) that increase(s) the lateral friction coefficient between the tubing and the casing while maintaining a low axial friction coefficient therebetween. 
         [0028]    More particularly, in order to limit the rate of buildup of axial compressive load in the horizontal section of an extended reach well, it is desirable to maintain a low friction coefficient in the axial direction. In one embodiment, the surface of a coiled tubing string is modified from a standard smooth cylindrical form that yields an isotropic frictional resistance in order to increase the frictional resistance to lateral motion while maintaining the low frictional resistance to axial motion.  FIG. 3  depicts one manner of achieving this anisotropic frictional resistance. In this case, tube  10  is provided with an outer surface  12  that is patterned such that there are axial “rails”  14  that run along the length of the tube.  FIG. 3  depicts the rails being triangular in cross-section and drawn as macroscopic features (on the millimeter level). However, much smaller length scales are also contemplated, in non-limiting examples, micron or nanometer scale. In addition, differently shaped cross-sections could be used. These rails  14  allow the tubing  10  to slide easily in the axial direction, but will provide enhanced resistance to lateral sliding motion. This enhanced resistance to lateral sliding motion will serve to delay the onset of buckling. It will be appreciated that in one embodiment, the rails  14  are not longitudinally continuous. 
         [0029]    In one embodiment, the rails  14  on the outer surface  12  of tube  10  are integral with the tube  10  itself. In another embodiment, the rails  14  are provided on a thin sleeve provided around and affixed to the outer surface  12  of the tube  10 . The thin sleeve may completely cover the outer surface  12  or may provide a partial patterned cover affixed to the outer surface  12 . In another embodiment, independently provided rails  14  are attached to the outer surface  12  of the tube. In embodiments, the rails  14  are adapted to permit the tubing  10  to slide easily in the axial direction, but to provide enhanced resistance to lateral sliding motion. 
         [0030]    In further embodiments, the surface of the wellbore casing is modified to increase the lateral frictional resistance. As seen in  FIG. 4 , a wellbore casing  50  is provided with an inner surface  52  that is patterned with axial “rails”  54  that run along the length of the wellbore casing. These rails  54  allow a standard smooth cylindrical tubing (not shown) to move in an axial direction with low frictional resistance while providing increased frictional resistance to lateral motion. As with the tubing surface modification of  FIG. 3 , the rails are depicted as being triangular in cross-section and drawn as macroscopic features, but much smaller length scales are contemplated. In addition, differently-shaped cross-sections could be used. 
         [0031]    In one embodiment, the rails  54  on the inner surface  52  of casing  50  are integral with the casing  50  itself In another embodiment, the rails  54  are provided with a thin sleeve provided around and affixed to the inner surface  52  of the casing  50 . The thin sleeve may completely cover the inner surface  52  or may provide a partial patterned cover affixed to the inner surface  52 . In another embodiment, independently provided rails  54  are attached to the inner surface  52  of the casing. In embodiments, the rails  54  are adapted to permit a tube to slide easily in the casing  50  in an axial direction, but to provide enhanced resistance to lateral sliding motion. 
         [0032]    In additional embodiments, both the tubing  10  and casing surface  50  could be modified in a complementary fashion in order to further enhance the resistance to lateral sliding motion. In a non-limiting example, if the tubing  10  shown in  FIG. 3  were placed inside the casing  50  shown in  FIG. 4 , the resulting combination would display a large resistance to lateral sliding motion. 
         [0033]    According to other embodiments, the onset of helical buckling may be delayed through modification of the bending stiffness of a tubing cross-section. More particularly, onset of buckling may be delayed through the use of tubing having anisotropic bending stiffness. Bending stiffness may be made anisotropic by appropriate design of the cross-section of the tubing. By way of example only, the cross-section of the tubing may be designed to be non-symmetrical (i.e., anisotropic), thereby permitting the tubing to bend more easily about one axis versus another. 
         [0034]    Helical buckling of an isotropic tube or cylindrical assembly occurs at a predictable level of axial compressive level and at a predictable wavelength or “natural wavelength.” By varying the anisotropy of a bending stiffness spatially with a wavelength incompatible with the natural wavelength of the helical buckle a delay occurs in the development of helical buckling, thus allowing further reach of a cylindrical assembly such as a tubing string. 
         [0035]    Embodiments of the subject disclosure comprise methods for providing a coiled tubing string that delays the onset of helical buckling. In one embodiment, a tube  110  has an anisotropic cross-section  110   a  at one location as seen in  FIG. 5A . An anisotropic cross-section will have different bending stiffness when bending in different directions. For example, the cross-section  110   a  of tube  110  depicted in  FIG. 5A  will bend more easily about axis  2 - 2  than  1 - 1 . The cross-section of tube  110  is shown with a circular outer surface  112  and an oval inner surface  114 . 
         [0036]    In further embodiments, the orientation of the anisotropy can vary along the length of tube.  FIG. 5B  illustrates a cross-section  110   b  taken further along the length of tube  110  of  FIG. 5A  with a different orientation.  FIG. 5B  depicts an orientation which is a 90° rotation of the orientation in  FIG. 5A . Spatial implementation may take a variety of forms, in non-limiting examples, these include varying smoothly, having a characteristic wavelength, random orientation or “jumps” in orientation. Any and all combinations of these spatial variations are contemplated and may suppress the onset of helical buckling The spatial variation can be tailored to a particular coiled tubing dimension and borehole diameter range so as to maximally delay the onset of helical buckling. 
         [0037]    Many other topologies for creating anisotropic stiffness are contemplated. In  FIG. 6 , a cross-section of tube  120  is shown where the outer wall surface  122  is non-concentric with the inner wall surface  124  of tube  120 , thereby providing tube  120  with an anisotropic cross-section. In  FIG. 7 , a cross-section of tube  130  is shown where the inner wall surface  134  is centrally located, but the outer wall  132  of tube  130  is oval in shape, thereby providing tube  130  with an anisotropic cross-section. 
         [0038]    In some situations, such as coiled tubing drilling, the coiled tubing string will be under a state of torsion. This will tend to cause helical buckle in which the spiral wraps in one sense. In these situations, a spatial distribution of the anisotropy which spirals in an opposite sense to this torsion sense may delay the onset of helical buckling. 
         [0039]    It will be appreciated that manufacturing of coiled tubing generally involves making a longitudinal weld along a uniform flat strip. The uniform flat strips are welded together with a bias weld to prepare the final flat strip. The final flat strip is then rolled and a longitudinal weld is manufactured making a tube of uniform outer diameter and inner diameter except for transition zones at bias weld where there may be a change from one uniform inner diameter to another uniform inner diameter. 
         [0040]    In one embodiment, the manufacture of the coiled tubing with anisotropic bending stiffness may involve the rolling of a strip having a non-uniform wall thickness (e.g., such as seen in  FIG. 8A ) into a tube having a uniform outer diameter and performing a longitudinal seam weld. In another embodiment, the manufacture of the coiled tubing with anisotropic bending stiffness may involve the rolling of a strip having a non-uniform wall thickness (e.g., such as seen in  FIG. 8A ) into a tube having a uniform inner surface diameter and performing a longitudinal seam weld. In both cases, the resulting tube will have an anistropic bending stiffness. 
         [0041]    In another embodiment, the manufacture of the coiled tubing with anisotropic bending stiffness may involve rolling a strip of material whose cross-sections change along the length of the strip and performing a longitudinal seam weld. Thus, by way of example, while the thickness of the strip in the cross-section of  FIG. 8A  is largest in the middle and smallest at the ends, the cross-section at a location, by way of example only one foot away, might have transitioned to having the thickness being largest at the ends and smallest at the middle as seen in  FIG. 8B . Effectively, looking lengthwise, the strip would appear to have a helical pattern to its thickness. 
         [0042]    In yet another embodiment, the manufacture of the coiled tubing with anisotropic bending stiffness may involve coiling a strip at an angle as depicted in  FIG. 8C , and welding the strip as depicted in  FIG. 8D  along a helix resultingly formed at adjacent edges of the strip to provide a tube as depicted in  FIG. 8E . The resulting tube may be coiled as desired. The weld itself may introduce the anisotropy to the tube, or the strip might have a varied thickness that introduces anisotropy. 
         [0043]    In certain embodiments, the pipe diameter is between 0.75 inches and 5 inches in outer diameter) and is spooled on a reel (as seen in  FIG. 8D ), and the pipe length when unspooled is at least one thousand feet long. The pipe has either an anisotropic bending stiffness, or an outer surface adapted to increase a coefficient of friction between its outer surface and an inner surface of a tubular path into which it is to be inserted in a first direction while maintaining or decreasing a coefficient of friction between its outer surface and the inner surface of the tubular path in a second direction. 
         [0044]    The apparatus and methods disclosed herein are equally applicable in other oilfield and non-oilfield industries. Non-limiting examples include optic cables, wireline cables, and slickline cables which may be inserted into various cylindrical assemblies, in non-limiting examples, coiled tubing or a wellbore. Non-oilfield applications include the use of embodiments of the subject disclosure in the medical field, non-limiting examples, include applications of stents and other medical devices. 
         [0045]    Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. By way of a non-limiting example only, while a cased wellbore has been shown as providing a tubular path, it will appreciated that the tubular path may be an uncased wellbore (borehole). Also, by way of a non-limiting example only, while a hollow structure (pipe) has been shown as being unwound and inserted into a tubular path, any elongated structure (typically having a length at least 1000 times its width) including an elongated solid structure (rod) may be unwound and inserted into a tubular path. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses, if any, are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.