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 . 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 . a typical example of axial load for a pipe as a function of measured depth is plotted in fig1 . the wellbore in which the pipe having the load depicted in fig1 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 fig1 , 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 ). 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 .” fig2 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 fig1 ( 4000 feet vertical section , followed by a transition section of 600 feet , followed by a horizontal section ). in fig2 , 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 fig2 , almost the entire length of tubing is under compression . 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 . 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 . fig3 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 . fig3 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 . 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 . in further embodiments , the surface of the wellbore casing is modified to increase the lateral frictional resistance . as seen in fig4 , 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 fig3 , 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 . 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 . 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 fig3 were placed inside the casing 50 shown in fig4 , the resulting combination would display a large resistance to lateral sliding motion . 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 . 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 . 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 fig5 a . 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 fig5 a 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 . in further embodiments , the orientation of the anisotropy can vary along the length of tube . fig5 b illustrates a cross - section 110 b taken further along the length of tube 110 of fig5 a with a different orientation . fig5 b depicts an orientation which is a 90 ° rotation of the orientation in fig5 a . 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 . many other topologies for creating anisotropic stiffness are contemplated . in fig6 , 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 fig7 , 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 . 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 . 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 . 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 fig8 a ) 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 fig8 a ) 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 . 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 fig8 a 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 fig8 b . effectively , looking lengthwise , the strip would appear to have a helical pattern to its thickness . 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 fig8 c , and welding the strip as depicted in fig8 d along a helix resultingly formed at adjacent edges of the strip to provide a tube as depicted in fig8 e . 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 . 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 fig8 d ), 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 . 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 . 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 .