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RELATED APPLICATIONS 
       [0001]    This application claims priority to application Serial No. 61/115,787 and is related to application Serial No. 61/115,779, filed concurrently herewith, entitled “Modifying Expansion Forces by Adding Compression.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       TECHNICAL FIELD OF THE INVENTION 
       [0003]    The present disclosure relates generally to a system and a method for expanding an expandable casing in a drilled hole. More particularly, the present invention relates to methods for reducing the amount of expansion force that is required to expand the casing. 
       BACKGROUND OF THE INVENTION 
       [0004]    Conventionally, when a wellbore is created, a number of casings are installed in the borehole to prevent collapse of the borehole wall and to prevent undesired outflow of drilling fluid into the formation or inflow of fluid from the formation into the borehole. The borehole is drilled in intervals whereby a casing which is to be installed in a lower borehole interval is lowered through a previously installed casing of an upper borehole interval. As a consequence of this procedure the casing of the lower interval typically has a smaller diameter than the casing of the upper interval. Thus, the casings are in a nested arrangement with casing diameters decreasing in downward direction. Cements is typically provided between the outer surfaces of the casings and the borehole wall to seal the casings from the borehole wall. 
         [0005]    As a consequence of this nested arrangement, a relatively large borehole diameter is required at the upper part of the wellbore. Such a large borehole diameter involves increased costs due to heavy casing handling equipment, large drill bits and increased volumes of drilling fluid and drill cuttings. In addition, the small diameter casing that is required at the bottom of the hole may not allow desired flow rates of drilling fluid. For these reasons, it may be desirable to expand the diameter of one or more strings of casing so as to reduce the diameter reduction(s) that would otherwise be necessary. Expandable casings are known in the art. 
         [0006]    Expanding the diameter of an upper casing interval allows lower casing intervals to have a greater diameter, since wider sections of pipe will fit through the expanded upper interval casing. Expansion of the casing may be accomplished by passing a mandrel through the casing, among other techniques. The mandrel is typically frustoconical in shape and has a diameter greater than the unexpanded diameter of the casing. In a bottom-up technique, the mandrel is typically placed at the bottom of the casing interval before the casing interval is inserted into the borehole. In some instances, the expandable casing may be lowered into the borehole on the mandrel. After the casing and the mandrel are placed into the borehole, the mandrel is drawn upward through the unexpanded casing, thereby expanding the casing. 
         [0007]    If the expandable casing is resting on and supported by the mandrel, applying an upward force on the mandrel will cause the casing to move upward. In other instances, the casing may not be supported on the mandrel, but the available upward force on the mandrel is insufficient to overcome the expansion force required to begin radially expanding the casing. In either case, it is desired to reduce the expansion force that is required. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The following figures form part of the present specification and are included to further demonstrate certain aspects of the present claimed subject matter, and should not be used to limit or define the present claimed subject matter. Consequently, a more complete understanding of the present embodiments and further features and advantages thereof will be acquired by referring to the following description taken in conjunction with the accompanying drawings, wherein: 
           [0009]      FIG. 1  is a schematic diagram depicting a system for expanding a pipe, according to one embodiment of the present invention; 
           [0010]      FIG. 2  is a schematic diagram depicting another system for expanding a pipe, according to a second embodiment of the invention; 
           [0011]      FIG. 3  is a schematic diagram depicting yet another system for expanding a pipe, according to a third embodiment of the invention; 
           [0012]      FIG. 4  is a schematic diagram depicting yet another system for expanding a pipe, according to a fourth embodiment of the present invention; and 
           [0013]      FIG. 5  is a schematic diagram depicting yet another system for expanding a pipe, according to a fifth embodiment of the invention. 
       
    
    
       [0014]    It is to be noted, however, that the appended drawings illustrate only certain embodiments of the present claimed subject matter and are, therefore, not to be considered limiting of the scope of the present claimed subject matter, as the present claimed subject matter will admit to other equally effective embodiments. 
         [0015]    It will be understood that the Figures are not to scale and are not intended to illustrate the size or relative sizes of the components. In addition, it will be understood that the concepts that are illustrated herein with respect to a vertical borehole are equally applicable to curved, deviated, and otherwise non-vertical boreholes. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring to  FIG. 1 , a first liner  124  is placed inside a borehole  126  within a formation  128 . If the well is offshore, borehole  126  is drilled from a rig resting on the seafloor, a floating rig, or other vessel. In that case, a riser  132 , comprising a long tube of steel from the sea floor to a surface vessel, allows drilling mud to be pumped into borehole  126  and returned to the surface. 
         [0017]    As is known in the art, first liner  124  has an upper end  134 , which may be fixed in the borehole by filling the annulus between first liner  124  and formation  128  with cement. Upper end  134  may be coupled to a blowout preventer (BOP)  138 , which can be closed in the event that excess formation pressure threatens to blow out the well. 
         [0018]    In a typical operation, a drilling cycle continues until the desired depth is reached, whereupon the drill bit is removed and first liner  124  is lowered into the borehole. First liner  124  is then expanded and/or cemented, if desired. The drill bit is then reinserted into borehole  126 , though first liner  124 , and a second drilling cycle begins and continues until the next desired depth is reached. The drill bit is again removed, and second liner  140  is inserted through first liner  124  and into borehole  126 . The outer diameter of second liner  140  is smaller than the inner diameter of first liner  124 , providing clearance as second liner  140  is passed through first liner  124 . Liner  140  has an upper end  144  and a lower end  146 . 
         [0019]    If desired, second liner  140  may be expanded, and the drilling cycles can be continued through first liner  124  and second liner  140 . Typically, one or more intervals of casing will already be positioned in borehole  126  before second liner  140  is placed in borehole  126 . 
         [0020]    Expansion of liner  140  may be carried out by pulling an expansion cone  102  upwardly through liner  140 . Alternatively, expansion may be carried out by providing a hydraulic expansion device that provides a radial expansion force and is moved incrementally through liner  140 . Regardless of how expansion is carried out, it is necessary to overcome the yield strength of the pipe in order to deform it to its expanded diameter. 
         [0021]    Expansion cone or mandrel  102  preferably includes a narrow portion  104  that can fit within liner  140  and a wide portion  106  that has a larger diameter than liner  140 . The wide portion  106  preferably has a diameter that is smaller than the inner diameter of first liner  124 , so that that mandrel  102  can be removed from the casing and drawn up to the surface after expanding liner  140 . 
         [0022]    Mandrel  102  is preferably suspended from a drill string  154  or other guide string, such as are known in the art, that passes through liner  124  and liner  140 . As it passes through liner  140 , mandrel  102  will plastically deform liner  140  radially outward, thereby increasing the inner diameter (and, generally, the outer diameter) of liner  140 . 
         [0023]    In accordance with one embodiment of the present invention, a pressure mechanism  114  is applied to liner  140  in order to facilitate expansion of liner  140 . For example, in the embodiment of  FIG. 1 , a ballast pipe  120  may be included at the upper end of second liner  140 . Ballast pipe  120  preferably remains within first liner  124  when second liner  140  has been lowered to its desired depth. The weight of ballast pipe  120  applies downward compressive force on the upper end of liner  140 . The weight of ballast pipe  120  and the weight of liner  140  itself result in a combined axial compressive load at the bottom of liner  140 . 
         [0024]    It has been found that applying an axial compressive load to an expandable pipe decreases the radial expansion force that is required to plastically deform the pipe. Thus, applying a ballast pipe  120  to the upper end of liner  140  results in a reduction of the required expansion force. When liner  140  is resting on mandrel  102 , the added weight of ballast pipe  120  also results in an increased expansion force applied by the expansion cone to the liner  140 . Thus, if the required expansion force is decreased and the applied expansion force is increased until the two become equal, the application of a ballast pipe or other weighting device to the upper end of liner  140  can be used to initiate radial expansion of liner  140 . Even if liner  140  is not resting on mandrel  102 , such as in cases where liner  140  is resting on the borehole bottom, the added weight of ballast pipe  120  still results in a reduction of the required expansion force. Thus, the use of a ballast pipe or other weighting device is advantageous regardless of whether liner  140  is supported on the expansion device and regardless of whether the expansion device is moving upwardly or downwardly through liner  140 . 
         [0025]    During expansion, liner  140  will have an expanded portion  156  and an unexpanded portion  158 . As mandrel  102  continues to be drawn upward from lower end  146  toward upper end  144 , expanded portion  156  will lengthen until there is nothing left of unexpanded portion  158 . Mandrel  102  is preferably sufficiently narrow to fit through first liner  124  and be retrieved from the surface when drawn upward by a pipe string or other device  154 . 
         [0026]    It should be noted that mandrel  102  need not move upward relative to second liner  140 ; downward movement is also contemplated. Similarly, and as discussed below, liner  140  may be pushed downward over mandrel  102 , or both items may move simultaneously relative to the borehole. Also, radial expansion force may be applied to liner  140  without use of a mandrel, such as through application of hydraulic pressure or mechanical force. If desired, explosives or high-pressure chemical reactions may also or alternatively be used to move mandrel  102  through the pipe. 
         [0027]      FIG. 2  is a schematic diagram depicting another system for expanding a pipe, and includes at least one aspect of the present invention. The various elements shown in  FIG. 2  are similar to like-numbered elements of  FIG. 1 . However, as shown in the system of  FIG. 2 , the ballast that is shown as a separate device  120  in  FIG. 1  may instead comprise part of liner  140 . For example, liner  140  will be slid entirely though first liner  124  until a desired portion  142  is below liner  124 . The portion  143  of liner  140  that lies within liner  124  functions as a weight resting on the lower portion  142  of liner  140 . As described above, the weight of upper portion  143  increases compressive force, reduces required expansion force, and may increase applied expansion force in lower portion  142 . In this embodiment, it is preferred to provide means, such as are known in the art, for severing the portion of the pipe that serves as ballast from the rest of the pipe, so that the ballast can be removed from the borehole. 
         [0028]    It will be understood that various other mechanisms for providing a weight or ballast pipe may be used. For instance, the ballast pipe may have a diameter that is unequal to the diameter of liner  140 , with the result that ballast pipe cannot rest directly on liner  140 . In these instances, the weight of the ballast pipe can be transferred to liner  140  by any suitable weight transfer mechanism at the interface between the ballast pipe and liner  140 . Devices for coupling the ballast pipe or weight to liner  140  include but are not limited to hooks, pegs, teeth, braces, or the like, which may engage corresponding holes, slots, ridges or the like, or otherwise engage liner  140 . The weight of ballast pipe  120  is thus preferably supported until mandrel  102  has passed fully through expandable portion  142 , whereupon the weight of ballast pipe  120  is transferred to mandrel  102  for removal from the borehole. 
         [0029]    Of course, if liner  140  is supported on the expansion device, it is preferred that the total downward force at bottom of second liner  140  not be so great that it overcomes the expansion force prematurely, or liner  140  would slide down over mandrel  102  before it was lowered to the desired position. Thus, either the ballast can be applied to the upper end of liner  140  after liner  140  has been positioned at the desired axial position in the borehole, or the ballast can be applied when liner  140  is not resting on the expansion device. In the former case, it will be preferred to provide some means for preventing the expanded liner  140  from falling downwardly into the borehole, such as by ensuring that the expanded liner  140  engages the borehole wall. Alternatively, the bottom of the expandable can be supported by something other than mandrel  102 , e.g. either the expandable is resting on the borehole bottom or the bottom of the expandable has been expanded (using a jack) and is “set” against the borehole wall. In that case, the added compression merely makes it easier to expand the expandable. 
         [0030]    Mandrel  102  may have a starting angle that provides a relatively large axial compression and a relatively small radial expansion to lower end  146  of liner  140  as mandrel  102  enters liner  140 . Mandrel  102  may also have an expansion angle that is more tapered than the starting angle and that provides a relatively smaller axial compression and relatively greater radial expansion than the starting angle as mandrel  102  moves through second liner  140 . A reverse situation is also possible: mandrel  102  may have a starting angle that is very tapered and that provides a relatively large radial expansion and only a relatively small axial compression to liner  140 . Mandrel  102  may also have an expansion angle that is provides more axial compression and less radial expansion than the starting angle. Mandrels with more than two angles are also contemplated. 
         [0031]    Ballast pipe  120  may comprise any suitable material and need not be expandable. Any weight or other means of providing an axial compression, force or pressure on second liner  140  may be used as ballast pipe  120 . Liner  140  is preferably fabricated of an expandable material. Thus, mandrel  102  simply carries ballast pipe  120  out of the borehole when mandrel  102  is withdrawn from the well. 
         [0032]      FIG. 3  is a schematic diagram depicting yet another system for expanding a pipe. The various elements shown in  FIG. 3  are similar to like-numbered elements of  FIG. 1  and  FIG. 2 . However, as shown in the system of  FIG. 3 , ballast pipe  120  may be replaced by other means of providing axial compression on liner  140 . If desired, for example, a pressure mechanism  114  may include a cup  122  (or a gripper, or a wedge) adjacent to upper end  144  of liner  140 . The application of fluid pressure behind (above) cup  122  will cause cup  122  to deform against the inside of liner  124 , forming a seal. Further pressure will cause cup  122  to bear on upper end  144  of liner  140 . In this manner, cup  122  can apply a compressive force to upper lip  144  of liner  140 , thereby resulting in the same benefits as ballast member  120 . Stationary while an upward compressive force or pressure is applied to lower lip  146  of second liner  140 . 
         [0033]    Referring now to  FIG. 4  pressure mechanism  114  includes an alternative mechanism for providing axial compression to liner  140 . In this embodiment, pressure mechanism  114  includes a first diaphragm  148 , which may be coupled to first liner  124 , and a second diaphragm  150 , which may be coupled to upper lip  144  of second liner  140 . First diaphragm  148  is preferably not coupled to string  154 . A hydraulic line  152  provides fluid access to the space  159  between first and second diaphragms  148 ,  150 . Pumping fluid through line  152  into space  159  results in the application of a compressive force to upper end  144  of liner  140 . In contrast to the embodiment shown in  FIG. 3 , the embodiment shown in  FIG. 4  does not require filling the entire volume of liner  124  with pressurized fluid. Hydraulic line  152  may comprise a hose or other suitable device, such as are known in the art. 
         [0034]    Referring now to  FIG. 5 , pressure mechanism  114  may alternatively include a upper diaphragm  148  that is coupled to mandrel  102 , rather than to first liner  124 . Hydraulic line  152  supplies fluid pressure to the space  159  between upper diaphragm  148  and lower diaphragm  150 . In this embodiment, the fluid pressure will force lower diaphragm  150  downward from first diaphragm  148 , while simultaneously forcing upper diaphragm  148  upward, thereby drawing mandrel  102  upward through liner  140 . Upper diaphragm  148  and lower diaphragm  150  cooperate to apply an axial compressive force to second liner  140 . 
         [0035]    In the embodiments shown in to  FIG. 4  and to  FIG. 5 , the application of hydraulic pressure will result in increased compressive force on liner  140 . 
         [0036]    In one implementation, the downward compressive force that is applied to liner  140  will approximately equal the upward axial force that is applied by the mandrel. Accordingly, the upward axial force applied by the mandrel and the compressive force in the second axial direction will provide a net zero axial force, such that the only net force on the pipe is radially outward. In another implementation, the compressive force that is applied in the second axial direction will be substantially greater than the upward axial force that is applied by the mandrel. In this case, if the liner is not resting on something (such as the borehole bottom), the downward force may be sufficient to move the pipe past the mandrel. In another implementation, the bottom of the pipe engages the borehole wall such that the wall applies a downward force in opposition to the upward force applied by the mandrel. In this case, the applied compressive facilitates expansion by reducing the required expansion force. In another, less desirable implementation, the upward axial force that is applied by the mandrel causes the mandrel to move upward through the pipe, while expanding the pipe. 
         [0037]    In some embodiments, a jack may be used to initiate deformation (i.e. movement of the mandrel relative to the pipe). The pressure mechanism serves to increase compressive force on the pipe at the expansion point, thereby reducing the expansion (jacking) force. 
         [0038]    Several implementations and embodiments have thus been described. It will be appreciated, however, that other implementations and embodiments will also be substituted within the scope of this disclosure. For example, the mandrel may be replaced with an electromechanical device (such as a motor) that can apply a radial force that is greater than the tension within the drill string, and that is also greater than the weight of the fluid in the well. The electromechanical device may also include a sensor that can detect cracks or other structural problems within the pipe, and may be able to adjust a magnitude of the radial force in accordance with an ability of the pipe to sustain the radial force without damage. 
         [0039]    Thus, although the invention has been described with reference to several exemplary embodiments, it is understood that the words that have been used are words of description and illustration, rather than words of limitation. Although the invention has been described with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed; rather, the invention extends to all functionally equivalent structures, methods, and uses such as are within the scope of the appended claims.

Summary:
A method for expanding a tubular in a borehole, the tubular having upper and lower ends, the system comprises a) applying a compressive load to the upper end of the tubular and b) expanding the tubular by moving an expansion device relative to the tubular while maintaining the compressive load. Step a) may include resting a weight on the upper end of the tubular or applying hydraulic pressure to the upper end of the tubular. The lower end of the tubular may engage the formation before step b) or as a result of step b).