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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     The present invention relates generally to drilling and completion of wellbores, and more particularly to use of a fluid-driven rotatable shoe at a bottom end of a casing string to cut or mill away obstructions in the wellbore that may otherwise interfere with, or prevent, the casing string from reaching the desired depth. 
     BACKGROUND OF THE INVENTION 
     In the oil and gas production industry, wellbores drilled into the earth to access hydrocarbons from subsurface formations are typically lined with metal tubulars lowered into the wellbore in an assembly of tubulars connected end to end to form a string. Such wellbore lining is generally referred to as casing, and is typically cemented into place once a desired depth has ben reached. Often, after such installation of a first casing string, the depth of the wellbore is extended by drilling a smaller bore through the bottom of the cemented-in casing. Further casing of smaller diameter than the first can then be run into the second bore through the first section of casing and then cemented in place in a similar process. 
     When running casing into a previously drilled wellbore, a casing string may encounter obstructions preventing it from reaching the desired depth, such as ledges, collapsed borehole sections, or other discontinuities of the wellbore. 
     Accordingly, there have been publications proposing to cut or mill away such obstructions during running of a casing string driving a rotatable casing shoe that is carried on the bottom end of the casing string and equipped with cutting edges or abrasive elements. 
     U.S. Pat. No. 7,849,927 and U.S. Patent Application Publication No. 2010/0032170 disclose such solutions, in which the rotatable shoe and the fluid-operated drive mechanism for same are sacrificial, i.e. intended to be left downhole during cementing of the casing string, and are drillable, soluble or degradable so as not to prevent further drilling of the wellbore past the bottom of the cemented casing. 
     Applicant has developed an improved rotatable shoe and drive configuration which can be used to reduce the parts and complexity of the fluid-operated drive mechanism and recover components thereof from downhole, thereby potentially lowering the cost of such a sacrificial tool. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided an apparatus for running a casing string into a wellbore, the apparatus comprising: 
     a housing arranged at an upper end for coupling to a bottom end of the casing string in a manner fluidly coupling an interior of the housing with an interior of the casing; 
     a shoe rotatably supported at a lower end of the housing; 
     a drive mechanism comprising a shaft extending internally along the housing toward the upper end thereof from a connection of the shaft to the shoe, and a ribbon coiling around and along the shaft toward the shoe in a manner radially spaced from the shaft above the connection thereof to the shoe to drive rotation of the shaft through action of a fluid on said ribbon under pumping of said fluid through the housing from the casing. 
     Preferably there is provided at least one fluid outlet port communicating the interior of the housing with an exterior thereof. 
     Preferably said at least one fluid outlet port includes at least one lower outlet port located in the shoe. 
     Preferably said at least one fluid outlet port includes at least one upper outlet port located in a wall of the housing. 
     Preferably the ribbon coils helically around the shaft. 
     Preferably there is provided a stabilizer coupled to an upper end of the shaft to locate an axis of said shaft within the housing. 
     Preferably the stabilizer comprises an outer circumference arranged to engage against an interior surface of the housing and an inner portion coupled to the upper end of the shaft, the inner portion comprising an inner opening communicating with a hollow interior of the shaft to enable fluid flow from the casing to the shoe through the shaft. 
     Preferably the stabilizer comprises outer openings disposed between the inner portion and the outer circumference to enable fluid flow into the interior of the housing from the casing to act on the ribbon to drive rotation of the shaft and the shoe. 
     Preferably the inner portion is annular around the inner opening and the stabilizer comprises radial arms extending outward from the annular inner portion between the outer openings to connect to the outer circumference. 
     Preferably the stabilizer comprises a fixed portion rigidly attached to the housing and a coupling engaged between the fixed portion and the shaft to allow relative rotation therebetween. 
     Preferably the coupling comprises a bushing. 
     Preferably there is provided at least one bearing mounted between the housing and one or both of the shaft and the shoe. 
     Preferably the shaft, the shoe and races of each bearing are drillable. 
     Preferably, expect for roller elements, all components of each bearing are drillable. 
     According to a second aspect of the invention there is provided a method for running a casing string into a wellbore, the method comprising: 
     running the casing string into the wellbore with a shoe at a bottom end thereof rotatably connected to the casing string by at least one bearing; 
     while running the casing string, rotating the shoe relative to the casing string by pumping fluid down the casing string to drive rotation of a fluid-driven rotor that is coupled to the shoe from thereabove; 
     when the casing string reaches a desired depth, cementing the casing string in the wellbore; 
     using a drill string, at least partially drilling out the fluid-driven rotor, races of the at least one bearing and the shoe; and 
     retrieving roller elements of the bearings by circulating fluid down through the drill string and back up to surface through an annulus between the cemented casing string and the drill string to carry said roller elements back to the surface through said annulus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, which illustrate exemplary embodiments of the present invention: 
         FIG. 1  is a vertical cross-sectional view of an apparatus of the present invention for clearing of wellbore obstructions while running a casing string. 
         FIG. 2  is an overhead plan view of a stabilizer of a rotational drive mechanism for a rotatable casing shoe of the apparatus of  FIG. 1 . 
         FIG. 3  is a schematic vertical cross-sectional view of the apparatus of  FIG. 1  during running in of the casing string. 
         FIG. 4  is a schematic vertical cross-sectional view of the apparatus of  FIG. 1  during cementing in of the casing string. 
         FIG. 5  is a schematic vertical cross-sectional view of the apparatus of  FIG. 1  during drilling out thereof after cementing of the casing string. 
         FIG. 6  is a schematic vertical cross-sectional view of the apparatus of  FIG. 1  after drilling fully therethrough to extend the wellbore to a further depth. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an apparatus or tool  10  of the present invention for clearing obstructions from a drilled wellbore while running in a casing string to be used to line the wellbore. The tool  10  features a hollow tubular housing  12  having an externally threaded upper end  14 . A coupling  16  internally threaded at each end receives the externally threaded upper end  14  of the housing  12  from below, and threadingly engages an externally threaded bottom end of a casing string  18  above the tool  10 , thereby coupling the tool housing  12  to the bottom end of the casing string  18 . 
     A stabilizer  20  is fixed onto the upper end  14  of the housing  12  and, with reference to  FIG. 2 , features an outer ring  22 , an inner ring  24  and a plurality radial arms  26  extending therebetween at angularly spaced positions around the inner ring  24 . The outer ring  22  defines a circular outer periphery or circumference of the stabilizer that sits at the inner surface of the housing&#39;s cylindrical wall, to which the outer ring is rigidly fixed. As shown in  FIG. 1 , an underside of the inner ring  22  features a cylindrical collar  28  that projects downward from the plane of the radial arms  26  and outer ring  22  in a manner concentric with the inner ring  24  at a distance outward from the ring&#39;s aperture or opening  30 . A bushing  32  is received and retained in the bore of the collar  28  to reside beneath the inner ring  24 . In addition to the central opening  30  defined by the inner ring, the stabilizer features a plurality of outer openings  34  positioned around the inner ring between the inner and outer rings, each outer opening spanning arcuately about the inner ring between an adjacent pair of the radial arms  26 . 
     Below the stabilizer  20 , the tool  10  features a rotor  36  rotatably disposed concentrically within the housing  12 . The rotor  36  features a central shaft  38  extending longitudinally on the shared axis of the casing  18  and tool housing  12 , and a helical ribbon  40  coiling around the shaft  38  at a radial distance outward therefrom. Radial support arms carry the ribbon  40  on the shaft at discrete spaced apart positions along the ribbon, i.e. spaced around and along the shaft axis. The top end of the central shaft  38  is received inside the bushing  32  of the stabilizer  20  so that the shaft  38 , and the helical ribbon  40  rigidly fixed thereto, are rotatable relative to the stabilizer and the surrounding tool housing  12 . The rotor shaft  38  is tubular, thus having a hollow interior that is open at each end of the shaft  38 . The opening at the top end of the rotor shaft  38  thus opens to the central opening  30  of the stabilizer, thereby fluidly communicating the hollow interior of the rotor shaft  38  with the interior of the casing  18  through the hollow interior passage of the coupling  16 . 
     The rotor  36  may be formed as a single, unitary piece in which the rotor shaft  38 , ribbon  40  and radial support arms therebetween are all seamlessly integral with one another, for example by machining the rotor from a single piece of stock material. The term ribbon is used to denote the shape of the helical element as a relatively thin band of material coiling around the rotor shaft, and is not intended to denote that this element is a flexible member. Rather, the rotor is made of a material that is sufficiently strong and rigid to be self-supporting and shape-consistent during use the tool to run in a casing string, but yet is drillable using available drill string bits for reasons described herein below. Known materials suitable for such application include aluminum, and drillable alloys and lower grade steels. In the illustrated embodiment, the inner and outer surface of the helical ribbon facing toward the shaft and the housing respectively are each axially oriented surfaces, i.e. parallel to the shaft axis in a direction moving downward therealong, but other embodiments may feature variations of the ribbon shape or orientation while still coiling around and downward along the shaft in a manner rotating the shaft under the action of fluid being pumped downwardly past the ribbon inside the housing. 
     Fixed to the bottom end of the rotor shaft  38  is a driven shaft  42  that continues from the rotor shaft  38  downward along the common longitudinal casing and tool axis. Like the rotor shaft  38 , the driven shaft  42  is tubular to allow fluid to flow longitudinally through it. The hollow interiors of the two shafts  38 ,  42  communicate with another through their connection at the top end of the driven shaft  42 . The driven shaft  42  extends downward past the bottom end  44  of the tool housing  12 , and at a distance upward from the housing&#39;s bottom end  44 , features a annular flange  46  projecting radially outward around the circumference of the shaft to position the circumference of the flange adjacent the inner surface of the tool housing  12 . In another embodiment, the assembling of two separate shaft sections may be avoided by instead employing a single, unitary, seamlessly integral shaft in place of the two interconnected shafts of the illustrated embodiment. 
     Below the shaft flange  46 , an annular rim  48  of the housing  12  juts a short distance radially inward from the inner surface of the housing&#39;s cylindrical wall toward the driven shaft  42 . The rim  48  is relatively small so as to leave an open annuls between the inner extent of the rim  48  and the outer periphery of the driven shaft  42 . Seated atop the rim  48  between the rim&#39;s annular upper surface and the underside of the shaft flange  46  is an upper bearing  50  having its outer race  50   a  fitted against the inner surface of the housing wall and its inner race  50   b  fitted against the outer circumference of the driven shaft  42 . Similarly, a lower bearing  52  situated at the underside of the rim  48  has its outer race  52   a  engaged with the inner surface of the housing wall and its inner race  52   b  engaged with the outer circumference of the driven shaft  42 . 
     Beneath the bottom end  44  of the tool housing  12 , a bullnose shaped shoe  54  is fixed to the bottom end of the driven shaft  42 , and due to the rotatable support of the rotor shaft  38  and driven shaft  42  by the stabilizer bushing  32  and upper and lower bearings  50 ,  52 , the shoe is rotatable relative to the housing  12 , together with the two shafts. The upper end of the shoe  54  features a central internal bore  56  into which the bottom end of the driven shaft  42  is fitted. The open bottom end of the hollow driven shaft  42  opens into the central bore  56  of the shoe  54 , and a plurality of lower outlet ports  58  each extend downward from the central interior bore  56  to the exterior of the shoe  54  at the rounded lower end thereof. Higher up the tool, upper outlet ports  60  extend through the wall of the housing  12  above the flange  46  of the driven shaft  42 . 
     With reference to  FIG. 3 , during running of the casing string  18  into a wellbore  70 , fluid is pumped down to the tool  10  through the inner bore of the casing string  18 , where some of the fluid passes through the central opening  30  of the stabilizer  20  into the rotor shaft  38 , and onward therefrom through the driven shaft  42  into the shoe  54 , where the fluid exits the tool through the lower outlet ports  58  at the tip of the shoe. The rest of the fluid from the casing string  18  enters the tool housing  12  through outer openings  34  of the stabilizer  20 . Moving down through the annulus  72  between the tool housing wall and the rotor shaft  38 , this fluid acts on the helical ribbon  40 , the downward force of the ribbon resulting in rotation of the rotor  36  in the downward coiling direction of the helical ribbon  40  (i.e. counterclockwise as viewed from above for the illustrated embodiment). 
     Coupled together, the driven shaft  42  and attached shoe  54  rotate together with the rotor shaft  38 . The engagement of the stabilizer  20  against the inner surface of the housing wall keeps the rotor shaft  38  centered in the housing during this rotation, and the close positioning of the circumference of the flange  46  of the driven shaft at the inner surface of the tool housing likewise keeps the driven shaft  42  centered. Cutting or abrasive elements on the exterior of the shoe  54  thus cut, mill or abrade away material the shoe runs into or brushes against during its decent in the wellbore. The fluid having acted on the rotor  36  exits the housing  12  of the tool  10  at the upper outlet ports  60 , from where the pumping pressure circulates it back up to surface through the annulus between the casing string and the surrounding wall of the wellbore. The other stream of fluid existing the tool through the lower outlet ports  58  in the shoe  54  dislodges or clears away cuttings or loosened material freed from the wellbore obstruction acted on the by the cutting, milling or abrading shoe  54 . 
     With reference to  FIG. 4 , once the casing string has been run to a desired depth, the circulation of fluid is ceased, and cement is instead pumped down the casing string, where through the ports  58 ,  60  of the tool  10 , it fills the bottom of the well bore and builds up inside the annular space outside the tool and the casing string, as well as inside the tool itself. Turning to  FIG. 5 , when the cement has solidified, a drill string  74  can be lowered through the cemented to drill out the tool to extend the wellbore  70  past the previously run casing to further depths. That is, the stabilizer, shafts, ribbon, bearing races and shoe are made of drillable material(s), such as aluminum, that can be drilled out with the cement that has hardened inside the tool housing  12 . During such drilling, the hardened cement inside the tool housing holds these components stationary against the rotational action of the drill string so that the drill bit  76  will work away these components rather than simply rotate them with the drill string after initial engagement by the working tip of the drill bit. 
     For strength during use of the tool, the roller elements  50   c ,  52   c  of the bearings  50 ,  52  may be made of a harder, stronger material that is not readily or easily drillable like the rotor  36 , driven shaft  42 , shoe  54  and bearing races. During drilling out of the tool  10 , the bearing races and driven shaft located inward thereof will be broken up by the drilling action, thus releasing the roller elements of the bearings under advancement of the drill bit in the downhole direction. IN the illustrated embodiment, the bearings, drill string and casing string are selected in a combination providing appropriate relative sizing between the bearing roller elements, the drill string outer diameter and the casing string inner diameter to allow production of the bearing roller elements to surface in the annular space between the drill string and the surrounding casing. That is, with reference to  FIG. 6 , when the drilling process has dislodged the bearing roller elements  50   c ,  52   c  from the bearing races, circulation of fluid down through the drill string and back up to surface through this annulus can carry the freed bearing roller elements back up to the surface for recovery. 
     In the illustrated embodiment, the drill bit  76  is sized to drill away the rotor shaft, the driven shaft and the inner races of the bearings, and the drill string shaft is smaller by an amount sufficient to accommodate the bearing roller elements in the in the annular space between the drill string shaft and any remnants of the drilled away components that may remain, for example including the helical ribbon  40  and remnants of the helical ribbon support arms on the rotor shaft and of the flange  46  of the driven shaft. The illustrated helical ribbon  40  occupies nearly the full inside diameter of the tool housing, and so drilling away the rotor shaft and parts of the ribbon support arms thereon leaves the helical behind, as shown in  FIGS. 5 and 6 . However, the ribbon is still preferably made of drillable material so as not to interfere with the drilling operation under contact thereof by the drill bit. 
     Drillable, rotatable casing shoes are known in the prior art, and accordingly limited detail is provided herein on the shoe of the tool, as different shoe shapes, structures, materials and cutting/milling/abrading elements may be employed, including those known from the prior art documents identified in the background section above, and other references that disclose rotatable drilling shoes that are used to actually drill the wellbore with the casing to avoid the need to first trip use a separate drill string to pre-form the wellbore before running the casing. In a known manner, the shoe may employ hard material at the outer circumference thereof for cutting performance, while using a softer readily drillable material for the central portion of the shoe that directly underlies the casing bore. In the illustrated embodiment, the maximum outer diameter of the shoe  54  is equal to the outer diameter of the cylindrical tool housing, which is also equal to the outer diameter of the casing. In other embodiments, the maximum outer diameter of the shoe  54  may be somewhat larger than the casing and tool diameter, for example equal to or slightly larger than the outer diameter of the casing-tool coupling  16 , which in the illustrated embodiment is slightly larger than the casing and tool housing. 
     Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without department from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.

Summary:
Apparatus for running a casing string into a wellbore features a housing arranged for coupling to the casing string in fluid communication with an interior thereof, a shoe rotatably supported at a lower end of the housing, and a drive mechanism. The drive mechanism features a shaft extending internally along the housing toward the upper end thereof from a connection of the shaft to the shoe, and a ribbon coiling around and along the shaft toward the shoe in a manner radially spaced from the shaft above the connection thereof to the shoe. The ribbon drive rotations of the shaft through action of a fluid on said ribbon under pumping of said fluid through the housing from the casing. The drive mechanism thus employs a simple structure of a rotor assembly spinning within the housing, without requiring an additional stator or housing-carried vanes.