The present invention provides a method and apparatus for increasing the drift diameter and improving the well path of the well bore, accomplished in one embodiment by cutting away material primarily forming surfaces nearer the center of the drift, thereby reducing applied power, applied torque and resulting drag compared to conventional reamers that cut into all surfaces of the well bore.

CROSS-REFERENCED APPLICATIONS 
     This application is a continuation of, and claims the benefit of the filing date of, U.S. patent application Ser. No. 13/517,870 entitled METHOD AND APPARATUS FOR REAMING WELL BORE SURFACES NEARER THE CENTER OF DRIFT, filed Jun. 14, 2012, which is a continuation of, and claims the benefit of the filing date of, U.S. patent application Ser. No. 13/441,230 entitled METHOD AND APPARATUS FOR REAMING WELL BORE SURFACES NEARER THE CENTER OF DRIFT, filed Apr. 6, 2012, which relates to, and claims the benefit of the filing date of, U.S. provisional patent application Ser. No. 61/473,587 entitled METHOD AND APPARATUS FOR REAMING WELL BORE SURFACES NEARER THE CENTER OF DRIFT, filed Apr. 8, 2011, the entire contents of which are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to methods and apparatus for drilling wells and, more particularly, to a reamer and corresponding method for enlarging the drift diameter and improving the well path of a well bore. 
     Description of the Related Art 
     Extended reach wells are drilled with a bit driven by a down hole motor that can be steered up, down, left, and right. Steering is facilitated by a bend placed in the motor housing above the drill bit. Holding the drill string in the same rotational position, such as by locking the drill string against rotation, causes the bend to consistently face the same direction. This is called “sliding”. Sliding causes the drill bit to bore along a curved path, in the direction of the bend, with the drill string following that path as well. 
     Repeated correcting of the direction of the drill bit during sliding causes friction between the well bore and the drill string greater than when the drill string is rotated. Such corrections form curves in the well path known as “doglegs”. Referring to  FIG. 1 a   , the drill string  10  presses against the inside of each dogleg turn  12 , causing added friction. These conditions can limit the distance the well bore  14  can be extended within the production zone, and can also cause problems getting the production string through the well bore. 
     Similar difficulties can also occur during conventional drilling, with a conventional drill bit that is rotated by rotating the drill string from the surface. Instability of the drill bit can cause a spiral or other tortuous path to be cut by the drill bit. This causes the drill string to press against the inner surface of resulting curves in the well bore and can interfere with extending the well bore within the production zone and getting the production string through the well bore. 
     When a dogleg, spiral path or tortuous path is cut by a drill bit, the relatively unobstructed passageway following the center of the well bore has a substantially smaller diameter than the well bore itself. This relatively unobstructed passageway is sometimes referred to as the “drift” and the nominal diameter of the passageway is sometimes referred to as the “drift diameter”. The “drift” of a passageway is generally formed by well bore surfaces forming the inside radii of curves along the path of the well bore. Passage of pipe or tools through the relatively unobstructed drift of the well bore is sometimes referred to as “drift” or “drifting”. 
     In general, to address these difficulties the drift diameter has been enlarged with conventional reaming techniques by enlarging the diameter  16  of the entire well bore. See  FIG. 1 a   . Such reaming has been completed as an additional step, after drilling is completed. Doing so has been necessary to avoid unacceptable increases in torque and drag during drilling. Such additional reaming runs add considerable expense and time to completion of the well. Moreover, conventional reaming techniques frequently do not straighten the well path, but instead simply enlarge the diameter of the well bore. 
     Accordingly, a need exists for a reamer that reduces the torque required and drag associated with reaming the well bore. 
     A need also exists for a reamer capable of enlarging the diameter of the well bore drift passageway and improving the well path, without needing to enlarge the diameter of the entire well bore. 
     SUMMARY OF THE INVENTION 
     To address these needs, the invention provides a method and apparatus for increasing the drift diameter and improving the well path of the well bore. This is accomplished, in one embodiment, by cutting away material primarily forming surfaces nearer the center of the drift. Doing so reduces applied power, applied torque and resulting drag compared to conventional reamers that cut into all surfaces of the well bore. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1 a  and 1 b    are a cross-section elevations of a horizontal well bore; 
         FIG. 2  is a representation of a well bore illustrating drift diameter relative to drill diameter; 
         FIG. 3  is a representation an eccentric reamer in relation to the well bore shown in  FIG. 2 ; 
         FIG. 4  is a magnification of the downhole portion of the top reamer; 
         FIG. 5  is illustrates the layout of teeth along a downhole portion of the bottom reamer illustrated in  FIG. 1 ; 
         FIG. 6  is an end view of an eccentric reamer illustrating the eccentricity of the reamer in relation to a well bore diameter; 
         FIG. 7  is an end view of two eccentric reamers in series, illustrating the eccentricity of the two reamers in relation to a well bore diameter; 
         FIG. 8  illustrates the location and arrangement of Sets 1, 2, 3 and 4 of teeth on another reamer embodiment; 
         FIG. 9  illustrates the location and arrangement of Sets 1, 2, 3 and 4 of teeth on another reamer embodiment; 
         FIG. 10  is a perspective view illustrating an embodiment of a reamer having four sets of teeth; 
         FIG. 11  is a geometric diagram illustrating the arrangement of cutting teeth on an embodiment of a reamer; 
         FIG. 12A-12D  illustrate the location and arrangement of Blades 1, 2, 3, and 4 of cutting teeth; 
         FIG. 13  is a side view of a reamer tool showing the cutting teeth and illustrating a side cut area; and 
         FIGS. 14A-14D  are side views of a reamer tool showing the cutting teeth and illustrating a sequence of Blades 1, 2, 3, and 4 coming into the side cut area and the reamer tool rotates. 
     
    
    
     DETAILED DESCRIPTION 
     In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, specific details, and the like have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the understanding of persons of ordinary skill in the relevant art. 
       FIG. 1  is a cross-section elevation of a horizontal well bore  100 , illustrating an embodiment of the invention employing a top eccentric reamer  102  and a bottom eccentric reamer  104 . The top reamer  102  and bottom reamer  104  are preferably of a similar construction and may be angularly displaced by approximately 180° on a drill string  106 . This causes cutting teeth  108  of the top reamer  102  and cutting teeth  110  of the bottom reamer  104  to face approximately opposite directions. The reamers  102  and  104  may be spaced apart and positioned to run behind a bottom hole assembly (BHA). In one embodiment, for example, the eccentric reamers  102  and  104  may be positioned within a range of approximately 100 to 150 feet from the BHA. Although two reamers are shown, a single reamer or a larger number of reamers could be used in the alternative. 
     As shown in  FIG. 1 , the drill string  106  advances to the left as the well is drilled. As shown in  FIG. 2 , the well bore  100  may have a drill diameter D1 of 6 inches and a drill center  116 . The well bore  100  may have a drift diameter D2 of 5⅝ inches and a drift center  114 . The drift center  114  may be offset from the drill center  116  by a fraction of an inch. Any point P on the inner surface  112  of the well bore  100  may be located at a certain radius R1 from the drill center  116  and may also be located at a certain radius R2 from the drift center  114 . As shown in  FIG. 3 , in which reamer  102  is shown having a threaded center C superimposed over drift center  114 , each of the reamers  102  (shown) and  104  (not shown) preferably has an outermost radius R3, generally in the area of its teeth  108 , less than the outermost radius R D1  of the well bore. However, the outermost radius R3 of each reamer is preferably greater than the distance R D2  of the nearer surfaces from the center of drift  114 . The cutting surfaces of each of the top and bottom reamers preferably comprise a number of carbide or diamond teeth  108 , with each tooth preferably having a circular cutting surface generally facing the path of movement P M  of the tooth relative to the well bore as the reamer rotates and the drill string advances down hole. 
     In  FIG. 1 , the bottom reamer  104  begins to engage and cut a surface nearer the center of drift off the well bore  100  shown. As will be appreciated, the bottom reamer  104 , when rotated, cuts away portions of the nearer surface  112 A of the well bore  100 , while cutting substantially less or none of the surface  112 B farther from the center of drift, generally on the opposite side of the well. The top reamer  102  performs a similar function, cutting surfaces nearer the center of drift as the drill string advances. Each reamer  102  and  104  is preferably spaced from the BHA and any other reamer to allow the centerline of the pipe string adjacent the reamer to be offset from the center of the well bore toward the center of drift or aligned with the center of drift. 
       FIG. 4  is a magnification of the downhole portion of the top reamer  102  as the reamer advances to begin contact with a surface  112  of the well bore  100  nearer the center of drift  114 . As the reamer  102  advances and rotates, the existing hole is widened along the surface  112  nearer the center of drift  114 , thereby widening the drift diameter of the hole. In an embodiment, a body portion  107  of the drill string  106  may have a diameter D B  of 5¼ inches, and may be coupled to a cylindrical portion  103  of reamer  102 , the cylindrical portion  103  having a diameter D C  of approx. 4¾ inches. In an embodiment, the reamer  102  may have a “DRIFT” diameter D D  of 5⅜ inches, and produce a reamed hole having a diameter D R  of 6⅛ inches between reamed surfaces  101 . It will be appreciated that the drill string  106  and reamer  102  advance through the well bore  100  along a path generally following the center of drift  114  and displaced from the center  116  of the existing hole. 
       FIG. 5  illustrates the layout of teeth  110  along a downhole portion of the bottom reamer  104  illustrated in  FIG. 1 . Four sets of teeth  110 , Sets  110 A,  110 B,  110 C and  110 D, are angularly separated about the exterior of the bottom reamer  104 .  FIG. 5  shows the position of the teeth  110  of each Set as they pass the bottom-most position shown in  FIG. 1  when the bottom reamer  104  rotates. As the reamer  104  rotates, Sets  110 A,  110 B,  110 C and  110 D  110 A,  110 B,  110 C and  110 D pass the bottom-most position in succession. The Sets  110 A,  110 B,  110 C and  110 D of teeth  110  are arranged on a substantially circular surface  118  having a center  120  eccentrically displaced from the center of rotation of the drill string  106 . 
     Each of the Sets  110 A,  110 B,  110 C and  110 D of teeth  110  is preferably arranged along a spiral path along the surface of the bottom reamer  104 , with the downhole tooth leading as the reamer  104  rotates (e.g., see  FIG. 6 ). Sets  110 A and  110 B of the reamer teeth  110  are positioned to have outermost cutting surfaces forming a 6⅛ inch diameter path when the pipe string  106  is rotated. The teeth  110  of Set  110 B are preferably positioned to be rotated through the bottom-most point of the bottom reamer  104  between the rotational path of the teeth  110  of Set  110 A. The teeth  110  of Set  110 C are positioned to have outermost cutting surfaces forming a six inch diameter when rotated, and are preferably positioned to be rotated through the bottom-most point of the bottom reamer between the rotational path of the teeth  110  of Set  110 B. The teeth  110  of Set  110 D are positioned to have outermost cutting surfaces forming a 5⅞ inch diameter when rotated, and are preferably positioned to be rotated through the bottom-most point of the bottom reamer  104  between the rotational path of the teeth  110  of Set  110 C. 
       FIG. 6  illustrates one eccentric reamer  104  having a drift diameter D3 of 5⅝ inches and a drill diameter D4 of 6 1/16 inches. When rotated about the threaded axis C, but without a concentric guide or pilot, the eccentric reamer  104  may be free to rotate about its drift axis C2 and may act to side-ream the near-center portion of the dogleg in the borehole. The side-reaming action may improve the path of the wellbore instead of just opening it up to a larger diameter. 
       FIG. 7  illustrates a reaming tool  150  having two eccentric reamers  104  and  102 , each eccentric reamer having a drift diameter D3 of 5⅝ inches and a drill diameter D4 of 6 1/16 inches. The two eccentric reamers may be spaced apart by ten hole diameters or more, on a single body, and synchronized to be 180 degrees apart relative to the threaded axis of the body. The reaming tool  150  having two eccentric reamers configured in this way, may be able to drift through a 5⅝ inch hole when sliding and, when rotating, one eccentric reamer may force the other eccentric reamer into the hole wall. An eccentric reaming tool  150  in this configuration has three centers: the threaded center C coincident with the threaded axis of the reaming toll  150 , and two eccentric centers C2, coincident with the drift axis of the bottom eccentric reamer  104 , and C3, coincident with a drift axis of the top eccentric reamer  102 . 
       FIGS. 8 and 9  illustrate the location and arrangement of Sets 1, 2, 3 and 4 of teeth on another reamer embodiment  200 .  FIG. 8  illustrates the relative angles and cutting diameters of Sets 1, 2, 3, and 4 of teeth. As shown in  FIG. 8 , Sets 1, 2, 3 and 4 of teeth are each arranged to form a path of rotation having respective diameters of 5⅝ inches, 6 inches, 6⅛ inches and 6⅛ inches.  FIG. 9  illustrates the relative position of the individual teeth of each of Sets 1, 2, 3 and 4 of teeth. As shown in  FIG. 9 , the teeth of Set 2 are preferably positioned to be rotated through the bottom-most point of the reamer between the rotational path of the teeth of Set 1. The teeth of Set 3 are preferably positioned to be rotated through the bottom-most point of the reamer between the rotational path of the teeth of Set 2. The teeth of Set 4 are preferably positioned to be rotated through the bottom-most point of the reamer between the rotational path of the teeth of Set 3. 
       FIG. 10  illustrates an embodiment of a reamer  300  having four sets of teeth  310 , with each set  310 A,  310 B,  310 C, and  310 D arranged in a spiral orientation along a curved surface  302  having a center C2 eccentric with respect to the center C of the drill pipe on which the reamer is mounted. Adjacent and in front of each set of teeth  310  is a groove  306  formed in the surface  302  of the reamer. The grooves  306  allow fluids, such as drilling mud for example, and cuttings to flow past the reamer and away from the reamer teeth during operation. The teeth  310  of each set  310 A,  310 B,  310 C, and  310 D may form one of four “blades” for cutting away material from a near surface of a well bore. The set  310 A may form a first blade, or Blade 1. The set  310 B may form a second blade, Blade 2. The set  310 C may form a third blade, Blade 3. The set  310 D may form a fourth blade, Blade 4. The configuration of the blades and the cutting teeth thereof may be rearranged as desired to suit particular applications, but may be arranged as follows in an exemplary embodiment. 
     Turning now to  FIG. 11 , the tops of the teeth  310  in each of the two eccentric reamers  300 , or the reamers  102  and  104 , rotate about the threaded center of the reamer tool and may be placed at increasing radii starting with the #1 tooth at 2.750″ R. The radii of the teeth may increase by 0.018″ every five degrees through tooth #17 where the radii become constant at the maximum of 3.062″, which corresponds to the 6⅛″ maximum diameter of the reamer tool. 
     Turning now to  FIGS. 12A-12D , the reamer tool may be designed to side-ream the near side of a directionally near horizontal well bore that is crooked in order to straighten out the crooks. As shown in  FIG. 12A-12D , 30 cutting teeth numbered 1 through 30 may be distributed among Sets  310 A,  310 B,  310 C, and  310 D of cutting teeth forming four blades. As plotted in  FIG. 11 , the cutting teeth numbered 1 through 8 may form Blade 1, the cutting teeth numbered 9 through 15 may form Blade 2, the cutting teeth numbered 16 through 23 may form Blade 3, and the cutting teeth numbered 24 through 30 may form Blade 4. As the 5¼″ body  302  of the reamer is pulled into the near side of the crook, the cut of the rotating reamer  300  may be forced to rotate about the threaded center of the body and cut an increasingly larger radius into just the near side of the crook without cutting the opposite side. This cutting action may act to straighten the crooked hole without following the original bore path. 
     Turning now to  FIG. 13 , the reamer  300  is shown with the teeth  310 A of Blade 1 on the left-hand side of the reamer  300  as shown, with the teeth  310 B of Blade 2 following behind to the right of Blade 1, the teeth  310 C of Blade 3 following behind and to the right of Blade 2, and the teeth  310 D of Blade 4 following behind and to the right of Blade 3. The teeth  310 A of Blade 1 are also shown in phantom, representing the position of teeth  310 A of Blade 1 compared to the position of teeth  310 D of Blade 4 on the right-hand side of the reamer  300 , and at a position representing the “Side Cut” made by the eccentric reamer  300 . 
     Turning now to  FIGS. 14A-14D , the extent of each of Blade 1, Blade 2, Blade 3, and Blade 4 is shown in a separate figure. In each of the  FIG. 14A-14D , the reamer  300  is shown rotated to a different position, bringing a different blade into the “Side Cut” position SC, such that the sequence of views  14 A- 14 D illustrate the sequence of blades coming into cutting contact with a near surface of a well bore. In  FIG. 14A , Blade 1 is shown to cut from a 5¼″ diameter to a 5½″ diameter, but less than a full-gage cut. In  FIG. 14B , Blade 2 is shown to cut from a 5⅜″ diameter to a 6″ diameter, which is still less than a full-gage cut. In  FIG. 14C , Blade 3 is shown to cut a “Full Gage” diameter, which may be equal to 6⅛″ in an embodiment. In  FIG. 14D , Blade 4 is shown to cut a “Full gage” diameter, which may be equal to 6⅛″ in an embodiment. 
     The location and arrangement of Sets of teeth on an embodiment of an eccentric reamer as described above, and teeth within each set, may be rearranged to suit particular applications. For example, the alignment of the Sets of teeth relative to the centerline of the drill pipe, the distance between teeth and Sets of teeth, the diameter of rotational path of the teeth, number of teeth and Sets of teeth, shape and eccentricity of the reamer surface holding the teeth and the like may be varied. 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.