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CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of co-pending application Ser. No. 09/392,043 as filed on Sep. 8, 1999 which in turn depends from and incorporates the subject matter of provisional application Serial No. 06/118,518 as filed on Feb. 3, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention is directed to downhole tools. More specifically, the present invention is directed to a bi-center drilling bit adapted to fit within and drill through a casing shoe without damage to the surrounding casing.  
           [0004]    2. Background  
           [0005]    Bi-center bits are adapted for insertion down a wellbore having a given diameter where, once in position, the rotation of the bi-center bit creates a borehole having a selectedly greater diameter than the borehole.  
           [0006]    In conventional bi-center bits, the bit is designed to rotate about a rototial axis which generally corresponds to the rotational axis defined by the drill string. Such conventional designs are further provided with cutting elements positioned about the face of the tool to reveal a low backrake angle so as to provide maximum cutting efficiency.  
           [0007]    Disadvantages of such conventional bi-center bits lie in their inability to operate as a cutting tool within their pass-through diameter while still retaining the ability to function as a traditional bi-center bit. In such a fashion, a conventional bi-center bit which is operated within casing of its pass-through diameter will substantially damage, if not destroy the casing.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention addresses the above and other disadvantages of prior bi-center drilling bits by allowing selective modification of the use of the tool within the borehole.  
           [0009]    In one embodiment, the present invention includes a drill bit body which defines a pilot section, a reamer section and a geometric axis. The pilot section defines a typical cutting surface about which is disposed a plurality of cutting elements. These elements are situated about the cutting face to generally define a second rotational axis separate from the rotational axis defined by the drill string as a whole. This second or pass-through axis is formed by the rotation of the bit about the pass-through diameter.  
           [0010]    In one embodiment, the pilot section may define a smaller diametrical cross-section so as to further prevent the possibility of damage to the borehole and/or casing when the bit is rotated about the pass-through axis. To further accomplish this goal, a gauge pad may also be situated on the drill bit body opposite the reamer. In yet other embodiments, cutters emphasizing a high back rake angle are employed on the peripheral cutting blades of the tool.  
           [0011]    The present invention presents a number of advantages over prior art bi-center bits. One such advantage is the ability of the bi-center bit to operate within a borehole or casing approximating its pass-through diameter without damaging the borehole or casing. In the instance of use in casing, the casing shoe may thus be drilled through.  
           [0012]    A second advantage is the ability of the same tool to be used as a conventional bi-center bit to create a borehole having a diameter greater than its pass-through diameter. In such a fashion, considerable cost savings may be observed since only one tool need be used where this tool need not be retrieved to the surface to modify its character of use.  
           [0013]    Other advantages of the invention will become obvious to those skilled in the art in light of the figures and the detailed description of the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a side view of a conventional bi-center drill bit;  
         [0015]    [0015]FIG. 2 is an end view of the working face of the bi-center drill bit illustrated in FIG. 1;  
         [0016]    FIGS.  3 A-C are end views of a bi-center bit as positioned in a borehole illustrating the pilot bit diameter, the drill hole diameter and pass through diameter, respectively;  
         [0017]    FIGS.  4 A-B illustrate a conventional side view of a bi-center bit as it may be situated in casing and in operation, respectively;  
         [0018]    [0018]FIG. 5 is an end view of a conventional bi-center bit;  
         [0019]    [0019]FIG. 6 illustrates a cutting structure brazed in place within a pocket milled into a rib of a conventional bi-center drill bit;  
         [0020]    [0020]FIG. 7 illustrates a schematic outline view of an exemplary bi-center bit of the prior art;  
         [0021]    [0021]FIG. 8 illustrates a revolved section of a conventional pilot section cutter coverage as drawn about the geometric axis;  
         [0022]    [0022]FIG. 9 illustrates a revolved section of a conventional pilot section cutter coverage as drawn about the pass-through axis;  
         [0023]    [0023]FIG. 10 illustrates a side view of one embodiment of the bi-center bit of the present invention;  
         [0024]    [0024]FIG. 11 illustrates an end view of the bi-center bit illustrated in FIG. 10;  
         [0025]    [0025]FIG. 12 illustrates a revolved section of the pilot section of the bi-center bit illustrated in FIG. 10, as drawn through the pass-through axis;  
         [0026]    [0026]FIG. 13 illustrates a revolved section of the pilot section of the bi-center bit illustrated in FIG. 10, as drawn through the geometric axis;  
         [0027]    [0027]FIG. 14 illustrates a graphic profile of the cutters positioned on the reamer section of the embodiment illustrated in FIG. 10.  
         [0028]    [0028]FIG. 15 illustrates a schematic view of the orientation of cutters in one preferred embodiment of the invention.  
         [0029]    While the present invention will be described in connection with presently preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents included within the spirit of the invention and as defined in the appended claims. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    FIGS.  1 - 9  generally illustrate a conventional bi-center bit and its method of operating in the borehole.  
         [0031]    By reference to these figures, bit body  2 , manufactured from steel or other hard metal, includes a threaded pin  4  at one end for connection in the drill string, and a pilot bit  3  defining an operating end face  6  at its opposite end. A reamer section  5  is integrally formed with the body  2  between the pin  4  and the pilot bit  3  and defines a second operating end face  7 , as illustrated. The term “operating end face” as used herein includes not only the axial end or axially facing portion shown in FIG. 2, but also contiguous areas extending up along the lower sides of the bit  1  and reamer  5 .  
         [0032]    The operating end face  6  of bit  3  is transversed by a number of upsets in the form of ribs or blades  8  radiating from the lower central area of the bit  3  and extending across the underside and up along the lower side surfaces of said bit  3 . Ribs  8  carry cutting members  10 , as more fully described below. Just above the upper ends of rib  8 , bit  3  defines a gauge or stabilizer section, including stabilizer ribs or gauge pads  12 , each of which is continuous with a respective one of the cutter carrying rib  8 . Ribs  8  contact the walls of the borehole that has been drilled by operating end face  6  to centralize and stabilize the tool  1  and to help control its vibration. (See FIG. 4).  
         [0033]    The pass-through diameter of the bi-center is defined by the three points where the cutting blades are at gauge. These three points are illustrated at FIG. 2 are designated “x,” “y” and “z.” Reamer section  5  includes two or more blades  11  which are eccentrically positioned above the pilot bit  3  in a manner best illustrated in FIG. 2. Blades  11  also carry cutting elements  10  as described below. Blades  11  radiate from the tool axis but are only positioned about a selected portion or quadrant of the tool when viewed in end cross section. In such a fashion, the tool  1  may be tripped into a hole having a diameter marginally greater than the maximum diameter drawn through the reamer section  5 , yet be able to cut a drill hole of substantially greater diameter than the pass-through diameter when the tool  1  is rotated about the geometric or rotational axis “A.” The axis defined by the pass-through diameter is identified at “B.” (See FIGS.  4 A-B.)  
         [0034]    In the conventional embodiment illustrated in FIG. 1, cutting elements  10  are positioned about the operating end face  7  of the reamer section  5 . Just above the upper ends of rib  11 , reamer section  5  defines a gauge or stabilizer section, including stabilizer ribs or kickers  17 , each of which is continuous with a respective one of the cutter carrying rib  11 . Ribs  11  contact the walls of the borehole that has been drilled by operating end face  7  to further centralize and stabilize the tool  1  and to help control its vibration.  
         [0035]    Intermediate stabilizer section defined by ribs  11  and pin  4  is a shank  14  having wrench flats  15  that may be engaged to make up and break out the tool  1  from the drill string (not illustrated). By reference again to FIG. 2, the underside of the bit body  2  has a number of circulation ports or nozzles  15  located near its centerline. Nozzles  15  communicate with the inset areas between ribs  8  and  11 , which areas serve as fluid flow spaces in use.  
         [0036]    With reference now to FIGS. 1 and 2, bit body  2  is intended to be rotated in the clockwise direction, when viewed downwardly, about axis “A.” Thus, each of the ribs  8  and  11  has a leading edge surface  8 A and  11 A and a trailing edge surface  8 B and  11 B, respectively. As shown in FIG. 6, each of the cutting members  10  is preferably comprised of a mounting body  20  comprised of sintered tungsten carbide or some other suitable material, and a layer  22  of polycrystalline diamond carried on the leading face of stud  38  and defining the cutting face  30 A of the cutting member. The cutting members  10  are mounted in the respective ribs  8  and  11  so that their cutting faces are exposed through the leading edge surfaces  8 A and  11 , respectively.  
         [0037]    In the conventional bi-center bit illustrated in FIGS.  1 - 9 , cutting members  10  are mounted so as to position the cutter face  30 A at an aggressive, low angle, e.g., 15-20° backrake, with respect to the formation. This is especially true of the cutting members  10  positioned at the leading edges of bit body  2 . Ribs  8  and  11  are themselves preferably comprised of steel or some other hard metal. The tungsten carbide cutter body  38  is preferably brazed into a pocket  32  and includes within the pocket the excess braze material  29 .  
         [0038]    As illustrated in profile in FIG. 7, the conventional bi-center bit normally includes a pilot section  3  which defines an outside diameter at least equal to the diameter of bit body  2 . In such a fashion, cutters on pilot section  3  may cut to gauge.  
         [0039]    The cutter coverage of a conventional bi-center bit may be viewed by reference to a section rotated about a given axis. FIG. 8 illustrates the cutter coverage for the pilot bit illustrated in FIGS.  1 - 2 . The revolved section identifies moderate to extreme coverage overlap of the cutters, with the maximum overlap occurring at the crown or bottommost extent of pilot section  3  when said pilot section  3  is rotated about geometric axis “A.” The cutter coverage illustrated in FIG. 8 should be compared with the absence of cutter coverage occurring when pilot section  3  is rotated about the pass-through axis “B.” (See FIG. 9.) Clearly, the bi-center bit illustrated in FIG. 9 would be inefficient if used in hard or resilient formations such as a casing shoe.  
         [0040]    When a conventional bi-center bit is rotated about its rotational axis “A,” the bit performs in the manner earlier described to create a borehole having a diameter larger than its pass-through diameter. (See FIGS.  4 A- 4 B.) This result is not desirable when the bit is used in casing to drill through a casing shoe since, while the shoe might be removed, the casing above the shoe would also be damaged. Consequently, it has become accepted practice to drill through a casing shoe using a conventional drill bit which is thereafter retrieved to the surface. A bi-center bit is then run below the casing to enlarge the borehole. However, the aforedescribed procedure is costly, especially in deep wells when many thousand feet of drill pipe may need be tripped out of the well to replace the conventional drilling bit with the bi-center bit. The bi-center bit of the present invention addresses this issue.  
         [0041]    One embodiment of the bi-center bit of the present invention may be seen by reference to FIGS.  10 - 15 . FIG. 10 illustrates a side view of a preferred embodiment of the bi-center bit of the present invention. By reference to the figures, the bit  100  comprises a bit body  102  which includes a threaded pin at one end  104  for connection to a drill string and a pilot bit  103  defining an operating end face  106  at its opposite end. For reasons discussed below, end face  106  defines a flattened profile. A reamer section  105  is integrally formed with body  102  between the pin  104  and pilot bit  103  and defines a second operating end face  107 .  
         [0042]    The operating end face  106  of pilot  103  is traversed by a number of upsets in the form of ribs and blades  108  radiating from the central area of bit  103 . As in the conventional embodiment, ribs  108  carry a plurality of cutting members  110 . The reamer section  105  is also provided with a number of blades or upsets  152 , which upsets are also provided with a plurality of cutting elements  110  which themselves define cutting faces  130 A.  
         [0043]    The embodiment illustrated in FIG. 10 is provided with a pilot section  103  defining a smaller cross-section of diameter than the conventional embodiment illustrated in FIGS.  1 - 8 . The use of a lesser diameter for pilot section  103  serves to minimize the opportunity for damage to the borehole or casing when the tool  100  is rotated about the pass-through axis “B.” 
         [0044]    In a conventional bit, cutters  110  which extend to gauge generally include a low backrake angle for maximum efficiency in cutting. (See FIG. 11.) In the bi-center bit of the present invention, it is desirable to utilize cutting elements which define a less aggressive cutter posture where they extend to gauge when rotating about the pass-through axis. In this connection, it is desirable that cutters  110  at the pass-through gauge and positioned on the leading and trailing blades  118  define a backrake angle of between 30-90 degrees with the formation. Applicant has discovered that a preferred backrake angle for soft to medium formations is 55 degrees. The orientation of cutting elements  110  to define such high backrake angles further reduces the potential for damage to casing  136  when the tool  110  is rotated about the pass-through axis “B.” 
         [0045]    In a preferred embodiment, bit  100  may be provided with a stabilizer pad  160  opposite reamer section  105 . Pad  160  may be secured to bit body  102  in a conventional fashion, e.g., welding, or may be formed integrally. Pad  160  serves to define the outer diametrical extent of tool  100  opposite pilot  103 . (See FIG. 10.) It is desirable that the uppermost extent  161  of pad  160  not extend beyond the top of cutters  110  on reamer blades  152 .  
         [0046]    When rotated in the casing, the tool  100  is compelled to rotate about pass-through axis “B” due to the physical constraints of casing  136 . Casing  136  is not cut since contact with tool  100  is about the three points defined by leading edges  118  and stabilizer pad  160 . As set forth above, edges  118  include cutting elements having a high backrake angle not suited to cut casing  136 . Likewise, pad  160  is not adapted to cut casing  136 . The cutters disposed elsewhere about operating face  107  incorporate a backrake angle of 15°-30° and thus are able to cut through the casing shoe. When the casing shoe has been cut, the tool  100  is able to rotate free of the physical restraints imposed by casing  136 . In such an environment, the tool reverts to rotation about axis “A.” 
         [0047]    The method by which the bi-center bit of the present invention may be constructed may be described as follows. In an exemplary bi-center bit, a cutter profile is established for the pilot bit. Such a profile is illustrated, for example, in FIG. 8 as drawn through the geometrical axis of the tool. The pass-through axis is then determined from the size and shape of the tool.  
         [0048]    Once the pass-through diameter is determined, a cutter profile of the tool is made about the pass-through axis. This profile will identify any necessary movement of cutters  110  to cover any open, uncovered regions on the cutter profile. These cutters  110  may be situated along the primary upset  131  or upsets  132  radially disposed about geometric axis “A.” 
         [0049]    Once positioning of the cutters  110  has been determined, the position of the upsets themselves must be established. In the example where it has been determined that a cutter  110  must be positioned at a selected distance r 1 , from pass-through axis “B,” an arc  49  is drawn through r 1  in the manner illustrated in FIG. 15. The intersection of this arc  49  and a line drawn through axis “A” determines the possible positions of cutter  110  on radially disposed upsets  132 .  
         [0050]    To create a workable cutter profile for a bi-center bit which includes a highly tapered or contoured bit face introduces complexity into the placement of said cutters  110  since issues of both placement and cutter height must be addressed. As a result, it has been found preferable to utilize a bit face which is substantially flattened in cross section. (See FIG. 10.) Once positioning of the upsets has been determined, the cutters  110  must be oriented in a fashion to optimize their use when tool  100  is rotated about both the pass-through axis “B” and geometric axis “A.” By reference to FIGS. 11 and 15, cutters  110  positioned for use in a conventional bi-center bit will be oriented with their cutting surfaces oriented toward the surface to the cut, e.g., the formation. In a conventional bi-center bit, however, cutters  110  so oriented on the primary upset  131  in the area  140  between axes “A” and “B” will actually be oriented 180° to the direction of cut when tool  100  is rotated about pass-through axis “B.” To address this issue, it is preferable that at least most of cutters  110  situated on primary upset  131  about area  140  be oppositely oriented such that their cutting faces  130 A are brought into contact with the formation or the casing shoe, as the case may be, when tool  100  is rotated about axis “B.” This opposite orientation of cutter  110  is in deference to the resilient compounds often comprising the casing shoe.  
         [0051]    Cutters  110  disposed along primary upset  131  outside of region  140  in region  141  are oriented such that their cutting faces  130 A are brought into at least partial contact with the formation regardless when rotated about axis “A.” Cutters  110  oppositely disposed about primary upset  131  in region  142  are oriented in a conventional fashion. (See FIG. 15.) Cutting or wear elements situated on blades which extend to or are proximate the pass-through gauge define a back angle, a skew angle and an angle between the line of contact on the cutting or wear element and the material to be drilled. This angle of contact is preferably between 5 and 45 degrees.  
         [0052]    Cutters  110  not situated on primary upset  131  oriented are disposed on radial upsets  132 . These cutters  110 , while their positioning may be dictated by the necessity for cutter coverage when tool  100  is rotated about axes “A” and “B,” as described above, are oriented on their respective upsets  132  or are skewed to such an angle such that at least twenty percent of the active cutter face  130  engages the formation when the bi-center bit is rotated about axis “A.” Restated as a function of direction of cut, the skew angle of cutters  110  is from 0°-80°.

Summary:
Although particular detailed embodiments of the apparatus and method have been described herein, it should be understood that the invention is not restricted to the details of the preferred embodiment. Many changes in design, composition, configuration and dimensions are possible without departing from the spirit and scope of the instant invention.