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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to drill bits useful for drilling oil, gas and water wells and methods for manufacturing such bits. More specifically, the present invention relates to a bi-center bit or two-piece bi-center bit which includes a sub-reamer section to aid in enhancing stability of the tool when rotated in the borehole. 
   2. Description of the Prior Art 
   A significant source of many drilling problems relates to drill bit and string instability. Bit and/or string instability probably occurs much more often than is readily apparent by reference to immediately noticeable problems. However, when such instability is severe, it places high stress on drilling equipment that includes not only drill bits but other downhole tools and the drill string in general. Common problems caused by such instability may include, but are not limited to, excessive torque, directional drilling control problems, and coring problems. 
   One typical approach to solving these problems is to over-design the drilling tool to thereby resist the stress. However, this solution is usually expensive and can actually limit performance. For instance, one presently commercially available drill bit includes reinforced polycrystalline diamond compact (“PDC”) members that are strengthened by use of a fairly large taper or frustoconical contour on the PDC member. The taper angle in this tool is smaller than the backrake angle of the cutter to allow the cutter to cut into the formation at a desired angle. While this design makes the PDC cutters stronger so as to reduce cutter damage, it does not solve the primary problem of bit instability. Thus, drill string problems, directional drilling control problems, and excessive torque problems remain. Also, because the PDC diamond table must be ground on all of the PDC cutters, the drill bits made in this manner are more expensive and less resistant to abrasive wear as compared to the same drill bit made with standard cutters. 
   Another prior art solution to bit instability problems is directed toward a specific type of bit instability that is generally referred to as bit whirl. Bit whirl is a very complicated process that includes many types of bit movement patterns or modes of motion wherein the bit typically does not remain centered within the borehole. The solution is based on the premise that it is impossible to design and build a perfectly balanced bit. Therefore, an intentionally imbalanced bit is provided in a manner that improves bit stability. One drawback to this method is that for it to perform properly in the borehole, the bit forces must be the dominant force acting on the bit. These bits are generally designed to provide for a cutting force imbalance that may range about 500 to 2000 pounds depending on bit size and type. Unfortunately, there are many cases where gravity or string movements create forces larger than the designed cutting force imbalance and therefore become the dominant bit forces. In such cases, the intentionally designed imbalance is ineffective to prevent the tool from becoming unstable and whirling in operation in the borehole. 
   Yet other attempts to reduce bit instability have incorporated devices that are generally referred to as penetration limiters. Penetration limiters work to prevent excessive cutter penetration into the formation that can lead to bit whirl or cutter damage. These devices may act to prevent not only bit whirl but also prevent radial bit movement or tilting problems that occur when drilling forces are not balanced. Furthermore, penetration limiters reduce bit rate of penetration of used too extensively but do not significantly improve stability of used too sparingly. Due to the variety of different applications, it is frequently difficult to determine to what extent penetration limiters should ve used. 
   While the above background has been directed to drill bits in general, more specific problems of bit instability are created in the instance of the bi-center bit. Bi-center bits have been used sporadically for over two decades as an alternative to undereamers. A desirable aspect to the bi-center bit is its ability to pass through a small hole and then drill a hole of a greater diameter. Problems associated with the bi-center bit, however, include those of a short life due to irregular wear patterns and excessive wear, the creation of a smaller than expected hole size and overall poor directional characteristics. 
   As in the instance of conventional drill bits, many solutions have been proposed to overcome the above disadvantages associated with instability and wear. One such proposed solution includes the use of penetration limiters to enhance the stability of the bi-center bit. As set forth above, penetration limiters prevent disadvantages in the reduction of the rate of penetration when a high number of limiters is used. Secondly, the geometry of a bi-center bit limits the number of positions for penetration limiters on one side of the bit. Placing more penetration limiters on one side of the bit can cause a force imbalance that makes the bit less stable. 
   Other proposed solutions include the use a stabilizer between the pilot and the reamer. The disadvantage of this is that it requires that the pilot bit produce a true size hole. Frequently, the pilot bit will create an oversized hole which prevents the stabilizer from contacting the hole wall or allows the bit to move laterally until the stabilizer does contact the hole wall which causes the reamer to produce an undersized hole. 
   As a result of these and other proposed problems, the bi-center bit has yet to realize its potential as a reliable alternative to undereaming. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above identified and other disadvantages usually associated with the instability and poor wear characteristics associated with drill bits, and more particularly, bi-center type downhole drilling tools. 
   The downhole tool of the present invention generally comprises a proximal end adapted to be operably coupled to the drill string, and a distal end. The proximal end typically comprises a threaded pin. A pilot bit is formed about the distal end face and includes a plurality of cutting elements, e.g., PDC cutting elements. A reamer section is formed on one side or quadrant of the body between the threaded pin and the pilot section. This reamer section also includes cutting elements disposed about one or more cutting blades or upsets. These cutting blades, if more than one, are configured about a radial arc of less than 180 degrees. 
   A sub-reamer section is positioned between the pilot and the reamer section. In one embodiment, the sub-reamer section includes cutting blades or upsets also radially distributed in an arc of at least 180 degrees, using the axis of rotation “AA” as the center, where the end points of these cutter blades extend to a distance from “AA” equal to one-half of the diameter of the maximum tool size. In this first embodiment, the cutting blades can be oriented about any radial position vis-a-vis the reamer section. 
   A second embodiment of the invention incorporates the same pilot and reamer sections as described in association with the first embodiment. In this embodiment, the sub-reamer section is provided with cutting blades where the maximum distance of the endpoints of these blades as measured from “AA” varies depending on the position of the blades vis-a-vis the reamer section. In this embodiment, the endpoints of the cutter blades extend a maximum distance from “AA” equal to one-half of the reamer drill diameter, but do not exceed one-half of the tool pass-through diameter, as measured from the pass-through axis “AB”. These blades preferably are oriented in a radial arc about the sub-reamer section which exceeds 180 degrees. 
   In yet another embodiment incorporating the same pilot and reamer sections as described in association with the first embodiment, the sub-reamer section is provided with cutting blades which define endpoints where the distance of these endpoints from “AA” varies depending on the position of the cutting blades vis-a-vis the reamer section. The endpoints of these blades extend to a maximum distance from “AA” which is equal to one-half of the tool pass-through diameter, which is measured from the pass-through axis “AB”. In this embodiment, the cutting blades describe a radial arc of less than 180 degrees, where this arc is disposed opposite the reamer section. 
   The present invention has a number of advantages over the prior art. One such advantage is enhanced stability in the borehole during a variety of operating conditions. Another advantage is improved wear characteristics of the tool. 
   In some applications, it is required that a pilot hole be drilled with another drilling tool before the bi-center is used or in the middle of a bi-center run. Conventional bi-centers can do this only if the pilot hole is the same size as the bi-center pilot bit or smaller. If the pilot hole is larger than the bi-center pilot bit then the bi-center will produce an undersized hole (the pilot bit will not center the bi-center in the hole). In most of these applications, the pilot hole is larger than the bi-center pilot, eliminating the use of the bi-center and forcing the use of less efficient tools. Since the sub-reamer has a larger cutting diameter than the pilot bit, a bi-center with a sub-reamer can be used in a pilot hole that is equal to or smaller than the cutting diameter of the sub-reamer. 
   The aforedescribed and other advantages of the present invention will become apparent by reference to the drawings, the description of the preferred embodiment and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a conventional bi-center drill bit; 
       FIG. 2  is a schematic view of the relative forces acting on a bi-center tool; 
       FIG. 3  is a side view of one embodiment of the bi-center tool of the present invention illustrating a sub-reamer section; 
       FIG. 4  illustrates an end view of the embodiment illustrated in  FIG. 3  illustrating the pass-through diameter, the maximum tool diameter and the sub-reamer diameter; 
       FIG. 5  is a schematic end view representation of a first preferred embodiment of the invention illustrating the reamer drill diameter, the tool pass-through diameter, the maximum tool size and the sub-reamer drill diameter; 
       FIG. 6  is a side view of the embodiment illustrated in  FIG. 5 ; 
       FIG. 7  illustrates a second preferred embodiment of the invention also illustrating the reamer drill diameter, pass-through diameter, maximum tool size and sub-reamer drill diameter; 
       FIG. 8  is a side view of the embodiment illustrated in  FIG. 7 ; 
       FIG. 9  illustrates a third preferred embodiment of the invention also illustrating the reamer drill diameter, pass-through diameter, maximum tool size and sub-reamer drill diameter; and 
       FIG. 10  is a side view of the embodiment illustrated in FIG.  9 . 
   

   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. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A. General Structure of the Bi-Center Bit 
     FIG. 1  depicts a conventional bi-center drill bit. A bit body  2 , manufactured from steel or other hard metal, has a threaded pin  4  at its proximal end for connection in the drill string (not shown), and a pilot bit  3  defining an operating end face  6  at its opposite or distal 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 face. 
   The operating end face  6  of pilot 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 pilot bit  3  and extending across the distal most portion and up along the lower side surfaces of said bit  3 . Blades  8  are provided with a plurality of cutting elements  10  which may include polycrystalline diamond compacts (“PDC”). Removed from the distal-most end, the pilot bit  3  defines a gauge or stabilizer section which includes stabilizer ribs or kickers  12 , each of which is continuous with a respective one of the upsets  8 . Stabilizer ribs  12  contact the wall of the borehole (not shown) that has been drilled by the rotation of operating end face  6  and thus function to centralize and stabilize the tool and to help control its vibration within the borehole. 
   By its nature, a bi-center tool such as the one illustrated in  FIG. 1  is inherently unstable in operation in the borehole. While a variety of designs and manufacturing techniques have been implemented to improve stability, these techniques have in some occasions been at the expense of optimum tool performance. 
     FIG. 2  schematically illustrates the forces acting on a bi-center downhole tool during its operation in the borehole. When the tool is operated in the borehole, the reamer section  5  exerts a large radial force in the direction indicated by arrow “A”. While the pilot bit  3  can be designed to create an oppositely disposed force, here represented by arrow “B”, the comparatively smaller size of the operating face of the pilot  3  versus that of the reamer  5  translates into a force imbalance where “B&lt;A”. Accordingly, a lesser but still significant resultant force is created away from the reamer section  5  in the direction indicated by arrow “A.” This imbalance of forces creates instability of the tool as it is rotated in the borehole. 
     FIGS. 3-4  illustrate a general embodiment of the bi-center bit of the present invention.  FIG. 3  illustrates a bit body  20  which again may be milled from a high-strength material, e.g., steel. The bit body defines a proximal and a distal end. A threaded pin  32  is formed about the proximal end whereas a pilot bit section  12  is formed about the distal end. A reamer section  14  is formed intermediate the pilot section  12  and the proximal end. 
   Each of the pilot  12  and reamer  14  sections include one or more radial cutting blades  16  and  46 , respectively. Each of these cutting blades includes an endpoint  17 . Each of these cutting blades is also provided with one or more cutting elements,  18  and  48 , respectively, as described above in relation to a standard bi-center bit. The cutting elements may be made of a polycrystalline diamond compact or other material suitable for cutting through formations. 
   The tool  20  defines a maximum tool diameter “MTD” (See FIG.  4 ). The maximum tool diameter “MTD” is that diameter measured from the rotational axis “AA” to the offside  35  of the reamer section  14 . The maximum tool diameter “MTD” therefore defines the largest permissible diameter of a tool positioned above or below the reamer section  14  that will enable the tool to be rotated in the borehole in an unobstructed manner. The tool  20  also defines a reamer drill diameter “RDD”. The reamer drill diameter “RDD” is that maximum diameter which the sub-reamer defines when rotated in the borehole about the rotational axis “AA”. 
   In this embodiment, a sub-reamer section  30  is disposed intermediate the pilot  12  and reamer sections  14 , as illustrated. The sub-reamer section  30  is also provided with one or more cutting blades  33  which are adapted to carry cutting elements  39 . The endpoints  31  of those blades are positioned at specific locations based on maximum tool diameter, pass-through diameter, and the reamer drill diameter. As described above, the rotational axis “AA” is that axis about which the tool  20  is rotated when not in casing. 
   The tool  20  also defines a pass-through axis “AB”. The pass-through axis is that axis about which the tool is rotated when in casing. The rotation of the tool about the pass-through axis “AB” defines a pass-through diameter designated “PTD”. 
   A first embodiment of the present invention may been seen by reference to  FIGS. 5-6 . In these Figures there is illustrated a bi-center tool  100  which includes a pilot bit section  102  and a reamer section  104  which are oriented about the tool in the fashion described above with respect to the general embodiment. The tool  100  defines a rotational axis “AA” and a maximum tool diameter “MTD”, as also defined above in relation to the general embodiment. 
   A sub-reamer  108  is positioned intermediate of the pilot  102  and reamer  104  sections. In this embodiment, the sub-reamer  108  is provided with a plurality of cutting blades  110  which define endpoints  111  which extend to a distance less than or equal to the maximum tool diameter “MTD”, as measured from the rotational axis “AA”. These cutting blades  110  are radially distributed in an arc greater than or equal to 180 degrees about the sub-reamer  180 . In a preferred embodiment, endpoints which extend the same distance from “AA” and generally extend about the full 360 degrees of the sub-reamer section  108 . Each of the cutting blades  110  may include one or more cutting elements  113 , e.g., PDC cutting elements, which may be affixed to cutting blades  110  in a conventional fashion. This embodiment has particular application in the use of a mid-reamer is used where the pilot bit is significantly smaller than the maximum tool size. 
   A second embodiment of the invention may be seen by reference to  FIGS. 7-8 . In these Figures is illustrated a bi-center tool  140  which includes a pilot section  142 , a reamer section  144  and a sub-reamer section  150  whose respective orientation has been described above. Tool  140  defines a rotational axis “AA” and a pass-through axis “AB”. The rotation of the tool about the pass-through axis “AB” defines a pass-through diameter “PTD”. The maximum tool diameter “MTD” and reamer drill diameter “RDD,” as defined above, are also illustrated. 
   In this embodiment, the cutting blades on the reamer section  144  describe an arc which further defines a midpoint “Q.” This midpoint “Q” can be determined by bisecting the linear distance between the endpoint  161  on the leading edge  160  of the first blade  162  and the endpoint  163  on the trailing edge  166  of the last blade  168 , as illustrated. 
   Consistent with previous embodiments, the sub-reamer section  150  is provided with a number of cutting blades  152 , each of which define endpoints  151 . Blades  152  on the sub-reamer  150  are formed in an arc where this arc is centered about a line passing through rotational axis “AA” and midpoint “Q”. 
   In this embodiment, the intersection of the reamer drill diameter “RDD” and the pass-through diameter “PTD” defines two points of contact which are collectively designated  160  (See FIG.  7 ). These contact points  160  divide an end-section of the tool  140  into two different zones or regions. Zone 1 is that zone or region opposite the reamer section  144  and is disposed between contact points  160 . Zone 2 is complimentary to Zone 1 and is thus aligned about reamer section  144  and centered about midpoint “Q”. 
   As set forth above, sub-reamer section  150  includes a plurality of cutting blades or upsets  152  which are radially oriented about the tool. In this embodiment, the endpoints  151  of these cutting blades  152  is determined by their position relative to Zones 1 and 2. Those blades  152  situated in Zone 1 have endpoints which do not extend beyond the pass-through diameter “PTD”. The endpoints of all cutting blades  152  situated in Zone 2 do not radially extend beyond the reamer drill diameter “RDD”. 
   In a preferred embodiment, cutting blades  152  extend radially in an arc of at least 180 degrees. In a second preferred embodiment, no cutting blades  152  on the sub-reamer section  150  are disposed directly opposite the main reamer blades. Main reamer blades are those blades whose endpoints extend to the reamer drill diameter (RDD). 
   A third embodiment of the invention is illustrated at  FIGS. 9-10  in which is disclosed a bi-center downhole tool  200  which includes a pilot section  202 , reamer section  204  and sub-reamer section  212 , as described above in relation to the prior embodiments. Tool  200  defines a rotational axis “AA” and a pass-through axis “AB”, as illustrated in both  FIGS. 9-10 . Consistent with prior embodiments, the tool defines a maximum tool diameter as indicated at “MTD”. The rotation of the tool about the pass-through axis “AB” defines a pass-through diameter “PTD”. The rotation of the reamer section  204  about the rotational axis defines a reamer drill diameter “RDD”. (See FIG.  9 ). 
   The cutting blades or upsets  206  disposed on the reamer section  204  describe an arc which further defines a midpoint “Q”. In this embodiment, this midpoint “Q” is also determined by bisecting the linear distance between the endpoint of the leading edge  220  of the first blade  222  and the endpoint of the trailing edge  226  of the last blade  230 , as illustrated. 
   Consistent with previous embodiments, the sub-reamer section  206  is provided with a number of cutting blades  212  which define endpoints  207 . Blades on the sub-reamer are formed in a radial arc where this arc is centered about a line passing through rotational axis “AA” and midpoint “Q”. 
   The intersection of the reamer drill diameter “RDD” and the pass-through diameter “PTD” defines contact points designated  260  (See FIG.  9 ). These contact points  260  again define two different zones. As described above, Zone 1 is formed opposite the reamer section  14 , where Zone 2 is that zone complementary to Zone 1 and centered about midpoint “Q.” In this embodiment, all cutting blades  212  disposed on the sub-reamer section  206  are disposed in Zone 1 and define a radial arc of less than 180 degrees. The endpoints  213  of these blades  212  does not extend beyond the pass-through diameter “PTD”. 
   The following example demonstrates the utility of the patented invention: In a given application, it is desired to cut 200 feet of core in the middle of a bi-center run. The bi-center tool used is a 10⅝×12¼ (pass through diameter×drill diameter). In this example, a conventional bi-center would typically have an 8″ diameter pilot bit. However, it is described to use an 8½″ core bit to cut the core. In this case, a bi-center with a sub-reamer can be designed with a sub-reamer that has a cutting diameter of 8¾. Once the core is cut, the conventional bi-center can ream open the section of cored hole but create a hole that is smaller than the desired 12¼ for the entire 200 feet of the cored hole. This is because the 8″ pilot bit is smaller than the 8½ pilot hole so the pilot bit cannot center the bi-center. The bi-center which includes the sub-reamer can create a 12¼ inch hole in this section of cored hole because the 8¾ inch sub-reamer is able to center the bi-center in the cored hole. 
   The foregoing disclosure and description of the invention is illustrative and explanatory thereof, and it will appreciated by those skilled in the art, that various changes in the size, shape and materials as well as in the details of the illustrated construction or combinations of features of the various bit or coring elements may be made without departing from the spirit of the invention.

Summary:
An improved bi-center with improved directional stability and wear resistance is disclosed, said bit optimally utilizing a plurality of shaped PDC cutting elements selectively situated about the cutting surfaces of the pilot and the reamer to produce a minimal force imbalance, where further said pilot bit and the reamer are force balanced to further reduce imbalance in the operation of the tool.