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BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to tools for forming bores in the earth, especially rock, and particularly to rotary drill bits for use in oil and gas exploration and mining.  
         [0002]     Drag bits for drilling through rock are typically outfitted with hard, durable cutters. To improve wear, the cutters often possess contact surfaces made from diamond, typically in the form of polycrystalline diamond compact (PDC). PDC is an extremely hard and wear resistant material.  
         [0003]     Although PDC cutters are known to have one of the lowest rates of wear when operated at cooler temperatures, thermal damage to the diamond layer of the cutter begins at temperatures of approximately 700 degrees Celsius. Thermal damage lowers wear resistance and the PDC cutters become more susceptible to abrasive wear and breakage from impact.  
         [0004]     Greater tangential cutter velocity causes more friction, thus generating more heat. Cutters moving at higher tangential velocities will thus tend to operate at higher temperatures. At some velocity, frictional heat reaches a level sufficient to cause cutter wear rates to accelerate, reducing the life of the cutters. In conventional PDC drag bits, the tangential velocity of a cutter, when measured relative to the material being cut, depends on the distance of the cutter from the center of rotation of the drill bit. For a given angular velocity, the tangential velocity of cutter increases with the distance of the cutter from the bit&#39;s axis of rotation. Thus, a PDC cutter&#39;s intolerance of high temperatures limits, in practice, the diameter of the bit.  
         [0005]     Increased application of force also generates more heat. Cutters require more force to penetrate harder rock. Cutters dragging through harder rock have higher wear rates due to the increased application of force. Therefore, the critical point at which the wear rate begins to accelerate is also a function of hardness of rock in addition to the rotational velocity of the drill bit to which the cutter is attached. In softer rocks, accelerated wear rates do not occur until higher rotational speeds are used; in harder rocks, acceleration of the wear rate occurs at much lower rotational speeds.  
         [0006]     A number of additional factors also shorten the life of PDC cutters.  
         [0007]     First, a cutter&#39;s abrupt contact with rock formations also increases the rate of wear of PDC cutters. Drilling with conventional PDC drag bits require application of weight and torque to a drill string to turn the drilling tool face and drive the face into the formation. Torque rotates the bit, dragging its PDC cutters through the formation being cut by the cutters. Dragging generates chips, which are removed by drilling fluids, thereby forming a bore or drilled hole. The drilling action causes a reverse, corresponding torque in the drill string. Because of the length of the drill string, the torque winds the drill string like a torsion spring. If a bit releases from consistent contact with the formation being drilled, the drill string will unwind and rotate backward. Changing the tension in the drill string causes the drill bit to come into irregular, abrupt contact either with the sides of the bore or the exposed formation surface being cut. These irregular contacts can cause impact damage to the cutters.  
         [0008]     Second, drill strings will also vibrate, sometimes severely. Under typical drilling conditions, a drill string rotates at 90 to 150 rpm. These vibrations can also damage a drill bit, including the cutters, as well as the drill pipe, MWD equipment, and other components in the drilling system.  
         [0009]     Third, “bit whirl” further contributes to impact loads on PDC cutters. This complex motion of the drill bit is thought to occur due to a combination of causes, including lateral forces on the drill bit due to vibration of the drill string vibration, heterogeneous rock formations, bit design, and other factors in combination with the radial cutting ability of PDC bits. Whirl of a drill bit in a bore subjects PDC cutters on the bit to large impact loads as the bit bounces against rock or other material in the bore. Cutters on these drill bits will lose large chips of PDC from impact, rather than from gradual abrasion of the cutter, thereby shortening the effective life of the cutters and the drill bit.  
         [0010]     Drilling tools disclosed in U.S. Pat. No. 6,488,103 of Dennis et al., and in U.S. application Ser. No. 10/988,722, filed Nov. 15, 2004, both of which are incorporated herein by reference, address these problems by reducing the thermal and impact stresses on the cutters. The tools employ a plurality of satellite mills surrounding a central pilot bit. The satellite mills reduce the tangential velocity of the cutters along the periphery of the bore hole.  
       SUMMARY OF THE INVENTION  
       [0011]     The invention pertains to an earth boring tool, or aspects thereof, having PDC cutters that overcomes one or more of these problems by combining on the same rotational axis a central bit and a relatively larger diameter reamer that extends beyond the central bit. In effect, the central bit bores the center of the hole and the reamer enlarges it. By turning the reamer at a relatively low angular velocity relative to the earth, the tangential velocity of the cutters on the reamer are kept low enough to reduce wear and other adverse affects associated with higher tangential velocities of the cutters. The central bit is allowed to rotate relatively faster, thereby permitting larger diameter holes to be bored without adversely affecting cutter performance or drilling rate. Cutting speed can be optimized, allowing the maximum efficiency without excess wear of the cutters.  
         [0012]     Several additional benefits are possible with such a tool. It will tend to create less vibration and chatter. Less force on the drilling tool is required for cutting. This in turn lowers the torque on the drill string, lessening the chance of the drill string of wrapping up. Lighter force applied to the tool also permits use of a lighter tubing having thinner walls to be used.  
         [0013]     Details of an example of such an earth boring tool are described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a highly simplified, schematic representation of the drive system for turning a pilot bit and reamer for the drilling tool shown in  FIGS. 2 and 3 .  
         [0015]      FIG. 2  is a side, elevation view of a first example of a drilling tool, sectioned its entire length along its axis.  
         [0016]      FIG. 3  is a side, elevation view of a second example of a drilling tool, sectioned its entire length along its axis.  
     
    
     DETAILED DESCRIPTION  
       [0017]     In the following description, like numbers throughout the figures refer to like elements.  
         [0018]     Briefly, in the following example of an earth boring tool, the pilot bit and reamer are each driven by a separate motor, thereby avoiding the complexities of gears and problems occasioned by them. Examples of such problems include structural complexities necessary to have seals, with the attendant potential for failure. If seals are not used, there is a substantial risk of gear failure or jamming that is not easily addressed by merely hardening the gears.  
         [0019]      FIG. 1  schematically illustrates the functional and spatial relationships between two motors  10  and  12 , a drill string  14 , reamer  16  and a central bit  18  within an earth boring tool  20 . The reamer and central bit each possess a plurality of cutters  21  along exterior cutting surfaces. The central bit drills a pilot hole; the sleeve-shaped or donut-shaped reamer, since it has an wider diameter, widens the hole. These cutters are wear-resistant, and may be made from carbide, polycrystalline diamond or other super hard materials. Motor  10 , the casing (not shown) of tool  20  and drill string  14  to which the tool is attached are in a fixed relationship and do not rotate with respect to each other. Motor  10  has a rotating output  22 , with which motor  12  is in a fixed relationship. Thus, motor  10  effectively turns motor  12 . Reamer  16  has a fixed relationship with the rotating output  22  as well as motor  12 . Whether the reamer is connected with motor  12  or output  22 , it is in effect turned by motor  10 . Rotating output  24  of motor  12  turns central bit  18 . Depending on the direction of rotation of the outputs of the motors, their rotations can be additive, meaning that angular velocity of the bit  18 , relative to the tool and the earth through which the tool is boring, is the sum of the angular velocity of the rotating output of each motor. Thus the central bit will naturally turn at a relatively higher rate of rotation than the reamer. The relative angular velocity of the pilot bit to the reamer will depend on the rate of rotation of the output  24  of motor  12 .  
         [0020]     Counter-rotating the reamer and drill bit will reduce torque on the string and stress on the cutting tool. With this configuration, the angular velocity of motor  12  must overcome the opposite rotation of the output  22  of motor  10 . Preferably, the central, high-speed bit rotates right, to tighten threaded connections, and the low-speed reamer turns left.  
         [0021]     Using conventional positive displacement motors (PDMs)—also called “mud motors”—for motors  10  and  12  permits the motors to be powered by drilling fluid pumped down a drilling string. With their axes aligned with each other and the tool, drilling fluid will flow from one into the next, and then out the end of the tool in a manner to cool the cutters and clear cutting debris. A central stator of the first mud motor, motor  10  in the preceding schematic, remains stationary with respect to the casing of the tool and the drill string. An outer, sleeve-shaped rotor functions as output  22 . This outer rotator is then coupled with an outer, sleeve-shaped stator of the second mud motor, which corresponds with motor  12 . The construction of the second motor is the inverse of the first mud motor: the stator, or stationary part, is disposed on the outside of the mud motor, with the rotor formed on an internal, rotating shaft. This inverse construction or arrangement allows the two motors to be coupled for drilling fluid to flow straight from one into the other. It also permits the reamer to be easily coupled to the rotor of the first motor or the stator of the second motor.  
         [0022]      FIGS. 2 and 3  illustrate details of two examples of such an earth boring tool. These examples share certain characteristics and elements, which will be discussed first. Unless otherwise noted or apparent from the context, each element is symmetrical about the tool&#39;s central axis  28 . Each has a top connection subassembly  30  having a threaded rod box  32  for connection to a drill string. Connected to the top connection subassembly by support pin  34 , is a flex joint  36 . The flex joint has fixed relationship with (i.e., does not substantially rotate with respect to) the drilling string and tool, and extends down into a main body of each of the tools. The main body is defined in part by an outer tool casing  39 .  
         [0023]     Mounted within the main body of each of the tools include an upper positive displacement motor (PDM)  40  and a lower PDM  42 . One purpose of PDM  40  is to provide a relatively low-speed rotational output for turning a reamer. One purpose of PDM  42  is to provide a relatively high-speed rotational output for turning a pilot bit. However, PDM  42  is rotated by PDM  40  and, therefore, the true angular velocity of the “high speed” PDM  42  may not necessarily be higher than the angular velocity of the output of the upper, “low speed” PDM  40 .  
         [0024]     The upper, low speed PDM is coupled to a lower end of the flex joint  36  in a substantially non-rotating or fixed relationship by attaching stator  44  to flex joint  36 . Rotor  46  of upper PDM  40 , which is an elastomer, rotates an outer body  48  of the upper PDM  40 . Fluid under pressure flows from the drilling string (not shown) into passage  50 , which in turn carries it to the upper PDM  40 , causing the rotor  48  and, thus also, body  52  of the upper PDM to turn. Small arrows throughout the figures indicate the direction of fluid flow during operation.  
         [0025]     Body  54  of the lower PDM  42  connects to body  52  of the upper PDM. This connection is, in the example, threaded, though other types of connections may be used. The connection causes the body of the lower PDM to rotate with the body of the upper PDM. Stator  56  of the lower PDM  42  is thus coupled to, and turns with, the rotor  48  of the upper PDM  40 . Rotor  58  of the lower PDM is connected to a flex shaft  59 , which in turn is connected to lower shaft  60 . The flex shaft provides, in essence, a flexible coupling between the output of the lower PDM and the lower shaft that accommodates the eccentric movement of the rotor  58  with respect to the center line of the tool. A drill bit  62 , on which a plurality of cutters (not shown) are mounted, is attached to the free end of shaft  60 . The shaft includes a passageway  64  through its center. A portion of the drilling fluid exiting the lower PDM is diverted through the passageway to the drill bit.  
         [0026]     Reamer  66  couples to body  54  of the lower PDM  42  through inner bearing housing  68 . In the illustrated embodiments, reamer  66  is attached to inner bearing housing  68  by a threaded connection, and the bearing housing  68  is connected to the body  54  of the lower PDM by a threaded connection.  
         [0027]     Several sets of radial bearings support rotating components within the body of the tool, namely radial bearing assemblies  70  and  78  support the relative rotation of the upper and lower PDMs in each of the tools, and radial bearing assemblies  71  and  73  support rotation of the lower shaft. Radial bearing assembly  70  includes a radial bearing  72  and a bearing wear surface layer  76  disposed between the tool casing  39  and upper bearing housing  74 . The upper bearing housing is connected to body  52  of the upper PDM, and thus rotates with the body of the upper PDM. Bearing assembly  78  includes a radial bearing  80  disposed between lower bearing housing  68  and outer bearing housing  82 . The outer bearing housing is connected to casing  39  of the tool, preferably by a threaded connection. Bearing assemblies  71  and  73  are located at opposite ends of the lower shaft  60 . They include radial bearings  75  and  79 , respectively, each with a wear surface  79 .  
         [0028]     A set of thrust bearings limit movement of rotating components along the axis of the tool. Upper thrust bearing assembly  84  include a pair a fixed bearings  86  and  88 , and a pair of moving bearings  90  and  92 , each having a wear surface  113 . Spacer  94  acting against radial bearing  72  prevents upward movement of the fixed bearing  86 , and thus also of thrust bearing assembly  84 . Locking nut  96  stops upward movement of the radial bearing. Ledge  98 , which is integrally formed in casing  39 , prevents downward movement of fixed bearing  88  and thus also of the thrust bearing assembly. Moving bearings  90  and  92  are trapped by the fixed bearings. Ledge  100  transfers the load on the rotating components to the thrust bearing assembly. Some amount of lateral movement of elements of the thrust bearing assembly is desirable, as it permits drilling fluid to migrate into and down through outer passageway  102 , through the upper radial bearing assembly and then through the upper thrust bearing assembly. Spacer  103  prevents downward movement of bearing wear surface layer  76 .  
         [0029]     Lower thrust bearing assembly  105  has a construction similar to that of the upper thrust bearing, with fixed bearings  106  and  108  and moving bearings  110  and  112 , each with a wear surface  113 . The thrust bearing is trapped by the set of radial bearings  71  and  73 , with shoulder or ledge  114  stopping upward lateral movement of the bearings. Spacers are used to space apart the bearings and facillitate flow of drilling fluids through the bearings. Spacer  116  keeps fixed bearings  106  and  108  spaced apart at the correct distance. Lock nut  118  screws onto a threaded interior surface of inner bearing housing  68  to prevent downward movement of the radial bearing assemblies  71  and  73  and thrust bearing assembly  105 . Like the other radial and thrust bearings, this thrust bearing assembly is also lubricated and cooled by drilling fluid. However, it is cooled by fluid exiting lower PDM  42 .  
         [0030]     It is preferred that at least the thrust bearings, due to expected high loading, be made of a wear resistant material, such as a polycrystalline diamond compact or similar material.  
         [0031]     The bearing assemblies are, in the example tools described above and shown in  FIGS. 2 and 3 , not sealed. Drilling fluid pumped through the tool lubricates and cools the bearings. As indicated by arrows, a portion of the drilling fluid flowing into the tool is diverted into for lubricating the radial bearing assemblies  70  and  87  and thrust bearing assembly  84 . The fluid travels through passageway  102  to bearing assembly  78  before it exits through opening  104  between the bottom end of outer bearing housing  82  and a shoulder of the inner bearing housing  68 . Similarly, a portion of drilling fluid exiting lower PDM  42  flows, as indicted by the arrows, through radial bearing assemblies  71  and  73 , and thrust bearing assembly  105 , before exiting the bottom of the tool.  
         [0032]     Referring now just to  FIG. 2 , central, high speed bit  62 , which turns at a high speed relative to the tool, extends beyond the end of tool, in front of the reamer. It forms a pilot hole having a relatively smaller diameter, and the reamer enlarges it. In the embodiment of  FIG. 3 , reamer  66  leads the central bit, the reamer first cutting an annular bore and then the central bit subsequently crushing the core.

Summary:
An earth boring tool includes two coaxially-aligned, positive displacement motors. One motor turns a pilot bit and the other turns a reamer concentric with the pilot bit. The central bit bores the center of the hole and the reamer enlarges it. The central bit is rotated relatively faster, while rotation of the larger diameter reamer is relatively slow. The tool can thus be used to bore larger diameter holes without slowing drilling rates or adversely affecting performance of the cutter elements due to higher tangential velocities.