Earth boring tool

Abstract

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.

1 . A drilling tool comprising: 
 a body have a central axis;    a central bit mounted for rotation about the central axis, on which is disposed a first plurality of cutters, the central bit having a first cutting diameter;    a sleeve-shaped reamer mounted for rotation about the central axis, on which is disposed a second plurality of cutters, the reamer having a second cutting diameter greater than the first cutting diameter;    a first motor coupled with the central bit for rotating the central bit;    a second motor coupled with the first motor and the reamer for rotating the first motor and reamer,    the first motor being configured to turn the central bit at a first angular velocity relative to the body of the drilling tool; the second motor being configured to rotate the reamer at a second angular velocity relative to the body, the magnitude of the first angular velocity being greater than the magnitude of the second angular velocity.    
   
   
       2 . The drilling tool of  claim 1 , wherein the first and second motors are positive displacement motors powered by drilling fluid pumped under pressure through the tool.  
   
   
       3 . The drilling tool of  claim 1 , wherein 
 the first motor is comprised of a first positive displacement motor (PDM); and    the second motor is comprised of a second positive displacement motor (PDM);    the second PDM having a internal stator having a non-rotational relationship with the body of the tool and an external rotor; the first PDM have an external stator coupled in fixed relationship with the external rotor of the second PDM and an internal rotor coupled with the central bit for rotation of the central bit.    
   
   
       4 . The drilling tool of  claim 3 , wherein the first and second PDMs are aligned such that the drilling fluid flowing into the tool flows into the second PDM, exits the second PDM and flows into the first PDM.  
   
   
       5 . The drilling tool of  claim 3 , further comprising unsealed bearings for supporting the first PDM within the body for rotation relative to the body, the bearings having surfaces comprised of a hard or super hard material and being lubricated by the drilling fluid.  
   
   
       6 . The drilling tool of  claim 5 , wherein the bearings are thrust bearings.  
   
   
       7 . The drilling tool of  claim 1 , wherein the reamer is connected to the first motor.  
   
   
       8 . The drilling tool of  claim 1 , wherein the first motor rotates a first direction and the second motor rotates a second direction.  
   
   
       9 . A drilling tool comprising: 
 a body have a central axis;    a central bit mounted for rotation about the central axis, on which is disposed a first plurality of cutters, the central bit having a first cutting diameter;    a sleeve-shaped reamer mounted for rotation about the central axis, on which is disposed a second plurality of cutters, the reamer having a second cutting diameter greater than the first cutting diameter;    a first positive displacement motor (PDM) coupled with the central bit for rotating the central bit;    a second PDM coupled with the first motor and the reamer for rotating the first PDM and reamer;    wherein 
 the first PDM being configured to turn the central bit at a first angular velocity relative to the body of the drilling tool; the second PDM being configured to rotate the reamer at a second angular velocity relative to the body, the magnitude of the first angular velocity being greater than the magnitude of the second angular velocity; and  
 the first and second PDMs are aligned such that drilling fluid flowing into the tool under pressure flows into the second PDM, exits the second PDM and flows into the first PDM; the second PDM having an internal stator having a non-rotational relationship with the body of the tool and an external rotor; the first PDM having an external stator coupled in fixed relationship with the external rotor of the second PDM and an internal rotor coupled with the central bit for rotation of the central bit.  
   
   
   
       10 . The drilling tool of  claim 9 , further comprising unsealed bearings for supporting the first PDM within the body for rotation relative to the body, the bearings having surfaces comprised of a hard or super hard material and being lubricated by the drilling fluid.  
   
   
       11 . The drilling tool of  claim 10 , wherein the bearings are thrust bearings.  
   
   
       12 . The drilling tool of  claim 9 , wherein the reamer is connected to the first PDM.  
   
   
       13 . The drilling tool of  claim 9 , wherein the first PDM rotates a first direction and the second PDM rotates a second direction opposite the first direction.