Patent Publication Number: US-6213229-B1

Title: Drilling motor drill bit reaming stabilizer

Description:
FIELD OF THE INVENTION 
     The invention is in the field of rotatable well drilling tools, and provides a well bore reamer and drill bit stabilizer for use in well drilling, particularly drilling using motor assemblies such as mud motors. The invention also provides methods for using the such a device in well drilling. 
     BACKGROUND OF THE INVENTION 
     Conventional well drilling technologies, particularly for drilling oil wells, involve the use of a rotatable drill bit fastened to the end of a drill string. The bit is attached to the string by a threaded pin (or dome) on the bit that is received in a threaded ‘bit box’ at the end of the string. The entire drill string, typically made up of a large number of threadably connected lengths of drill pipe, is rotated at the derrick to turn the bit in the hole. A wide variety of bits have been developed for use in different geological formations, such as the rock bits available from Smith International of Houston, Tex., USA. 
     A variety of drilling tools have been developed for attachment between the drill string and the bit to stabilize the bit in conventional straight-line drilling. U.S. Pat. No. 5,474,143 issued Dec. 12, 1995 to Smith International discloses a reaming drill bit stabilizer adapted for use in conventional drilling, in which the drill string is rotated to turn the bit. As disclosed therein, that reaming stabilizer is particularly suited to ensuring that the drill bit is stabilized and drills in a substantially straight line. In some circumstances, however, it is desirable to drill a well that deviates from a straight line. 
     Mud motors are particularly suited to directional drilling, in which the well bore is deliberately made to deviate from a straight line. In some formations, for example, it is desirable to drill horizontal wells, or a number of horizontal wells that are generally parallel to one another. Mud motors are typically capable of directional drilling because the motor is not straight. An example of such a motor is shown in U.S. Pat. No. 4,667,751 issued May 26, 1987 to Smith International. The bend in the mud motor may be oriented towards any wall of the well bore by turning the drill string. Once the motor is appropriately oriented, the motor may be operated to turn the bit, sliding the string forward as the well is excavated. The housing of the mud motor may be protected by stabilizers (which may be known as concentric stabilizers) which are not rotated by the motor, but may rotate with the whole motor for example when the motor is being oriented (see for example U.S. Pat. No. 4,667,751 issued to Smith International May 26, 1987). Concentric stabilizers are available, for example, for use with the DYNA-DRILL™ brand motors available from Smith International. Such stabilizers are typically slightly under-gage (smaller in diameter than the bit), since they serve to protect and in some cases to orient the motor. 
     Bits designed for drilling are generally not suited to reaming out a previously drilled bore. If they are to be used for reaming, such bits should generally be operated with reduced weight on the bit and at relatively low rpm., which may raise the cost and reduce the efficiency of drilling. 
     SUMMARY OF THE INVENTION 
     The invention provides a reaming stabilizer for use with a drilling motor. In one embodiment the stabilizer is made up as part of a drilling assembly comprising a motor, a reaming stabilizer and a drill bit. The motor is adapted to be attached to a drill string, the motor includes a drive shaft with a mandrel mounted at the bottom end of the shaft. The reaming stabilizer is attached to the shaft of the motor by the mandrel, so that the stabilizer may be rotatably driven by the motor. The drill bit is attached to the stabilizer to be rotatably driven by the stabilizer. The stabilizer is sized so that the overall length of the drilling assembly is increased by a maximum of about 16 inches. Greater lengths may reduce motor bearing life, adversely affect the steering of the tool or result in drive shaft breakage with consequent loss of the drill bit and stabilizer in the hole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational partially sectioned view of a drilling assembly of the invention, showing a motor attached to a reaming stabilizer which is in turn attached to a drill bit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the invention provides a drilling assembly comprising a motor  10  with a mandrel  12 , a reaming stabilizer  14  and a drill bit  16 . The downhole drilling motor  10  has a top end and a bottom end. The top end of the motor is adapted for connection to a drill string, typically by being provided with a conical threaded connector  18  at its top end. The motor  10  may be any of a wide variety of motor types, such as directional motors, straight-hole motors or steerable motors. Such motors  10  have a rotatable longitudinal drive shaft  11 . The drive shaft  11  is connected to a longitudinal cylindrical mandrel  12  on the bottom end of the motor  10 . The distal end of the mandrel  12  (in operation, the end furthest from the dill string) has an axial, internally threaded mandrel box  20 . The box  20  is typically where a drill bit  16  would be attached to the motor  10 , and may accordingly be referred to as the ‘bit box’. The whole motor  10  is typically adapted to permit drilling fluid to flow through the motor  10 , to drive it and to provide drilling fluid to the bit  16 . 
     The invention provides a longitudinal, cylindrical, reaming stabilizer  14  with a top end adapted to be threadably connected to the mandrel box  20 . The stabilizer  14  has longitudinal flutes  28  and lands  30 . The flutes  28  are adapted to allow fluid flow back past the stabilizer  14  (for this reason the flutes  28  may be referred to as ‘junk slots’). The lands  30  define the outer transverse stabilizer  14  diameter. The lands  30  and flutes  28  may be spirally arranged. In one embodiment, there are 6 lands  30  and 6 flutes  28 . 
     The diameter of the stabilizer  14  is typically greater than the diameter of the mandrel  12 , so that the stabilizer  14  protects the mandrel  12  from wear. The bottom end of the stabilizer  14  has an axial, internally threaded stabilizer box  22 , for attachment of the drill bit  16 . The stabilizer  14  is typically provided with an axial channel  24  to permit drilling fluid to flow through the stabilizer  14  into the drill bit  16 . 
     The drill bit  16  is threadably received in the stabilizer box  22 . Preferably, the stabilizer box  22  is provided with shoulders  26  that extend the lands  30  towards the bit  16 , which may enhance the bit stabilizing effect of the stabilizer  14 , improving bit performance and life. The transverse diameter of the drill bit  16  is substantially the same as the transverse diameter of the stabilizer  14 . Accordingly, when the drill bit  16  wears down so that it goes under size, the stabilizer  14  will engage the hole and function as a reamer. 
     The stabilizer  14  is sized so that the drill bit  16  is longitudinally spaced apart from the distal end of the mandrel  12  by the stabilizer  14  by no more than about 16 inches, a distance shown as L 1  in FIG. 1, or in alternative embodiments 10 inches, or 8 inches, and preferably from 5 to 8 inches. Preferably the stabilizer  14  is long enough to accept a ‘sub breaker’ tool that is adapted to accept the tongs typically used for assembly of the drill string components. 
     The lands  30  on the stabilizer  14  may be provided with a plurality of hardened inserts  32  extending outwardly from the lands  30 . In which case, the outer edges of the inserts  32  will define the transverse diameter of the stabilizer  14 . The hardened inserts  32  may be made with a hardened surface, such as a polycrystaline diamond or tungsten carbide. Preferably, the inserts  32  extend outwardly from the lands  30 , defining between them spaces for material to flow past the inserts  32 , which may help to reduce the torque added to the motor  10  once the stabilizer  14  begins to engage the hole in a reaming action. Preferably, the inserts  32  project from the lands  30  by between about ¼ inch and about ¾ inch, and the inserts  32  have a hemispherical conformation, these characteristics may help to ensure that the stabilizer  14  does not ‘bite’ into the hole too aggressively, which could cause an undesirable amount of torque and could also deform the well bore in an undesirable manner. The hardened inserts  32  may be removably mounted in the stabilizer  14  by brazing, for example by silver brazing the inserts  32  into a hole is about 4 to 5 thousandths of an inch oversize for the inserts  32 . Such inserts  32  may be removed by melting the brazing to facilitate re-use of the tool. Alternatively, the inserts  32  may be tight fit in the stabilizer  14  in holes that are 3 to 4 thousandths of an inch under size hole. 
     The lands  30  may be provided with steps  34 . In one embodiment, the steps  34  positioned towards the bottom end of stabilizer  14  extend outwardly from stabilizer  14  less than steps  34  positioned towards the top end of stabilizer  14 . Hardened inserts  32  of the type discussed above may be mounted on the steps. 
     The use of the drilling tool of the invention provides an unexpected advantage, particularly when used in conjunction with rotatable cone bits operated with mud motors. When the drill bit  16  wears to a diameter less than the diameter of the reamer-stabilizer  14  tool of the invention, the hardened inserts  32  on the external surface of the reamer-stabilizer  14  will begin to engage the well bore, reaming the bore to maintain well gage. As the reamer-stabilizer  14  engages the well bore, the torque required to drive the bit  16  and the reamer-stabilizer  14  will generally begin to rise. When the bit  16  is being driven by a mud motor  10 , this requirement for more torque to drive the bit  16  may be perceived by the driller as an increase in the pressure of the drilling fluid required to drive the motor  10 . This requirement for increased torque provides the driller with an indication that the bit  16  has gone under-gage. At an appropriate point following this indication, the driller may then withdraw the string to replace the bit  16 , to avoid prolonged uneconomical drilling with a worn bit  16  and reduce the risk of bit failure. In many cases the reamer-stabilizer  14  will not be worn down as quickly as the bit  16 , so that the reamer-stabilizer  14  of the invention may be reused with a succession of bits. 
     Although various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way.