Abstract:
A roller cone drill bit having selected left hand walk is disclosed. The bit includes a bit body adapted to couple to a drill string, and a plurality of legs depending from the bit body. Each leg has a roller cone rotatably mounted thereon. Each cone includes a plurality of cutting elements thereon. At least one cone has a nose point of its profile displaced radially inward with respect to a nose point of the profile of the other cones. A roller cone drill bit having selected right hand walk is also disclosed. The bit include a bit body adapted to couple to a drill string, and a plurality of legs depending from the bit body. Each leg has a roller cone rotatably mounted thereon, each cone including a plurality of cutting elements thereon defining a profile. At least one cone has a nose point of the profile displaced radially outward with respect to a nose point of the profile of the other cones.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The invention relates generally to the field of roller cone drill bits used to drill wellbores through the earth. More particularly, the invention relates to roller cone drill bits having predetermined directional drilling characteristics.  
           [0005]    2. Background Art  
           [0006]    Roller cone drill bits are commonly used to drill wellbores through earth formations. A typical roller cone bit includes a bit body adapted to be coupled to a drill string, and having one or more roller cones each rotatably mounted on a bearing journal depending from the bit body. The roller cones have a plurality of cutting elements, such as milled steel teeth ,or hard or super hard material inserts disposed at selected locations about the cones. As the bit body is rotated by the drill string, the cutting elements on the roller cones come into scraping and crushing contact with the bottom and side of the wellbore, cutting through the earth formations.  
           [0007]    It is known in the art to drill certain wellbores along a predetermined trajectory other than vertical for the purpose of penetrating selected earth formations at a subsurface position different from the surface position of the wellbore. Many types of directional drilling tools are known in the art for drilling wellbores along selected trajectories. It is well known in the art that drill bits have certain tendency to “walk” or change the azimuthal direction of the wellbore as drilling progresses. Bit walk, when uncontrolled, can have an effect on the efficiency and accuracy of directional drilling operations. Polycrystalline diamond compact (PDC) bits are known in the art which have predictable walk tendencies. Such bits are described in U.S. Pat. No. 5,937,958 issued to Mensa-Wilmot et al. Bits having predictable walk tendencies can improve the efficiency and accuracy of directional drilling operations, for reasons well explained in the &#39; 958  patent.  
           [0008]    Certain earth formations, however, are more effectively drilled using roller cone bits than by using PDC bits. What is needed, therefore, is a roller cone bit having selected walk tendencies.  
         SUMMARY OF INVENTION  
         [0009]    One aspect of the invention is a roller cone drill bit having a selected amount of left hand walk. The bit includes a bit body adapted to couple to a drill string, and a plurality of legs depending from the bit body. Each leg has a roller cone rotatably mounted thereon. Each cone includes a plurality of cutting elements thereon. At least one cone has a nose point of its profile displaced radially inward with respect to a nose point of the profile of the other cones.  
           [0010]    In one embodiment, the leg on which the at least one cone is mounted is arranged so that the cone axis offset is greater than on the other cones. In one embodiment, the leg on which the at least one cone is mounted is arranged so that has a smaller journal angle than the other legs. In one embodiment, the at least one cone has larger cutting elements than on the other cones. In one embodiment, the at least one cone has more aggressive cutting elements than on the other cones.  
           [0011]    Another aspect of the invention is a roller cone drill bit having a selected amount of right hand walk. The bit includes a bit body adapted to couple to a drill string, and a plurality of legs depending from the bit body. Each leg has a roller cone rotatably mounted thereon. Each cone includes a plurality of cutting elements thereon. At least one cone has a nose point of its profile displaced radially outward with respect to a nose point of the profile of the other cones.  
           [0012]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    [0013]FIG. 1 shows one example of a roller cone bit according to the invention.  
         [0014]    [0014]FIG. 2 shows a profile view of a roller cone having a nose point at a conventional location along the profile.  
         [0015]    [0015]FIG. 3 shows a profile view of a roller cone having a nose point located toward the rotational axis of the bit.  
         [0016]    [0016]FIG. 4 shows a profile view of a roller cone having a nose point located toward the gage side of the bit. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The invention in its various aspects includes a roller cone drill bit having a plurality of roller cones, typically but not exclusively three such cones, each rotatably mounted on a bit body. In the various aspects of the invention, at least one of the roller cones has one or more of the following design aspects selected to be different than the other cones on the bit. These design aspects include axial position of the roller cone, lateral position of the “nose point” of the roller cone, type and aggressiveness of the cutting elements on the roller cone, journal angle of the roller cone, roller cone axis offset, lateral force generated by the roller cone during drilling and circumferential force generated by the roller cone during drilling. By selecting one or more of these design aspects to be different on one roller cone than on the other roller cones, a roller cone bit may be designed to have a selected amount of right hand or left hand walk during drilling, or neutral walk tendency during drilling. It should be understood that the individual aspects of the invention described herein are not exclusive of each other and in various embodiments of a bit made according to the invention, any one or more of the various aspects may be combined to provide a bit with a selected amount of right hand or left hand walk, or neutral walk tendency if such a bit is needed.  
         [0018]    A general example of a roller cone bit which may include one or more of the various aspects and embodiments of the invention is shown in FIG. 1. The bit  10  includes a bit body  14  formed from steel or other high strength material. The bit body  14  typically includes a coupling  13  for joining the bit body  14  to a drill string (not shown). Roller cones  20 ,  30 ,  40  are rotatably mounted to the bit body  14 . The roller cones  20 ,  30 ,  40 , include various forms of cutting elements  16  thereon.  
         [0019]    A profile view of one of the roller cones  20  is shown in FIG. 2. The cone  20  in FIG. 2 is an “insert” type, meaning that the cutting elements are inserts such as formed from tungsten carbide, but this is not intended to limit the invention. The cutting elements may also be milled steel teeth, or any other form of cutting element known in the art for use in roller cone bits. The cutting elements may include heel row elements  21 , gage row elements  22  and interior row elements  23 . An axially lowermost point  12 A defined by a “profile” traced by the cutting elements is referred to as the “nose point”. The nose point  12 A of the cone  20  in FIG. 2 is in a selected radial location along the “profile” of the cone  20 , positioned between the longitudinal axis  11  of the bit and the gage edge  15  of the bit. The nose points on the other cones (see FIG. 1) are typically located in similar radial locations between the axis  11  and the gage side  15 . As is known in the art, having similarly radially positioned nose points on all the cones typically results in a drill bit having a slight “right hand” walk characteristic.  
         [0020]    [0020]FIG. 3 shows an embodiment of a roller cone  30  according to one aspect of the invention. The cone in FIG. 3 also includes heel row elements  31 , gage row elements  32  and interior row elements  33 , similar to the cone in FIG. 2. The nose point  12 B of the cone  30  of FIG. 3, however, is positioned closer to the longitudinal axis  11  of the drill bit than the selected position of the nose point ( 12 A in FIG. 2) of the cone in FIG. 2. Having at least one roller cone with such a nose point is one requirement to provide the bit with a tendency to “left hand” walk. By appropriate selection of the position of the nose point  12 B on at least one cone, combined with other design selections, the drill bit may be designed to have a selected amount of left hand walk, right hand walk or neutral walk tendencies.  
         [0021]    One of the cutting elements, shown at  33 A in FIG. 3, extends at least out to, or on some cases may extend past the longitudinal axis  11  of the bit. Such a cutting element is typically referred to as a “spear point” with reference to a milled tooth cutting element bit, but the effect is similar for bits in which the cutting elements are insert-type. In one aspect of the invention, a bit having a selected amount of left hand walk preferably includes at least one cutting element on at least one roller cone which extends to or past the longitudinal axis of the bit.  
         [0022]    [0022]FIG. 4 shows an embodiment of a roller cone  40  according to another aspect of the invention. The cone in FIG. 4 also includes heel row elements  41 , gage row elements  42  and interior row elements  43  as is the case for the other cones ( 20  in FIG. 2 and  30  in FIG. 3). The nose point  12 C of the cone of FIG. 4, however, is positioned further from the longitudinal axis  11  of the drill bit, and closer to the gage side  15 , than the selected position of the cone of FIG. 2. Having at least one cone with a nose point  12 C moved closer to the gage side  15  provides the bit with a strong tendency to “right hand” walk. By appropriate selection of the position of the nose point on at least one cone, along with other design selections, the drill bit may be designed to have a selected amount of left hand walk, right hand walk or neutral walk tendencies.  
         [0023]    In another aspect of the invention, selected ones of the cutting elements, such as  43 A in FIG. 4, may be larger diameter, greater extension from the cone surface, and/or more aggressive in shape than other cutting elements on the same or the other roller cones. Such cutting elements  43 A may provide the drill bit with selected right hand or selected left hand walk tendencies because they have varying degrees of aggressiveness on different portions of the bit, providing the bit with selected amount of torque in certain positions on the bit.  
         [0024]    Referring again to FIG. 2, the cone  20  is mounted on the bit body ( 14  in FIG. 1) so that the rotational axis  20 A of the cone  20  defines an angle with respect to the bit axis  11 . This angle θ is referred to as the “journal angle”. The other cones ( 30  and  40  in FIG. 1) are also mounted at a selected journal angle. Typical prior art bits have the same journal angle for all the roller cones. In another aspect of the invention, the journal angle θ is made higher on one roller cone than on the other roller cones. Typically, bits known in the art for drilling “soft” formations have a journal angle of about 32.5 degrees. “Hard” formation bits have a journal angle of about 36 degrees, and “very hard” formation bits have a journal angle of about 39 degrees. In certain embodiments of a bit according to the invention, the one roller cone can have a journal angle of less than 32.5 degrees for soft formations, less than 36 degrees for hard formations, and less than 39 degrees for very hard formations, while the journal angle on the other cones is selected as conventional for the type of formation being drilled. Decreasing the journal angle on the at least one cone may provide the bit with increased tendency to left hand walk. In general, this aspect of the invention includes selecting a journal angle on one cone that is less than the journal angle on the other cones to provide a selected amount of left hand walk tendency.  
         [0025]    Conversely, in another embodiment, the journal angle θ on the one cone may be made larger than the journal angle on the other cones. Making the journal angle larger on one roller cone may increase the tendency of the bit to right hand walk.  
         [0026]    In another aspect of the invention, the axes of rotation of the cones,  20 A in FIG. 2, 30A in FIG. 3 and  40 A in FIG. 4, are oriented generally in the direction of the longitudinal axis  11  of the bit (as is apparent by reviewing FIG. 1 and is well known in the art). However, the axes of rotation  20 A,  30 A,  40 A are typically not coplanar with the longitudinal axis  11 , but each extend along a line which is displaced from the axis  11  by a selected amount. This selected amount is known as the cone axis “offset”. As is known in the art, the amount of cone axis offset for any particular bit is selected according to the type of formation being drilled and the structure of the cutting elements on the bit. In another aspect of the invention, one roller cone may have a larger offset than the other cones so as to increase the amount of left hand walk tendency of the drill bit. In a different embodiment, the amount of offset of one cone may be made smaller than the offset of the other cones to increase the amount of right hand walk tendency of the bit. The amount of offset is related to the amount of torque developed by the cone. The amount of torque will affect the tendency of the bit to right hand or left hand walk.  
         [0027]    Another aspect of the invention is illustrated in FIG. 1. Each one of the roller cones  20 ,  30 ,  40  on typical prior art roller cone bits is positioned longitudinally along the bit so that the profile of each roller cone (see FIGS. 2, 3 and  4 ) are at approximately the same longitudinal position. In this aspect of the invention, the axial position of one of the roller cones, for example L2 for cone  20  may be displaced with respect to the axial positions of the other cones (L3 for cone  30  and L4 for cone  40 ) to provide the bit with a selected amount of right hand or left hand walk.  
         [0028]    Yet another aspect of the invention concerns arranging the cutting elements on at least one cone to provide the bit with a selected amount of net lateral force generated during drilling. The lateral force includes two components, the radial and circumferential components. Preferably, this aspect of the invention is combined with having the net lateral force be in a direction corresponding to the position of one roller cone having one or more of the previously described aspects of the invention. For example, a bit may have the cutting elements arranged on the roller cones to develop a net lateral force in a direction corresponding to the azimuthal position (with respect to the axis  11 ) of one roller cone having a nose point closer to the longitudinal axis ( 11  in FIG. 2) than the other cones to increase the amount of left hand walk. As another example, the cutting elements may be arranged on one or more of the cones to develop a net lateral force in a direction corresponding to one roller cone having a higher journal angle than the other roller cones.  
         [0029]    The actual amount of right hand or left hand walk which will be developed by any bit made according to the above aspects of the invention may be determined by simulating the drilling response of the bit using a simulation program described in co-pending application Ser. No. 09/524,088 filed on Mar. 13, 2000 and assigned to the assignee of the present invention. In a method of designing a drill bit to have a selected walk characteristic according to another aspect of the invention, a bit may be designed with an initial set of design parameters. The drilling response of the bit having the initial parameters is simulated. One or more of the design aspects (parameters) described above may be modified or adjusted on one roller cone, and the response of the bit simulated, until a desired amount of right hand or left hand walk is developed by the bit during drilling.  
         [0030]    Various embodiments of the invention may provide a bit which has selected amounts of tendency to right hand walk or left hand walk during directional drilling operations. As will be readily appreciated by those skilled in the art, such selective walk tendencies may reduce the need for “slide” drilling with steerable motors in order to change the trajectory of a wellbore during directional drilling operations.  
         [0031]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.