Abstract:
This rotary drill bit provides a plurality of spaced polycrystalline diamond compact cutter elements arranged at spaced 120° concentric rings around the face of the drill bit and a secondary, redundant deeper set polycrystalline diamond compact cutter elements placed to commence scuffing the face of a bore upon wear of the primary cutter element, thereby providing extended continuous service in hard rock drill applications previously unattainable with conventional PDC bits.

Description:
BACKGROUND OF INVENTION 
       [0001]    The present invention relates to a drill bit for use in drilling a bore in a hard-rock formation; and more specifically, to a rotary drill bit having a plurality of hardened cutting elements providing a polycrystalline diamond compact (PDC) crown coating on each spaced carbide insert button for scuffing a bore hole face and providing a secondary shadowing or redundant hardened PDC crowned cutting element to assume the scuffing action at the same radial groove created by the primary cutter element, after wear removes the PDC crown from the primary button. Button drill bits have long been known to be useable for drilling in formations for percussion-type bits. Some have suggested shaped button drill bits for rotary drill bit applications having cutting edges for cutting into the rock face. Others have provided enlarged buttons for crushing engagement of the well bore face. As an example, U.S. Pat. No. 4,109,731 discloses a drill bit having a plurality of PDC cutter element disposed on a bit shank, but which does not attempt to systematically scour the face of the bore and only seeks to minimize the chipping forces experienced by the brittle PDC element. In the &#39;731 patent, the cutters are angled and mounted on compression fit posts to maximize the support given the cutter element. The PDC elements have long been known to withstand direct engagement with the well bore face but easily chip when minimally stressed on an angle to the cutter body. References herein to PDC cutter elements shall mean polycrystalline diamond compact crowns or coverings on carbide inserts. The present arrangement seeks to maximize the scouring force while aligned with the longitudinal orientation of each PDC cutter element, thereby maximizing wear and minimizing chipping. The spacing of the elements and the generous fluid flow around them prevents chipping forces from prematurely fracturing the PDC elements. 
       SUMMARY OF INVENTION 
       [0002]    This rotary drill bit comprises a body having a proximal end adapted to connect to a drill string with a longitudinal passage accommodating a fluid flow through a face on a distal end of the drill bit. The rotary drill bit is populated with PDC elements which are ballistic conical buttons providing a minimal radius point to contact a bore face with a scuffing motion, thus removing a minimal amount of hardened rock in concentric tracks around the bore. These PDC elements comprise: a central PDC element with a coating of PDC creating a composite crown on a hardened element connected on the face adjacent a longitudinal axis of the drill bit; a first plurality of evenly spaced PDC crowned elements connected on the face in concentric rings providing a single point of contact on each concentric ring; and, at least three full-gauged PDC elements peripherally and evenly spaced around the face and angled from the longitudinal axis of the drill bit for contacting a bore. 
         [0003]    This rotary drill bit can also have a second and redundant plurality of PDC elements, here each spaced about 180° from the first plurality of spaced PDC elements but placed on the same concentric radial ring, save and except for the central cutter element directly adjacent to the longitudinal axis, which has no redundant element. Each of these redundant elements has a surface profile substantially equal to the depth of the surface thickness of the PDC crown on the hardened elements of the first plurality of spaced PDC elements and there may be one or more evenly spaced plurality of PDC elements on each concentric ring. The spacing of each redundant PDC element, from one concentric ring to the next outer concentric ring, is about 120°, in the same manner as the spacing of the first plurality of elements. 
         [0004]    At the outer edge of the rotary drill bit, the peripherally spaced full-gauged PDC crowned elements are angled away from the longitudinal axis at about 30°. As each cutter element on a concentric ring scores the wellbore face, the hardened PDC composite crown of the first plurality of spaced elements wears down. Upon full wear of the crown of each primary PDC element, the second plurality of spaced PDC elements assumes the scuffing of the well bore in place of the first on each concentric ring. The outermost elements are placed at an angle of about 30° away from the longitudinal axis to continue scuffing the well bore face as well as to prevent excessive wear on the edge of the drill bit as drilling continues into the bore. 
         [0005]    This disclosure also provides a method for rotary drilling of hard rock providing the steps of attaching a rotary bit having radially-spaced, longitudinally-aligned ballistic conical PDC crowned carbide elements arranged in a manner that permits only a single ballistic conical element to engage a borehole face at a radius from the central longitudinal axis of the drill bit; turning the rotary bit to engage the borehole face at a central location scouring a central groove in the borehole face; increasing the forward movement of the rotary drill bit to progressively advance the crowned carbide elements against the borehole face; and, clearing borehole cuttings from the face of the borehole utilizing the hydraulic pressure from a plurality of jetting nozzles in the face of the rotary bit. The velocity of the longitudinal movement of the drill string and bit can be increased after a desired borehole width is achieved, by increasing both the speed of rotation and increasing the weight on the bit, all in a manner well known in the drilling industry. Since each radial groove is being cut in the borehole face by a single cutter, the borehole face erodes quickly from both the deepening of the grooves formed by the PDC cutter elements and the collapse of the intervening peaks from one groove to the next outer groove. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an end or face view of an embodiment of the present invention. 
           [0007]      FIG. 2  is cross-sectional view of a portion of the drill bit of the present invention showing the relative placement of a jetting nozzle passage of one view. 
           [0008]      FIG. 3  is a second cross-sectional view of a portion featuring the drill bit of the present embodiment showing the relative placement of the second (and third) jetting nozzle passage. 
           [0009]      FIG. 4  is a cross-sectional schematic view depicting the central or primary cutter element adjacent the central axis of the drill bit face. 
           [0010]      FIG. 5  is a cross-sectional schematic view of additional spaced cutter elements in a cross-sectional plane showing redundant cutter elements providing a shortened profile which become active upon wear of the primary cutter element at the same radius. 
           [0011]      FIG. 6  is an additional, cross-sectional schematic view showing the redundant cutter elements providing a shortened profile which becomes active upon wear of the primary cutter elements. 
           [0012]      FIG. 7  is a cross-sectional schematic view of the outer cutter elements showing a redundant cutter element and the relative location of an outer PDC element preventing wear on the outer diameter of the drill bit. 
           [0013]      FIG. 8  is a cross-sectional view of the drill bit and the bore hole face showing the relative scuffing action of the longitudinal PDC cutter elements creating grooves in the well bore face. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a detailed schematic layout of the drill bit face  10  of the present application showing the arrangement of the polycrystalline diamond compact cutters (PDC) of the present embodiment. Although applicant has described these as PDC elements, other hard faced cutter element inserts might be substituted, such as cubic boron nitride (PCBN), depending on the hardness of the rock being bored, without departing from the spirit or intent of this disclosure. Junk slots  12  and a plurality of jetting nozzle passages  14  and  14 ′ permit generous use of fluid to both cool the drill bit and flush cuttings away from the bore face. 
         [0015]    The central cutter element  16  is affixed to the drill bit adjacent the central longitudinal axis  11  of the drill bit. Central cutter element  16  is the first to engage the face of the bore and scuffs and scores the face. Since the angle of incidence of the central cutter element  16  is virtually at 90° to the rock face, no or little chipping force is experienced by this cutter element. Upon sufficient penetration of the central cutter  16 , a second cutter element  18  begins to engage the bore face. The outer diameter of this second primary cutter element  18  is located a radial distance from the longitudinal center axis  11  of the drill bit equivalent to the central cutter  16  diameter, and rotated about 120° from the radius on which the central cutter is located. In  FIG. 1 , the central cutter element  16  is located on line  4 - 4  and the secondary primary cutter element  18  is located on line  5 - 5  rotated 120° from the line  4 - 4 . 
         [0016]    Except for the central cutter element  16 , each subsequent primary cutter element, beginning with the second, has a secondary or twin, redundant or shadow cutter element shown as broken-lined elements. Accordingly, the redundant cutter  18 ′ is located on a concentric radial ring (not shown) occupied by the primary cutter  18  and is located 180° from the primary cutter  18  at the same radial distance from the longitudinal central axis  11  of the drill bit. Each redundant cutter is attached at a distance of about 0.050″ (0.13 cm) deeper into the face of the drill bit equivalent to the depth of the PDC crown on the primary cutter, thus allowing the secondary or redundant cutter to assume and continue the scuffing action commenced by the action of the primary cutter as wear erodes the PDC crown of the prior cutter. 
         [0017]    Accordingly, PDC element  20  is located on a radius outside element  18  and rotated as described 120°. Shadow or redundant cutter element  20 ′ is on the same radius as its primary  20 , and is placed on the same radius at 180° from the primary  20 . Similarly, PDC element  22  is located on the next radial ring further from the axis and is 120° from element  20 . Its redundant cutter element  22 ′ is on the same radius at 180° from the primary cutter element  22 . Similarly, in this embodiment, primary cutter elements  24 ,  26  and  28  are placed at 120° from each other and their respective secondary shadow cutter elements  24 ′,  26 ′ and  28 ′ are placed 180° on the same radius from their respective primary cutter elements. 
         [0018]    The concentric lines shown of  FIG. 1  are flats to facilitate machining and should not be construed as equivalent to the radial concentric rings for placement of the PDC crowned elements on the bit face  10 . Each of the concentric rings of cutter elements (not shown) is arranged in the same manner disposing the primary and redundant cutters around the drill bit face until the outer periphery of the drill bit is reached. Around the periphery, at least three evenly spaced PDC hardened crown elements  30 ,  32  and  34  are positioned on a 30° angle from the central longitudinal axis of the drill bit as will be more accurately shown in  FIGS. 2 and 3 . Secondary or redundant cutters  36 ,  38 , and  40 , angled at the same 30° angle, can be inserted to the same PDC-crown-related depth to provide backup contact with the bore wall during drilling in order to prevent wear on the outer edges of the drill bit. 
         [0019]      FIG. 2  is a first cross-sectional view of the drill bit shank showing the relative placement of the central  16  and primary  18 - 30  PDC cutter elements and a first passage  14  for fluid jetting through the drill bit through its longitudinal passage  56 . The first cutter element  16  is the central cutter and the subsequent cutter elements are the primary elements of each successive concentric ring spaced from the longitudinal axis of the drill bit. Additional hardened buttons  42  as shown in  FIGS. 2 and 3  can be inserted to center the drill bit within the bore and prevent excessive wearing as the drill bit moves through the bore.  FIG. 2  also discloses a breaker slot  54  found on most drill bit shanks  50  permitting the makeup of the drill bit, all in a manner well known in the drilling industry by connecting the male threaded surface  52  to a drill bit sub (not shown). 
         [0020]      FIG. 3  is a second cross-sectional view of the drill bit shank  50 , again showing the relative placement of the PDC cutter elements  16 - 30  and a fluid passage  14 ′ connected to the longitudinal passage  56 , identical for both the second and third nozzle passages shown in  FIG. 1  directed at differing angles from the longitudinal axis of the drill bit. These three nozzles  14  and  14 ′ permit abundant fluid flow around the sparsely populated drill bit face moving cuttings through the junk slots  12  identified in  FIG. 1  and thereafter up the annulus to the surface. This abundant fluid flow also allows cooling of the cutter elements; thereby allowing the use of PDC cutter elements which become unstable and deteriorate at temperatures between 600° and 800° C. 
         [0021]      FIG. 4  is a schematic and exaggerated cross-sectional view through the line  4 - 4  of  FIG. 1  showing the central cutter element and the relative placement of other cutter elements spaced on other concentric rings within the drill bit face at the forth and seventh concentric ring as shown in  FIG. 1 . 
         [0022]      FIG. 5  is a schematic and exaggerated cross-sectional view through the line  5 - 5  showing the relative placement of the other cutter elements spaced on the first and fourth concentric rings described in  FIG. 1 . 
         [0023]      FIG. 6  is a schematic and exaggerated cross-sectional view through the line  6 - 6  showing the relative placement of the other cutter elements spaced at the second and fifth concentric rings shown in  FIG. 1 . 
         [0024]    Located around the periphery of the drill bit, as more fully shown in  FIGS. 1 and 7  are the at least three evenly spaced cutter elements which are angled at the 30° from the longitudinal axis previously described. Redundant secondary periphery cutters, again recessed into the drill bit to a depth approximating the height of the PDC crown on each primary angled cutter element are positioned to take over and continue serving to limit wear to the drill bit shank, at the full-gauge preventing excessive wear on the edge of the drill bit as drilling continues into the bore. PDC buttons  42  can be inserted around the full diameter of the drill bit shank to minimize wear of the bit from the bore wall. These wear buttons  42  can be placed at a variety of positions along the drill bit shank in a manner well known in the art. 
         [0025]      FIG. 8  is a cross-sectional view of the rotary drill bit bore hole face B showing the scoured grooves G rubbed in the face of the bore by the rotary movement of the PDC cutters in each distinct groove. Here, PDC cutters of line  4 - 4 , more clearly shown in  FIGS. 1 and 4 , are shown scuffing grooves in the bore face B. The peaks P and P′ formed between the successive grooves G disintegrate and crumble when the central cutter  16  and adjacent cutters  22 ,  22 ′,  28  and  28 ′ score the interior surfaces of each groove allowing the rock to be swept from the bore hole by the flushing drill fluid directed at them. Similar action of grooving, peak fracture and removal occur on the alternative cross-sectional planes of cutters shown in  FIGS. 5-7 . The forward motion of the drill bit into the bore B continuously deepens each groove G thereby causing crumbling and fracture of the alternative peak points P and P′ which are rapidly removed from the drill bit face by the hydraulic sweep of the jetting nozzles. The swarf examined from tests run with this bit show fines originating from each groove and a larger particulate rubble from the collapsed peaks. Removal of material is both fast and effective, and wear is minimal, in the testing performed on this new and unique rotary drill bit. 
         [0026]    Fabricators can immediately note that as the size of the drill bit made larger, exposing a greater area cut by the drill bit, the number of PDC elements increases only linearly and not in proportion to the total area of the drill bit face. This feature makes this drill bit less expensive to scale up for use in larger diameter holes; distinguishing it from other PDC drill bit products, which require additional coverage area of the PDC cutter elements to increase the size of the drill bit. The amount of space between adjacent cutter elements is believed to minimize the chipping occurring from the swarf cut from the bore face since large pathways are provided to move the cuttings into the junk slots and up the annulus. The redundant cutter elements are believed to extend the cutting life of this bit in hard rock. Testing performed in 35 Ksi granite suggests exceptional wear characteristics might be expected in drilling hard-rock formations. 
         [0027]    Each of the foregoing aspects of the invention may be used alone or in combination with other such aspects. The embodiments described herein are exemplary only and are not limiting of the invention, and modifications thereof can be made by one skilled in the art without departing from the spirit or teachings of this invention. Many variations and modifications of the embodiments described herein are thus possible and within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein.