Abstract:
Rock detritus created by a drag bit cutter shearing subterranean formation material may flow under the cutter and attach itself to the side surface of the cutter barrel by differential pressure-induced sticking, and dilate. This attached material, confined by hydrostatic pressure, can create and strengthen a barrier between the cutter and the virgin rock being cut. The detritus barrier absorbs bit weight and reduces cutter efficiency by impairing contact of the cutter with the virgin rock formation. Increasing the friction between the rock detritus and the side surface of the cutter barrel inhibits detritus flow, reduces build up, and allows hydrostatic pressure to contribute to, rather than inhibit, the cutting process. Similar beneficial results may be obtained when hydrostatic pressure drilling fluid is permitted to communicate through holes in the side surface of the cutter, or through an otherwise permeable side surface alleviating detritus sticking due to differential pressure effects.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to drill bits for drilling subterranean formations and, more specifically, to cutters for drilling such formations and drill bits so equipped. 
       BACKGROUND OF THE INVENTION 
       [0002]    Rotary drag bits have been use for subterranean drilling for many decades, and various sizes shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements, or cutters. When drilling certain subterranean formations, a properly designed drag bit can provide an improved rate of penetration (ROP) over a tri-cone bit. 
         [0003]    Rotary drag bit performance has been improved significantly with the introduction of polycrystalline diamond compact (PDC) cutting elements, usually configured with a substantially planar PDC table formed onto a cemented tungsten carbide substrate under high temperature and high pressure conditions. PDC tables are formed into various shapes, including circular, semicircular, and tombstone, which are the most commonly used configurations. Additionally the PDC tables can be formed so that a peripheral edge, or edge portion, of the table is coextensive with the sidewall of the supporting tungsten carbide substrate, or the PDC table may overhang the substrate sidewall slightly, forming a “lip” at the trailing edge of the table. In some instances, such as when a portion of the PDC table adjacent the cutting face has been leached of the metal catalyst used to stimulate diamond-to-diamond bonding during formation of the PDC table, a lip may form during drilling due to more rapid wear of the unleached portion of the PDC table to the rear of the leached portion. PDC cutters have provided drill bit designers with a wide variety of potential cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives not possible with natural diamond or smaller synthetic diamond cutters. 
         [0004]    While rotary drag bits provide better ROP than tri-cone bits under many conditions, the performance of rotary drag bits can still be improved. Researchers in the industry have recognized that controlling buildup of recompacted rock cuttings, or detritus, on the cutting face of a PDC cutter is a significant factor affecting cutting performance. Methods used to manage detritus buildup on PDC table cutting faces include mechanical, hydraulic and chemical means of attacking the recompacted detritus. 
         [0005]    The aforementioned lip configuration on PDC cutting elements has been observed to improve cutting efficiency by reducing detritus buildup on the sidewall of the cutting element to the rear of the PDC table, but the operative mechanism for this observed phenomenon has not been understood. Moreover, configuring a PDC cutting element with, or to form, a protruding lip adds cost to cutting element fabrication and the increased cost of such cutting elements may not be perceived to be commensurate with the benefits obtained for many applications. 
         [0006]    What is needed are straightforward, cost-effective improvements to rotary drag bit cutters to inhibit flow and buildup of detritus over the side surface of the cutter adjacent the formation being cut, to remove recompacted detritus from the side surface of the cutter earlier in the buildup cycle, or both. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Embodiments of the invention demonstrate that modifications to the structure of PDC cutting elements or cutters, such as varying the topography of the side surface of the cutter barrel or increasing its permeability at least in an area adjacent the formation being cut, can achieve beneficial results by inhibiting the flow and buildup of detritus on the side surface, or by effectively removing detritus buildup. 
         [0008]    These structural configurations appear to counteract “differential sticking,” which may be described as the tendency of detritus cut from the formation and flowing past a cutter between the cutter and the adjacent formation to adhere to the surface of the cutter due to hydrostatic pressure acting on the detritus. Such differential sticking is avoided because these structural configurations of the cutter barrel enable hydrostatic pressure to invade between the side surface and any closely proximate detritus. 
         [0009]    Embodiments of the invention include various structures to increase provide a varying topography for the side surface of the cutter barrel. 
         [0010]    One approach to providing a varying side surface topography comprises texturing or roughening the side surface of the cutter barrel. A texture can be cast, milled, or cut into the side surface and may comprise ridges, grooves, cross-hatching, bumps, divots, dimples or holes. Roughening can be accomplished with sandblasting, beadblasting, shot-peening, or by adding hardfacing to the side surface by welding techniques. 
         [0011]    Another approach to varying side surface topography may include adding structures to the side surface. It is contemplated that bars, discs, triangles, cubes, rods or balls formed from a wear-resistant material such as tungsten carbide, PDC elements, TSP (thermally stable PDC) elements, or a combination of such materials may be used. The structures, depending upon their composition, may be welded, brazed or cemented directly to the side surface or to compatible sockets formed in the side surface. 
         [0012]    As yet another approach, particles of a wear-resistant material such as tungsten carbide, natural diamond or synthetic diamond may be applied to, or included in, the material used to form the side surface of the cutter barrel, or incorporated in an insert secured in a recess in the side surface. 
         [0013]    In all of the foregoing cases, the varying side surface topography promotes access of ambient hydrostatic drilling fluid pressure in the vicinity of the cutter to the side surface and specifically between detritus closely proximate the side surface and the side surface itself, which prevents differential sticking of detritus flowing past the side surface of the cutter. 
         [0014]    A further approach to effectively reduce the amount of detritus buildup on the side surface of the cutter barrel is to increase the permeability of the side surface to permit the ambient hydrostatic drilling fluid pressure in the vicinity of the cutter to communicate through the side surface to the area between the side surface and any detritus in close proximity, and prevent differential sticking. 
         [0015]    The permeability can be improved by establishing a pattern of holes or apertures on the side surface of the cutter barrel or by forming the side surface of the cutter barrel from a porous, or permeable, material. The holes or porous material place the side surface of the cutter barrel in the vicinity of the formation in communication with the drilling fluid filtrate under hydrostatic pressure. Thus, the drilling fluid adjacent the side surface of the cutter barrel will lubricate the side surface and offset any tendency of the hydrostatic pressure adjacent the side surface to cause differential sticking. Since the hydrostatic pressure in the vicinity of the side surface of the cutter barrel is substantially equalized on the cutter side and the formation side of any detritus contacting the cutter barrel, the flow of drilling fluid (or the rotation of the bit moving through the drilling fluid) will break away any cut formation material stuck on, or compressed to, the side surface earlier in a detritus buildup cycle. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0016]    The foregoing and other advantages of the invention will become apparent upon reading the following detailed description with reference to the drawings, in which: 
           [0017]      FIG. 1A  is a Particle Flow Code (PFC) model of a cutter barrel assembly with detritus buildup on the bottom surface; 
           [0018]      FIG. 1B  is a PFC model of a cutter barrel assembly with a cutter including a lip and no detritus buildup on the bottom surface; 
           [0019]      FIG. 2A  is a PFC model of a cutter barrel assembly with the detritus forming an obtuse angle between the bottom surface and the pressure boundary of the compacted detritus; 
           [0020]      FIG. 2B  is a PFC model of a cutter barrel assembly with detritus forming an acute angle between the bottom surface and the pressure boundary of the compacted detritus; 
           [0021]      FIG. 3A  is a PFC model of a cutter barrel assembly where the coefficient of friction for the bottom surface is low; 
           [0022]      FIG. 3B  is PFC model of a cutter barrel assembly where the coefficient of friction for the bottom surface is high; 
           [0023]      FIG. 4  depicts a conventional rotary drag bit including one embodiment of the present invention; 
           [0024]      FIG. 5A  is a section view of cutter barrel assembly including structure disposed in sockets formed in the bottom surface; 
           [0025]      FIG. 5B  is a section view of a cutter barrel assembly including balls or cylinders attached to the bottom surface; 
           [0026]      FIG. 5C  is a section view of a cutter barrel assembly including abrasive particles interstitial with the cutter barrel assembly; 
           [0027]      FIG. 5D  is a section view of a cutter barrel assembly where the bottom surface includes a texture or has been roughened; and 
           [0028]      FIG. 5E  is a section view of a cutter barrel assembly where holes or nozzles, in communication with pressurized drilling fluid filtrate, are disposed on the bottom surface. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    It has been found that the recompacted rock detritus can have a confined strength on the same order of magnitude as virgin rock, and Particle Flow Code (PFC) models used in Discrete Element Modeling (DEM) of rock formations show that most of the energy in rock cutting using a fixed cutter is expended while extruding the recompacted detritus. Particle Flow Code is produced by Itasca Consulting Company of Minneapolis, Minn. 
         [0030]    Additionally, PFC models show that the flow of detritus under the cutter (between the cutter and the formation being cut) is equally as important as the flow of detritus on the cutter face. This role of detritus flow affecting the cutting mechanism, and the consequent potential for differential sticking to the cutter barrel, which impairs cutter access to the formation being drilled and significantly reduces cutting efficiency, has previously gone unrecognized in the art. Innovations that affect the flow of detritus under the cutter offer opportunity to enhance cutting efficiency. 
         [0031]    When detritus material flows adjacent to a surface of a cutting element or cutter, it can differentially stick to the surface; this is true both of the recompacted cuttings or chips flowing on the cutting face of the cutter and those flowing under the cutter and across the side surface of the cutter barrel adjacent to the formation being cut. Particle Flow Code (PFC) models of rock characteristics show that the differential sticking of detritus material flowing under a cutter can be a significant factor governing cutting efficiency in certain subterranean formations and, perhaps, the single most significant factor in relatively impermeable formations such as all shales, and most carbonates. In such formations, where both the rock and the detritus are relatively impermeable, this recompacted particulate material creates a barrier between the cutter and the virgin rock. Downhole pressure compacts and strengthens the detritus material into the barrier, causing it to absorb bit weight and reduce cutter efficiency. 
         [0032]    The pore pressure inside the detritus is typically lower than the hydrostatic pressure of the surrounding drilling fluid, because of dilation of the detritus, so the hydrostatic pressure pushes the detritus against the side surface of the cutter barrel. The nature of drilling fluid, or “mud,” prevents penetration of the fluid into the particulate detritus mass, initiating and exacerbating this problem. 
         [0033]      FIGS. 1A and 1B  show PFC models of a PDC cutter  10  cutting rock. As shown, the bit body carrying the PDC cutter  10  comprising a tungsten carbide substrate  20  having a diamond table  12  formed thereon is traveling in a left to right direction, cutting into virgin rock  62  (below line  60 ), shearing the rock and forming detritus  64 . A portion of the detritus  64  is extruded up the cutting face of diamond table  12  of the PDC cutter  10 , forming a cuttings chip  68 . In each of  FIGS. 1A and 1B , some detritus  64  flows under the cutter  10 . The black dots at the surface of the detritus  64  on the cutting face and under the PDC cutter  10  as well as on the surface of the virgin rock  62  represent a pressure boundary between, respectively, the detritus  64  and rock  62  and the surrounding drilling fluid pumped into the borehole and under hydrostatic pressure. In  FIG. 1A  the detritus  64  flowing under the cutter  12  is differentially sticking to the side surface  14  of the cutter barrel  10  and inhibiting cutting. In contrast,  FIG. 1B  includes a diamond table  12  that overhangs or forms a lip  16  beyond the adjacent surface of the tungsten carbide substrate  20 . In this model very little detritus  64  flows under the cutter  10  and no detritus  64  is sticking to the side surface  14 . This beneficial effect is attributed to the ability of the lip  16  to inhibit the flow of detritus  64 . A clear side surface  14  allows the hydrostatic pressure to penetrate the detritus  64  at the lip  16  of diamond table  12 , contributing to the efficiency of the cutting process. 
         [0034]    Additionally, when detritus flows under a cutter during drilling, the degree of sticking of detritus to the cutter barrel has been observed to effect a clearing mechanism under appropriate circumstances. Initially, the detritus will form a deposit that continues to gather material until the buildup is large enough and configured in a shape that allows ambient hydrostatic pressure between the detritus and the side surface of the cutter barrel and alleviate differential sticking. As the cutter advances under these circumstances, the material buildup is sheared away from the side surface of the cutter barrel, temporarily enhancing cutting efficiency. 
         [0035]    Each of  FIGS. 2A and 2B  show a cutter  10  moving from the left toward the right with the diamond table  12  taking a cutting  68 . The detritus  64  is shown to be flowing under the cutter  12  in both instances. However, the image of  FIG. 2A  depicts an undesirable situation in terms of the buildup of detritus  64 . As the detritus  64  flows under cutter  10 , it begin to differentially stick due to hydrostatic pressure pushing it against side surface  14 , forming a compacted mass  66  on the side surface  14 . The compacted mass  66  creates an obtuse angle  54  with the side surface  14 . In this detritus configuration, the hydrostatic pressure (shown as a vectors by arrows  52 ), which acts perpendicular to the pressure boundary  50 , forces and holds the compacted mass  66  against the bottom surface  14 . However, as shown in  FIG. 2B , if movement of the detritus  64  adjacent side surface  14  is arrested, rather than the detritus  64  being permitted to slide on, stick to, and be compacted on, side surface  14 , the angle  54  between the compacted mass  66  and the bottom surface  14  becomes acute, as shown in  FIG. 2B . Once the detritus forms an acute angle  54  with side surface  14 , the hydrostatic pressure  52  along pressure boundary  50  wedges between and forces any compacted mass  66  away from the side surface  14 , releasing the differential pressure-initiated bond between the detritus  64  and the side surface  14  of cutter  10 . As the total mass flow of detritus  64  past the cutter  10  continues during the drilling process, if the detritus  64  cannot slip easily along side surface  14 , then the detritus  64  will form the aforementioned acute angle  54  with side surface  14  and hydrostatic pressure will continue its beneficial penetration into the region between the side surface  14  and the detritus  64 , wedging and spreading the gap therebetween on a substantially continuous basis. 
         [0036]    It is common in the drilling industry to polish cutting faces of PDC cutters to attempt to limit detritus buildup by providing a low-friction surface on which the detritus, forming a cuttings chip, may easily side. However, PFC models show that, contrary to conventional thinking, higher coefficients of friction may be used to inhibit detritus buildup on cutter barrels.  FIGS. 3A and 3B  are PFC models showing cutters  10  where the friction coefficient of the bottom surface  14  has been manipulated. For the model shown in  FIG. 3A , the coefficient is set arbitrarily low (0.1) and in for the model in  FIG. 3B  the coefficient is set arbitrarily high (2.0). In  FIG. 3A  the detritus  64  is shown to be flowing under the side surface  14  of cutter  10  and differentially sticking, forming a compacted mass  66  on the side surface  14 . This compacted mass  66  of detritus  64  absorbs bit weight and enables the hydrostatic pressure  52  to continue buildup of detritus  64 . In contrast, the PFC model with a high coefficient of friction shown in  FIG. 3B  shows no differential sticking. This allows the cutting edge of diamond table  12  to substantially fully contact the virgin rock surface  60  and the hydrostatic pressure  52  to penetrate between the detritus  64  and side surface  14  proximate the cutting edge of diamond table  12  and act beneficially to lift the detritus  64  away from the side surface  14 , inhibiting buildup. The PFC model tests shown in  FIGS. 3A and 3B  were repeated numerous times with different bit clearance angles  18  (the angle between the side surface  14  of the cutter  10  and the direction of cut into adjacent, underlying formation material), including tests with the clearance angle as low as 5 degrees. All tests provided consistent, repeatable results confirming the phenomenon illustrated in  FIGS. 3A AND 3B . 
         [0037]    Referring to  FIG. 4 , a conventional fixed-cutter rotary drill bit  300  includes a bit body  302  that has generally radially projecting and longitudinally extending wings or blades  304 , which are separated by junk slots  306 . A plurality of PDC cutters  10  are provided on the leading faces of the blades  304  extending over the face  308  of the bit body  302 . The face  308  of the bit body  302  includes the surfaces of the blades  304  that are configured to engage the formation being drilled, as well as the exterior surfaces of the bit body  302  within the channels and junk slots  306 . The plurality of PDC cutters  10  may be provided along each of the blades  304  within pockets  310  formed in the blades  304 , and may be supported from behind by buttresses  312 , which may be integrally formed with the bit body  302 . 
         [0038]    The drill bit  300  may further include an API threaded connection portion  314  for attaching the drill bit  300  to a drill string (not shown). Furthermore, a longitudinal bore (not shown) extends longitudinally through at least a portion of the bit body  302 , and internal fluid passageways (not shown) provide fluid communication between the longitudinal bore and nozzles  316  provided at the face  308  of the bit body  302  and opening onto the channels leading to junk slots  306 . 
         [0039]    During drilling operations, the drill bit  300  is positioned at the bottom of a well bore hole and rotated while weight on bit is applied and drilling fluid is pumped through the longitudinal bore, the internal fluid passageways, and the nozzles  316  to the face  308  of the bit body  302 . As the drill bit  300  is rotated, the PDC cutters  10  scrape across, and shear away, the underlying earth formation. The formation cutting mix with and are suspended within the drilling fluid and pass through the junk slots  306  and up through an annular space between the wall of the bore hole and the outer surface of the drill string to the surface of the earth formation. 
         [0040]    The inventor contemplates that embodiments of the cutter of the invention will be used on rotary drag bits as described above and including without limitation core bits, bi-center bits, and eccentric bits, as well as on fixed cutter drilling tools of any configuration, including without limitation reamers or other hole opening tools. Accordingly, the terms “rotary drag bit” and “apparatus for subterranean drilling” as used herein encompasses all such apparatus. 
         [0041]    Each of  FIGS. 5A-5E  is a partial section view of an embodiment of a cutter according to the present invention, each cutter embodiment including a cutter barrel  110  comprising a supporting substrate having a PDC table  112  formed thereon and a side surface  114  which, when the cutter is positioned on a rotary drag bit, is adjacent to the formation being cut. 
         [0042]      FIG. 5A  is a partial section view including structures  140 A disposed in sockets formed in, or disposed on, the side surface  114  of cutter barrel  110 . The structures  140 A may be configured as bars, discs, triangles, cubes or rods, which are welded, brazed or cemented into reciprocal sockets formed in the side surface  114 . The structures  140 A may be formed using a hard, erosion- and abrasion-resistant material such as tungsten carbide, PDC or TSP. Structures  140 A will increase friction between the detritus cut from the formation and the side surface  114 . 
         [0043]      FIG. 5B  depicts balls or cylinders  140 B secured to the side surface  114  of cutter barrel  110 . The balls or cylinders  140 B will increase friction between the side surface  114  and the detritus. The cylinders or balls  140 B may be cemented, welded or brazed directly on the side surface  114 , or may be secured in sockets formed in the side surface  114 . The balls or cylinders  140 B may comprise a wear-resistant material such as tungsten carbide, PDC or TSP. 
         [0044]      FIG. 5C  depicts abrasive particles  140 C carried on side surface  114  of cutter barrel  110 . The abrasive particles  140 C can be tungsten carbide, natural diamond, or synthetic diamond. The abrasive particles  140 C may be cemented, welded or brazed on the bottom surface  114  or the abrasive particles  140 C may be cast or otherwise incorporated directly into the barrel material. The abrasive particles  140 C may also be formed into an insert by a process such as casting or sintering. The insert can then be disposed in a complementary receptacle in side surface  114 . Embodiments where the abrasive particles  140 C are integral with the side surface  114  provide an additional advantage in that, as the side surface  114  wears, new abrasive particles will be exposed. Further, it is known in the art to coat diamond grit with a single layer of metal, or multiple layers, which coatings may be used to bond the aforementioned natural or synthetic diamond particles to side surface  114 , or integrally with the material (conventionally tungsten carbide) of cutter barrel  110  during formation thereof. 
         [0045]    The section of side surface  114  of cutter barrel  110  shown in  FIG. 5D  includes a textured or patterned topography or has been roughened, at  140 D, to provide an irregular surface. The texture  140 D can be cast, milled, or cut into the side surface  114  and may comprise ridges, grooves, cross-hatching, bumps, divots, dimples or holes. Roughening can be achieved by sandblasting, beadblasting, shot-peening, or by welding a hardfacing material to the side surface  114 . 
         [0046]    As will be readily appreciate by those of ordinary skill in the art, the foregoing embodiments, which may be said to increase frictional characteristics of the side surface  114 , hinder the formation of the previously-described obtuse angle between detritus and the side surface  114 , maintaining access of hydrostatic pressure to the area therebetween. 
         [0047]      FIG. 5E  is a partial section view of the side surface  114  of cutter barrel  110  including holes or apertures  140 E opening thereonto. High pressure filtrate in the form of drilling fluid under ambient pressure communicating through the holes or apertures  140 E will equalize pressure with that tending to press detritus against side surface  114 , largely prevent detritus buildup on the side surface  114  and break away any significant deposit that begins to form. In lieu of the relatively large holes or apertures  140 E, a portion of cutter barrel  110  may be formed to be substantially porous or permeable, as illustrated in broken lines  140 E′, or a porous insert (such as a porous, sintered body) disposed in a recess in the cutter barrel  110 , to provide access by high pressure drilling fluid from the drill bit interior to side surface  114 . 
         [0048]    The foregoing embodiment maybe described as hindering differential sticking by allowing hydrostatic pressure in the vicinity of the cutter to communicate into the area between the side surface  114  and proximate detritus. 
         [0049]    While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.