Patent Publication Number: US-6338390-B1

Title: Method and apparatus for drilling a subterranean formation employing drill bit oscillation

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods of drilling subterranean formations using rotary-type drag bits, and more particularly to such methods employing an oscillating drill bit for more effective removal of formation chips from around the drill bit using drilling fluid. 
     2. Description of Related Art 
     Fixed-cutter rotary drag bits have been employed in subterranean drilling for many decades with various sizes, shapes and patterns of natural and synthetic diamonds used on drag bit crowns as cutting elements. Rotary drag-type drill bits typically comprise a bit body having a shank for connection to a drill string and an inner channel for supplying drilling fluid to the face of the bit through nozzles or other apertures. Drag bits may be cast and/or machined from metal, typically steel, or may be formed of a powder metal (typically tungsten carbide (WC)) infiltrated at high temperatures with a liquified binder material (typically copper-based) to form a matrix. Such bits may also be formed with layered-manufacturing technology, as disclosed in U.S. Pat. No. 5,433,280, which is assigned to the assignee of the present invention and incorporated herein by reference. 
     The bit body typically carries a plurality of cutting elements which is mounted directly on the face of the bit body or on carrier elements. The cutting elements are positioned adjacent fluid courses which allow cuttings (i.e., formation chips) generated during drilling to flow from the cutting elements to and through junk slots on the gage of the bit. The cuttings then move to the borehole annulus above the bit. Cutting elements may be secured to the bit by preliminary bonding to a carrier element, such as a stud, post, or cylinder, which is, in turn, inserted into a pocket, socket, recess or other aperture in the face of the bit and mechanically or metallurgically secured thereto. 
     One type of drag bit includes polycrystalline diamond compact (PDC) cutters typically comprised of a diamond table (usually of circular, semi-circular or tombstone shape) which presents a generally planar cutting face. A cutting edge (sometimes chamfered or beveled) is formed on one side of the cutting face which, during boring, is at least partially embedded into the formation so that the formation impacts at least a portion of the cutting face. As the bit rotates, the cutting face contacts the formation and a chip of formation material shears off and rides up the surface of the cutting face. When the bit is functioning properly, the chip breaks off from the formation and is transported out of the borehole via circulating drilling fluid. Another chip then begins to form in the vicinity of the cutting edge, slides up the cutting face of the cutting element, and breaks off in a similar fashion. Such action occurring at each cutting element on the bit removes formation material over the entire gage of the bit, and thereby causes the borehole to become progressively deeper. 
     In some subterranean formations, PDC cutting elements are very effective in cutting the formation as the drag bit rotates and the cutting edge of the cutting element engages the formation. However, in certain formations exhibiting plastic behavior, such as highly pressurized deep shales, mudstones, siltstones, some limestones and other ductile formations, the formation chips have a marked tendency to adhere to the leading surface of the bit body and the cutting face of the cutting element. 
     When formation chips adhere to the cutting elements, fluid courses or junk slots of the drill bit, the accumulated mass of chips impedes the flow of drilling fluid to the cutters and impedes the flow through the fluid courses and junk slots resulting in the reduction of cooling efficiency of the drilling fluid. Additionally, adherence of formation chips at or near the cutting faces of the cutting elements can actually prevent chips from sliding over the cutting face resulting in reduced cutting efficiency. 
     When these formation chips adhere to the cutting face of a cutting element, they tend to collect and build up as a mass of cuttings ahead of and adjacent to the point or line of engagement between the cutting face of the PDC cutting element and the formation, potentially increasing the net effective stress of the formation being cut. The buildup of formation chips moves the cutting action away from and ahead of the edge of the PDC cutting element and alters the failure mechanism and location of the cutting phenomenon so that cutting of the formation is actually effected by the built-up mass, which obviously is quite dull. Thus, the efficiency of the cutting elements, and hence of the drag bit itself, is drastically reduced. 
     Undesired adhesion of formation cuttings to the PDC cutting elements has long been recognized as a problem in the subterranean drilling art. A number of different approaches have been attempted to facilitate removal of formation cuttings from the cutting face of PDC cutting elements. For example, U.S. Pat. No. 5,582,258 to Tibbitts et al., assigned to the assignee of the present invention and herein incorporated by this reference, includes a chip breaker formed adjacent the cutting edge of the cutting elements to impart strain to a formation chip by bending and/or twisting the chip and thereby increasing the likelihood that the chip will break away from the face of the bit. Other approaches to solving the problem of formation chip removal include U.S. Pat. No. 4,606,418 to Thompson which discloses cutting elements having an aperture in the center thereof which feeds drilling fluid from the interior of the drill bit onto the cutting face to cool the diamond table and to remove formation cuttings. 
     U.S. Pat. No. 4,852,671 to Southland discloses a diamond cutting element which hag a passage extending from the support structure of the cutting element to the extreme outermost portion of the cutting element, which is notched in the area in which it engages the formation being cut so that drilling fluid from a plenum on the interior of the bit can be fed through the support structure and to the edge of the cutting element immediately adjacent the formation. U.S. Pat. No. 4,984,642 to Renard et al. discloses a cutting element having a ridged or grooved cutting face on the diamond table to promote the break-up of formation chips, or in the case of a machine tool, the break-up of chips of material being machined, and enhance their removal from the cutting face. The irregular topography of the cutting face assists in preventing balling or clogging of the drag bit by reducing the effective surface or contact area of the cutting face, which also reduces the pressure differential of the formation chips being cut. U.S. Pat. No. 5,172,778 to Tibbitts et al., assigned to the assignee of the present application, employs ridged, grooved, stair-step, scalloped, waved and other alternative non-planar cutting surface topographies to permit and promote the access of fluid in the borehole to the area on the cutting element cutting face immediately adjacent to and above the point of engagement with the formation. Such a non-planar cutting surface helps to equalize differential pressure across the formation chip being cut and thus reduce the shear force which opposes chip movement across the cutting surface. 
     U.S. Pat. No. 4,883,132 to Tibbitts, assigned to the assignee of the present application, discloses a novel drill bit design providing large cavities between the face of the bit and the cutting elements engaging the formation. Formation cuttings entering the cavity area are thus unsupported and more likely to break off for transport up the borehole. In addition, clearing of the cut chips is facilitated by nozzles aimed from behind the cutting elements (taken in the direction of bit rotation) so that the chips are impacted in a forward direction to break off immediately after being cut from the formation. U.S. Pat. No. 4,913,244 to Trujillo, assigned to the assignee of the present invention, discloses bits which employ large cutters having associated therewith directed jets of drilling fluid emanating from specifically oriented nozzles placed in the face of the bit in front of the cutting elements. The jet of drilling fluid is oriented so that the jet impacts between the cutting face of the cutting element and a formation chip as it is moving along the cutting face to peel the chip away from the cutting element and toward the gage of the bit. Likewise, GB 2,085,945 to Jurgens provides nozzles that direct drilling fluid toward the cutting elements to flush away cuttings generated by the cutting elements. 
     U.S. Pat. No. 5,447,208 to Lund et al., assigned to the assignee of the present invention, discloses a superhard cutting element having a polished, low friction, substantially planar cutting face to reduce chip adhesion across the cutting face. U.S. Pat. No. 5,115,873 to Pastusek, assigned to the assignee of the present application, discloses yet another manner in which formation cuttings can be removed from a cutting element by use of a structure adjacent to and/or incorporated with the face of the cutting element to direct drilling fluid to the face of the cutting element and behind the formation chip as it comes off the formation. 
     It has also been disclosed in the art that drilling systems which employ cycloidal sonic energy as a method of drilling cause highly effective cutting action on the bottom and particularly the adjacent side walls of the bottom portion of the well bore by virtue of the cycloidal drilling action. Typically, such vibratory drilling systems employ orbiting mass oscillators to generate vibratory energy. Such orbiting mass oscillators may employ orbiting rollers which are rotatably driven around the inner race wall of a housing, as disclosed in U.S. Pat. No. 4,815,328 to Bodine, or an unbalanced rotor, the output of which is coupled to a drill bit, as disclosed in U.S. Pat. No. 4,261,425 to Bodine. U.S. Pat. No. 5,562,169 to Barrow discloses a sonically driven drill bit employing an oscillator adapted to transmit sinusoidal pressure waves through the drill pipe. 
     None of the foregoing approaches to cutting element and bit design have been completely successful in facilitating chip removal from the face of the cutting element. Moreover, it will be appreciated by those skilled in the art that many of the foregoing approaches require significant modification to the cutting elements themselves, to the structure carrying the cutting elements on the bit face, and/or to the bit itself. Thus, many of the foregoing approaches to the problem require significant expenditures which substantially raise the price of the drill bit. In addition, due to required cutter placement on certain styles and sizes of bits, many of the prior art hydraulic chip removal arrangements are unsuitable for general application. Moreover, those bits employing vibrating drilling systems do not address the problem of chip removal. Accordingly, it would be extremely desirable to provide the industry with a solution to the impairment to the cutting mechanism caused by chip adhesion, which solution could be economically employed in any drill bit regardless of size or style, and regardless of the type of formation which might be encountered by the drill bit. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, drilling apparatus is provided for effecting a drilling method in which formation chips are produced with varying thicknesses to promote fracturing of the formation chips, thereby avoiding the buildup of formation chips near the bit body and facilitating removal of the formation chips from the bit face. Formation chips having various thicknesses are produced by selectively modifying the degree to which the cutting elements of the bit contact and cut the formation. Selective modification of the degree to which cutting elements contact the formation is achieved in the present invention by essentially modifying the axial and/or rotational/torsional movement of the drill bit, portions of the drill bit or the cutting elements attached to the drill bit. 
     The present invention provides apparatus for drilling a subterranean formation employing, by way of example only, a rotary-type drag bit comprising a bit body having a plurality of longitudinally extending blades, where adjacent blades define fluid courses with communicating junk slots therebetween. A plurality of cutting elements is attached to the blades, each cutting element including a cutting face oriented toward a fluid course. Upon rotation of the drill bit in a subterranean formation, formation chips cut by the cutting elements slide across the cutting elements, into the fluid courses and through the junk slots. The formation chips are then flushed into the annulus of the borehole. 
     In accordance with the drilling methods of the present invention, movement of the drill string, bit body or cutting elements is modified in a manner which introduces weak points into the formation chips as they are cut from the formation. That is, varying thicknesses are introduced into each formation chip as it is cut, thereby facilitating preferential breaking of the chip. In one embodiment, the bit is structured to oscillate torsionally as it rotates to produce alternating, relatively thicker and thinner sections of the chip such that each thicker chip portion is more likely to break away from the rest of the chip along the thinner portions of the chip by the force of drilling fluid contacting the chip. The broken formation chips enable their removal from the bit body and the borehole. Oscillation can be achieved by, for example, vibrating a near-bit sub or the bit shank using, for example, unbalanced rotating masses or an oscillating motor having an unbalanced rotor. In addition, such torsional oscillations may be produced at the surface by using a slip clutch in a near-bit sub, at the top drive, or in association with the rotating table. A pulsing hole wall brake, which cyclically engages and disengages the wall of the well bore, or a near-bit sub having a rotational transmission device which cyclically engages and disengages the drill bit may also oscillate the rotational velocity of the rotating drill bit. In harder formations, a cavitation jet which creates an irregular turbulent flow of drilling fluid around the bit, the flow direction of which oscillates, may cause vibration and, thus, may cause rotational oscillation of the bit relative to the well bore. Finally, a drill bit having individually oscillating cutting elements induced by increasing and decreasing drilling fluid pressure to the cutting elements may be employed to achieve the desired torsional oscillation. 
     In another embodiment of the invention, the bit is vertically oscillated relative to the longitudinal axis of the bit such that the load on the drill bit is cyclically increased and decreased to effect alternating deeper and relatively more shallow cuts into the formation, thus varying the thickness of formation chips generated by the cutting elements. Such vertical oscillations may be affected by varying the weight on bit (WOB) at the top drive. In addition, vertical oscillations may be accomplished by employing a fluid pulse to cyclically create alternating higher and lower hydrostatic pressures in the bit to cause variable degrees of contact with the formation. This may be accomplished by employing a valve and fluid jet assembly on a near-bit sub to “pulse” the drill bit vertically or at an angle, or by employing a valve and a piston-like assembly in or above the drill bit to cyclically vary the depth of cut (DOC) of the drill bit into the formation. In addition, a drill bit which is resiliently attached to the drill string, such as by a spring-loaded bit sub or piston-like bit sub which can vertically oscillate the bit relative to its longitudinal axis, can cyclically vary the depth of cut of the bit into the bottom of the borehole to produce formation cuttings of different thicknesses. Vertical oscillation in the cutting elements may also be produced by structuring a bit having adjustable blades. 
     In yet another embodiment of the invention, both vertical and torsional oscillation may be imposed on the drill bit by combining devices that produce vertical oscillation with those that produce torsional oscillation. Likewise, drill bit oscillation that is neither completely torsional nor completely vertical, but at some angle to the longitudinal axis of the drill bit, may be produced by combining devices herein describe or by operating a single device, such as a fluid pulse, at an angle to the longitudinal axis of the drill bit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention: 
     FIG. 1 is a view in elevation of a rotary-type drill bit in accordance with the present invention; 
     FIG. 2 is a partial view in cross-section of a formation chip being cut by a cutting element on a drill bit using a prior art method of drilling: 
     FIG. 3 is a partial view in cross-section of a formation chip being cut by a cutting element on a drill bit using a first embodiment of a drilling method in accordance with the present invention; 
     FIG. 4 is a partial view in cross-section of a formation chip being cut by a cutting element on a drill bit using a second embodiment of a drilling method in accordance with the present invention; 
     FIG. 5 is a view in elevation of an exemplary drilling apparatus having a motorized mechanism for providing vertical movement of the drill string to provide modified chip formation in accordance with the present invention; 
     FIG. 6 is a view in elevation and in partial cross-section of a second embodiment of a rotary-type drill bit in accordance with the present invention; 
     FIG. 7 is a view in elevation and in partial cross-section of a third embodiment of a rotary-type drill bit in accordance with the present invention; 
     FIG. 8 is a view in elevation and in partial cross-section of a fourth embodiment of a rotary-type drill bit in accordance with the present invention; 
     FIG. 9 is a view in elevation and in partial cross-section of a fifth embodiment of a drill bit in accordance with the present invention; 
     FIG. 10 is a view in elevation and in partial cross-section of a sixth embodiment of the present invention structured to provide vertical oscillation to the drill bit; 
     FIG. 11 is a view in elevation and in partial cross-section of a seventh embodiment of the present invention structured to provide movement in the cutting elements relative to the drill bit; 
     FIG. 12 is a view in elevation and in partial cross-section of an eighth embodiment of the present invention structured to provide torsional oscillation in the drill bit; 
     FIG. 13 is a partial view in cross-section of a drill bit blade illustrating a ninth embodiment of the present invention structured to provide movement in the cutting elements; 
     FIG. 14 is a partial view in longitudinal cross-section of one half of a drill bit illustrating a tenth embodiment of the present invention also structured to provide movement in the cutting elements; and 
     FIG. 15 is a view in elevation and in partial cross-section of an eleventh embodiment of the present invention also structured to provide movement in the cutting elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A typical rotary-type drill bit  10 , as shown in FIG. 1, comprises a bit body  12 , attached at the proximal end  16  thereof to a near-bit sub member  14 , and a bit crown  18  located at the distal end  20  of the drill bit  10 . The bit crown  18  includes a plurality of longitudinally extending blades  22  with a fluid course  23  positioned between each adjacent pair of blades  22 . Each fluid course  23  has a communicating junk slot  24  which is also positioned between adjacent blades  22 . Along each blade  22 , proximate the distal end  20  of the bit  10 , a plurality of cutting elements  25  is attached to the leading edge  27  of each blade  22  and oriented to cut into a subterranean formation upon rotation of the bit  10 . Each fluid course  23  is specifically defined by a first side wall  26 , a second side wall  28  and a bottom  30 . The first side wall  26  provides a surface adjacent the cutting face  29  of each cutting element  25 . 
     In conventional drilling, as formation chips are cut by the cutting elements  25 , the chips slide over the cutting face  29  of each cutting element  25 , across the side wall  26  adjacent the cutting elements  25  and into the corresponding fluid course  23 . In ideal conditions, drilling fluid directed through the fluid course  23  removes the chips from the cutting elements  25  and provides substantially clean cutting faces  29  during drilling. In some situations, such as drilling formations that exhibit plastic characteristics, the formation chips may tend to stick or adhere to the cutting face  29  of the cutting elements  25  and the adjacent side wall  26  of the fluid course  23 . Accordingly, drilling fluid flowing through the fluid course  23  may not adequately lift the formation chips from the side wall  26  for flushing away from the bit  10 . 
     As illustrated in FIG. 2, a typical method of drilling into a subterranean formation  40  employs both rotation of the bit  10  and weight on bit (WOB) to force the cutting element  25  into the formation  40 . Rotation of the drill bit  10  typically continues at substantially the same rate during drilling of the formation  40 . In many plastic formations, such as the aforementioned highly pressured or deep shales, mudstones, siltstones, some limestones and other ductile formations, a formation chip  42  cut by the cutting element  25  may actually be an elongated, substantially pliable chip  42  that will effectively flow over the cutting face  29  and adhere to the side wall  26  of the fluid course  23 . As the formation  40  is cut, the pliable chips  42  cut by the cutting element  25  may build up in the fluid course  23 , and eventually build up over the cutting face  29  of the cutting element  25 , effectively balling the drill bit  10  and preventing it from efficiently drilling into the formation  40 . 
     To overcome such problems as described in conventional drilling methods, the drill bit  10  and, thus the cutting elements  25  are oscillated in the present invention to create a formation chip  50  which has both relatively thick portions  52  and relatively thin portions  54 , as illustrated in FIG.  3 . In a first method of drilling in accordance with the present invention, illustrated in FIG. 3, the drill bit  10  and cutting elements  25  are rotatably and/or torsionally oscillated to create a formation chip having thick portions  52  and thin portions  54 . As the thin portion  54  extends over the cutting face  29  of the cutting element, the thick portion  52  is left substantially unsupported such that drilling fluid contacting the leading thick portion  52  can break it from the next following thick portion  52  along the interconnecting thin portion  54 . Thus, the chip  50  is broken into smaller sections before it can adhere to and build up on the side wall  26  of the fluid course  23  or on the cutting face  29 . FIG. 3 illustrates a formation chip  50  having a thick portion  52  of substantial longitudinal length relative to the size of the cutting face  29  of the cutting element  25 . Notably, increasing the frequency of oscillations causes the formation  40  to be cut in a manner which pulverizes the formation chips so that they can be carried away by the drilling fluid. 
     In some drilling operations, several different types of formations are encountered, ranging from relatively hard formations to relatively pliable formations. The rate of penetration of the bit  10  into the formation may typically be slower through hard formations and faster through softer formations. Knowing the pliability of the formation  40  at any given time, the various thick portions  52  and thin portions  54  of the formation chip  50  can be substantially predicted for a given WOB and rotational speed. Accordingly, when a formation  40  is encountered where balling of the bit  10  is of concern (i.e., adhesion of the formation chips  50  to the cutting elements  25  and bit body  12 ), the bit  10  may be selectively oscillated to produce a desired formation chip  50  profile, and when the bit  10  reaches a harder formation, the frequency of oscillation may be reduced or eliminated as necessary. Thus, the frequency of oscillation may be adjusted to optimize chip production for each of the different types of formation. 
     In FIG. 4, a second method according to the present invention is illustrated. In this method, a formation chip  50  having relatively thick portions  52  and relatively thin portions  54  is generated by the cutting element  25  under conditions where the normal force or WOB driving the bit  10  axially into the formation is cyclically varied. Accordingly, the cutting element  25  moves vertically or longitudinally relative to the formation  40  in a cyclical manner, cutting a depth D 1  to produce the thick portions  52  of the formation chip  90  and a depth D 2  to produce the thin portions  54  of the formation chip  50 . In a similar fashion to that illustrated in FIG. 3, the thick portions  52  will break away from the rest of the formation chip  50  relatively easily and will break sequentially along the intervening thin portions  54 . 
     The oscillating movement of the cutting elements, drill bit or drill string in the present invention to produce the desired profile of formation chips (i.e., alternating thick and thin portions) may be accomplished in various ways. FIG. 5, which schematically illustrates a formation drilling assembly, shows a drill string  60  positioned in a borehole  62  as it would be during a drilling operation. At the lower terminal end of the drill string  60  is a drill bit  10  positioned to cut into the formation. The drill string  60  is operatively connected to a rotary drive unit  64  which imparts rotational movement to the drill string  60  and, thus, to the drill bit  10 . Axial oscillation of the drill bit  10  to produce formation chips  50 , as shown in FIG. 4, may be achieved by imposing an axial oscillation or movement in the drill string  60 . Such axial oscillation may be induced, for example, by securing the rotary drive unit  64  to a support  66  using a resilient mechanism  68  (e.g., springs) which allows the drill string  60  to cyclically oscillate in a vertical direction  70 . The vertical oscillation imposed on the drill string  60  is translated to the drill bit  10 , causing the drill bit  10 , and thus the cutting elements, to contact the formation at varying depths to produce a formation chip  50  as shown in FIG.  4 . Oscillation of the drill string  60  may also be achieved in a similar manner by cyclically varying the WOB imposed on the drill string above ground. 
     Vertical oscillation necessary to produce the formation chips  50  shown in FIG. 4 may also be achieved by imposing oscillation in the drill bit  10 . A number of mechanisms may be employed to achieve oscillation in the drill bit  10 , a representative sampling of which are illustrated in FIGS. 6-10. In the assembly illustrated in FIG. 6, for example, the drill bit  10  is attached to a near-bit sub  76  which houses a spring mechanism  78  for effecting an oscillating movement in the drill bit  10  in the direction of arrow  70 . The drill bit  10  is attached to the near-bit sub  76  by any conventional structure, such as by securement of the threaded pin  80  of the drill bit  10  into a correspondingly threaded box  82  extending from the near-bit sub  76 . 
     The spring mechanism  78  may comprise a shank  83  which is slidably positioned through an opening  84  formed in the bottom of a retainer housing  86  of the near-bit sub  76 . The retainer housing  86  is, in turn, secured to an upper housing  88  of the near-bit sub  76 . The retainer housing  86  and upper housing  88  may be joined, for example, at joint  89  by a weld, although other forms of securement may be used. The retainer housing  86  may preferably be formed with at least one keyway  90  extending about the opening  84  of the retainer housing  86  into which may be positioned a spline  92  radiating from the shank  83 . The positioning of the spline  92  in the keyway go prevents the shank  83  from rotating relative to the retainer housing  86  during normal drilling operations. However, elimination of the keyway  90  may provide a slip clutch between an upper member  94  of the spring mechanism  78  and the shank  83  thereby providing torsional movement in the drill bit  10  as well. 
     The upper member  94  is sized to be retained inside the retainer housing  86  and is secured to the upper housing  88  of the near-bit sub  76 . As illustrated, the upper member  94  of the spring mechanism  78  may be separately formed and secured to the upper housing  88  by, for example, a weld at a contact interface  96  between the upper member  94  and the upper housing  88 . Other equally suitable means of securement may be employed however. Alternatively, the upper housing  88  and upper member  94  may be integrally formed as a single unit. The upper member  94  is configured with a centrally-located fluid channel  100  which communicates with a fluid channel  102  of the near-bit sub  76 . The shank  83  is also configured with a fluid channel  104  which is in fluid communication with the fluid channel  100  of the upper member  94  to deliver drilling fluid to the drill bit  10 . The upper member  94  is structured with a collar  106  which is slidably positioned within the fluid channel  104  of the shank  83  to prevent fluid from entering into the spring mechanism  78 . A structure other than a collar  106  may be suitably employed to achieve a resilient seal between the upper member  94  and the shank  83 . 
     The upper member  94  is configured with a flange  108  which is sized to be snugly received into the retainer housing  86 . The flange  108  is structured to retain an o-ring  109  about the circumference thereof to provide a seal between the upper member  94  and the retainer housing  86 . Likewise, the shank  83  is configured with a flange  110  which is snugly, but slidably received into the retainer housing  86  and which is positioned to contact an inner shoulder  112  of the retainer housing  86 . The flange  110  is also structured to retain an o-ring  111  about the circumference thereof to provide a seal between the shank  83  and the retainer housing  86 . An annular space  114  is formed between the flange  108  of the upper member  94  and the flange  110  of the shank  83  and a spring  116  is positioned about the upper member  94  and shank  83  within the annular space  114 . The spring  116  has a high degree of rigidity which, in the non-drilling state, keeps the upper member  94  spaced from the shank  83 , thereby providing a space  118  therebetween. Other resilient members, such as a rubber pad located within the space  118  formed between the upper member  94  and the shank  83 , may be employed to resiliently maintain the upper member  94  in spaced relationship to the shank  83 . 
     In operation, the shank  83  is maintained at a distance from the upper member  94  by the rigidity of the spring  116 . However, with a cyclical increase in the WOB imposed on the drill string or near-bit sub  76 , the spring  116  becomes slightly compressed, thereby allowing the shank  83  to slidably move toward the upper member  94 , and the space  118  therebetween is reduced. Thus, the drill bit  10  may be caused to oscillate in an axial direction  70 . Because there is inherent vibration of the drill bit  10  during drilling, the associated forces will facilitate the oscillation of the drill bit  10 . Accordingly, the drill bit  10  can axially oscillate relative to the upper housing  88 , and thus the drill string, resulting in the production of a formation chip  50  having relatively tick portions  52  and relatively thin portions  54  as illustrated in FIG. 4. A resilient sleeve  120  positioned about the shank  83  and pin  80  of the drill bit  10  allows the drill bit  10  to move axially while keeping debris from contacting the shank  83 . 
     In a second embodiment of a drill bit  10  structured to axially oscillate, illustrated in FIG. 7, the drill bit  10  may be attached to a near-bit sub  76  which is structured to house an alternative type of spring mechanism  124 . The near-bit sub  76  may be structured with a retainer housing  126  sized to receive the spring mechanism  124  therein. The retainer housing  126  is secured to an upper housing  127  of the near-bit sub  76 . The spring mechanism  124  in this embodiment comprises a body  128  positioned within the retainer housing  126  and a shank  130  extending from the body  128  through a central opening  132  of the retainer housing  126  through which the shank  130  is slidably received. The retainer housing  126  may be formed with at least one keyway  131  which is sized to receive a corresponding spline  133  formed on the shank  130  of the spring mechanism  124 . The spline  133  is vertically slidable within the keyway  131  to allow the spring mechanism  124  to impart axial oscillation to the drill bit  10 , but prevents rotation of the drill bit  10  relative to the near-bit sub  76  during drilling operations. 
     The body  128  of the spring mechanism  124  is configured with a flange  134  which is sized to fit snugly circumferentially within the retainer housing  126 . The flange  134  is structured to receive an o-ring  136  which maintains a seal between the retainer housing  126  and the flange  134  of the spring mechanism  124 . The body  128  is also formed with a portion adjacent the flange  134  which has an outer perimeter surface  135  that is of less circumferential dimension than the circumferential dimension of the flange  134 , thereby providing an annular space  138  about the body  128 . A rigid spring  140  is positioned within the annular space  138  and about the body  128  of the spring mechanism  124 . 
     The body  128  and shank  130  of the spring mechanism  124  are configured with a fluid channel  142  which receives drilling fluid moving from a fluid channel  144  of the near-bit sub  76  and delivers the drilling fluid to the drill bit  10 . The body  128  is also sized so that a gap  146  is provided between the bottom surface  147  of the upper housing  127  of the near-bit sub  76  and the upper surface  148  of the body  128 . The body  128  is also sized so that when drilling is not taking place, the rigid spring  140  maintains the body  128  of the spring mechanism  124  in spaced relation to the internal shoulder  149  of the retainer housing  126 . During drilling operations, drilling fluid flowing through the fluid channel  144  of the near-bit sub  76  fills the gap  146  and flows trough the fluid channel  142  of the spring mechanism  124 , While an amount of hydrostatic pressure results from the flow of drilling fluid, the spring  140  is normally sufficiently rigid to maintain the body  128  at a spaced distance from the internal shoulder  149  of the retainer housing  126 . However, vertical oscillation of the drill bit  10  may be produced by selectively and alternatively increasing and decreasing the flow of drilling fluid through the fluid channel  144  to thereby generate a pulsing action, or axial oscillation, in the drill bit  10 . A resilient sleeve  145  may be positioned about the shank  130  of the spring mechanism  124  to prevent fluid and debris from contacting the shank  130 . 
     In a third embodiment illustrated in FIG. 8, the hydrostatic pressure provided by the drilling fluid moving through the near-bit sub  76  is used to produce axial oscillation in the drill bit  10  using a pressure release mechanism  150 . The pressure release mechanism  150  is housed within the near-bit sub  76  and comprises a shank portion  152  slidably received within an opening  154  formed in the bottom of a retainer housing  156  of the near-bit sub  76 . The retainer housing  156  is secured to an upper housing  158  of the near-bit sub  76 . The retainer housing  156  is formed with at least one keyway  160  extending radially outward from the opening  154  and is sized to slidably receive a spline  162  formed in the shank portion  152 . The spline  162  is able to move vertically within the keyway  160  as the shank portion  152  oscillates, but the spline  162  and keyway  160  keep the shank portion  152  from rotating relative to the near-bit sub  76 . A resilient sleeve  163  may be positioned about the shank portion  152  to keep fluid and debris from the opening  154  of the retainer housing  156 . 
     The pressure release mechanism  150  includes a valve member  164  which is secured to the shank portion  152 . The valve member  164  includes a plunger-like portion  166 , the circumferential dimension of which allows the valve member  164  to fit snugly and slidably within the retainer housing  156  of the near-bit sub  76 . The valve member  164  is also structured with an upstanding hollow throat  168 , which is in axial alignment with the fluid channel  170  of the upper housing  158  of the near-bit sub  76 , and is positioned to be slidably receivable in the fluid channel  170 . The hollow throat  168  is sized in circumferential dimension to provide an annular space  172  between the hollow throat  168  and the fluid channel  170  for movement of drilling fluid therethrough. The hollow throat  168  defines a fluid channel  174  which is positioned to receive drilling fluid from the fluid channel  170  of the upper housing  158  of the near-bit sub  76  and is in fluid communication with a fluid channel  176  formed in the plunger-like portion  166  and a fluid channel  178  formed through the shank portion  152 . Thus, drilling fluid is able to move through the axially aligned series of fluid channels  170 ,  174 ,  176 ,  178  to deliver fluid to the drill bit  10  and is able to move through the annular space  172  formed about hollow throat  168  to fill a chamber  180  defined by the retainer housing  156 , upper housing  158  and valve member  164 . 
     In operation, as drilling fluid moves through the drill string and through the near-bit sub  76 , a greater portion of drilling fluid moves through the hollow throat  168  to the drill bit  10  while a smaller portion of drilling fluid moves through the annular space  172  to fill the chamber  180  with drilling fluid. As the chamber fills and pressure in the chamber  180  increases, the valve member  164  is forced downward, which also results in the shank portion  152  being forced downward. At least one opening  182  formed in the retainer housing  156  provides an opening through which drilling fluid may escape when the valve member  164  is forced downward a sufficient distance to allow the plunger-like portion  166  of the valve member  164  to clear the opening  182 . Thus, when sufficient pressure builds within the chamber, the valve member  164  is moved downward a sufficient distance to allow drilling fluid to escape the chamber  180  and pressure is released, causing the valve member  164  to move axially upward again until sufficient pressure builds in the chamber  180  again to produce a release in drilling fluid from the chamber  180 . A sufficient amount of pressure build-up and release is generated to produce oscillation of the drill bit  10  to provide a cutting of the formation as shown in FIG.  4 . 
     In a fourth embodiment illustrated in FIG. 9, axial oscillation of the drill bit  10  is induced by use of an oscillation mechanism  186  which employs the pressure of drilling fluid moving through the drill string to cause a vibration or oscillation of the drill bit  10  in the direction of arrow  70 . The oscillation mechanism  186  may be any suitable device which can operate to impose oscillation of the drill bit  10  relative to the drill string or, as shown, relative to a near-bit sub  76 . By way of example, one such device may be an oscillation valve  188  positioned within the fluid channel  190  of a shank  192  slidably positioned within the opening  194  of a retainer housing  196  of a near-bit sub  76 . The shank  192  is secured to the drill bit  10  by any conventional device, such as threaded securement of the pin  80  of the drill bit  10  to a correspondingly threaded box  198  of the shank  192 . 
     The shank  192  is slidably movable through an opening  194  in the retainer housing  196 , but the upper limit of movement of the shank  192  is defined by a stop member  200  housed within the retainer housing  196 . The stop member  200  may preferably be configured to fit snugly within the retainer housing  196  and provide a fluid seal between the stop member  200  and the retainer housing  196 , except for a fluid channel  202  formed in the center of the stop member  200  which is axially aligned with the fluid channel  190  of the shank  192 . Vertical movement of the shank  192  is also limited by the movement of a spline  204  of the shank  192  within a corresponding keyway  206  formed in the retainer housing  196  in radial position about the opening  194 . There may be at least one such keyway  206  formed in the retainer housing  196 . The spline  204  not only limits the axial movement of the shank  192  by contacting the bottom surface  208  of the stop member  200 , but prevents rotation of the shank  192  during drilling. 
     In operation, drilling fluid moving through the drill string (not shown) enters a fluid channel  210  formed in the upper housing  212  of the near-bit sub  76  and fills a chamber  214  defined by the upper housing  212 , the retainer housing  196  and the stop member  200 . Weight imposed on the drill bit  10  by the drill string, or WOB, causes the shank  192  to contact the stop member  200 . Additionally, as drilling fluid continues to move through the fluid channel  202  of the stop member  200  and into the fluid channel  190  of the shank  192 , the fluid pressure forces the shank  192  away from the stop member  200 , thereby providing a space  216  between the stop member  200  and the shank  192 . Fluid fills the space  216  and exerts sufficient pressure to provide a cushioning effect between the stop member  200  and the shank  192 . Drilling fluid moving through the oscillation mechanism  186 , here represented as an oscillation valve  188 , causes the shank  192  to vibrate or oscillate in the direction of arrow  70 . The shank  192  oscillates enough to provide contact with the formation in the manner shown in FIG. 4 to produce formation cuttings  50  of the type shown in FIG.  4 . Again, a resilient sleeve  218  may be positioned about the shank  192  to keep debris and fluid from clogging the opening  194  of the retainer housing  196 . 
     In a fifth embodiment of the invention illustrated in FIG. 10, the drill bit  10  may be made to vertically oscillate by providing at least one vibration mechanism  220  which receives electrical signals from above-ground. One possible method of providing vibration in the drill bit  10  is shown in FIG. 10 where one or more electrically-driven vibrating pistons  222  are housed within a near-bit sub  76 . The drill bit  10  is connected to a retainer cylinder  224  of the near-bit sub  76  by any suitable device, such as threaded securement of the pin  80  of the drill bit  10  with a correspondingly threaded box  226  of the retainer cylinder  224 . The retainer cylinder  224  is structured with a centrally-located fluid channel  232  which delivers drilling fluid to the drill bit  10 . The retainer cylinder  224  is further structured with a centrally-located upstanding collar  228  having an outwardly-extending flange  230 . 
     The near-bit sub  76  may further comprise an articulating cylinder  234  structured with a central channel  236  which is axially aligned with the fluid channel  232  of the retainer cylinder  224  to communicate drilling fluid from the drill string  60  to the drill bit  10 . The articulating cylinder  234  is secured to an end plate  238  of the near-bit sub  76  which, in turn, may be fitted with a threaded pin  240  or other device for securement of the near-bit sub  76  to the next adjacent section of the drill string  60 . The articulating cylinder  234  may be configured with a collar  242  which is sized to extend into the fluid channel  232  of the retainer cylinder  224  and register thereagainst so that fluid moving through the central channel  236  of the articulating cylinder  234  and the fluid channel  232  does not flow between the retainer cylinder  224  and the articulating cylinder  234 . The articulating cylinder  234  is further configured with an inwardly-extending flange  244  which is axially aligned with the flange  230  of the retainer cylinder  224  and is spaced therefrom. A resilient and compressible ring  246  is positioned between the flange  230  of the retainer cylinder  224  and the inwardly-extending flange  244  of the articulating cylinder  214  to cushion the movement of the retainer cylinder  224  relative to the articulating cylinder  234  and maintain the spacing between the flange  230  and the inwardly-extending flange  244 , as described more fully below. 
     The articulating cylinder  234  may generally be structured with a smaller circumferential dimension than the retainer cylinder  224 , thereby providing an annular space  248  about the articulating cylinder  234  in which the vibrating pistons  222  may reside as shown. Alternatively, the articulating cylinder  234  may be structured with openings sized in length and diameter sufficient to house the vibrating pistons  222  therein. The vibrating pistons  222  are positioned so that a vibrating tip  250  of the piston  222  contacts an upper surface  252  of the retainer cylinder  224 . In operation, as an electrical signal is sent via appropriate wiring  254  to each vibrating piston  222 , the tip  250  of each piston  222  contacts the upper surface  252  of the retainer cylinder  224  and causes a momentary downward force on the retainer cylinder  224 , and thus the drill bit  10 . The outwardly-extending flange  230  of the retainer cylinder  224  is momentarily forced toward the inwardly-extending flange  244  of the articulating cylinder  234 , such movement being cushioned by the resilient ring  246 . As the electrical signal is intermittently discontinued, the ring  246  forces the inwardly-extending flange  244  of the articulating cylinder  234  away from the flange  230  of the retainer cylinder  224  again. The intermittent application of power to the vibrating pistons  222  causes an axial vibration in the drill bit  10  which, in turn, produces a formation chip  50  as shown in FIG.  4 . 
     While the previously described embodiments of the invention have illustrated how vertical oscillation of the drill bit  10  may be produced by movement of the drill bit  10  relative to a near-bit sub  76 , FIG. 11 illustrates how relative axial oscillation of bit components can also be produced to achieve formation chips  50 , as shown in FIG. 4, by providing a drill bit  10  which is structured with a bit crown  270  which is movable in relation to the bit shank  272 . Specifically, the bit shank  272  is configured with an annular groove  274  which encircles the lower portion of the bit shank  272 . The annular groove  274  is sized to receive a resilient split ring  276 . The bit crown  270  is provided with an annular race  278  which is positioned to align with the annular groove  274  of the bit shank  272  when the bit crown  270  is secured to the bit shank  272  as shown. The annular race  278  is sized to receive a portion of the resilient split ring  276  such that the split ring  276  resides within both the annular groove  274  and the annular race  278 . As shown, the depth  280  of the annular race  278  is greater than the width of the resilient split ring  276  so that the bit crown  270  is capable of moving in an axial direction  70 , as suggested by the broken lines shown. 
     The bit crown  270  is formed with a plurality of fluid passageways  282  which extends from the exterior  284  of the bit crown  270  to a plenum  286  defined between the bit crown  270  and the bit shank  272 . In operation, when drilling fluid is delivered through the central channel  288  of the bit shank  272  to the plenum  286  for communication through the fluid passageways  282 , and when the pressure within the plenum increases a sufficient amount to overcome the WOB exerted on the bit crown  270 , the bit crown  270  is forced downward away from the bit shank  270  which, in turn, causes the cutting elements  25  to extend further into the formation. Pulsing action in the drilling fluid causes fluctuating increases and decreases in pressure within the plenum  286 , thereby providing a vertical oscillation in the bit crown  270  relative to the bit shank  272 . 
     FIG. 12 illustrates a different embodiment of the present invention where the varying degree to which the drill bit impacts the formation is provided by torsional oscillation  72  of the bit  10 , Torsional oscillation in the bit  10  may be accomplished by providing a pulsing hole wall brake  300  variably positionable within a near-bit sub  76  to oscillate between a wall-engaged position  302 , represented by broken lines, and a wall disengaged position  304 . In the wall-disengaged position  304 , the brake  300  is slidably movable within the near-bit sub  76  for residence therewithin such that the outer surface  306  of the brake  300  is substantially flush with the outer surface  308  of an upper segment  310  of the near-bit sub  76 . The brake  300  is secured to the near-bit sub  76 , though slidably movable relative thereto, by a pair of threaded fasteners  314 ,  316  which are secured to the upper segment  310  and a lower segment  312 , respectively, of the near-bit sub  76 . In addition, a fastener  318 , such as a bolt or other suitable device, may be employed to prevent rotation of the lower segment  312  relative to the upper segment  310  during drilling. The threaded fasteners  314 ,  316  are positioned through holes  320 ,  322  formed in the hole wall brake  300  and each fastener is encircled by a coiled spring  324 ,  326  which biases the brake  300  against the head  328 ,  330  of either threaded fastener  314 ,  316  during slidable movement of the brake  300  from the wall-engaging position  302  to the wall-disengaging position  304 , 
     Housed within the upper segment  310  and retained against the lower segment  312  is an offset orbiting member  334 , having a centerline  336  which is offset from the centerline  338  of the upper segment  310 . The orbiting member  334  is provided with a radial race  340  into which an upwardly extending protrusion  342  extends to maintain rotation of the orbiting member  334  about the centerline  338  of the upper segment  310 . The orbiting member  334  is provided with a fluid course  344  extending the longitudinal length of the orbiting member  334  and which is in fluid communication with the fluid passage  346  of the upper segment  310  and with the fluid passage  348  of the lower segment  312 . Flow of drilling fluid through the fluid course  344  of the orbiting member  334  causes the orbiting member to rotate, thereby effecting a spiral rotation of the fluid course  344 . With rotation of the orbiting member  334 , the brake  300  is intermittently forced outward toward the wall (not shown) of the formation to engage the wall. As the orbiting member  334  rotates further, the hole wall brake  300  returns to its wall-engaging position  302 . Engagement of the brake  300  with the formation may also be encouraged by cyclically varying fluid pressure moving through the fluid passage  146  into the fluid course  144  of the orbiting member  334 . With intermittent movement of the brake  300  from a wall-engaging position  302  to a wall-disengaging position  304 , torsional oscillation of the drill bit  10  is provided to, in turn, provide a variable cut in the formation. 
     As illustrated in FIG. 13, other bit configurations may be employed to impart torsional oscillation, represented by arrow  72 , to the bit  10  or, more precisely, portions thereof. In this embodiment, the bit  10  may be provided with a plurality of movable cutting elements  25  positioned along the leading edge  27  of each blade  22  of the bit  10 . Each cutting element  25  has a cutting face  360  and a support  362 , and further comprises a stem  364  which is housed within a socket  366  formed in the blade  22  of the bit  10  in a piston-like arrangement. The socket  366  is sized and shaped to receive a piston member  368  which is secured to the stem  364 . A cylindrical sleeve  370  encircles the stem  364  and is held within the socket  366  by a split retaining ring  372 . The stem  364  is slidably movable relative to the cylindrical sleeve  370 . The stem  364  is provided with a circumferential groove  374  which houses an O-ring  376  to seal the stem  364  relative to the cylindrical sleeve  370 . The socket  366  is in fluid communication with a fluid passageway  378  which receives drilling fluid from the drill string (not shown). When the fluid passageway  378  is pressurized by the flow of drilling fluid through the drill bit  10 , the cutting element  25  is forced outwardly from the leading edge  27  of each blade  22  of the bit  10 . By modulating the pressure of the drilling fluid exerted in the fluid passageway  378 , the cutting element  25  may be oscillated relative to the blade  22 , thereby achieving a chip formation as shown in FIG.  4 . 
     Another method of achieving torsional oscillation in the drill bit  10  is illustrated in FIG. 14, which illustrates one half of a drill bit  10  in cross section. In this embodiment, the blades  22  (only one being shown) of the drill bit  10  are movable relative to a bit body  400 , which comprises a combined bit shank  402  and bit crown  404 . The bit  10  includes a central fluid channel  406  which delivers drilling fluid into a plenum  408  formed in the bit crown  404 . Although not specifically shown in FIG. 14, the bit  10  is also structured with fluid passageways which communicate with the exterior of the bit  10  to deliver drilling fluid into the formation. In the illustrated embodiment, the blades  22  of the bit  10  are formed with a conventional structure comprising a gage portion  410  and a crown, or bottom portion  412 , which is positioned to engage the bottom of the formation during drilling. Cutting elements  25  are attached to each blade  22  in a conventional manner. 
     The bit body  400  is structured with a plurality of recesses  414  (only one being shown) which is sized and shaped to receive a blade  22  in a slidingly movable manner relative thereto, as suggested by the broken lines. Notably, the recesses  414  are sized such that blade  22  fits snugly into the recess  414  to avoid infiltration of dirt or other potentially clogging debris between the blade  22  and the recess  414 . Each blade  22  is attached to the bit body  400  by a suitable device which allows the blade  22  to move outwardly from the bit body  10  in response to, for example, an increase in fluid pressure exerted within the plenum  408 . By way of example only, the movable blade  22  may be secured to the bit body  400  at the crown  404  by a fastener  416 , such as a pin or bolt, which is positioned through an opening  418  in the bit body  400  and which extends into the blade  22  for securement thereto. The fastener  416  may be configured with a head  420  which is sized or shaped to respond to increases in pressure within the plenum such that the head  420 , and thus the fastener  416 , may be forced outwardly from the plenum responsive to such pressure increases. Movement of the fastener  416  forces the blade  22  outwardly as well to drive the cutting elements into the formation. Thus, when the pressure in the plenum  408  overcomes the WOB exerted on the drill bit  10 , and/or when the WOB exerted on the bit  10  is varied, the blades  22  are cyclically driven into the formation to produce a formation chip  50  as shown in FIG.  4 . 
     Movement of a portion of the drill bit  10  to achieve a variably shaped formation chip may be accomplished as illustrated in FIG. 15 where the drill bit  10  is again comprised of a separate bit shank  500  and bit crown  502  which are secured to each other in a movable manner, thereby allowing the bit crown  502  to move relative to the bit shank  500 . This embodiment is distinguishable from the embodiment shown in FIG. 11 by providing a bit crown  502  which is outwardly or laterally movable, in the direction of arrow  506 , from the bit shank  500 . Thus, the bit crown  502  of this embodiment is comprised of a plurality of crown sections  508  which is slidably movable relative to each other along a lateral surface  510  as the bit crown  502  expands responsive to a pressure exerted from within the bit  10 . It should be noted that the expansion of the bit crown  502  is relatively small (e.g., outward movement of from about one millimeter to about 5 millimeters) and the tolerances between the articulating crown sections  508  of the bit crown  502  are so small that the infiltration of dirt or other clogging material between the crown sections  508  is prevented. 
     The separate crown sections  508  comprising the bit crown  502  are each attached to the bit shank  500  by a fastener  512 , such as a bolt or other suitable device, which is positioned through an opening  514  formed through the upper portion  516  of the section  508 . The fastener  512  is secured at one end  518  to the bit shank  500  and may, for example, be threadingly engaged with a suitably sized and threaded opening  520  therein. The outer end  522  of the opening  514  is enlarged to accommodate the head  524  of the fastener  512  and provides a shoulder  526  against which the head  524  of the fastener comes in contact as the crown section  508  is moved outwardly under pressure. A spring  528  is positioned about a portion of the fastener and is biased between the opening  520  in the bit shank  500  and the fastener  512  to provide resilient movement of the crown section  508  relative to the bit shank. O-rings  530 ,  532  may be positioned between the crown section  508  and the bit shank  500  to provide a fluid-tight seal therebetween. 
     As drilling fluid moves through a central fluid channel  536  formed through the bit shank  500  and fills the plenum  538 , pressure in the plenum increases. The drilling fluid moves through a plurality of fluid passageways  540  formed in the crown sections  508  to provide fluid to the formation. When hydrostatic pressure within the plenum increases to a point where the pressure overcomes the WOB, the crown sections  508  move outwardly in the direction of arrow  506  to contact the formation at a greater depth. Further varying the WOB, in concert with a cyclical variation of the fluid pressure, causes the cutting elements  25  to contact the formation in a manner to produce formation chips as shown in FIG.  4 . 
     While the methods of achieving vertical and torsional oscillation of drill bits have been illustrated and described herein with respect to specific examples, those skilled in the art will appreciate that the structures and methods generally described may be adapted for use in a variety of situations or may be adapted to use with other types of bits, such as, for example, the drill bit having a tilting bit crown disclosed in U.S. Pat. No. 5,595,254 to Tibbitts and assigned to the assignee of the present invention. Thus, those skilled in the art will appreciate that one or more features of the illustrated embodiments may be combined with one or more features from another to form yet a further combination within the scope of the invention as described and claimed herein. Moreover, while certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the invention disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.