Boring or penetrating the earth – Bit or bit element – Noncutting portion forwardly of cutting portion
Reexamination Certificate
1998-08-05
2001-11-27
Lillis, Eileen D. (Department: 3673)
Boring or penetrating the earth
Bit or bit element
Noncutting portion forwardly of cutting portion
C175S393000, C175S401000, C175S408000, C175S431000
Reexamination Certificate
active
06321862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to rotary bits for drilling subterranean formations. More specifically, the invention relates to fixed cutter or so-called “drag” bits suitable for directional drilling, wherein tandem gage pads are employed to provide enhanced stability of the bit while drilling both linear and non-linear borehole segments, and leading surfaces of the trailing or secondary gage pads in the tandem arrangement, and optionally trailing surfaces thereof, are provided with discrete, negatively-raked cutters or other cutting structures to remove ledging on the borehole sidewall.
2. State of the Art
It has long been known to design the path of a subterranean borehole to be other than linear in one or more segments, and so-called “directional” drilling has been practiced for many decades. Variations of directional drilling include drilling of a horizontal or highly deviated borehole from a primary, substantially vertical borehole, and drilling of a borehole so as to extend along the plane of a hydrocarbon-producing formation for an extended interval, rather than merely transversely penetrating its relatively small width or depth. Directional drilling, that is to say, varying the path of a borehole from a first direction to a second, may be carried out along a relatively small radius of curvature as short as five to six meters, or over a radius of curvature of many hundreds of meters.
Perhaps the most sophisticated evolution of directional drilling is the practice of so-called navigational or steerable drilling, wherein a drill bit is literally steered to drill one or more linear and non-linear borehole segments as it progresses using the same bottomhole assembly and without tripping the drill string.
Positive displacement (Moineau) type motors as well as turbines have been employed in combination with deflection devices such as bent housings, bent subs, eccentric stabilizers, and combinations thereof to effect oriented, nonlinear drilling when the bit is rotated only by the motor drive shaft, and linear drilling when the bit is rotated by the superimposed rotation of the motor shaft and the drill string.
Other steerable bottomhole assemblies are known, including those wherein deflection or orientation of the drill string may be altered by selective lateral extension and retraction of one or more contact pads or members against the borehole wall. One such system is the AutoTrak™ system, developed by the INTEQ operating unit of Baker Hughes Incorporated, assignee of the present invention. The bottomhole assembly of the AutoTrak™ system employs a non-rotating sleeve through which a rotating drive shaft extends to drive a rotary bit, the sleeve thus being decoupled from drill string rotation. The sleeve carries individually controllable, expandable, circumferentially spaced steering ribs on its exterior, the lateral forces exerted by the ribs on the sleeve being controlled by pistons operated by hydraulic fluid contained within a reservoir located within the sleeve. Closed loop electronics measure the relative position of the sleeve and substantially continuously adjust the position of each steering rib so as to provide a steady side force at the bit in a desired direction.
In any case, those skilled in the art have designed rotary bits and, specifically, rotary drag or fixed cutter bits to facilitate and enhance “steerable” characteristics of bits, as opposed to conventional bit designs wherein departure from a straight, intended path, commonly termed “walk”, is to be avoided. Examples of steerable bit designs are disclosed and claimed in U.S. Pat. No. 5,004,057 to Tibbitts, assigned to the assignee of the present invention.
Prevailing opinion for an extended period of time has been that bits employing relatively short gages, in some instances even shorter than gage lengths for conventional bits not intended for steerable applications, facilitate directional drilling. The inventors herein have recently determined that such an approach is erroneous, and that short-gage bits also produce an increased amount of borehole irregularities, such as sidewall ledging, spiraling of the borehole, and rifling of the borehole sidewall. Excessive side cutting tendencies of a bit may lead to ledging of a severity such that downhole tools may actually become stuck when traveling through the borehole.
Elongated gage pads exhibiting little or no side cutting aggressiveness, or the tendency to engage and cut the formation, may be beneficial for directional or steerable bits, since they would tend to prevent sudden, large, lateral displacements of the bit, which displacements may result in the aforementioned so-called “ledging” of the borehole wall. However, a simplistic elongated gage pad design approach exhibits shortcomings, as continuous, elongated gage pads extending down the side of the bit body may result in the trapping of formation cuttings in the elongated junk slots defined at the gage of the bit between adjacent gage pads, particularly if a given junk slot is provided with less than optimum hydraulic flow from its associated fluid passage on the face of the bit. Such clogging of only a single junk slot of a bit has been demonstrated to cause premature bit balling in soft, plastic formations. Moreover, providing lateral stabilization for the bit only at the circumferentially-spaced locations of gage pads comprising extensions of blades on the bit face may not be satisfactory in all circumstances. Finally, enhanced stabilization using elongated gage pads may not necessarily preclude all ledging of the borehole sidewall.
Thus, there is a need for a drill bit which provides good directional stability, as well as steerability, precludes lateral bit displacement, enhances formation cuttings removal from the bit, and maintains borehole quality.
BRIEF SUMMARY OF THE INVENTION
The present invention comprises a rotary drag bit, preferably equipped with polycrystalline diamond compact (PDC) cutters on blades extending above and radially to the side beyond the bit face, wherein the bit includes tandem, non-aggressive gage pads in the form of primary or longitudinally leading gage pads which may be substantially contiguous with the blades, and secondary or longitudinally trailing gage pads which are at least either longitudinally or rotationally discontinuous with the primary gage pads. Such an arrangement reduces any tendency toward undesirable side cutting by the bit, reducing ledging of the borehole sidewall.
The discontinuous tandem gage pads of the present invention provide the aforementioned benefits associated with conventional elongated gage pads, but provide a gap or aperture between circumferentially adjacent junk slots in the case of longitudinally discontinuous pads so that hydraulic flow may be shared between laterally-adjacent junk slots.
In the case of rotationally-offset, secondary gage pads, there is provided a set of rotationally-offset secondary junk slots above (as the bit is oriented during drilling) the primary junk slots, each of which secondary junk slots communicates with two circumferentially adjacent primary junk slots extending from the bit face, the hydraulic and cuttings flow from each primary junk slot being divided between two secondary junk slots. Thus, a relatively low-flow junk slot is not completely isolated, and excess or greater flows in its two laterally-adjacent junk slots may be contributed in a balancing effect, thus alleviating a tendency toward clogging of any particular junk slot.
In yet another aspect of the invention, the use of circumferentially-spaced, secondary gage pads rotationally offset from the primary gage pads provides superior bit stabilization by providing lateral support for the bit at twice as many circumferential locations as if only elongated primary gage pads or circumferentially-aligned primary and secondary gage pads were employed. Thus, bit stability is enhanced during both linear and non-linear drilling, and any tendency toward undesirable side cutting by the bit i
Beuershausen Christopher C.
Costo, Jr. Robert J.
Dykstra Mark W.
Illerhaus Roland
Norris James A.
Baker Hughes Incorporated
Lee Jong-Suk
Lillis Eileen D.
Trask & Britt
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