Drag-bit drilling with multi-axial tooth inserts

Details Drag-bit drilling with multi-axial tooth inserts Drag-bit drilling with multi-axial tooth inserts

1999-05-13

2001-10-16

Schoeppel, Roger (Department: 3672)

Boring or penetrating the earth

Bit or bit element

Forked rotary nontracking

C175S413000, C175S432000

Reexamination Certificate

active

06302224

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to drag-type drill bits, methods, and systems, and more particularly to replaceable teeth used in such bits.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling. In conventional vertical drilling a drill bit is mounted on the end of a drill string (drill pipe plus drill collars), which may be several miles long. At the surface a rotary drive turns the string, including the bit at the bottom of the hole, while drilling fluid (or “mud”) is pumped through the string.
When the bit wears out or breaks during drilling, it must be brought up out of the hole. This requires a process called “tripping”: a heavy hoist pulls the entire drill string out of the hole, in stages of (for example) about ninety feet at a time. After each stage of lifting, one “stand” of pipe is unscrewed and laid aside for reassembly (while the weight of the drill string is temporarily supported by another mechanism). Since the total weight of the drill string may be hundreds of tons, and the length of the drill string may be tens of thousands of feet, this is not a trivial job. One trip can require tens of hours, and this is a significant expense in the drilling budget. To resume drilling the entire process must be reversed. The bit's durability is very important, to minimize round trips for bit replacement during drilling.
The bit's teeth must crush or cut rock. The necessary forces are supplied by the “weight on bit” (WOB) which presses the bit down into the rock, and by the torque applied at the rotary drive. The WOB and torque are controlled to match the bit type, size, and drilling conditions, but the WOB may in some cases be 100,000 pounds or more. However, the forces actually seen at the drill bit are not constant: the rock being cut may have harder and softer portions (and may break unevenly), and the drill string itself can oscillate in many different modes. Thus the drill bit must be able to operate for long periods under high stresses in a remote environment.
Background: Drag-Type Bits
The simplest type of bit is a “drag” bit, where the entire bit rotates as a single unit. The body of the bit holds fixed teeth, which are typically made of an extremely hard material, such as e.g. tungsten carbide faced with polycrystalline diamond compact (PDC). The body of the bit may be steel, or may be a matrix of a harder material such as tungsten carbide.
As the drillstring is turned, the teeth of the drag bit are pushed through the rock by the combined forces of the weight-on-bit and the torque seen at the bit. (The torque at the bit will be somewhat less than the rotary torque, due to drag along the length of the drill string. The torque at the bit may also contain a dynamic component due to oscillation modes of the drill string). Since the weight-on-bit and the rotary torque are controlled by the driller, the net thrust vector seen at the tooth face will be slightly uncertain; but the normal range of torque and WOB values will imply only a relatively small range of angular uncertainty for each tooth's net force vector. (The rate-of-penetration and the hardness of the formation also have some effect on the orientation of the thrust vector seen at the tooth.) Thus each tooth can be aligned to an expected thrust direction, within a cone of a few degrees of uncertainty.
Background: Failure Modes of PDC-Type Teeth
The drilling environment is a harsh one, with high shock loading, high temperatures, and abrasive fluid flows. Even with modern superhard materials (such as PDC facings on a tungsten carbide body), drilling contractors often must perform expensive “trips” merely to replace drill bits.
All drill bit teeth can be expected to fail eventually. However, an important question is: How do they fail? PDC-type drill bit teeth have at least three important failure modes, as illustrated schematically in
FIGS. 20A-20C
. (These failure modes are illustrated for the bullet-type tooth of
FIG. 20
, but are relevant to many other tooth types as well.)
The most innocuous mode, illustrated in
FIG. 20A
, is inward abrasive wear of the cutting face. The side of the tooth's superhard face
2040
is gradually eroded inward, so that portion
2000
′ of the tooth's volume is gradually removed.
A less welcome failure mode, illustrated in
FIG. 20B
, is fracture. The force on the tooth's face is not distributed evenly, so it is possible for failure in shear to occur (where part of the face, and the part of the body behind it, breaks away from the rest of the tooth). This is a particularly damaging failure mode, since the separated tooth fragment
2000
″ is likely to be encountered by the next tooth behind it. The separated tooth fragment
2000
″, unlike the rock being drilled, is just as hard and has just as high a yield stress as the tooth behind it. Thus the separated tooth fragment
2000
″ has some chance of breaking the following tooth also. There is thus some chance of a “chain reaction,” where trash from one broken tooth causes tooth breakage to propagate to corresponding locations all the way around the bit.
An even more unwelcome failure mode, illustrated in FIG.
20
C, is “prying out” failure, where all or most of a single tooth's volume is removed from its socket. The single mass of tooth material has an even better chance of damaging the following tooth.
Background: Angled Teeth
Some attempts have been made to use angled teeth in drag bits.
FIG. 18
shows a conventional drill bit tooth
1810
which contains two nonparallel axes. This design has not come into wide use. The cantilevered front portion
1820
of the tooth provides a weak spot where large fragments or stray trash can exert outward forces; such outward forces can cause the inside of the bend to begin to fail in tension, and cracks can then propagate quickly. Even without trash or cuttings wedging under the front portion of the tooth, transient impacts at the face of the tooth can also translate to a net outward torque at the bend of the tooth, and this can lead to rapid failure. Thus such teeth are susceptible to failure modes which include being levered up, or failing in tension at the inner radius of the angle, or failing in shear across the shank.
FIG. 19
shows a different conventional drill bit tooth which has less of its length protruding from the body. This bit contains a face
1920
bent at an angle of roughly 90 degrees to the shank
1910
of the tooth. Here the very sharp angle between the cutting face and the shank produces a point of stress concentration, which is conducive to possible failure. Moreover, the thickness of material through which a shear-failure crack or defect must propagate through is at most the thickness of the tooth's shank
1910
. Such teeth are often backed by a portion of the steel bit body, but still the failure resistance is less than optimal. When the tooth starts to fail, its resultant cutting radius changes rapidly.
Background: “Bullet”-Type Teeth
FIG. 20
shows a sectional view of bullet-type drill bit tooth
2000
as disclosed in a sample embodiment of commonly-owned U.S. Pat. No. 5,558,170 to Thigpen et al. This patent, which is hereby incorporated by reference, describes (among other teachings) a drag-type drill bit in which the teeth are cylindrical, with a hemispherical back end
2050
for seating into a milled pocket. Typically the body
2010
of the tooth is a hard strong material, such as cemented tungsten carbide, and its front end is typically a flat circle which is coated with a superhard material
2040
such as a polycrystalline diamond compact (“PDC”). By using a spherical mill, an open cylindrical pocket with a spherically-shaped end surface can be machined into a steel bit body
2060
to provide a reasonably close fit to such a tooth, and the tooth can be brazed into the pocket to form a high-strength joint. By designing the pocket so that its sidewalls extend up to partially enclose the top of the tooth
2000
, some resis

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