Boring or penetrating the earth – Bit or bit element – Rolling cutter bit or rolling cutter bit element
Reexamination Certificate
2000-06-08
2003-09-02
Bagnell, David (Department: 3672)
Boring or penetrating the earth
Bit or bit element
Rolling cutter bit or rolling cutter bit element
C175S331000, C175S378000
Reexamination Certificate
active
06612384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bit designs.
2. Background Art
Roller cone drill bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells.
FIG. 1
shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig
10
used to turn a drill string
12
which extends downward into the well bore
14
. Connected to the end of the drill string
12
is a roller cone-type drill bit
20
, shown in further detail in FIG.
2
.
Referring to
FIG. 2
, roller cone drill bits
20
typically comprise a bit body
22
having an externally threaded connection at one end
24
, and a plurality of roller cones
26
(usually three as shown) attached at the other end of the bit body
22
. The cones
26
are able to rotate with respect to the bit body
22
. Disposed on each of the cones
26
of the bit
20
is a plurality of cutting elements
28
typically arranged in rows about the surface of each cone
26
.
The cutting elements
28
on a roller cone
26
may include primary cutting elements, gage cutting elements, and ridge cutting elements. Primary cutting elements are the cutting elements arranged on the surface of the cone such that they contact the bottomhole surface as the bit is rotated to cut through the formation. Gage cutting elements are the cutting elements arranged on the surface of the cone to scrape the side wall of the hole to maintain a desired diameter of the hole as the formation is drilled. Ridge cutting elements are miniature cutting elements typically located between primary cutting elements to cut formation ridges that may pass between the primary cutting elements to protect the cones and minimize wear on the cones due to contact with the formation. The cutting elements
28
may be tungsten carbide inserts, superhard inserts, such as polycrystalline diamond compacts, or milled steel teeth with or without hardface coating.
Significant expense is involved in the design and manufacture of drill bits to produce bits which have increased drilling efficiency and longevity. For more simple bit designs, such as those for fixed cutter bits, models have been developed and used to design and analyze bit configurations which exhibit balanced forces on the cutting elements of the bit during drilling. Fixed cutter bits designed using these models have been shown to provide faster penetration and long life.
Roller cone bits are more complex than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones, wherein each roller cone independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the cones rotate independently of each other, the rotational speed of each cone of the bit can be different from the rotation speed of the other cones. The rotation speed for each cone of a bit can be determined from the rotational speed of the bit and the effective radius of the “drive row” of the cone. The effective radius of the drive row is generally related to the radial extent of the cutting elements that extend axially the farthest from the axis of rotation of the cone, these cutting elements generally being located on a so-called “drive row”. Adding to the complexity of roller cone bit designs, the cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modem roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as shown for example in FIG.
3
A and further detailed in U.S. Pat. No. 5,372,210 issued to Harrell. Intermeshing of the cutting elements on roller cone bits is desirable to enable high insert protrusion to achieve good rates of penetration while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits substantially constrains cutting element layout on the bit, thereby further complicating the designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, accurate models of roller cone bits have not been widely developed or used to design roller cone bits. Instead, roller cone bits have largely been developed through trial and error. For example, if cutting elements on one cone of a prior art bit wore down faster that the cutting elements on another cone of the bit, a new bit design would be developed by simply adding more cutting elements to the faster worn cone in hopes of reducing the wear of each cutting element on that cone. Trial and error methods for designing roller cone bits have led to roller cone bits which have an imbalanced distribution of force on the bit. This is especially true for roller cone bits having cutting elements arranged to intermesh between adjacent cones.
Using a method for simulating the drilling performance of roller cone bits drilling earth formations, described in a U.S. patent application filed Ser. No. 09/524,088 in the United States on Mar. 13, 2000, entitled “Method for Simulating the Drilling of Roller Cone Drill Bits and its Application to Roller Cone Drill Bit Design and Performance” and assigned to the assignee of this invention, prior art roller cone bits were analyzed and found to typically unequally distribute the axial force on the bit between the cones, such that the axial forces on two cones differ by more than 200%. Such an unequal distribution of force between the cones results in an unequal distribution of stress, strain, wear, and premature failure of the cone or cones carrying the largest load(s) during drilling. Additionally, prior art roller cone bits typically have significant imbalances in the distribution of the volume of formation cut between the cones. In such prior art bits, the volume of formation cut by each cone, typically, differs by more than 75%, wherein the volume cut by one cone was 75% more than the volume of formation cut by each of the other cones on the bit. Prior art bits also have substantial imbalance between the amount of work performed by each of the cones on the bit.
Additionally, prior art bits with cutting elements arranged to intermesh between adjacent cones have significant differences in the number of cutting elements on each cone in contact with the formation during drilling. Prior art bits also typically have large differences in the projected area of cutting elements in contact with formation on each cone, and in the depths of penetration achieved by the cutting elements on each cone. As a result, the projection area of cutting element contact for each cone greatly differs in typical prior art bit designs. Additionally, the cutting elements on each cone of prior art bits typically achieve unequal depths of penetration for each cone. In some prior art designs, the unequal cutting element penetration depth between the cones is partially due to the bottomhole profile formed by the bit during drilling. Additionally for typical prior art bits, the axial force on the bit is distributed in a multi modal profile and the forces on corresponding rows of each cone may significantly differ. Further, prior art bits often have cutting elements arranged about the surface of each cone such that forces acting on corresponding cutting elements on each cone significantly differ. Using drill bits which have multi-modal force distributions, or an unequal distribution of force between corresponding rows of the cones or corresponding cutting elements of the cones may result in a bottomhole profile formed by the bit that is multi-modal which may contribute to the unequal cutting element penetration depth and an imbalanced distribution of force on the bit between the cones.
One example of a prior art bit considered effective in the drilling wells is shown in
FIGS. 3A-3D
. This drill bit com
Huang Sujian
Singh Amardeep
Xiang Ying
Bagnell David
Rosenthal & Osha L.L.P.
Smith International Inc.
Walker Zakiya
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