Metal tools and implements – making – Blank or process – Drill
Reexamination Certificate
2002-12-23
2004-03-30
Watts, Douglas D. (Department: 3724)
Metal tools and implements, making
Blank or process
Drill
C175S331000
Reexamination Certificate
active
06711969
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, employing superabrasive cutters exhibiting continuously varying cutter backrake angles along different locations or zones on the face of the bit, the variations being tailored to improve the transition between portions of the bit which may contain different cutter backrake angles as well as optimize the performance of the drill bit.
2. State of the Art
Conventional rotary-type earth-boring drill bits typically include cutting elements, or “cutters,” arranged thereon so as to facilitate the cutting away of a subterranean formation in a desired manner. Cutters, typically including polycrystalline diamond compacts (PDCs), are oriented in cutter pockets of the bit, which are oriented so as to protect the cutter and provide clearance at the trailing edge of the cutter as it moves axially while drilling. The angle at which a cutting face of a cutter is oriented relative to a wall of a bore hole being formed is referred to as “rake.” If the angle between a bore hole surface and a cutter face is 90°, the rake is said to be neutral, or zero degrees. If the angle between the cutting face of a cutter and the adjacent surface of the bore hole being formed is less than 90°, the rake angle is negative, and is typically termed “backrake.” The amount of backrake is equal to the angle the cutting face of the cutter is tilted from the neutral rake position. For example, a cutter oriented with its cutting face at a 70° angle to the adjacent surface of the bore hole being formed has a 20° backrake (90°−70°=20°). When the rake angle between the cutting face of a cutter and the adjacent bore hole surface is greater than 90°, the cutter is oriented with a positive, or aggressive, rake angle, or a “frontrake,” which is measured in a similar manner to that in which backrake is measured.
Recent laboratory testing and modeling have demonstrated that cutter backrake angles may affect drilling performance characteristics. Specifically, increasing the backrake angle of a cutter appears to improve drilling performance after the cutter begins to wear. The wear flat of a cutter oriented at a larger backrake angle is smaller than the wear flat of a cutter oriented at a smaller (i.e., closer to neutral) backrake angle for a given amount of diamond volume removed. This means that as the diamond begins to wear away from the cutter, cutters oriented at larger backrake angles have smaller “flat” areas than do cutters oriented at smaller backrake angles. Smaller wear flats on cutters essentially provide a more effective cutting geometry. A sharp cutter (i.e., small wear flat) contacts a formation with less area and the same amount of force, thereby inducing larger stresses in the formation, increasing cutting efficiency. In addition, it has been found that orienting cutters to have larger backrake angles does not detrimentally affect the performance of the bit as cutter wear increases. Moreover, cutters that are oriented to have larger backrake angles typically provide better impact resistance than cutters that are oriented to have smaller backrake angles.
Although the aforementioned increased impact resistance and advantageous wear flat behavior is beneficial, the detriment to large backrake angles is that more weight on bit (WOB) is required to drill at a given rate of penetration (ROP). Therefore, generally, an all-encompassing increase in cutter backrake angles may cause the drill bit to require such a great WOB so as to render the bit undrillable.
Cutter rake not only affects the relationship between the ROP and the WOB but also determines the aggressiveness of the bit. Thus, the rakes of the cutters on a drag bit can affect the performance and drilling characteristics of the bit. The cutters on many drag bits are oriented so as to be backraked due to the increased fracture resistance of cutters with relatively large backrakes.
Current PDC drag bit design typically includes cutters oriented at different backrake angles depending upon their locations upon the bit. For example, cutters that are located within about a third of the bit radius from the bit's longitudinal axis are typically oriented with nominal 15° backrake angles. Cutters located in the shoulder area of the bit are oriented with backrake angles of about 20°. Cutters that are positioned near the gage section of the bit are typically oriented so as to have even higher backrake angles, for instance, about 30°. This discontinuous change in cutter backrake angle abruptly changes cutter behavior and performance between each area of the bit. This discontinuity may be exaggerated by the effective rake angles of the cutters.
Each cutter located on a bit crown at a given radial distance from the longitudinal axis of the bit will traverse a helical path upon rotation of the bit. The geometry (pitch) of the helical path is determined by the ROP of the bit (i.e., the rate at which the bit drills into a formation) and the rotational speed of the bit. Mathematically, it can be shown that the helical angle traversed by a cutter relative to a horizontal plane (i.e., a plane normal to the longitudinal axis of the bit) depends upon the distance the cutter is spaced apart from the longitudinal axis of the bit. For a given ROP and rotary speed, cutters located closer to the longitudinal axis have greater helical angles than those of cutters positioned greater distances from the longitudinal axis of the bit. Essentially, the greatest change in helical angles occurs for cutters positioned about 1½ inches to about 2 inches from the bit's longitudinal axis. In this region, the helical angles of the cutters during rotation of the bit vary from near 90° for cutters nearest the longitudinal axis of the bit to about 7° for cutters positioned about 2 inches from the longitudinal axis. The change in helical angle for cutters spaced about 2 inches from the longitudinal axis up to the bit gage is relatively small.
Effective cutter backrake is the angle between the cutter and the formation after correcting for the aforementioned helical angle during drilling (i.e., subtracting the helical angle of a cutter during drilling from the rake angle of the cutter). Since cutters may be at different radial locations, their cutting speeds will vary linearly with their radial position. This phenomenon of variance in “effective rake” of a cutter with radial location, bit rotational speed, and ROP is known in the art and a more detailed discussion thereof may be found in U.S. Pat. No. 5,377,773, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein in its entirety by this reference.
Planar state of the art PDCs, as well as thermally stable products (TSPs) and other known types of cutters, are typically set at a given backrake angle on the bit face to enhance their ability to withstand axial loading of the bit, which is caused predominantly by the downward force applied to the bit during drilling, WOB. By comparing the effective backrake of a cutter, it is easy to see that cutters positioned within about 2 inches of the longitudinal axis of a bit are angled more aggressively than more distantly positioned cutters with the same or similar actual backrake angles.
As a result of the different effective rake angles of cutters that are oriented on a bit so as to have the same actual rake angles, these cutters wear differently, depending upon their radial distances from the longitudinal axis of the bit. Attempts have been made to correct for this problem through cutter redundancy, but the effectiveness of cutter redundancies is limited by the number of blades on the bit and by space constraints.
U.S. Pat No. 5,979,576 to Hansen et al. (hereinafter “Hansen”), assigned to the assignee of the present invention, discloses anti-whirl drag bits with “flank” cutters placed in a so-called “cutter-devoi
Harris Thomas M.
Lund Jeffrey B.
Meiners Matthew J.
Baker Hughes Incorporated
TraskBritt
Watts Douglas D.
LandOfFree
Methods for designing rotary drill bits exhibiting sequences... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Methods for designing rotary drill bits exhibiting sequences..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods for designing rotary drill bits exhibiting sequences... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3283993