Patent Application
20150233186
This invention is related in general to the field of drill bits. More particularly, the invention is related to a rotary drill bit with crushing cutters in the center or cone region of the bit to advance a borehole.
In a typical drilling operation, a drill bit is rotated while being advanced into a rock formation. There are several types of drill bits, including roller cone bits, hammer bits and drag bits. There are many kinds of bits and cutters with different features and cutter configurations.
Drag bits typically include a body with a plurality of arms or blades extending from the body. The bit can be made of steel alloy, a tungsten matrix or other material. Drag bits typically have no moving parts and are cast or milled as a single-piece body with cutters brazed into the blades of the body. Each blade supports a plurality of discrete cutting elements that contact, shear and/or crush the rock formation in the wellbore as the bit rotates to advance the borehole. Cutters on the shoulder of drag bits effectively enlarge the borehole initiated by cutters on the nose and in the cone, or center, of the drill bit.
Cutters mounted on blades of the drag bit can be made from any durable material, but are conventionally formed from a tungsten carbide backing piece, or substrate, with a front facing table comprised of a diamond material. The tungsten carbide substrates are formed of cemented tungsten carbide comprised of tungsten carbide particles dispersed in a cobalt binder matrix. The diamond table, which engages the rock formation, typically comprises polycrystalline diamond (“PCD”) directly bonded to the tungsten carbide substrate, but could be any hard material. The PCD table provides improved wear resistance, as compared to the softer, tougher tungsten carbide substrate that supports the diamond during drilling.
Cutters shearing the rock in the borehole are typically received in recesses along the leading edges of the blades. The drill string and the bit rotate about a longitudinal axis and the cutters mounted on the blades sweep a radial path in the borehole, failing rock. Bit dynamics in operation can be complex with several overlapping modes of motion that in specific circumstances can damage portions of the bit.
Cutters designed to shear rock formations are principally loaded predominantly normal to the PCD face and are subject to damage when loaded in the opposite direction. Drag bits tend to whirl in the wellbore, increasing the diameter of the hole beyond the diameter of the bit. During whirl, the bit rotates in one direction but bit whirl can generate significant rotational motion in the opposite direction producing a complex movement of the bit. Cutters on the blades near the center of the bit, close to its longitudinal axis, move in a reverse direction to the rotation of the bit and can sustain damage as the diamond tables can be pulled away and separated from the substrate. Cutters distributed on the blades of the bit further from the center are not subject to this reverse rotation and damage.
If there were no cutters in the center of the bit, as the bit advances, a portion of the formation would extend from the bottom of the wellbore as a column. Once the column extended a sufficient distance to contact the bit body, it would bear on the surface of the bit, limiting forward progress and penetration of the bit. Many systems have been developed for drag bits and roller cone bits to break down this center column during drilling including downward extending tools that cycle to impact the column. These systems make the bit structure more complex with moving parts, making the manufacturing process more intricate and reducing reliability of the bit.
The center, or cone, portion of a bit tends to cut less efficiently than the peripheral portion of the bit, due to lower rotational surface speed, and less rock is removed in the center cone area. A roller cone drill bit with truncated cones spaced from each other and downward facing cutter inserts at the center of the bit is disclosed in U.S. Pat. No. 5,695,019 in an effort to improve penetration. A column created by the spaced cones removing surrounding material is fractured by the downward facing cutters.
The present invention generally pertains to drilling operations where a rotating bit with cutters advances a wellbore in the earth. The bit is attached to the end of a drill string and is rotated to fail the rock in the wellbore. Cutters on arms or blades of a bit contact the formation and fail the rock of the borehole by shearing or crushing. In one aspect of the invention, a drill bit rotates about a bit longitudinal axis to advance a borehole and comprises a bit body with a pin connection at the upper end to allow attachment of the bit to the drill string, a recess at a leading end of the bit, and a profile which includes a recess in the center or cone of the bit. One or more cutters are mounted in the recessed cone portion of the bit profile, with the PCD or hard cutting surface pointing outward from the bit body. The cutting element or elements in the recess fracture the column of rock extending upward into the recess, formed as the borehole is advanced.
In another aspect of the invention, a drill bit has blades that converge close to the cone or central region of the bit profile that define a central recess. Three cutters are positioned at least partially within the recess with each cutter having a hard material, commonly PCD, contact face extending from the face and a cutter longitudinal axis facing generally parallel to the bit longitudinal axis.
In another aspect of the invention a drill bit includes cutters mounted in abase of the recess for advancing a borehole. Each cutter faces generally in the direction of the advancing borehole and inward with a positive back rake in the range of 30 and 70 degrees and a side rake in the range of 45 and 135 degrees in a negative direction in relation to a surface perpendicular to the longitudinal axis.
In another aspect of the invention, a drill bit comprises a bit body with blades, a face in a recess facing in the direction of advancement of the borehole and a longitudinal axis. First cutters mounted in the recess face generally in the direction of advancement of the borehole, and second cutters mounted on the blades facing generally in the direction of bit rotation about the longitudinal axis.
In another aspect of the invention, a drill bit to be fixed to a drill string at a mounting end of the bit comprises a recessed face at a leading end of the bit (in the cone of the bit) and a conduit from the face to a side of the bit body. Cutters with a base end are mounted in the recess of the bit body. The contact ends of these cutters extend from the face proximate the bit longitudinal axis to fracture a column portion of the rock extending toward the recess surface as the borehole is advanced. The fractured portions of the column pass through the conduit and up the borehole annulus.
Other aspects, advantages, and features of the invention will be described in more detail below and will be recognizable from the following detailed description of example structures in accordance with this disclosure.
Bits used in downhole boring operations such as for gas and oil exploration operate at extreme conditions of heat and pressure often miles underground. The rate of penetration of the bit in creating the wellbore is critical to a cost effective drilling operation. The rate of penetration depends on several factors including the density of the rock the wellbore passes through, the configuration of the bit and the weight on bit (WOB) among others.
Drag bits include most often, PDC cutters mounted on arms or blades of the bit that engage the surfaces of the wellbore to fail the rock in the wellbore. Each cutter is retained in a recess of the blade and secured by brazing, welding or other method. The cutters are distributed along the blades and each cutter is oriented with back rake and side rake to optimally engage the wellbore. Drilling fluid is pumped down the drill string through outlets, or nozzles, in the bit to flush the rock cuttings away from the bit and up the wellbore annulus.
The bit is advanced into the hole by the weight of the drill string and bottom hole assembly bearing on the bit. The WOB may be increased by adding collars to the drill string. The WOB can be decreased by pulling the top of the drill string at the surface. The cutters in the cone, nose and lower shoulder regions of the profile at the bit leading end have the greatest load pressing them against the bottom of the wellbore and generally engage the wellbore simultaneously. The cutters on the upper shoulder of the bit widen the borehole created by the cutters on the bit face.
The drag bit tends to have some lateral movement as it advances, cutting a larger diameter hole than the size of the bit. The bit then tends to whirl in the oversize wellbore causing the cutters in the center, or cone, of the bit to move in the opposite direction to the bit rotation. Whirling results in the bit's longitudinal axis moving in a circular path in the opposite direction to the simultaneous bit rotation around the longitudinal axis LA. Depending on the frequency of whirling and the radius of the circle of rotation of the bit axis, a number of the cutters inside the whirl radius will rotate backwards at a rotational speed proportional to the whirl frequency. The interface between the table of the cutter and the substrate of the cutter secured to the bit is weak in tension and the diamond table is subject to separation from the substrate under tension. Even momentary or transitory reverse rotation of the cutter can cause serious damage to the cutters reducing effectiveness of the bit.
Bits can be designed without cutters in the center of the bit face to limit exposure of the cutters to reverse motion during operation that would damage the cutters. This void in the center portion of the bit close to the axis of rotation does not therefore cut rock. As the bit rotates during operations the cutters sweep a path to fail the rock in front of them. A column of rock forms in the center area free of cutters. Once the column of rock extends far enough so as to contact the body of the bit in cutter free area, the rate of progress of the bit is significantly reduced.
Bits, cutters, other components and features are generally represented in
Center of bit rotation 22 coincides with the bit longitudinal axis LA and the center of the recess 24 about the longitudinal axis. Recess 24 may be defined by the ends 16A of blades 16. The area within recess 24 at leading end 12B is subject to reverse rotation from whirl of the bit that can damage the borehole cutters. Shoulder cutters 20 are positioned on blades generally outside recess 24 and face generally in the direction of rotation of the bit. Cutters 28 (referred to as column cutters) are mounted to the bit body 12 at a face 24A of recess 24. Column cutters 28 facing generally downward contact the top of column of rock 4A that is formed in recess 24. Column cutters can use the same construction as shoulder cutters. Alternatively, column cutters can use a different construction, different dimensions and/or different materials than shoulder cutters where the construction and materials perform a similar function.
With a downward and inward orientation, the column cutters apply an optimized force to the column as the bit rotates and progresses downhole through the rock to effectively fracture the rock of the column. Fracturing of the rock results in larger broken pieces of the column than the fine material produced by borehole cutters at the face of the borehole. Column 4A has a different fracture strength than the bulk material the bit advances through as it is unconfined due to the lack of any surrounding, uncut rock. The borehole walls tend to fail due to small fractures that result from the angle of attack of the shoulder cutter 20 and surrounding support for the rock. Column 4A tends instead to fracture in larger chips or chunks when impacted by the column cutters 28. Column 4A does not have the support of the surrounding material and the walls of the column form a large area without support that is relatively easy to fracture.
The size of the wellbore and whirl of the bit in the wellbore depend on a number of factors including the bit design, the density of the rock in the wellbore, the rotation speed and rate of penetration of the bit. The forces that initiate whirling of the bit are opposed by the friction of the shoulder cutters and by the development of column 4A as well as other factors.
Recess 24 can be any shape.
The end of the blade typically terminates with one or more shoulder cutters so the blade material is not subject to excessive abrasion or erosion. For a roller cone bit, the recess radius can be defined by the distance, measured radially, from the longitudinal axis to the closest tooth or insert mounted on a cone. Alternatively, the recess 24 can be an irregular shape defined by blades that terminate or end at different distances from the longitudinal axis.
A single column cutter can be used and configured to eject discrete rock chips 4B of column 4A in one direction from the front of the bit. Typically the single column cutter is positioned to sweep the rock within the recess and the flow of drilling fluid flushes the debris through the preferred channel. A single downward facing column cutter tends to form a bearing face which can detrimentally generate off center rotation or whirl of the cutter. In a preferred configuration three cutters oriented inward and downward to limit the initiation of bit whirl and efficiently fracture column 4A. With three column cutters, when the bit deflects laterally as may happen during whirling, a column cutter on the opposite side engages column 4A and generates a correcting force to re-center the bit. The use of more cutters in the recess, specifically located, could resist any lateral motion even more effectively. The arrangement of two column cutters could also be used.
Multiple downward facing column cutters are compatible with steering of the bit for directional drilling. As the bit is oriented or pointed to turn the bit, the column cutters on the side away from the turn direction contact the column 4A and the column cutters on the side towards the turn direction are offset from the column. This increases the force applied to one side of the column during steering fracturing one side of the column. The intact side of the column continues to extend into the recess until the column cutters on the opposite side again contact the column.
The reference face for column cutters 28 is an upward face 28A at the top of rock column 4A and forward of the face 24A. In most operations column cutters 28 can have a back rake in an inclusive range of 30 to 70 degrees in a positive direction and may have a side rake in an inclusive range of 45 to 135 degrees in a negative direction. Column cutters 28 preferably have a back rake in an inclusive range of 35 to 55 degrees in a positive direction and may have a side rake in an inclusive range of 70 to 90 degrees in a negative direction. Column cutters 28 more preferably have a back rake in an inclusive range of 40 to 50 degrees in a positive direction and more preferably have a side rake in an inclusive range of 75 to 85 degrees in a negative direction. Never the less, other back rake and side rake angles outside the noted ranges could be used depending on the use of the bit. In
The more preferred rake angles generally orient the cutter face downward in the direction of advancement of the borehole and the side rake skews the cutter inward toward the axis of bit rotation. This orientation of the column cutters applies an optimized downward force to fracture and fail the rock of the column in discrete chips that are flushed to the surface for analysis together with a radial or lateral force that centers the bit and resists radial displacement or whirl of the bit. The offset of the column cutter axes to face inward provides the lateral centering force urging the bit to continue to rotate about its center axis in a stable fashion as more lateral force is required to push the bit off its path. This optimized orientation protects the cutters from damage in case of reverse rotation, or bit whirl, by limiting impact forces on the table of the column cutter that can fail the polycrystalline diamond or separate the table from the substrate. The orientation of the column cutters 28 are distinct from the orientation of the borehole cutters 20 and reflect the different functions for the cutters and different stresses they are subject to.
The shoulder cutters oriented in the direction of rotation on the blade pulverize the rock to a fine consistency as the bit drills. The rock cuttings are flushed to the surface during operation by the drilling fluid circulated through the drill string and out of the bit's nozzles. Monitoring performance of the drilling operation includes analyzing the materials brought to the surface to determine the constituents and physical properties of the failed rock. The fine consistency of the cuttings created by the borehole cutters limits the kinds of geological analysis that can be performed at the surface. The rock column can be failed by crushing or fracturing in order to produce larger discrete chips, chunks or micro cores. These discrete chips then pass with the drilling fluid into the annulus of the borehole to the surface. Intact discrete chips of rock are analyzed to determine the strength of the rock and other properties of the rock.
Typically in bits without conduits the rock portions are flushed in the waterways between the blades of the bit where they risk reduction in size by the borehole cutters on the blades. In passing through the conduit 30 rock portions 4B bypass borehole cutters 20 and avoid further reduction allowing larger chips to pass to the surface for analysis. A supply channel 32 is shown intersecting with conduit 30. Fluid channel 32 injects drilling fluid into conduit 30 to generate a positive flow and flush rock portions 4B through the conduit. Other channel and conduit configurations are possible. The fluid channel 32 in some embodiments may be omitted altogether.
It should be appreciated that although selected embodiments of the representative column cutters are disclosed herein, numerous variations of these embodiments may be envisioned by one of ordinary skill that do not deviate from the scope of the present disclosure. This presently disclosed invention lends itself to use for steel and tungsten carbide matrix bits as well as a variety of styles and materials of cutters.
It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. Each example defines an embodiment disclosed in the foregoing disclosure, but any one example does not necessarily encompass all features or combinations that may be eventually claimed. Where the description recites “a” or “a first” element or the equivalent thereof, such description includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated.
| Number | Date | Country | |
|---|---|---|---|
| 61942492 | Feb 2014 | US |
