GB2385618A

GB2385618A - Device for drilling a subterranean formation with variable depth of cut

Info

Publication number: GB2385618A
Application number: GB0308401A
Authority: GB
Inventors: Gordon A Tibbitts
Original assignee: Baker Hughes Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 1999-01-12
Filing date: 1999-12-24
Publication date: 2003-08-27
Anticipated expiration: 2019-12-24
Also published as: GB2384016A; GB0308541D0; GB2384016B; GB2384018B; GB2384017A; GB0308401D0; GB0308501D0; GB2385350B; GB2385351A; GB2385080B; GB0308538D0; GB0308498D0; GB2385618B; GB2385351B; GB2384017B; GB2385350A; GB0308535D0; GB2384018A; GB0308500D0; GB2385080A

Abstract

A rotary-type drag bit for drilling a subterranean formation which comprises a bit body 400 configured for attachment to a downhole end of a drill string and comprised of a bit shank 402, a crown 404, a blade 22 movably secured to the bit body, a cutting element 25 secured to the bit body by the blade and positioned to contact a formation, wherein the blade is attached and movable relative to the bit crown to provide a variable depth of cut by the cutting element, and further comprises an apparatus associated with the bit body for producing the variable depth of cut by the cutting element where the apparatus comprises a movable fastener 416 positioned through the blade and movable in response to an increase in pressure within the bit body.

Description

23856 1 8

ROTARY DRAG DRILLING DEVICE

WITH VARIABLE DEPTH OF CUT

The present invention relates generally to a drilling device for drilling subterranean formations using rotary-type drag bits, and more particularly to a drilling device employing an oscillating drill bit for more effective removal of formation chips from around the drill bit using drilling fluid.

Fixed-cutter rotary drag bits have been employed in subterranean drilling for many decades with various sizes, shapes and patterns of natural and synthetic diamonds used on drag bit crowns as cutting elements. Rotary drag-type drill bits typically comprise a bit body having a shank for connection to a drill string and an inner channel for supplying drilling fluid to the face of the bit through nozzles or other apertures. Drag bits may be cast and/or machined from metal, typically steel, or may be formed of a powder metal (typically tungsten carbide (WC)) infiltrated at high temperatures with a liquified binder material (typically copper- based) to form a matrix. Such bits may also be formed with layered- manufacturing technology, as disclosed in U.S. Pat. No. 5,433,280, which is assigned to the assignee of the present invention and incorporated herein by reference.

The bit body typically carries a plurality of cutting elements which is mounted directly on the face of the bit body or on carrier elements. The cutting elements are positioned adjacent fluid courses which allow cuttings (i.e., formation chips) generated during drilling to flow from the cutting elements to and through junk slots on the gage of the bit. The cuttings then move to the borehole annulus above the bit.

Cutting elements may be secured to the bit by preliminary bonding to a carrier element, such as a stud, post, or cylinder, which is, in turn, inserted into a pocket, socket, recess or other aperture in the face of the bit and mechanically or metallurgically secured thereto.

One type of drag bit includes polycrystalline diamond compact (PDC) cutters typically comprised of a diamond table (usually of circular, semicircular or

tombstone shape) which presents a generally planar cutting face. A cutting edge (sometimes chamfered or beveled) is formed on one side of the cutting face which, during boring, is at least partially embedded into the formation so that the formation impacts at least a portion of the cutting face. As the bit rotates, the cutting face contacts the fortnation and a chip of formation material shears off and rides up the surface of the cutting face. When the bit is functioning properly, the chip breaks off from the formation and is transported out of the borehole via circulating drilling fluid.

Another chip then begins to form in the vicinity of the cutting edge, slides up the cutting face of the cutting element, and breaks off in a similar fashion. Such action occurring at each cutting element on the bit removes formation material over the entire gage of the bit, and thereby causes the borehole to become progressively deeper. In some subterranean formations, PDC cutting elements are very effective in cutting the formation as the drag bit rotates and the cutting edge of the cutting element engages the formation. However, in certain formations exhibiting plastic behaviour, such as highly pressurized deep shales, mudstones, siltstones, some limestones and other ductile formations, the formation chips have a marked tendency to adhere to the leading surface of the bit body and the cutting face of the cutting element.

When formation chips adhere to the cutting elements, fluid courses or junk slots of the drill bit, the accumulated mass of chips impedes the flow of drilling fluid to the cutters and impedes the flow through the fluid courses and junk slots resulting in the reduction of cooling efficiency of the drilling fluid. Additionally, adherence of formation chips at or near the cutting faces of the cutting elements can actually prevent chips from sliding over the cutting face resulting in reduced cutting efficiency. When these formation chips adhere to the cutting face of a cutting element, they tend to collect and build up as a mass of cuttings ahead of and adjacent to the point or line of engagement between the cutting face of the PDC cutting element and the formation, potentially increasing the net effective stress of the formation being

cut. The buildup of formation chips moves the cutting action away from and ahead of the edge of the PDC cutting element and alters the failure mechanism and location of the cutting phenomenon so that cutting of the formation is actually effected by the built-up mass, which obviously is quite dull. Thus, the efficiency of the cutting elements, and hence of the drag bit itself, is drastically reduced.

Undesired adhesion of formation cuttings to the PDC cutting elements has long been recognized as a problem in the subterranean drilling art. A number of different approaches have been attempted to facilitate removal of formation cuttings from the cutting face of PDC cutting elements. For example, U.S. Pat. No. 5,582,258 to Tibbitts et al., assigned to the assignee of the present invention and herein incorporated by this reference, includes a chip breaker formed adjacent the cutting edge of the cutting elements to impart strain to a formation chip by bending and/or twisting the chip and thereby increasing the likelihood that the chip will break away from the face of the bit. Other approaches to solving the problem of formation chip removal include U.S. Pat. No. 4,606, 418 to Thompson which discloses cutting elements having an aperture in the center thereof which feeds drilling fluid from the interior of the drill bit onto the cutting face to cool the diamond table and to remove formation cuttings.

U.S. Pat. No. 4,852,671 to Southland discloses a diamond cutting element which hag a passage extending from the support structure of the cutting element to the extreme outermost portion of the cutting element, which is notched in the area in which it engages the formation being cut so that drilling fluid from a plenum on the interior of the bit can be fed through the support structure and to the edge of the cutting element immediately adjacent the formation. U.S. Pat. No. 4,984,642 to Renard et al. discloses a cutting element having a ridged or grooved cutting face on the diamond table to promote the break-up of formation chips, or in the case of a machine tool, the break-up of chips of material being machined, and enhance their removal from the cutting face. The irregular topography of the cutting face assists in preventing balling or clogging of the drag bit by reducing the effective surface or

contact area of the cutting face, which also reduces the pressure differential of the formation chips being cut. U.S. Pat. No. 5,172,778 to Tibbitts et al., assigned to the assignee of the present application, employs ridged, grooved, stair-step, scalloped, waved and other alternative non-planar cutting surface topographies to permit and promote the access of fluid in the borehole to the area on the cutting element cutting face immediately adjacent to and above the point of engagement with the formation.

Such a non-planar cutting surface helps to equalize differential pressure across the formation chip being cut and thus reduce the shear force which opposes chip movement across the cutting surface.

U.S. Pat. No. 4,883,132 to Tibbitts, assigned to the assignee of the present application, discloses a novel drill bit design providing large cavities between the face of the bit and the cutting elements engaging the formation. Formation cuttings entering the cavity area are thus unsupported and more likely to break off for transport up the borehole. In addition, clearing of the cut chips is facilitated by nozzles aimed from behind the cutting elements (taken in the direction of bit rotation) so that the chips are impacted in a forward direction to break off immediately after being cut from the formation. U.S. Pat. No. 4,913,244 to Trujillo, assigned to the assignee of the present invention, discloses bits which employ large cutters having associated therewith directed jets of drilling fluid emanating from specifically oriented nozzles placed in the face of the bit in front of the cutting elements. The jet of drilling fluid is oriented so that the jet impacts between the cutting face of the cutting element and a formation chip as it is moving along the cutting face to peel the chip away from the cutting element and toward the gage of the bit. Likewise, GB 2,085,945 to Jurgens provides nozzles that direct drilling fluid toward the cutting elements to flush away cuttings generated by the cutting elements.

U.S. Pat. No. 5,447,208 to Lund et al., assigned to the assignee of the present invention, discloses a superhard cutting element having a polished, low friction, substantially planar cutting face to reduce chip adhesion across the cutting face. U.S. Pat. No. 5,115,873 to Pastusek, assigned to the assignee of the present application,

s discloses yet another manner in which formation cuttings can be removed from a cutting element by use of a structure adjacent to and/or incorporated with the face of the cutting element to direct drilling fluid to the face of the cutting element and behind the formation chip as it comes off the formation.

It has also been disclosed in the art that drilling systems which employ cycloidal sonic energy as a method of drilling cause highly effective cutting action on the bottom and particularly the adjacent side walls of the bottom portion of the well bore by virtue of the cycloidal drilling action. Typically, such vibratory drilling systems employ orbiting mass oscillators to generate vibratory energy. Such orbiting mass oscillators may employ orbiting rollers which are rotatably driven around the inner race wall of a housing, as disclosed in U.S. Pat. No. 4,815,328 to Bodine, or an unbalanced rotor, the output of which is coupled to a drill bit, as disclosed in U.S. Pat. No. 4,261,425 to Bodine. U.S. Pat. No. 5,562, 169 to Barrow discloses a conically driven drill bit employing an oscillator adapted to transmit sinusoidal pressure waves through the drill pipe.

None of the foregoing approaches to cutting element and bit design have been completely successful in facilitating chip removal from the face of the cutting element. Moreover, it will be appreciated by those skilled in the art that many of the foregoing approaches require significant modification to the cutting elements themselves, to the structure carrying the cutting elements on the bit face, and/or to the bit itself. Thus, many of the foregoing approaches to the problem require significant expenditures which substantially raise the price of the drill bit. addition, due to required cutter placement on certain styles and sizes of bits, many of the prior art

hydraulic chip removal - arrangements are unsuitable for general application.

Moreover, those bits employing vibrating drilling systems do not address the problem of chip removal. Accordingly, it would be extremely desirable to provide the industry with a solution to the impairment to the cutting mechanism caused by chip adhesion, which solution could be economically employed in any drill bit regardless of size or

style, and regardless of the type of formation which might be encountered by the drill bit. According to the present invention there is provided an earth drilling device for variably contacting an earth formation, comprising: a rotary drag bit body configured for attachment to a downhole end of a drill string and comprised of a bit shank and crown; at least one blade movably secured to said bit body; at least one cutting element secured to said bit body by said at least one blade and positioned to contact an earth formation, wherein said crown of said bit body is attached and movable relative to said bit shark to provide a variable depth of cut by said at least one cutting element into said earth formation during drilling; and apparatus associated with said bit body for producing said variable depth of cut by said at least one fixed cutting element into said earth formation during drilling, said apparatus comprising a movable fastener positioned through said at least one blade and movable responsive to an increase in pressure within said bit body.

In accordance with the present invention, drilling apparatus is provided for effecting a drilling method in which formation chips are produced with varying thicknesses to promote fracturing of the formation chips, thereby avoiding the buildup of formation chips near the bit body and facilitating removal of the formation chips from the bit face. Formation chips having various thicknesses are produced by selectively modifying the degree to which the cutting elements of the bit contact and cut the fortnation. Selective modification of the degree to which cutting elements contact the formation is achieved in the present invention by essentially modifying the torsional oscillation of portions of the drill bit and the cutting elements attached to the drill bit.

The present invention provides apparatus for drilling a subterranean formation employing, by way of example only, a rotary-type drag bit comprising a bit body having a plurality of longitudinally extending blades, where adjacent blades define fluid courses with communicating junk slots therebetween. A plurality of cutting

elements is attached to the blades, each cutting element including a cutting face oriented toward a fluid course. Upon rotation of the drill bit in a subterranean formation, formation chips cut by the cutting elements slide across the cutting elements, into the fluid courses and through the junk slots. The formation chips are then flushed into the annulus of the borehole.

In accordance with the present invention, movement of cutting elements is modified in a manner which introduces weak points into the formation chips as they are cut from the formation. That is, varying thicknesses are introduced into each formation chip as it is cut, thereby facilitating preferential breaking of the chip. In one embodiment, the bit is structured to oscillate torsionally as it rotates to produce alternating, relatively thicker and thinner sections of the chip such that each thicker chip portion is more likely to break away from the rest of the chip along the thinner portions of the chip by the force of drilling fluid contacting the chip. The broken formation chips enable their removal from the bit body and the borehole.

The bit may also be vertically oscillated relative to the longitudinal axis of the bit such that the load on the drill bit is cyclically increased and decreased to effect alternating deeper and relatively more shallow cuts into the coronation, thus varying the thickness of formation chips generated by the cutting elements. Such vertical oscillations may be affected by varying the weight on bit (WOB) at the top drive. In addition, vertical oscillations may be accomplished by employing a fluid pulse to cyclically create alternating higher and lower hydrostatic pressures in the bit to cause variable degrees of contact with the formation. This may be accomplished by employing a valve and fluid jet assembly on a near-bit sub to "pulse" the drill bit vertically or at an angle, or by employing a valve and a piston-like assembly in or above the drill bit to cyclically vary the depth of cut (DOC) of the drill bit into the formation. In addition, a drill bit which is resiliently attached to the drill string, such as by a spring-loaded bit sub or piston-like bit sub which can vertically oscillate the bit relative to its longitudinal axis, can cyclically vary the depth of cut of the bit into the bottom of the borehole to produce formation cuttings of different thicknesses.

/ Vertical oscillation in the cutting elements may also be produced by structuring a bit having adjustable blades.

Both vertical and torsional oscillation may be imposed on the drill bit by combining devices that produce vertical oscillation with those that produce torsional oscillation. In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:

FIG. l is a view in elevation of a conventional rotary-type drill bit in accordance with the present invention; FIG. 2 is a partial view in crosssection of a formation chip being cut by a cutting element on a drill bit using a prior art method of drilling:

FIG. 3 is a partial view in cross-section of a formation chip being cut by a cutting element on a drill bit in accordance with the present invention; FIG. 4 is a partial view in cross-section of a formation chip being cut by a cutting element on a drill bit which is axially oscillated; FIG. 5 is a view in elevation of an exemplary drilling apparatus having a motorized mechanism for providing vertical movement of the drill string to provide a modified chip formation; and FIG. 6 is a partial view in longitudinal cross-section of one half of a drill bit illustrating an embodiment of the present invention also structured to provide movement in the cutting elements; and A typical rotary-type drill bit 10, as shown in FIG. 1, comprises a bit body 12, attached at the proximal end 16 thereof to a near-bit sub member 14, and a bit crown 18 located at the distal end 20 of the drill bit 1 O. The bit crown 18 includes a plurality of longitudinally extending blades 22 with a fluid course 23 positioned between each

adjacent pair of blades 22. Each fluid course 23 has a communicating junk slot 24 which is also positioned between adjacent blades 22. Along each blade 22, proximate the distal end 20 of the bit 10, a plurality of cutting elements 25 is attached to the leading edge 27 of each blade 22 and oriented to cut into a subterranean formation upon rotation of the bit 10. Each fluid course 23 is specifically defined by a first side wall 26, a second side wall 28 and a bottom 30. The first side wall 26 provides a surface adjacent the cutting face 29 of each cutting element 25.

In conventional drilling, as formation chips are cut by the cutting elements 25, the chips slide over the cutting face 29 of each cutting element 25, across the side wall 26 adjacent the cutting elements 25 and into the corresponding fluid course 23. In ideal conditions, drilling fluid directed through the fluid course 23 removes the chips from the cutting elements 25 and provides substantially clean cutting faces 29 during drilling. In some situations, such as drilling formations that exhibit plastic characteristics, the formation chips may tend to stick or adhere to the cutting face 29 of the cutting elements 25 and the adjacent side wall 26 of the fluid course 23.

Accordingly, drilling fluid flowing through the fluid course 23 may not adequately lift the formation chips from the side wall 26 for flushing away from the bit 10.

As illustrated in FIG. 2, a typical method of drilling into a subterranean formation 40 employs both rotation of the bit 10 and weight on bit (WOB) to force the cutting element 25 into the formation 40. Rotation of the drill bit 10 typically continues at substantially the sane rate during drilling of the formation 40. In many plastic formations, such as the aforementioned highly pressured or deep shales, mudstones, siltstones, some limestones and other ductile formations, a formation chip 42 cut by the cutting element 25 may actually be an elongated, substantially pliable chip 42 that will effectively flow over the cutting face 29 and adhere to the side wall 26 of the fluid course 23. As the formation 40 is cut, the pliable chips 42 cut by the cutting element 25 may build up in the fluid course 23, and eventually build up over the cutting face 29 of the cutting element 25, effectively balling the drill bit 10 and preventing it from efficiently drilling into the formation 40.

To overcome such problems as described in conventional drilling methods, the drill bit 10 and, thus the cutting elements 25 are oscillated in the present invention to create a formation chip 50 which has both relatively thick portions 52 and relatively thin portions 54, as illustrated in FIG. 3. In a first method of drilling in accordance with the present invention, illustrated in FIG. 3, the drill bit 10 and cutting elements 25 are torsionally oscillated to create a formation chip having thick portions 52 and thin portions 54. As the thin portion 54 extends over the cutting face 29 of the cutting element, the thick portion 52 is left substantially unsupported such that drilling fluid contacting the leading thick portion 52 can break it from the next following thick portion 52 along the interconnecting thin portion 54. Thus, the chip 50 is broken into smaller sections before it can adhere to and build up on the side wall 26 of the fluid course 23 or on the cutting face 29. FIG. 3 illustrates a formation chip 50 having a thick portion 52 of substantial longitudinal length relative to the size of the cutting face 29 of the cutting element 25. Notably, increasing the frequency of oscillations causes the formation 40 to be cut in a manner which pulverizes the formation chips so that they can be carried away by the drilling fluid.

In some drilling operations, several different types of formations are encountered, ranging from relatively hard formations to relatively pliable formations.

The rate of penetration of the bit 10 into the formation may typically be slower through hard formations and faster through softer formations. Knowing the pliability of the formation 40 at any given time, the various thick portions 52 and thin portions 54 of the formation chip 50 can be substantially predicted for a given WOB and rotational speed. Accordingly, when a formation 40 is encountered where balling of the bit 10 is of concern (i.e., adhesion of the formation chips 50 to the cutting elements 25 and bit body 12), the bit 10 may be selectively oscillated to produce a desired formation chip 50 profile, and when the bit 10 reaches a harder formation, the frequency of oscillation may be reduced or eliminated as necessary. Thus, the frequency of oscillation may be adjusted to optimize chip production for each of the different types of formation.

In FIG. 4, a second drilling method which may be combined with the present invention is illustrated. In this method, a formation chip 50 having relatively thick portions 52 and relatively thin portions 54 is generated by the cutting element 25 under conditions where the normal force or WOB driving the bit 10 axially into the formation is cyclically varied. Accordingly, the cutting element 25 moves vertically or longitudinally relative to the formation 40 in a cyclical manner, cutting a depth D 1 to produce the thick portions 52 of the formation chip 90 and a depth D2 to produce the thin portions 54 of the formation chip 50. In a similar fashion to that illustrated in FIG. 3, the thick portions 52 will break away from the rest of the formation chip 50 relatively easily and will break sequentially along the intervening thin portions 54.

Oscillating movement of cutting elements, the drill bit or drill string in the present invention to produce the desired profile of formation chips (i.e., alternating thick and thin portions) may be accomplished in various ways. FIG. 5, which schematically illustrates a formation drilling assembly, shows a drill string 60 positioned in a borehole 62 as it would be during a drilling operation. At the lower terminal end of the drill string 60 is a drill bit 10 positioned to cut into the formation.

The drill string 60 is operatively connected to a rotary drive unit 64 which imparts rotational movement to the drill string 60 and, thus, to the drill bit 10.

A drill bit 10 in accordance with the present invention is illustrated in FIG. 6, which illustrates one half of a drill bit 10 in cross section. The blades 22 (only one being shown) of the drill bit 10 are movable relative to a bit body 400, which comprises a combined bit shank 402 and bit crown 404. The bit 10 includes a central fluid channel 406 which delivers drilling fluid into a plenuTn 408 formed in the bit crown 404. Although not specifically shown in FIG. 14, the bit 10 is also structured with fluid passageways which communicate with the exterior of the bit 10 to deliver drilling fluid into the formation. In the illustrated embodiment, the blades 22 of the bit 10 are formed with a conventional structure comprising a gage portion 410 and a crown, or bottom portion 412, which is positioned to engage the bottom of the

formation during drilling. Cutting elements 25 are attached to each blade 22 in a conventional manner.

The bit body 400 is structured with a plurality of recesses 414 (only one being shown) which is sized and shaped to receive a blade 22 in a slidingly movable manner relative thereto, as suggested by the broken lines. Notably, the recesses 414 are sized such that blade 22 fits snugly into the recess 414 to avoid infiltration of dirt or other potentially clogging debris between the blade 22 and the recess 414. Each blade 22 is attached to the bit body 400 by a suitable device which allows the blade 22 to move outwardly from the bit body 10 in response to, for example, an increase in fluid pressure exerted within the plenum 408. By way of example only, the movable blade 22 may be secured to the bit body 400 at the crown 404 by a fastener 416, such as a pin or bolt, which is positioned through an opening 418 in the bit body 400 and which extends into the blade 22 for securement thereto. The fastener 416 may be configured with a head 420 which is sized or shaped to respond to increases in pressure within the plenum such that the head 420, and thus the fastener 416, may be forced outwardly from the plenum responsive to such pressure increases. Movement of the fastener 416 forces the blade 22 outwardly as well to drive the cutting elements into the formation. Thus, when the pressure in the plenum 408 overcomes the WOB exerted on the drill bit 10, andlor when the WOB exerted on the bit 10 is varied, the blades 22 are cyclically driven into the formation to produce a formation chip 50 as shown in FIG. 4.

While the methods of achieving oscillation of drill bits have been illustrated and described herein with respect to specific examples, those skilled in the art will appreciate that the structures and methods generally described may be adapted for use in a variety of situations or may be adapted to use with other types of bits, such as, for example, the drill bit having a tilting bit crown disclosed in U.S. Pat. No. 5,595,254 to Tibbitts and assigned to the assignee of the present invention.

Claims

1. An earth drilling device for variably contacting an earth formation, comprising: a rotary drag bit body configured for attachment to a downhole end of a drill string and comprised of a bit shank and crown; at least one blade movably secured to said bit body; at least one cutting element secured to said bit body by said at least one blade and positioned to contact an earth formation, wherein said crown of said bit body is attached and movable relative to said bit shank to provide a variable depth of cut by said at least one cutting element into said earth formation during drilling; and apparatus associated with said bit body for producing said variable depth of cut by said at least one fixed cutting element into said earth formation during drilling, said apparatus comprising a movable fastener positioned through said at least one blade and movable responsive to an increase in pressure within said bit body.

in Amendments to the claims have been filed as follows 1. An earth drilling device for variably contacting an earth formation, comprising: a rotary drag bit body configured for attachment to a downhole end of a drill string and comprised of a bit shank and crown; at least one blade movably secured to said bit body; at least one cutting element secured to said bit body by a at least one blade and positioned to contact an earth formation; apparatus associated with said bit body for producing said variable depth of cut by said at least one cutting element into said earth formation during drilling, said apparatus comprising a movable fastener positioned through said at least one blade and movable responsive to an increase in pressure within said bit body.