Abstract:
A reaming apparatus for enlarging a borehole, including a tubular body having one or more longitudinally and generally radially extending blades circumferentially spaced thereabout. Each of the blades carries highly exposed cutting elements, on the order of fifty percent exposure, on its profile substantially all the way to the gage. At least one of the blades is a primary blade for cutting the full or drill diameter of the borehole, while one or more others of the blades may be secondary blades which extend a lesser radial distance from the body than the primary blade. A secondary blade initially shares a large portion of the cutting load with the primary blade while the borehole size is in transition between a smaller, pass through diameter and drill diameter. It functions to enhance the rapidity of the transition while balancing side reaction forces, and reduces vibration and borehole eccentricity. After drill diameter is reached, cutting elements on the secondary blade continue to share the cutting load over the radial distance they extend from the body.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to enlarging the diameter of a subterranean borehole, and more specifically to enlarging the borehole below a portion thereof which remains at a lesser diameter. 
     2. State of the Art 
     It is known to employ both eccentric and bi-center bits to enlarge a borehole below a tight or undersized portion thereof. 
     An eccentric bit includes an extended or enlarged cutting portion which, when the bit is rotated about its axis, produces an enlarged borehole. An example of an eccentric bit is disclosed in U.S. Pat. No. 4,635,738. 
     A bi-center bit assembly employs two longitudinally-superimposed bit sections with laterally offset axes. The first axis is the center of the pass through diameter, that is, the diameter of the smallest borehole the bit will pass through. This axis may be referred to as the pass through axis. The second axis is the axis of the hole cut as the bit is rotated. This axis may be referred to as the drilling axis. There is usually a first, lower and smaller diameter pilot section employed to commence the drilling, and rotation of the bit is centered about the drilling axis as the second, upper and larger diameter main bit section engages the formation to enlarge the borehole, the rotational axis of the bit assembly rapidly transitioning from the pass through axis to the drilling axis when the full diameter, enlarged borehole is drilled. 
     Rather than employing a one-piece drilling structure such as an eccentric bit or a bi-center bit to enlarge a borehole below a constricted or reduced-diameter segment, it is also known to employ an extended bottomhole assembly (extended bi-center assembly) with a pilot bit at the distal end thereof and a reamer assembly some distance above. This arrangement permits the use of any standard bit type, be it a rock bit or a drag bit, as the pilot bit, and the extended nature of the assembly permits greater flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot bit so that the pilot hole and the following reamer will traverse the path intended for the borehole. This aspect of an extended bottomhole assembly is particularly significant in directional drilling. 
     While all of the foregoing alternative approaches can be employed to enlarge a borehole below a reduced-diameter segment, the pilot bit with reamer assembly has proven to be the most effective overall. The assignee of the present invention has, to this end, designed as reaming structures so-called &#34;reamer wings&#34; in the very recent past, which reamer wings generally comprise a tubular body having a fishing neck with a threaded connection at the top thereof, and a tong die surface at the bottom thereof, also with a threaded connection. The upper mid-portion of the reamer wing includes one or more longitudinally-extending blades projecting generally radially outwardly from the tubular body, the outer edges of the blades carrying superabrasive (also termed superhard) cutting elements, commonly termed PDC&#39;s (for Polycrystalline Diamond Compact). The lower mid-portion of the reamer wing may include a stabilizing pad having an arcuate exterior surface of the same or slightly smaller radius than the radius of the pilot hole on the exterior of the tubular body and longitudinally below the blades. The stabilizer pad is characteristically placed on the opposite side of the body with respect to the reamer blades so that the reamer wing will ride on the pad due to the resultant force vector generated by the cutting of the blade or blades as the enlarged borehole is cut. The aforementioned reamer wing as described and as depicted herein is not acknowledged or admitted to constitute prior art to the invention described and claimed herein. 
     While the aforementioned reamer wing design has enjoyed some success, it has been recognized by the invention herein that the device, as presently constructed, may not effectively and efficiently address the problem or task of achieving a rapid transition from pass through to full hole or &#34;drill&#34; diameter which closely tracks the path of the pilot bit and which does not unduly load the blades or bottomhole assembly during the transition. Since the reamer wing may have to re-establish a full diameter borehole multiple times during its drilling life in a single borehole, due to washouts and doglegs of the pilot hole, rapid transitioning ability when reaming is restarted as well as a robust design which can accommodate multiple transitions without significant damage is desirable. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a reamer wing having one or more blades, at least one of which comprises a primary blade for cutting the full diameter of an enlarged borehole and another of which may comprise a secondary blade for enhancing the transition from the pass through diameter to the enlarged full diameter cut by the primary blade. 
     More specifically, the invention includes at least a body having a longitudinally-extending primary blade extending generally radially therefrom, the radial extent of the primary blade substantially defining the gage of the enlarged borehole to be cut by the reamer wing, and an optional, longitudinally-extending secondary blade circumferentially spaced from the primary blade and having a radial extent of less than that of the primary blade as measured from the drilling axis. The blades carry cutting elements along the outer edges or profiles thereof, the cutting elements extending from proximate the body to the outermost extent of the blade. Moreover, the cutting elements are substantially exposed on the order of one-half of their cutting face height all the way up the flank of the blades to, and optionally including, the gage. At least some of the cutting elements are also aggressively raked relative to conventional, prior art rakes, to promote engagement with the formation. Thus, the cutting elements as positioned and exposed cut away the formation to an extent that the gage pads on the main blade or blades do not have to cut laterally. Alternatively, contentional gage pads without cutting elements may be completely eliminated and cutting elements may be placed along the entire gage extent of one or more blades. 
     In such a manner, when multiple blades are employed, the loading of certain cutting elements on the blades can be shared, and the cutting elements at the flank and shoulder of the secondary blade accelerate the removal of formation material while minimizing side reaction forces, to speed the transition of the primary blade to its full-diameter reaming task in rotating about the drilling axis of the reamer wing. 
     The invention also contemplates a method of enlarging a borehole by rotating a body carrying first and second radially-extending cutting means thereon, and engaging the borehole with the first cutting means extending to a first radius from a rotational center for the diameter of the enlarged borehole and subsequently engaging the borehole with the second cutting means extending to a second radius from the drilling axis substantially equal to one-half of drill diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 3 comprise schematic partial sectional elevations of a bottomhole assembly including a reamer wing as employed in one aspect of the present invention, the bottomhole assembly being shown in pass through condition (FIG. 1), in start up condition (FIG. 2) and in a normal drilling mode for enlarging the borehole (FIG. 3); 
     FIG. 4 comprises a side elevation of an exemplary reamer wing in accordance with the present invention; 
     FIG. 5 comprises an enlarged bottom elevation of the reamer wing of FIG. 4; 
     FIG. 6 comprises an enlarged schematic of the profile of the primary and secondary reamer blades of the reamer of FIG. 4, rotated into a single plane about the drilling axis to illustrate cutting element position, exposure and coverage; and 
     FIG. 7 comprises an enlarged schematic of an alternative blade profile and cutting element disposition according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 through 3 depict an exemplary bi-center bottomhole assembly 10 in which the reamer wing of the present invention may be employed. 
     Commencing with FIG. 1 and moving from the top to the bottom of the assembly 10, one or more drill collars 12 are suspended from the distal end of a drill string extending to the rig floor at the surface. Pass through stabilizer (optional) 14 is secured to drill collar 12, stabilizer 14 being sized equal to or slightly smaller than the pass through diameter of the bottomhole assembly 10, which may be defined as the smallest diameter borehole through which the assembly may move longitudinally. Another drill collar 16 (or other drill string element such as an MWD tool housing or pony collar) is secured to the bottom of stabilizer 14, below which reamer wing 100 according to the present invention is secured via tool joint 18, which may be a 65/8inch API joint. Another API joint 22, for example a 41/2inch API joint, is located at the bottom of the reamer wing 100. Upper pilot stabilizer 24, secured to reamer wing 100, is of an O.D. equal to or slightly smaller than that of the pilot bit 30 at the bottom of the assembly 10. Yet another, smaller diameter drill collar 26 is secured to the lower end of pilot stabilizer 24, followed by a lower pilot stabilizer 28 to which is secured pilot bit 30. Pilot bit 30 may be either a rotary drag bit or a tri-cone, so-called &#34;rock bit&#34;. The bottomhole assembly 10 as described is exemplary only, it being appreciated by those of ordinary skill in the art that many other assemblies and variations may be employed. 
     It should be noted that there is an upper lateral displacement 32 between the axis of pass through stabilizer 14 and that of reamer wing 100, which displacement is provided by the presence of drill collar 16 therebetween and which promotes passage of the assembly 10, and particularly the reamer wing 100, through a borehole segment of the design pass through diameter. 
     For purposes of discussion, the following exemplary dimensions may be helpful in understanding the relative sizing of the components of the assembly for a particular pass through diameter, pilot diameter and drill diameter. For a pass through diameter of 10.625 inches, a pilot diameter of 8.500 inches and a maximum drill diameter of 12.250 inches (the full bore diameter drilled by reamer wing 100) would normally be specified. In the bottomhole assembly 10, for the above parameters: 
     (a) drill collar 12 may be an eight inch drill collar; 
     (b) drill collar 16 may be a thirty foot, eight inch drill collar; 
     (c) drill collar 26 may be a fifteen foot, 63/4inch drill collar; and 
     (d) pilot bit 30 may be an 81/2inch bit. 
     In pass through condition, shown in FIG. 1, the assembly 10 is always in either tension or compression, depending upon the direction of travel, as shown by arrow 34. Contact of the assembly 10 with the borehole wall 50 is primarily through pass through stabilizer 14 and reamer wing 100. The assembly 10 is not normally rotated while in pass through condition. 
     FIG. 2 depicts the start-up condition of assembly 10, wherein assembly 10 is rotated by application of torque as shown by arrow 36 as weight-on-bit (WOB) is also applied to the string, as shown by arrow 38. As shown, pilot bit 30 has drilled ahead into the uncut formation to a depth approximating the position of upper pilot stabilizer 24, but reamer wing 100 has yet to commence enlarging the borehole to drill diameter. As shown at 32 and at 40, the axis of reamer wing 100 is laterally displaced from those of both pass through stabilizer 14 and upper pilot stabilizer 24. In this condition, the reamer wing 100 has not yet begun its transition from being centered about a pass through center line to its drilling mode center line which is aligned with that of pilot bit 30. 
     FIG. 3 depicts the normal drilling mode of bottomhole assembly 10, wherein torque 36 and WOB 38 are applied and, upper displacement 32 may remain as shown, but generally is eliminated under all but the most severe drilling conditions. Lower displacement 40 has been eliminated as reamer wing 100 is rotating about the same axis as pilot bit 30 in cutting the borehole to full drill diameter. 
     It is the rapidity and efficiency of the transition from start up as shown in FIG. 2 to normal drilling as shown in FIG. 3 to which the present invention primarily pertains. 
     FIGS. 4, 5 and 6 illustrate exemplary reamer wing 100 according to the present invention in greater detail. Reamer wing 100 comprises a tubular body 102 having an axial bore 104 therethrough. Tubular body 102 includes a fishing neck portion 106 at the upper exterior thereof, and a tong die surface 108 at the lower exterior thereof. Reamer wing 100 is secured in a bottom hole assembly such as 10, described above, via API threaded connections of the type indicated. 
     Circumferentially-spaced primary blades 110 and 112 and secondary blades 114 and 116 extend longitudinally and generally radially from body 102 on the upper mid-portion thereof. Body 102 and blades 110-116 are preferably formed of steel, and the blades may be integral or welded to the body. It should be noted that the number of blades depicted is exemplary only, and that as many as five or more blades may be employed on a reamer wing according to the invention. The larger the required diameter of the enlarged borehole, the larger number of blades being generally dictated. Depending on the number of blades used, one or more passages (not shown) may extend from bore 104 to the surface of body 102 to direct drilling fluid to the blades and cutting elements thereon via nozzles (not shown), such technology being well known in the drilling art. 
     Optional stabilizer pad 118 is located on the lower mid-portion of body 102 generally diametrically opposite in location to primary blades 110 and 112 and closely therebelow, as shown in FIG. 4 and 5. Stabilizer pad 118 is provided with an arcuate exterior surface which is of the same or slightly less radius than the radius of the pilot hole drilled by the pilot bit, the radius being measured from the centerline 120 of tubular body 102. Placement of stabilizer pad 118 is dictated by the resultant lateral force vector generated by the blades during transition from start-up condition to and during drilling of the drill diameter hole so that the pad rides on the borehole wall as the blades cut the transition and ultimate drill diameter. If reamer wing 100 is employed with a steerable bottomhole assembly, it is likely that stabilizer pad 118 would be omitted. The exterior of stabilizer pad 118 may be faced with tungsten carbide bricks and/or diamond wear surfaces, as known in the art for conventional stabilizers. 
     As shown in FIGS. 4-6, primary blades 110 and 112 extend radially outward from drilling axis or centerline 120 a greater distance than secondary blades 114 and 116. Referring to FIG. 6, it can be seen that primary blades 110 and 112 define one profile, and that secondary blade 114 defines a second, more slender profile. Blade 116 is a main blade which has been cut back in lateral extent (see FIG. 5) to fit the pass through diameter. Looking at FIG. 5, it can be seen that both primary and secondary blades carry cutting elements 122 at their lower and radially inner extents which will continue to actively cut after full drill diameter is reached. However, due to the radially smaller extent of the secondary blades, cutting elements 124 and 126 on the flank of secondary blade 114 will only cut during the transition from start up to full drill diameter, after which they will no longer contact the borehole sidewall, at which time cutting elements 128 and 130 on primary blades 110 and 112 will still be active. In other words, a major function of secondary blade 114 and cutting elements 124 and 126 is to effectuate as rapid and smooth transition as possible to full drill diameter by permitting reamer wing 100 to remove more formation material per revolution and with lower side reaction forces and thus less lateral disruption of assembly rotation than if only primary blades were employed. 
     Referring to FIG. 6, the large degree of exposure of cutting elements 122, 124, 126, 128 and 130 beyond the blade profiles is readily visible, the exposure approximating one-half or 50% of cutting element height. This is in contrast to the prior art, wherein extremely small cutting element exposures were generally employed, and gage-position cutting elements such as 124, 126 and 130, if employed at all, were not exposed above or beyond the blade profile. As shown in FIG. 6, cutting elements 130 along the gage segment of a blade may employ flat cutting edges parallel to the gage. Moreover, the cutting elements of the reamer wing of the invention are set at a reduced negative back rake, such as 20° or even 10°, as opposed to the prior art practice of using a 30° design negative back rake. It is also contemplated that neutral rake cutting elements may be employed, or combinations of positive and negative back rake cutting elements. 
     Looking specifically to FIG. 5, the various operational stages of reamer wing 100 can be related to pass through and drill diameters, pass through and drill centerlines, and the transition therebetween. Pass through centerline 130 is the centerline of the pass through diameter 132, the smallest diameter through which reamer wing 100 may pass longitudinally. As the bottomhole assembly (such as 10) is placed in operation, with torque and WOB applied, reamer wing 100 is rotated about a centerline which begins to shift from 130 to 120 along transition line 134, which is not stationary but obviously rotates as reamer wing 100 itself rotates. As can readily be seen from FIG. 5, at commencement of rotation the presence of secondary blade 114 provides a balance to the cutting forces acting on reamer wing 100 and thus reduces vibration tendencies and impact on the cutting elements. Circles 136 and 138 illustrate the progression from pass through to drill diameter at the half and three-quarters open stages. Circle 140 illustrates full drill diameter, which is drilled about centerline 120 by primary blades 110 and 112. As can be seen in FIG. 3, during drilling of the drill diameter, stabilizer pad 118 rides against the pilot bit-sized borehole wall below the enlarged borehole segment 142 drilled by primary blades 110 and 112. Also as shown in FIG. 3, while the face and lower flank cutting elements of all the blades are in continuous engagement with the formation, neither of the secondary blades 114 and 116 or any other portion of reamer wing 100, except for the primary blades, 110 and 112 on the same radial plane as the primary blades will normally contact the borehole sidewall during drilling after the borehole is enlarged to drill diameter. While not so readily apparent, it will also be appreciated that trailing primary blade 112 will not be engaged with the formation until drill diameter is reached and the reamer wing 100 is rotating about center-line 120. 
     It is contemplated that other modifications may be made to reamer wing 100 to enhance its effectiveness. For example, neutral or even slightly positive back rake cutting elements may be employed on one or both secondary blades to more rapidly pull the blades into the formation to open the borehole, back rake angle being determined with respect to a line perpendicular to the blade profile at a particular cutting element radial location. Side rake of the cutting elements, being the angle thereof tangent to the profile or the angle thereof with respect to the radial line extending from the drilling axis through the cutting element location, may also be altered to aggressively engage the formation. The gage pad on both primary and secondary blades may be eliminated altogether, as shown on exemplary alternative primary blade 160 in FIG. 7, and cutting elements employed over the entire outer extent of the blades intended for contact with the borehole during the reaming operation. Alternatively, a very short gage pad may be employed to protect the uppermost cutting element while tripping into and out of the borehole. Material may be removed from the trailing edges of all the blades as shown at 150 on FIG. 5 to enhance clearance at pass through diameter and transition to drill diameter. 
     Many other additions, deletions and modifications of the invention as described and illustrated herein may be made without departing from the scope of the invention as hereinafter claimed.