EP1039095B1

EP1039095B1 - Downhole drill bit

Info

Publication number: EP1039095B1
Application number: EP00302231A
Authority: EP
Inventors: Coy M. Fielder; Rogerio H. Silva
Original assignee: Diamond Products International Inc
Current assignee: ReedHycalog LP
Priority date: 1999-03-19
Filing date: 2000-03-20
Publication date: 2005-05-18
Anticipated expiration: 2020-03-20
Also published as: DE60020185T2; EP1039095A2; EP1039095A3; DE60020185D1

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to downhole tools. More specifically, the present invention is directed to a stabilized bi-center drill bit and methods for its manufacture.

2. Description of the Prior Art

A significant source of many drilling problems relates to drill bit and string instability, of which there are many types. Bit and/or string instability probably occurs much more often than is readily apparent by reference to immediately noticeable problems. However, when such instability is severe, it places high stress on drilling equipment that includes not only drill bits but also downhole tools and the drill string in general. Common problems caused by such instability may include, but are not limited to, excessive torque, directional drilling control problems, and coring problems.
One typical approach to solving these problems is to over-design the drilling product to thereby resist the stress. However, this solution is usually expensive and can actually limit performance in some ways. For instance, one presently commercially available drill bit includes reinforced polycrystalline diamond compact ("PDC") members that are strengthened by use of a fairly large taper, or frustoconical contour on the PDC member. The taper angle is smaller than the backrake angle of the cutter to allow the cutter to cut into the formation at a desired angle. While this design makes the PDC cutters stronger so as to reduce cutter damage, it does not solve the primary problem of bit instability. Thus, drill string problems, directional drilling control problems, and excessive torque problems remain. Also, because the PDC diamond table must be ground on all of the PDC cutters, the drill bits made in this manner are more expensive and less resistant to abrasive wear as compared to the same drill bit made with standard cutters.
Another prior art solution to bit instability problems is directed toward a specific type of bit instability that is generally referred to as bit whirl. Bit whirl is a very complicated process that includes many types of bit movement patterns or modes of motion wherein the bit typically does not remain centered within the borehole. The solution is based on the premise that it is impossible to design and build a perfectly balanced bit. Therefore, an intentionally imbalanced bit is provided in a manner that improves bit stability. One drawback to this method is that for it to work, the bit forces must be the dominant force acting on the bit. The bits are generally designed to provide for a cutting force imbalance that may range about 500 to 2000 pounds depending on bit size and type. Unfortunately, there are many cases where gravity or string movements create forces larger than the designed cutting force imbalance and therefore become the dominant bit forces. In such cases, the intentionally designed imbalance is ineffective to prevent the bit from becoming unstable and whirling.
Yet another attempt to reduce bit instability requires devices that are generally referred to as penetration limiters. Penetration limiters work to prevent excessive cutter penetration into the formation that can lead to bit whirl or cutter damage. These devices may act to prevent not only bit whirl but also prevent radial bit movement or tilting problems that occur when drilling forces are not balanced.
As discussed in more depth hereinafter, penetration limiters should preferably satisfy two conditions. Conventional wisdom dictates that when the bit is drilling smoothly (i.e., no excessive forces on the cutters), the penetration limiters must not be in contact with the formation. Second, if excessive loads do occur either on the entire bit or to a specific area of the bit, the penetration limiters must contact the formation and prevent the surrounding cutters from penetrating too deeply into the formation.
Prior art penetration limiters are positioned behind the bit to perform this function. The prior art penetration limiters fail to function efficiently, either partially or completely, in at least some circumstances. Once the bit becomes worn such that the PDC cutters develop a wear flat, the prior art penetration limiters become inefficient because they begin to continuously contact the formation even when the bit is drilling smoothly. In fact, a bit with worn cutters does not actually need a penetration limiter because the wear flats act to maintain stability. An ideal penetration limiter would work properly when the cutters are sharp but then disappear once the cutters are worn.
Another shortfall of prior art penetration limiters is that they cannot function of the bit is rocked forward, as may occur in some types of bit whirling or tilting. The rear positioning of prior art penetration limiters results in their being lifted so far from the formation during bit tilting that they become ineffective. Thus, to be most effective, the ideal penetration limiter would be in line with the cutters rather than behind or in front. However, this positioning takes space that is used for the cutters.
While the above background has been directed to drill bits in general, more specific problems of bit instability are created in the instance of the bi-center bit. Bi-center bits have been used sporadically for over two decades as an alternative to undereaming. A desirable aspect to the bi-center bit is its ability to pass through a small hole and then drill a hole of a greater diameter. Problems associated with the bi-center bit, however, include those of a short life due to irregular wear patterns and excessive wear, the creation of a smaller than expected hole size and overall poor directional characteristics.
As in the instance of conventional drill bits, many solutions have been proposed to overcome the above disadvantages associated with instability and wear. For example, the use of penetration limiters has also been employed to enhance the stability of the bi-center bit. However, the prior art has not addressed the difficulties associated with the placement of such penetration limiters to properly stabilize the bi-center bit, which by its design, is inherently unstable. Penetration limiters in more traditional applications have been simply placed behind multiple cutters on each blade and only the exposure of the cutters above the height of the penetration limiter was felt critical to producing proper penetration limiter qualities. Additional considerations, however, are involved with the placement of shaped cutters on a bi-center bit which must contemplate the cutting force of both the reamer and the pilot bit.
Current manufacturing methods are limited in their ability to direct more cutting force toward the primary blades on the reamer, resulting in high forces directed to the side of the pilot that is opposite the primary reamer blades. This force imbalance can cause the bi-center bit to exhibit undesirable directional problems, high torque, undersized hole, and broken cutters.
As a result of these and other proposed problems, the bi-center bit has yet to realize its potential as a reliable alternative to undereaming.
EP 0058061 discloses a tool for an underground formation. The tool is an eccentric hole opener and has eccentric portion, the interface of which has cutting elements, such as poly-crystalline diamond compacts. The tool thus has a body with a proximal end adapted for connection to a drill string and distal end which defines a pilot section. There is an intermediate reamer section which has a greater diameter than the pilot section.
The present invention seeks to provide an improved downhole tool and an improved method of fabricating a downhole tool.
According to one aspect of this invention there is provided a downhole tool comprising a body defining a proximal end adapted for connection to a drill string and a distal end and where said distal end defines a pilot section and an intermediate reamer section and where both the pilot section and reamer section are provided with upsets in the form of ribs or blades each having a plurality of cutting elements defining cutting surfaces and where the body defines a rotational diameter and a pass-through diameter wherein that stabilising wings are coupled to said body opposite said reamer section or said pilot section.
Preferably the stabilising wings define the outer diametrical extend of the tool.
Conveniently the stabilising wings are provided with cutting elements having a backrake angle in the range of 25°- 75°.
Advantageously said stabilising wings define a radial length which is determined as a function of the outside diameter of the tool.
Preferably the radial length is defined as the difference between the outside diameter of the pilot section and the pass-through diameter of the tool.
Conveniently said stabilising wings is provided with high angle cutting elements.
Advantageously the pass-through diameter is defined by two upsets of the reamer, termed the leading and trailing upsets, and one further point on the body, the cutting elements on the leading and trailing upsets defining a backrake angle of between 25° and 75° with the formation.
Preferably the reamer section comprises a plurality of upsets radiating from the tool axis about a selected portion of the tool when viewed in cross section, and the stabilising wings comprise a plurality of wings which extend from the body opposite said upsets.
Conveniently the stabilising wings have a mass selected to offset imbalance forces created by the reamer section on rotation of the tool.
The invention also provide a method of fabricating a downhole tool, said downhole tool comprising a body defining a proximal end adapted for connection to a drill string and a distal end, where the distal end defines a pilot section and an intermediate reamer section, and where both the pilot section and reamer section are provided with upsets in the form of ribs or blades each having a plurality of cutting elements defining cutting surfaces and where the body defines a rotational diameter and a pass-through diameter, wherein stabilising wings provided on said body opposite said reamer or said pilot section, the upsets of the pilot section and the reader section being disposed around a rotational axis "A" and a pass-through axis "B", the method comprising:
establishing a pass-through axis for the tool;
creating a cutter profile for the pilot section about the pass-through axis;
situating the cutters on the upsets to cover open regions identified in the cutter profile;
positioning the upsets by evaluating the distance of each given cutter from the pass-through axis "B" and the rotational axis "A"; and
orienting the cutters to optimise their use when the tool is rotated about either axis "A" or axis "B" such that the cutting faces of the cutters are brought into at least partial contact with the formation.
A preferred embodiment of the invention includes a pilot bit having a hard metal body defining a proximal end adapted to be operably coupled to the drill string, and an end face provided with a plurality of cutting elements, and a reamer section integrally formed on one side of the body between the proximal end and the end face. The resulting bi-center bit is adapted to be rotated in the borehole in a generally conventional fashion to create a hole of a larger diameter than through which it was introduced.
The pilot bit diameter is typically the same size as the max tool size. The max tool size is a diameter determined by the tools that are to be used directly above the bit and generally correspond to common motor diameters (this is a known factor in designating bi-center bits). In the new method the pilot bit is reduced in diameter by an amount that relates to the amount force required to improve the design (smaller pilot equals more force that can be directed to the reamer).
The wings that are added are designed to drill the formation that is encountered between the reduced pilot diameter and the max tool diameter. Since a smaller pilot results in larger wings and it is the cutters on the wing that creates the force direct at the primary reamer blades then a smaller pilot gives us the ability to direct more force towards the reamer.
In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which:
FIGURE 1 is a side view of a conventional bi-center drill bit;
FIGURE 2 is an end view of the working face of the bi-center drill bit illustrated in Figure 1;
Figures 3A-C are end views of a bi-center bit as positioned in a borehole illustrating the pilot diameter or maximum tool size, the drill hole diameter and pass through diameter, respectively;
Figures 4A-B illustrate a conventional side view of a bi-center bit as it may be situated in casing and in operation, respectively;
Figure 5 is an end view of a conventional bi-center bit;
Figure 6 illustrates a cutting structure brazed in place within a pocket milled into a rib of a conventional bi-center drill bit;
Figure 7 illustrates a schematic outline view of an exemplary bi-center bit of the prior art;
Figure 8 illustrates a side view of one embodiment of the bi-center bit of the present invention;
Figure 9 illustrates an end view of the bi-center bit illustrated in Figure 8;
Figure 10 illustrates a revolved section of the pilot section of the bi-center bit of the present invention;
Figure 11 illustrates a top view of one embodiment of the bi center bit of the invention as it might be applied to drill out a casing shoe;
Figure 12 illustrates a side view of the embodiment illustrated in Figure 11;
Figure 13 illustrates a revolved section of the bi-center bit illustrated in Figure 9, as drawn through the geometric axis;
Figure 14 illustrates a graphic profile of the cutters positioned on the reamer section of the embodiment illustrated in Figures 11-12;
Figure 15 illustrates a schematic view of the orientation of cutters in one preferred embodiment of the invention.
While the present invention will be described in connection with presently preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents included within the spirit of the invention and as defined in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figures 1-7 generally illustrate a conventional bi-center bit and its method of operating in the borehole.
By reference to these figures, bit body 2, manufactured from steel or other hard metal, includes a threaded pin 4 at one end for connection in the drill string, and a pilot bit 3 defining an operating end face 6 at its opposite end. A reamer section 5 is integrally formed with the body 2 between the pin 4 and the pilot bit 3 and defines a second operating end face 7, as illustrated. The term "operating end face" as used herein includes not only the axial end or axially facing portion shown in Figure 2, but also contiguous areas extending up along the lower sides of the bit 1 and rcamer 5.
The operating end face 6 of bit 3 is transversed by a number of upsets in the form of ribs or blades 8 radiating from the lower central area of the bit 3 and extending across the underside and up along the lower side surfaces of said bit 3. Ribs 8 carry cutting members 10, as more fully described below. Just above the upper ends of rib 8, bit 3 defines a gauge or stabilizer section, including stabilizer ribs or gauge pads 12, each of which is continuous with a respective one of the cutter carrying rib 8. Ribs 8 contact the walls of the borehole that has been drilled by operating end face 6 to centralize and stabilize the tool 1 and to help control its vibration. (See Figure 4).
The pass-through diameter of the bi-center is defined by the three points where the cutting blades are at gauge. These three points are illustrated at Figure 2 are designated "x," "y" and "z." Reamer section 5 includes two or more blades 11 which are eccentrically positioned above the pilot bit 3 in a manner best illustrated in Figure 2. Blades 11 also carry cutting elements 10 as described below. Blades 11 radiate from the tool axis but are only positioned about a selected portion or quadrant of the tool when viewed in end cross section. In such a fashion, the tool 1 may be tripped into a bore hole having a diameter marginally greater than the maximum diameter drawn through the reamer section 5, yet be able to cut a drill hole of substantially greater diameter than the pass-through diameter when the tool 1 is rotated about the geometric or rotational axis "A." The axis defined by the pass-through diameter is identified at "B." (See Figures 4A-B.)
In the conventional embodiment illustrated in Figure 1, cutting elements 10 are positioned about the operating end face 7 of the reamer section 5. Just above the upper ends of rib 11, reamer section 5 defines a gauge or stabilizer section, including stabilizer ribs or gauge pads 17, each of which is continuous with a respective one of the cutter carrying rib 11. Ribs 11 contact the walls of the borehole that has been drilled by operating end face 7 to further centralize and stabilize the tool 1 and to help control its vibration.
Intermediate stabilizer section defined by ribs 11 and pin 4 is a shank 14 having wrench flats 15 that may be engaged to make up and break out the tool 1 from the drill string (not illustrated). By reference again to Figure 2, the underside of the bit body 2 has a number of circulation ports or nozzles 15 located near its centerline. Nozzles 15 communicate with the inset areas between ribs 8 and 11, which areas serve as fluid flow spaces in use.
With reference now to Figures 1 and 2, bit body 2 is intended to be rotated in the clockwise direction, when viewed downwardly, about axis "A." Thus, each of the ribs 8 and 11 has a leading edge surface 8A and 11A and a trailing edge surface 8B and 11B, respectively. As shown in Figure 6, each of the cutting members 10 is preferably comprised of a mounting body 20 comprised of sintered tungsten carbide or some other suitable material, and a layer 22 of polycrystalline diamond carried on the leading face of stud 38 and defining the cutting face 30A of the cutting member. The cutting members 10 are mounted in the respective ribs 8 and 11 so that their cutting faces are exposed through the leading edge surfaces 8A and 11, respectively.
In the conventional bi-center bit illustrated in Figures 1-7, cutting members 10 are mounted so as to position the cutter face 30A at an aggressive, low angle, e.g., 15-20° backrake, with respect to the formation. This is especially true of the cutting members 10 positioned at the leading edges of bit body 2. Ribs 8 and 11 are themselves preferably comprised of steel or some other hard metal. The tungsten carbide cutter body 38 is preferably brazed into a pocket 32 and includes within the pocket the excess braze material 29.
As illustrated in profile in Figure 7, the conventional bi-center bit normally includes a pilot section 3 which defines an outside diameter at least equal to the diameter of bit body 2. In such a fashion, cutters on pilot section 3 may cut to gauge. (See Figure 3.)
One embodiment of the bi-center bit of the present invention may be seen by reference to Figures 8-9. Figure 8 illustrates a side view of a preferred embodiment of the bi-center bit 30 of the present invention. By reference to the figures, the bit 30 comprises a bit body 32 which includes a threaded pin at one end 34 for connection to a drill string and a pilot bit 33 defining an operating end face 36 at its opposite end. A reamer section 35 is integrally formed with body 32 between the pin 34 and pilot bit 33 and defines a second operating end face 37.
The operating end face 36 of pilot 33 is traversed by a number of upsets in the form of ribs and blades 38 radiating from the central area of bit 33. As in the conventional embodiment, ribs 38 carry a plurality of cutting members 40. The reamer section 35 is also provided with a number of blades or upsets 42, which upsets are also provided with a plurality of cutting elements 40 which themselves define cutting faces.
The general embodiment illustrated in Figure 8-9 is provided with a pilot section 33 defining a smaller cross sectional diameter than the conventional embodiment illustrated in Figures 1-7. The extent to which the pilot is smaller is determined as a function of the force improvement needed in the direction opposite the pilot 33.
Except where specified, the method of construction of the general embodiment of the bi-center bit is the same as that described in the parent applications. The method for construction of particular embodiments of the invention are set forth below.
In the embodiment illustrated in Figure 8, pilot 33 defines a bald or bare spot 44 where cutters 40 have been removed. Spot 44 is defined about an upset located about the midpoint of the arc defined by the reamer section 35. The purpose of removing cutters from this area of the pilot 33 is to lessen the forces about this upset on the pilot 33 toward wings 50, as will be described further below. Area 44 is created by removing cutter 40 situated in an area between 90 and 45 degrees along the upset as viewed in cross section.
In a preferred embodiment, bit 30 may be provided with stabilizer wings 50 opposite reamer section 35. Wings 50 may be secured to bit body 32 in a conventional fashion, e.g., welding, or may be formed integrally. Wings 50 serve to define the outer diametrical extent of tool 30 opposite pilot 33. (See Figure 8.) Wings 50 are preferably provided with cutters 42 having a backrake angle in the range of 25-75°.
The length of wings 50, as measured in a radial direction, is a function of the outside diameter of the tool 30 as a whole. In this connection, it is desired that this length, identified as L₂ in Figure 9, be the difference between the outside diameter of the pilot 33 and the pass through diameter of the tool 30. Alternately, wings 50 may adopt a length intermediate the pass-through differential.
Wings 50 may be axially situated approximately opposite reamer section 35, as illustrated. Alternatively, wings 50 may be disposed about pilot section 33, the goal being to offset imbalance forces created by reamer section 35. In this connection, wings 50 can be affixed on the reamer 35 or pilot 33. The use of a high imbalance is substantially meaningless on drill bits (non bi-centers) and has thus not been considered since it was thought to be impossible to accomplish on a bi-center. The force balancing method of the present invention makes this a possibility and provides significant improvement over a low imbalance. The key is that the higher force imbalance would be directed towards the pilot.
There is a major need in bi-center bits for using high angle cutters to manipulate the imbalance forces since there is no other method which produces an imbalance which is this low.
By adding high angle cutters on wings 50, the amount of force directed toward reamer 35 is significantly increased, thereby decreasing the percent imbalance of the tool as a whole. The "percent imbalance" is the most important result and smaller numbers are better. Using these high angle cutters 42 in the fashion described above may result in a tool imbalance of some 7.5% to as low as 0%, or in some case may allow the force to be directed towards the reamer section 35. The result of force balancing using conventional methods yields on imbalance of 17.37%. A value of 0 to 5 % is considered good for designing a drill bit and is typically not possible in a bi-center bit.
A force imbalance of 12.29% is the result of reducing the pilot diameter and adding wings opposite the primary reamer blades. (High angle cutters are not used in this example.) This is similar to bits produced in the past using a smaller pilot and wings. Although this number is better, it is far from what is considered acceptable. By changing the wing cutters from normal angle to high angle cutters, the imbalance drops to 7.5%. (Calculated results not shown.) For the preferred method in the invention disclosure containing all four steps, smaller pilot, wings, high angle cutters on the wings and the removal of cutters on one side of the pilot. The result here is 4.41 %, which is now in the "good" range of values.
The bi-center bit of the present invention may enjoy a number of adaptations. One such adaptation is an embodiment to drill through a casing shoe, such as is illustrated in Figures 11-12.
The embodiment illustrated in Figures 11-12 is also provided with a pilot section 103 defining a smaller cross-sectional diameter than the conventional embodiment illustrated in Figures 1-7. The use of a lesser diameter for pilot section 103 serves to minimize the opportunity for damage to the borehole or casing when the tool 100 is rotated about the pass-through axis "B."
In a conventional bit, cutters 110 which extend to gauge generally include a low backrake angle for maximum efficiency in cutting. (See Figure 1.) In the bi-center bit of the present invention, it is desirable to utilize cutting elements which define a less aggressive cutter posture where they extend to gauge if rotation about the pass-through axis is desired. In this connection, it is desirable in this particular embodiment that cutters 110 on the leading and trailing blades 118 at gauge define a backrake angle of between 25-75 degrees with the formation.
The method by which the shoe cutter embodiment of the bi-center bit of the present invention may be constructed may be described as follows. In an exemplary bi-center bit, a cutter profile is established for the pilot bit. The pass-through axis is the then determined from the size and shape of the tool.
Once the pass-through diameter is determined, a cutter profile of the tool is made about the pass-through axis. This profile will identify any necessary movement of cutters 110 to cover any open, uncovered regions on the cutter profile. These cutters 110 may be situated along the primary upset 131 or upsets 132 radially disposed about geometric axis "A." (See Figure 15.)
Once positioning of the cutters 110 has been determined, the position of the upsets themselves must be established. In the example where it has been determined that a cutter 110 must be positioned at a selected distance r₁, from pass-through axis "B," an arc 49 is drawn through r, in the manner illustrated in Figure 15. The intersection of this arc 49 and a line drawn through axis "A" determines the possible positions of cutter 110 on radially disposed upsets 151.
To create a workable cutter profile for a bi-center bit which includes a highly tapered or contoured bit face introduces complexity into the placement of said cutters 110 since issues of both placement and cutter height must be addressed. As a result, it has been found preferable to utilize a bit face which is substantially flattened in cross section. (See Figure 12.)
Once positioning of the upsets has been determined, the cutters 110 must be oriented in a fashion to optimize their use when tool 100 is rotated about both the pass-through axis "B" and geometric axis "A." By reference to Figures 9 and 15, cutters 110 positioned for use in a conventional bi-center bit will be oriented with their cutting surfaces oriented toward the surface to the cut, e.g., the formation. In a conventional bi-center bit, however, cutters 110 so oriented on the primary upset 131 in the area 110 between axes "A" and "B" will actually be oriented 180° to the direction of cut when tool 100 is rotated about pass-through axis "B." To address this issue, it is preferable that at least most of cutters 110 situated on primary upset 131 about area 110 be oppositely oriented such that their cutting faces 130A are brought into contact with the formation or the casing shoe, as the case may be, when tool 100 is rotated about axis "B."
Cutters 110 disposed along primary upset 131 outside of region 110 in region 141 are oriented such that their cutting faces 130A are brought into at least partial contact with the formation regardless of the axis of rotation. Cutters 110 oppositely disposed about primary upset 131 in region 142 are oriented in a conventional fashion. (See Figure 15.)
Cutters 110 not situated on primary upset 131 oriented are disposed on radial upsets 132. These cutters 110, while their positioning may be dictated by the necessity for cutter coverage when tool 100 is rotated about axes "A" and "B," as described above, are oriented on their respective upsets 132 or are skewed to such an angle such that at least twenty percent of the active cutter face 130 engages the formation when the bi-center bit is rotated about axis "A." Restated as a function of direction of cut, the skew angle of cutters 110 is from 0°-80°.
Although particular detailed embodiments of the apparatus and method have been described herein, it should be understood that the invention is not restricted to the details of the preferred embodiment. Many changes in design, composition, configuration and dimensions are possible without departing from the scope of the instant invention, as defined in the appended claims.

Claims

A downhole tool comprising a body (32) defining a proximal end (34) adapted for connection to a drill string and a distal end and where said distal end defines a pilot section (33) and an intermediate reamer section (35) and where both the pilot section (33) and reamer section (35) are provided with upsets in the form of ribs or blades (38,42) each having a plurality of cutting elements (40) defining cutting surfaces and where the body defines a rotational diameter and a pass-through diameter characterised in that stabilising wings (50) are coupled to said body opposite said reamer section (35) or said pilot section (33).
The downhole tool of Claim 1 wherein the stabilising wings (50) define the outer diametrical extend of the tool.
The downhole tool of Claim 1 or 2 wherein the stabilising wings are provided with cutting elements (40) having a backrake angle in the range of 25°- 75°.
The downhole tool of any one of the preceding Claims wherein said stabilising wings (50) define a radial length which is determined as a function of the outside diameter of the tool.
The downhole tool of Claim 4 wherein the radial length is defined as the difference between the outside diameter of the pilot section (33) and the pass-through diameter of the tool (32).
The downhole tool of any one of the preceding Claims wherein said stabilising wings (50) is provided with high angle cutting elements (40).
The downhole tool of any one of the preceding Claims wherein the pass-through diameter is defined by two upsets of the reamer, termed the leading and trailing upsets, and one further point on the body, the cutting elements on the leading and trailing upsets defining a backrake angle of between 25° and 75° with the formation.
The downhole tool of any one of the preceding Claims wherein the reamer section (35) comprises a plurality of upsets (42) radiating from the tool axis about a selected portion of the tool when viewed in cross section, and the stabilising wings (50) comprise a plurality of wings (50) which extend from the body opposite said upsets.
The downhole tool of any one of the preceding Claims wherein the stabilising wings (50) have a mass selected to offset imbalance forces created by the reamer section on rotation of the tool.
A method of fabricating a downhole tool, said downhole tool comprising a body (32) defining a proximal end (34) adapted for connection to a drill string and a distal end, where the distal end defines a pilot section (33) and an intermediate reamer section (35), and where both the pilot section (33) and reamer section (35) are provided with upsets in the form of ribs or blades (38,42) each having a plurality of cutting elements (40) defining cutting surfaces and where the body defines a rotational diameter and a pass-through diameter, wherein stabilising wings (50) provided on said body opposite said reamer (35) or said pilot section (33), the upsets (38,42) of the pilot section (33) and the reamer section (35) being disposed around a rotational axis "A" and a pass-through axis "B", the method comprising:

establishing a pass-through axis for the tool;

creating a cutter profile for the pilot section about the pass-through axis;

situating the cutters on the upsets to cover open regions identified in the cutter profile;

positioning the upsets by evaluating the distance of each given cutter from the pass-through axis "B" and the rotational axis "A"; and

orienting the cutters to optimise their use when the tool is rotated about either axis "A" or axis "B" such that the cutting faces of the cutters are brought into at least partial contact with the formation.