GB2052609A - Well deviation control tool and method of manufacture thereof - Google Patents

Abstract

A tool is incorporated in a drill string in order to minimize the deviation of drill bits utilized in drilling well bore-holes at substantial depths. The tool (10) comprises a single section of thick-wall drill collar positioned in the drill string directly above the bit (11). The tool preferably is provided in a length similar to that of a conventional drill collar, for example, 30 feet and comprises upper and lower portions (12, 14) having circular cross sections and adapted to being disposed centrally of the borehole, and an offset middle portion (16) connected to the upper and lower portions by suitable bends (20, 22) with the middle portion having a reduced sectional modulus along the axis of the bends. The tool facilitates the drilling of straight boreholes by causing the segment of the drill string directly above the bit to rotate about the axis of the hole rather than about the axis of the drill string. A method for manufacturing the tool is disclosed. <IMAGE>

Description

SPECIFICATION Well deviation control tool and method of manufacture thereof The invention relates to tools for controlling the deviation of drill bits during the drilling of boreholes.

It is well known that during the drilling of well boreholes to substantial depths, the drill bit will often tend to deviate from a straight course, especially in drilling through layers of varying hardnesses and densities. Conventional practice is to drill a well rotating from the surface a long tubular string with a drill bit on the end. The drill bit drills by supporting part of the weight of the drill string on the bit with the remainder of the weight supported from the surface. This causes a large contact force between the bit and the formation and this contact force, or "bit weight", can be rotated and cause the formation or earth below to be crushed and ground away.

The drill string is very flexible because of its long length and thus buckles against the side of the borehole at a relatively low bit weight. Since the drill string diameter is smaller than the hole diameter (to provide a return passage for drilling fluid and to keep the drill string from becoming stuck in the borehole), the lower portion of the drill string is not straight. In fact, at some distance above the bit, the drill string actually contacts the borehole wall. This distance (illustrated dimension L shown in Figure 1) depends on hole angle, bit weight, drill string properties, and other factors.

Because the lower portion of the drill string is not straight, the contact force between the bit and the formation (bit weight) is in general not in line with the axis of the hole. The force is usually at an angle X (Figure 1) with respect to the borehole.

This angle X does not change as the drill string rotates, and, since the bit drills in the direction of this resultant force, the hole tends to deviate.

Conventional methods for control of this deviation slow down the drilling process and are generally expensive.

It has been proposed to maintain the bit in the center of the borehole by "packing" the hole with various forms of stabilizers. This approach has the disadvantage of requiring the insertion of a number of stabilizers in the drill string and has the further disadvantage that a stabilizer works for only one hole size and has little, if any, use in areas of washouts or when the hole encounters a cavity in the formation. It is well known that stabilizers do little to prevent hole deviation in hard formations. It has also been proposed to provide an "elbow" in a selected number of collars above the bit whereby the elbow will swing by action of centrifugal force in a circular path around the borehole in wiping engagement with the sidewall of the borehole.Known attempts at utilizing these methods have proved ineffective in practice and have not gained widespread acceptance in the drilling industry.

Consequently, there is an acute need for reliable and simple means for controlling the deviation of drill bits during the drilling of boreholes.

In accordance with the present invention, there is provided a tool for controlling the deviation of well boreholes. In a particular embodiment, the tool comprises a thick-wall pipe section having upper, middle and lower portions; the upper and lower portions comprising substantially on-line thick-wall pipe sections having means at the ends thereof for connection is a drill string; the middle portion being offset from the upper and lower portions and providing a substantially straight borehole-engaging surface and having an area moment of inertia (i.e., cross sectional modulus) in one direction less than the area moment of inertia in a direction perpendicular thereto; and a pair of bend portions integrally connecting the upper and lower portions to the middle portions, the bend portions providing bends in the direction of the reduced sectional modulus.

In accordance with this embodiment, the tool is connected in the drill string directly above the bit so that the rotation of the tool is about the axis of the hole. That is, during one complete revolution of the drill string, the tool moves completely around the periphery of the borehole so that the angle of the bit weight with respect to the borehole varies completely around the borehole such that the average angle is zero. Averaged around one revolution, the resultant bit weight is exactly in line with the borehole and thus a straight hole is drilled.

It is preferred that the length of the tool be substantially the same as the length of a conventional drill collar, for example, thirty feet. it will be appreciated that hard facing may be provided on the borehole-engaging surface although in some applications hard facing will not be required since the engagement of the tool with the wall of the borehole will be spread over a substantial surface area. In a preferred embodiment, the tool is tapered from a circular cross sectional configuration in the upper and lower portions to the cross sectional configuration presenting a reduced sectional modulus in one direction which is found in the middle portion.

Preferably, this tapering is achieved in the bend portions so that the length of the middle portion may be maximized to a length on the order of twelve to twenty feet, with a length of approximately fifteen feet being preferred.

Tools manufactured in accordance with the present invention may have application to a range of borehole sizes. When a tool used with holes of the smallest diameter within its range, the middle portion of the tool will remain substantially straight and engage the wall of the borehole along substantially its entire length. With hole sizes at the upper end of the range, for example, with hole sizes from T to 1 1- inches larger in diameter than the smallest hole in the range, the middle portion of the tool will bow outwardly in the direction of reduced sectional modulus in order to engage the wall of the borehole.

One advantage of the tool is that, unlike stabilizers, it adapts to a range of borehole sizes.

The tool has the further advantage of working in washout situations or situations where the hole encounters a cavity in the formation. The tool has a still further advantage of being operable to maintain the direction of boreholes being drilled at substantial angles, for example, angles up to 45 degrees off vertical.

In accordance with the present invention there is also provided a method of manufacturing a tool for incorporation into a drill string above the bit to minimize well deviation, comprising the steps of providing a thick-wall drill collar having a circular cross section, a centrally disposed mud passage and connections at the ends thereof for joining the collar and a drill string; milling tapers in the collar at preselected distances from the ends thereof while leaving discrete links of collar at the ends for remaining centrally disposed in the borehole when connected in the drill string; milling the middle portion between the tapers to remove material from opposing sides of the mud passage, thereby providing a reduced sectional modulus in one direction; and bending the collar in the tapered regions to offset the middle portion from the ends, with the direction of the reduced sectional modulus of the middle portion being in the same direction as the bends to assure that any bending of the middle portion under load is in the direction of the bends.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic illustration of a conventional drilling operation according to the prior art wherein the drill string is buckled and rests against the side of the borehole at a distance Lfrom the bottom of the borehole thereby causing the borehole to deviate at an angle X.

Figure 1 A is a cross section view taken substantially along the line lA-lA of Figure 1 schematically illustrating that the axis of rotation of the drill bit of Figure 3 is the center, C, of the drill string.

Figures 2 and 3 are schematic illustrations of the lower portion of a drill string incorporating the tool of the present invention.

Figures 2A and 3A are cross section views taken substantially along lines 2A-2A and 3A-3A of Figures 2 and 3, respectively, schematically illustrating that the axis of rotation of the drill bit is the center of the borehole.

Figure 4 is a view of the deviation control tool of the present invention positioned within a borehole.

Figure 5 is a side view of the tool illustrated in Figure 4.

Figure 6 is an enlarged section view of the upper portion of the tool taken substantially along line 6-6 of Figure 4.

Figure 7 is an enlarged section view of the middle portion of the tool taken substantially along line 7-7 of Figure 4.

Figure 8 is a view superimposing a section view of the middle portion of the tool on a section view of the upper portion of the tool to illustrate the extent of the bend of the tool and the offset of the mud passage.

Figure 9 is a view of the tool before the two bends are formed to offset the middle portion.

Figure 10 illustrates a single hard facing region on the middle portion of the tool.

Figure 11 is a detailed perspective of a portion of the tool showing one of the tapers before bending.

Figure 12 is a view of the tool in an unloaded state positioned in a larger borehole than that shown in Figure 4.

Figure 13 is a view of the tool and borehole of Figure 12 with the tool loaded.

Figures 14 and 15 are views similar to Figure 7 illustrating first and second alternative cross sectional configurations for the middle portion of the tool.

The principle of operation of the present invention will first be described with reference to Figures 1, 1 A, 2, 2A, 3 and 3A. Figure 1 illustrates the lower portion of a prior art drill string wherein the weight of the drill string causes a drill collar near the bit to bend and engage the wall of the borehole at a distance L from the bottom of the borehole. It has been found that as the drill string rotates, the drill collar remains against the same side of the borehole as shown in Figure 1 A so that the drill bit drills at an angle X from its intended course.

Figures 2 and 3 schematically illustrate the tool of the present invention connected in the drill string directly above the bit. The tool is shown in two positions 180 degrees apart. It will be noted with reference to Figures 2A and 3A that the offset portion of the tool swings around the borehole in engagement with the wall so that the angle of the bit weight varies completely around the borehole. While the drill bit will be drilling at an angle X' from its intended course at any time, the average angle with respect to the intended course is zero, i.e., the resultant bit weight is in line with the borehole.

Referring to Figures 4-8, there is illustrated a preferred embodiment of a well deviation control tool 10 constructed in accordance with the present invention. In the illustrated embodiment, tool 10 takes the form of a relatively thick-wall pipe section adapted for securement in the drill string directly above the drill bit 1 Tool 10 may approximate in length a conventional thick-wall drill collar typically disposed directly above the drill bit, for example, a collar having a length on the order of 28 to 32 feet, with 30 feet being the most commonly used length.

Collar 10 includes an upper portion 12, a lower portion 14 and an offset middle portion 16. Upper.

and lower portions 12, 14 are provided at their ends with conventional connections for assembling tool 10 in the drill string in the manner well known to those skilled in the art, i.e., by connection to an adjacent collar 1 7 and to bit 11.

As illustrated in Figures 4 and 7, upper and lower portions 12, 14 are circular in cross section and are adapted to being centered within the borehole.

Upper and lower portions 12, 14 preferably extend only a small portion of the overall length of tool 10 and, in the illustrated embodiment, each portion 12, 14 is on the order of three feet in length.

Middle portion 1 6 is formed integral with upper and lower portions 12, 14 by means of a pair of Sbend portions 20, 22. As best shown in Figure 4, middle portion 1 6 forms a crank-like configuration having a substantially straight elongate surface (generally designated by reference numeral 23) adapted to engage the well borehole along a substantial portion of the length of tool 10, for example, for a distance on the order of twelve to twenty feet, with a distance of approximately fifteen feet being preferred for a tool having an overall length on the order of thirty feet.Bend portions 20, 22 not only include the bends which serve to offset middle portion 1 6 from the center of the borehole, but also include a tapering of the tool from the circular cross sections in portions 12, 14 (Figure 6) to the reduced cross section in portion 16 (Figure 7).

The tapering and bend portions 20, 22 and the reduced cross section of middle portion 16 is preferably achieved by milling material from a conventional stock piece of drill collar. In a preferred method of manufacturing tool 10, a thick-wall drill collar having a central mud passage 40 is first subjected to a milling operation wherein the cross sectional configurations of the bend portions 20, 22 and the middle portion 16 are formed by milling material from the stock collar. In order to form the illustrated tool, tapers are first milled in the bend portions 20, 22 to form tapers 30 as shown in Figure 11 (only one taper shown).

Next, the middle portion 16 is formed by milling flat 32 on one side of the collar and an identical flat on the other side of the collar, preferably at the same distance D from the mud passage 40. The corners of the outwardly projecting flat are then milled back to form flats 42, 44 while leaving a central flat 46 (Figure 7). Figure 9 shows the tool after the milling is complete but before the bends are formed.

Following the milling operation, bends are formed in the previously tapered regions, portions 20 and 22, in order to offset the middle portion 16 in the manner best shown in Figures 4 and 8. The bends may be formed, for example, by bending the tool over a jig by means of a hydraulic ram. As shown in Figure 8, the middle portion 16 is substantially offset from the centerline of portions 12, 14. In fact, it has been found that excellent results are achieved where the entire mass of portion 1 6 is offset on one side of the centerline of portions 1 2, 14.

In a preferred embodiment of the present invention made from a 5 inch O.D. drill collar, the dimensions of the tool are as follows: length, thirty feet; outside diameter of portions 12 and 14, five inches; diameter of mud passage 40, two inches; length of portions 12 and 14, three feet; length of bend and taper portions 20 and 22,42 feet; length of middle portion 16, fifteen feet; offset of mud passage in portions 12 and 14 from mud passage in portion 1 6 (dimension 0 in Figure 9), 19 inches; dimension of portion 1 6 in direction of reduced sectional modules (dimension P in Figure 9), three inches.This tool having been made from 5 inch O.D. drill collar may be utilized with hole sizes on the order of 6 inches to 7 inches, or even larger. It has been found that tools formed from 6 inch O.D. drill collar may be utilized with hole sizes on the order of 7 inches to 8f inches and tools manufactured from 7 2 inch O.D. drill collar may be utilized with hole sizes on the order of 9 2 to 9w inches. While the precise sizes may vary, it is believed that a number of tools of different sizes are needed to best accommodate the various hole sizes which may range from approximately 6 to 1 5 inches.

As set forth above, a single size of the tool may be used for a given range of hole sizes. For the smallest acceptable hole in the range, e.g., a inch hole for the 5 inch tool described above, the borehole-engaging surface will substantially engage the wall throughout its length. However, for holes of a larger size or in situations where the hole is enlarged by a washout or where the hole encounters a cavity in the formation, the borehole engaging portion of the tool will not continuously engage the wall unless it flexes outwardly.

Figure 12 illustrates the situation where the tool is used in a hole having a diameter at the larger end of the range of hole diameters for which it is designed. As shown in Figure 12, a gap 50, for example on the order of 3/8 inch, is found between the middle portion 1 6 and the wall of the borehole when the tool is unloaded. Upon loading of the tool as shown in Figure 13, the middle portion 1 6 will bow outwardly in the direction of reduced sectional modulus (the direction of the bends) to form a convex wall engaging portion between the bends. The bow typically will be rather small, for example, only sufficient to bridge of 3/8 inch gap, so that a substantial part of portion 16 may engage the borehole wall.

It can be seen that the tool of the present invention has the advantage of being applicable to a range of hole sizes and of having utility even in washout situations of in situations where the hole encounters a cavity in the formation. The tool has the further advantage of eliminating the need for stabilizers in the area immediately above the drill bit.

Since the middle portion of the tool will bear against the wall of the borehole, it is preferred to provide hard facing material on the boreholeengaging surfaces 42, 44, 46. In this regard, the hardfacing may comprise wear pads secured, e.g.

by welding, to the surfaces 42, 44, 46. However, it is preferred to apply lengthwise hardfacing to the tool in strips in spaced apart areas, for example, in the five areas 60 illustrated in Figures 9 and 10. A preferred form of hardfacing is provided by utilizing a welding rod comprising fine tungsten carbide particles and a soft metal base and depositing the material on the tool by arc welding.

Gas welding may be used, but arc welding is preferred because it generally involves the application of less heat to the pipe. Application of hardfacing strips having a thickness on the order of 1/1 6 inch to 1/8 inch is preferred. While the pattern of hardfacing shown in Figure 10 is believed to give excellent results, this pattern is not required. What is important in most applications is that the hardfacing strips run lengthwise on the tool rather than circumferentially.

Figure 14 illustrates a first alternative embodiment for the cross sectional configuration of the middle portion 16 of the tool. According to this embodiment, the borehole-engaging surfaces of portion 1 6 comprise five flats which are milled into the tool. It will be noted that with this configuration, as opposed to the three-flat configuration discussed above, the borehole engaging surfaces more nearly approximate an arc and, therefore, the center of gravity of the middle portion 16 may be offset somewhat more than with the previously illustrated embodiment.

However, it has been found that the three-flat configuration provides an adequate offset and is considerably less expensive to manufacture than a tool with more flats. It will be noted that the hard facing strips found on both the three-flat (Figure 7) and five flat (Figure 14) configurations are distributed throughout the wall engaging surfaces; however, it is particularly important that the hardfacing strips be applied at the corners between the flats to minimize the rapid wear at these points.

Figure 1 5 illustrates a second alternative embodiment for the cross sectional configuration of middle portion 1 6. This cross sectional configuration, resembling the shape of a banana, has the advantage of providing a boreholeengaging surface which is in the form of an arc and substantially approximates the curvature of the borehole with which it is used. It is believed that this configuration allows the maximum offset of the center of gravity of the middle portion; however, the forming of this cross section from a single piece of drill collar would be considerably more expensive than the two previously illustrated embodiments.

While the present invention has been disclosed in connection with several illustrated embodiments, it will be appreciated that numerous modifications may be made without departing from the spirit or scope of the invention.

Claims (1)

1. A well deviation control tool comprising: a thick-wall pipe section having upper, middle and lower portions; said upper and lower portions comprising substantially on-line thick-wall pipe sections having means at the ends thereof for connection in a drill string; said middle portion being offset from the upper and lower portions and providing a substantially straight boreholeengaging surface and having a cross-sectional modulus in one direction substantially less than the sectional modulus in the direction perpendicular thereto: and a pair of bend portions integrally connecting the upper and lower portions to the middle portion, said bend portions being formed in the direction of said reduced sectional modulus.

2. A tool as claimed in claim 1 wherein the entire mass of the middle portion is offset on one side of the center-line of the upper and lower portions.

3. A tool as claimed in claim 1 or claim 2 wherein said borehole engaging surface comprises at least three flats.

5. A tool as claimed in claim 1 or claim 2 wherein said borehole engaging surface comprises an arc approximating the curvature of the borehole for which the tool is intended.

6. A well deviation control tool substantially as hereinbefore described with reference to, and as illustrated in, Figures 2 to 13, or Figures 2 to 13 when modified as illustrated in Figure 14 or Figure 15, of the accompanying drawings.

7. A method of manufacturing a tool for incorporation into a drill string above the bit to minimize well deviation, comprising the steps of: providing a thick-wall drill collar having a circular cross section, a centrally disposed mud passage and connections at the ends thereof for joining the collar in a drill string; milling tapers in the collar at preselected distances from the ends thereof while leaving discrete lengths of collar at the ends for remaining centrally disposed in the borehole when connected in the drill string; milling the middle portion between the tapers to remove material from opposing sides of the mud passage, thereby providing a reduced sectional modulus in one direction; and bending the collar in the regions of the tapers to offset the middle portion from the ends, with the direction of the reduced sectional modulus of the middle portion being in the same direction as the bends to assure that any bending of the middle portion under load is in the direction of the bends.

8. A method as claimed in claim 7 wherein said tool has an overall length of the order of 28 to 32 feet, an outside diameter of the order of 5 to 7 inches and the middle portion thereof has a length in the range from 12 to 20 feet.

9. A method of manufacturing a tool for incorporation into a drill string above the bit, the method being substantially as hereinbefore described with reference to Figures 2 to 1 5 of the accompanying drawings.

10. A tool manufactured by a method as claimed in any of claims 7 to 9.