US20080202769A1

US20080202769A1 - Well Wall Gripping Element

Info

Publication number: US20080202769A1
Application number: US11/680,602
Authority: US
Inventors: Wade D. Dupree; Falk W. Doering
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 2007-02-28
Filing date: 2007-02-28
Publication date: 2008-08-28
Also published as: WO2008104898A2; WO2008104898A3; CN101338653A

Abstract

A gripping element for incorporation into a track of a downhole tractor. The gripping element may be configured with an arcuate or curved surface in order to roughly match the arcuate nature of a well through which the downhole tractor is advanced. Due to the matching arcuate surfaces of the gripping element and the wall of the well, problems associated with uneven wear on the tracks may be avoided. Furthermore, damage to the well wall may be minimized as a result of the substantially matching surfaces of the well wall and the track made up of such gripping elements. Additionally, the gripping element may be made up of primarily a monolithic body in order to avoid challenges presented by some segmented body configurations.

Description

FIELD OF THE INVENTION

Embodiments described relate to downhole tractors for use in the oilfield industry. A variety of downhole tractor components are discussed. In particular, embodiments of gripping element links for tracks of a downhole tractor are described in detail.

BACKGROUND OF THE RELATED ART

Driving mechanisms in the oilfield industry, such as downhole tractors, may be employed in conjunction with the completion and operation of hydrocarbon wells. For example, downhole tractors may be used to convey equipment such as logging tools for gathering and recording geologic information relative to the well, ultimately optimizing its productivity. Delivering a tool with a driving mechanism in this manner may be particularly beneficial where the well is highly deviated or horizontal. This is because a downhole tractor may be adept at driving a tool through more non-vertical or tortuous well configurations where the option of dropping the tool down vertically is unavailable.
The above described downhole tractor may operate by way of rotatable tracks configured to contact the wall of the well and rotate thereagainst to advance the downhole tractor with respect thereto. For example, the downhole tractor may be equipped with a centralized housing coupled to tracks interfacing opposite sides of the well wall. The tracks may be made up of a plurality of links similar to a chain. Like a conventional farm tractor, or tank, the tracks may then be rotated with the links pressed against the well wall to achieve advancement of the tractor driving mechanism within the hole. In this manner, an instrument that is attached to the tractor, such as the above-noted logging tool, may be conveyed within the well.
The above-described links are generally between about 1 and 4 cm in width and made up multiple plates (for example, see plates 359 and 360 of FIG. 3). The plates themselves may be of stainless steel or other durable material and less than about 4 mm thick. Limiting the thickness of the plates in this manner allows the plates to be formed by fairly inexpensive conventional stamping techniques. Nevertheless, between about 4 and 10 plates may be stacked across a given link in order to provide the indicated 1 to 4 cm link width.
Unfortunately, while the downhole tractor may be adept at driving a tool through a variety of well configurations, the links of the tracks are fairly susceptible to wear and breakdown. In addition to the limited thickness of the plates, the teeth of the plates (see again FIG. 3, teeth 310) are aligned and form a substantially straight line across the link width. This straight line arrangement of the teeth causes the plates to wear unevenly over a period of use. This is due to the circumferential nature of the well wall. When the straight line arranged teeth attempt to contact the arcuate well, only the plates at the outer edges of the links contact the well wall (for example at locations 315), thus causing these plates to wear at a greater rate than those at a more longitudinally central location of the links. As a result these plates are susceptible to fracture sooner than the remainder of the plates.
In addition, this prior art link provides inefficient traction against the well wall due to the large area across the link's width that does not touch the well wall. This remains true until either the teeth 310 of the outermost plates 359 dig into and damage the well wall; or the teeth 310 of the outermost plates 359 wear down to the point the next outermost plates touch the well wall (i.e. the plates 360 immediately adjacent to the outermost plates 359).
The rate or degree of this wear or wear into the well wall itself may be a factor of the amount of load that is being conveyed by the downhole tractor. This load is substantially determinative of the amount of force being applied to the interface of the tracks and the well wall. Given that the downhole tractor is configured to drive relatively large loads such as logging tools, the degree of wear is likely to be quite significant. Thus, as a track rotatably advances across the surface of the well wall uneven contact therebetween may eventually result in track wear and/or failure, and damage to the well wall.
A considerable amount of cost in terms of downtime and equipment repair may be associated with a track failure as noted above during operation. Furthermore, even in advance of complete track failure, uneven wear of the track and its links results in an inherent inefficiency of operation for the downhole tractor. For example, even in advance of complete track failure, the track may be left with outer edges that are ineffective for the purpose of tractor advancement within the borehole. Wearing of this nature may result in the inefficiency of decreased gripping ability of the track perhaps even leading to its slippage. This in turn may also result in added damage to the well wall.
As opposed to uneven wear on the track as described above, wear may be directed at the well wall due to its circumferential nature. This may be of even greater concern than track wear in circumstances where the downhole tractor is intended to encounter open well configurations. That is, the downhole tractor may be required to come into direct contact with the soil formation. Thus, the downhole tractor may traverse a variety of soil consistencies, including soft, more easily degradable portions of the well wall. Uneven application of force at the edges of the links in such well areas may lead to dig in and shearing damage to the well wall. Unlike damage to the track, well wall damage at a location potentially thousands of feet from the surface may not be repaired by mere removal of the downhole tractor and replacement of its track.

SUMMARY

A driving mechanism for interfacing a well wall is provided. The mechanism includes a gripping element for contacting the well wall wherein the element is configured with an arcuate surface selected based on an arcuate character of the well wall.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a downhole tractor employing an embodiment of a gripping element having an arcuate surface and disposed within a well.

FIG. 2 is a cross-sectional view of the gripping element of the downhole tractor of FIG. 1 shown in contact with a well wall.

FIG. 3 is a cross-sectional view of a prior art embodiment of a gripping element of a downhole tractor within the well of FIG. 1.

FIG. 4 is a cross-sectional view of a gripping element according to another embodiment of the invention.

FIG. 5 is a perspective view of one embodiment of gripping element according to the present invention.

FIG. 6 is a top view of a plurality of the gripping elements of FIG. 5 connected to form a track.

FIGS. 7 and 8 are gripping elements according to alternative embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments are described with reference to certain gripping element links for tracks of a downhole tractor. Focus is drawn to gripping element links having a monolithic or integrally formed body. However, a variety of gripping element link configurations may be employed. Regardless, embodiments described herein include gripping element links having an arcuate surface selected based on an arcuate character displayed by the surface of a well wall.
Referring now to FIG. 1, an embodiment of a driving mechanism in the form of a downhole tractor 101 is shown within a well 197 of substantially horizontal or deviated configuration. In the embodiment shown, the well 197 is of an open configuration such that tracks 175 of the downhole tractor 101 may be in direct contact with the geologic formation 199 surrounding the well 197. However, in other embodiments, the downhole tractor 101 may be driven through a borehole casing lining a closed well.
The downhole tractor 101 of FIG. 1 may be advanced through the well 197 by way of the noted tracks 175, which rotate against a wall 195 of the well 197 in order to convey logging tools and other devices which are attached to the tractor 101 through the well 197. In the embodiment of FIG. 1, tracks 175 are shown to one side of a central housing 125 from which they may be deployed. However, anywhere from about 1 to about 4 tracks 175 (or even more if desired) may be disposed on various embodiments of the downhole tractor 101. A logging tool or another appropriate tool may be coupled to an end of the central housing 125 and conveyed through the well 197 by operation of the downhole tractor 101.
The above-noted central housing 125 may be equipped with an expansion mechanism such as opening arms 150, which expand the tracks 175 into engagement with the well wall 195 to allow for the tracks 175 to rotate thereagainst to propel the tractor 101. Alternatively the opening arms 150 may be moved inwardly toward the central housing 125 to disengage the tracks 175 from the well wall 195. A wide variety of such conventional expansion mechanism configurations and other actuators may be available for the deployment and retraction of the tracks 175.
Regardless of the particular expansion or retraction mechanisms employed, the downhole tractor 101 may be configured to traverse an irregularly shaped well. For example, in the case of an open well 197 of less than consistent diameter, the expansion mechanism and retraction mechanisms may work in concert by conventional means to ensure a substantially consistent force for engagement of the track 175 against the well wall 195 throughout a downhole tractor 101 conveyance operation.
A downhole tractor 101 such as that of FIG. 1 may be adept at traversing horizontal or highly deviated wells as a result of the provided tracks 175. As shown in FIG. 1, in a deployed state, the tracks 175 extend from the central housing 125 and travel in a belt-like manner adjacent the well wall 195. Additional arms or other mechanisms (not shown) may be employed to ensure consistent contact of a track 175 against the well wall 195.
Additionally, a gear box may be contained within the central housing 125 for transferring power to drive sprockets (not shown) and rotating the tracks 175 by conventional means. The tracks 175 may also be driven by a rotable screw or other means. Regardless of the particular driving technique employed, the tracks 175 are adept at conforming to and gripping the well wall 195, open or otherwise. Thus, the downhole tractor 101 may be effectively driven through the well 197 by rotation of the tracks 175 as described.
In one embodiment, the above described tracks 175 are made up of a plurality of gripping elements or links 100. Each of these gripping elements 100 is provided with teeth 110 for contacting and gripping the well wall 195. As detailed with reference to FIGS. 2 and 3, the gripping elements 100 of FIG. 1 are configured with an arcuate surface 201 displaying a radius that may be configured based on the diameter of the well 197 itself. Thus, unlike prior art gripping elements 300 of a conventional downhole tractor, the gripping elements 100 of FIG. 1 are particularly configured to minimize wear on the well wall 195, as well as the tracks 175 themselves, and to increase the area of contact between the gripping element teeth 110 and the well wall 195 across the width of the gripping element 100 as the downhole tractor 101 is driven through the well 197.
This minimization of wear on the well wall 195 and tracks 175 is attained in a manner that avoids compromise of the gripping character of the gripping elements 100. For example, as detailed further below, the gripping elements 100 are able to maintain teeth 110 of sufficient sharpness and durable character for proper advancement of the downhole tractor 101 during operation. Nevertheless, in spite of the retained characteristics of the teeth 110, the likelihood of uneven wearing on the tracks 175 or degradation of the well wall 195 is minimized due to the arcuate surface 201 of the gripping elements 100 (see FIG. 2).
Continuing now with reference to FIG. 2, a cross-sectional view of one of the above described gripping elements 100 is shown in contact with a well wall 195. This depiction reveals a cross-section of the gripping element 100 having a monolithic or integrally formed body 200 which contacts the well wall 195 with its arcuate surface 201. In one embodiment, this arcuate surface 201 is at the apex of the tooth 110 of the gripping element 100. The monolithic body 200 may be of a conventional ceramic, composite material or metal of suitable durability.
As shown in FIG. 2, the monolithic body 200 terminates at a tooth 110 of the gripping element 100 which is configured for gripping the well wall 195 at an interface 275 thereof. Also apparent in FIG. 2 is the substantially continuous nature of the arcuate surface 201 of the tooth 110 and the manner in which it appears to match the arcuate character displayed by the well wall 195. As alluded to above, the arcuate surface 201 of the gripping element 100 substantially reduces the amount of wear on the element 100 as well as on the well wall 195.
Continuing with reference to FIGS. 2 and 3, the gripping element 100 of the embodiments described herein is depicted in contrast to a prior art gripping element 300. As shown, each gripping element 200, 300 includes pins 250, 350 securing rollers 225, 325 through the body of the element 200, 300. However, as indicated above, the monolithic body 200 of the gripping element 200 of FIG. 2 includes an arcuate surface 201 for contacting a well wall, whereas the plates across the width of the prior art gripping element 300 have teeth 310 that are arranged in a straight line. The results of this distinction may be significant as detailed below.
Unlike the substantially matching and smooth interface 275 between the arcuate surface 201 and the well wall 195 as shown in FIG. 2, the prior art gripping element 300 is made up of side 359 and interior 360 plates that leave the corners 315 of a flat surface to contact the well wall 195 (see FIG. 3). Thus, a prior art interface 375 includes a large area of non-contact between the well wall 195 and the prior art gripping element 300. As a result, the force that the prior art gripping element 300 applies to the well wall 195 is directed exclusively through the noted corners 315. Therefore, wear on the prior art gripping element 300 is focused disproportionately to the side plates 359 as the gripping element 300 pulls a load through the well 197. Thus causing a tremendous amount of stress at areas 390 of the well wall 195.
The above-noted disproportionate application of force through the side plates 359 of the prior art gripping element 300 may result in deterioration of the side plates 359, the degree and rate of which may be dependent upon factors such as the amount of load pulled by the gripping element 300 as well as the durability of the side plates 359. Regardless, the prior art gripping element 300 is prone to more rapid and extensive deterioration as compared to the gripping element 100 of embodiments employing an arcuate surface 201 as described herein. Thus, tracks employing such prior art gripping elements 300 may more readily become inefficient and ultimately more prone to failure.
In addition to wear at the edges of the prior art gripping element 300, the well wall 195 itself is susceptible to wear and damage by these edges (i.e. the side plates 359). This may be of particular concern in the case of an open well 197 as shown in FIGS. 1-3, where the well wall 195 includes bare and potentially soft geologic formation 199 prone to damage by application of such localized forces. This can be seen in FIG. 3 where the damaged locations 390 of the formation 199 have resulted from the disproportionate amount of localized force applied thereto by the side plates 359 of the prior art gripping element 300. Scratching and shearing damage into the well wall 195 is evident.
Continuing with reference to FIGS. 2 and 3, wearing and damage to the gripping element 100 and the well wall 195 may be substantially avoided by the presence of an arcuate surface 201 which contacts the well wall 195 as proposed by embodiments of the present invention. The arcuate surface 201 is configured in light of the arcuate nature or character of the well 197. In fact, the arcuate surface 201 may be configured with a radius based on the particular arcuate dimensions of the given well 197 (e.g. its diameter).
For example, in one embodiment, the well 197 is about 8.5 inches in diameter, as formed by a conventional 8 inch drill bit. Therefore, the gripping element 100 may be configured with an arcuate surface 201 displaying about a 4.25 inch radius. Thus, the arcuate surface 201 will be of a shape that substantially matches the shape of the well wall 195. As a result, the force of the gripping element 100 against the well wall 195 during operation of the tractor 101 may be substantially evenly distributed. Thus, uneven wearing of the gripping element 100 is unlikely and the possibility of damage to the well wall 195 from the operation is minimized. In fact, minimizing the possibility of damage to the well wall 195 in this manner, may improve the viability of open wells with softer formations that might otherwise be considered inoperable for tractoring operations.
While the above is described with reference to an 8.5 inch diameter well 197, other well sizes are common such as those that are about 6.5 inches in diameter and those that are about 10.5 inches in diameter drilled from bits that are roughly of the 6 inch and 10 inch variety respectively. Thus, embodiments of the gripping element 100 may be configured with an arcuate surface 201 displaying a radius that is about 3.25 inches or about 5.25 inches to match such well sizes. Additionally, a host of other arcuate surface 201 radii may be employed depending on the diameter of the well 197. So long as the arcuate surface 201 is configured based on the arcuate character of the well 197 involved, benefits of the described embodiments may be realized.
As indicated, the arcuate surface 201 may be selected based on the diameter of the well 197 involved. However, as detailed further below, this does not necessarily require that the arcuate surface 201 be entirely continuous or of substantially the same radius as that of the well 197 in order to minimize damage to the well wall 195 or wear at the edges of the arcuate surface 201. For example, the presence of an arcuate surface 201 sufficient to provide majority contact at the interface 275 between the gripping element 100 and the well wall 195 may be one of several manners of employing an arcuate surface 201 configured based on the arcuate character of the well 197. As detailed below, this remains so even with a minimal degree of dig into the wall 195 by the element 100.
By way of example, given the above scenario of an 8.5 inch diameter (4.25 inch radius) open well 197 through a soft formation 199, significant benefit may be realized from employing a non-matching arcuate surface 201, for example, displaying a 3.25 inch radius. That is, in spite of the arcuate surface 201 having a radius that is about an inch different from that of the well 197, less than about 1 mm of dig into the soft well wall 195 will result in the majority of the arcuate surface 201 being in stable direct contact with the well wall 195. This is unlike the flat surface of the prior art gripping element 300 which would require deterioration of the well wall 195 across the majority of the interface 375 in order to provide majority contact between the flat surface and the well wall 195 (see FIG. 3). Therefore, with the significant difference in these two scenarios in mind, the arcuate surface 201 of the embodiment described here may still be referred to as substantially matching the arcuate shape of the well 197 (i.e. in spite of the noted inch difference in radii). Stated another way, an arcuate surface 201 having a radius that is within about an inch of a radius of the well 197 may be considered substantially matching the arucate shape of the well wall 195.
As shown in FIG. 2, the gripping element 100 may be made up of primarily a monolithic or integrally formed body 200 providing an arcuate surface 201 based on the dimensions of the well 197. However, as alluded to above, providing an arcuate surface 201 based on the well does not necessarily require an entirely continuous surface. For example, the prior art gripping element 300 fails to provide an arcuate surface at its interface 375 with the well wall 195. Nevertheless, a new embodiment of a gripping element 400 may be provided with such a segmented body. For example, the gripping element 400 may be formed by a plurality of stamped plates 459-462 held together by a pin 250 and rollers 225, wherein each successive interior plate 460-462 is incrementally taller than its outer-more adjacent plates. As such, the innermost plates 462 are taller than plates 461, which are taller than plates 460, which are taller than side plates 459. Thus, in combination, the plates 459-462 form an arcuate surface 401 across the width of the gripping element 400 formed by the plates, wherein the arcuate surface 401 is configured based on the arcuate character and dimensions of the well 197. Notes that although, the gripping element 400 of FIG. 4 is depicted as having eight stamped plates 459-462, in alternative embodiments, the gripping element 400 may have any desired number of plates arranged in any appropriate lateral arrangement. For example, a lateral spacing may exist between every other plate.
In an embodiment shown in FIG. 5, a monolithic body 500 is provided having a male region 575, a female region 525 and a middle region 550, wherein both the male 575 and female 525 regions include multiple arcuate shaped teeth 510. Such bodies 500 may be connected end to end as shown in FIG. 6 to form a continuous chain or track 675. Such regions may be connected by a pin 250 and rollers 225. As shown, the male region 575 of each body 500 is sized to fit within a female region 525 of each adjacent body 500. In addition, the teeth on the pinned male 575 and female 525 regions may be aligned such that across the width of the track 675, the male 575 and female 525 regions combine to form substantially continuous teeth 510. Note also that the width of the track 675 contains only two junctions 680 between pinned bodies 500. This is in comparison to the seven junctions 380 formed by the plates of the prior art gripping element 300, the significance of which is described below.
In still another embodiment, the gripping element may include a monolithic body that nevertheless displays a degree of discontinuity at the arcuate surface thereof. For example, an additional roller may be positioned at the center of the male region 575 of the monolithic body. This may aid the track 675 in its translation across the well wall 195 and enhance the interaction of the track 675 and the tractor's driving mechanism. However, in such a circumstance a channel would be provided into the surface of the track 675 in order to accommodate the added roller at each gripping element. However, such a channel would traverse the arcuate surface of each gripping element leaving a gap or break in contact between the arcuate surface and the well wall 195.
In spite of the slight interruption of physical contact between the arcuate surface and the well wall 195 in the embodiment described above, the arcuate surface may still be considered as configured based on the arcuate character of the well wall 195. In fact, the arcuate surface 201 may be said to be substantially matching the dimensions of the well 197. For example, in one such embodiment with a central roller, the arcuate surface is configured to make continuous contact with the well wall 195 at between about 50% and about 97% of the interface even with the presence of the indicated channel. As a result, the probability of wear at the outer edges of the gripping element or damage to the well wall 195 by the gripping element may be minimized. Thus, as with other embodiments described above, substantial benefit may be realized in spite of a degree of discontinuity in the arcuate surface.
Continuing now with reference to FIG. 6, an upper surface of the track 675 for use with the tractor 101 of FIG. 1 is shown. In such an embodiment, the entirety of the upper surface of the track 675 is arcuate, and specifically each tooth 510 which extends across the width of the track 675 is arcuate. As such, each tooth 510 forms the arcuate surface 200 shown in FIG. 2. The series of “y-shaped” gripping elements 500 reveals a host of teeth 510 as indicated above for gripping and pulling along a well wall 195 such as shown in FIG. 2. In fact, in the embodiment shown, each gripping element 500 includes teeth 510 at its middle region 550, female region 510, and male region 575. The “y-shaped” gripping elements 500 are aligned akin to overlapping chain links to form the track 675.
In an alternate embodiment, such as that shown in FIG. 7, the gripping elements 700 are composed of alternating male 775 (i.e. “t-shaped”) and female 725 (i.e. “H-shaped”) sections. In such an embodiment the male 775 and female 725 sections of the gripping elements 700 may be alternatingly positioned to form a track for use with the tractor 101 of FIG. 1. In another alternative embodiment, such as that shown in FIG. 8, each gripping element 800 contains both female and male interlocking portions.
Referring back to FIGS. 5 and 6, of the regions 550, 525, and 575 described above, the middle region 550 includes the widest portion of the monolithic body 200 and is the segment of the gripping element 100 that is depicted in FIG. 2. A tooth 110 traversing this region 550 of the gripping element 100 is formed entirely by a sole gripping element 100 without contribution from adjacent elements 100. Thus, such a tooth 110 may be advantageously devoid of lateral interfacing, a benefit detailed further below.
Referring again now to FIGS. 2 and 3, advantages of employing a gripping element 100 having a monolithic body 200 in comparison to a gripping element 300 with a body formed of separate plates 359, 360, are described. That is, in addition to readily providing a substantially uninterrupted arcuate surface 201, other benefits may be realized by employing a gripping element 100 made up primarily of a monolithic body 200.
As shown in FIG. 3, the prior art gripping element 300 includes a body that is made up of stacked plates 359, 360. As a result, a plurality of lateral interfaces 380 are present across the element 300. Each such interface 380 presents a location available for the accumulation of dirt and debris and friction which may affect the performance of a downhole tractor operation. For example, in the case of the prior art gripping element 300 shown in FIG. 3, debris would be able to reach all the way to the interface of the pin 350 which is configured for rotation through the plates 359, 360.
The efficiency of such rotation may be affected by the described accumulation of debris. Given that the tractor operation may be focused at an open well wall 195 in direct contact with earth formations 199, the possibility of accumulation of debris in this manner may be very likely. For example, it would be common for corrosive chemicals, dust, suspended sand, rock, mud, drilling fluid additives and other debris to be present within a conventional open hydrocarbon well 197. Use of a monolithic body 200 as described above minimizes the interfacing 380 that is found within the body of a prior art gripping element 300 as shown in FIG. 3. For example, in the embodiment of FIGS. 5 and 6, the main body 550 of each gripping element 500 contains no interfaces, while the male 575 and female 525 regions interface at only two places across the track 675 width.
Even without the accumulation of debris, the presence of such a high amount of lateral interfacing in the prior art gripping element 300 of FIG. 3 means that the pin 350 is forced to traverse a host of plates 359, 360. Use of a monolithic body 200 on the other hand, as shown in FIG. 2, provides added stability and avoids the possibility of independent translation or rotation of plates 359, 360. As a result, the efficiency of pin rotation is not frictionally compromised by competitively shifting plates 359, 360 of the body of the gripping element 100. By providing the monolithic body 200 to the gripping element 100 the challenges presented by a significant amount of lateral interfacing as described here may be largely avoided.
The embodiments described hereinabove significantly address problems of wear and breakdown of tracks and gripping element links thereof for a downhole tractor. In particular, problems of uneven wear on gripping elements resulting from the arcuate character of a well wall are substantially avoided along with the problem of wear on the well wall imposed by the gripping element. As a result, occurrences of track failure or inefficiency during a downhole tractoring operation may be minimized and the integrity of the well wall maintained. This may be of significant benefit in the case of open wells, especially those traversing softer formations. By protecting the integrity of the well wall and the gripping elements, time spent on equipment repair in the field may be saved. Furthermore, the availability of tractoring embodiments described herein may improve the viability of open wells through softer formations previously thought inoperable for tractoring operations.
The preceding description has been presented with reference to presently preferred embodiments of the invention. Persons skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structures and methods of operation can be practiced without meaningfully departing from the principle, and scope of this invention. Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope.

Claims

1. A driving mechanism for interfacing a well wall with a gripping element, said gripping element having an arcuate surface selected based on an arcuate character of the well wall, wherein an interfacing of the gripping element with the well wall causes the driving mechanism to move relative to the well wall.

2. The driving mechanism of claim 1 wherein said gripping element comprises at least one tooth at said arcuate surface for the interfacing.

3. The driving mechanism of claim 1 wherein said gripping element comprises a body that is one of a monolithic and a segmented variety.

4. The driving mechanism of claim 3 further comprising:

a rotable pin through said body; and

rollers secured at ends of said rotable pin.

5. The driving mechanism of claim 1 wherein said gripping element is of a y-shaped configuration comprising a male region and a female region.

6. The driving mechanism of claim 5 wherein each said region further comprises more than one tooth at a surface thereof.

7. The driving mechanism of claim 1, wherein the driving mechanism is a downhole tractor and said gripping element forms a portion of a track on the downhole tractor.

8. The driving mechanism of claim 7 wherein said track is a first track, the downhole tractor further comprising between about 1 and about 3 additional tracks.

9. The driving mechanism of claim 7 wherein said downhole tractor further comprises a central housing comprising:

an expansion mechanism for engaging said track with the well wall.

10. The driving mechanism of claim 9 further comprising an extension coupled to said central housing for accommodating a downhole tool.

11. A downhole tractor for use within a well and comprising a gripping element having a monolithic body with an arcuate surface configured based on an arcuate character of a wall of the well, the downhole tractor to effect an interface between the arcuate surface and the wall, which causes the downhole tractor to move relative to the well wall.

12. The downhole tractor of claim 11 wherein the arcuate surface is substantially continuous across a width of the gripping element.

13. The downhole tractor of claim 11 wherein the arcuate surface is configured to contact the wall through between about 50% and about 97% of the interface.

14. A gripping element for use on a track of a downhole tractor, the gripping element comprising an arcuate surface to interface a well wall, said arcuate surface substantially matching an arcuate shape of the well wall.

15. The gripping element of claim 14 wherein said arcuate surface displays a radius that is within about one inch of a radius of the well wall.

16. The gripping element of claim 14, wherein the arcuate shape of the well wall is in the form of a diameter, said diameter chosen from the group consisting of about 6.5 inches, about 8.5 inches and about 10.5 inches, and wherein said arcuate surface of the gripping element is in the form or a radius, said radius chosen from the group consisting of about 3.25 inches, about 4.25 inches and about 5.25 inches.

17. A downhole tractor comprising:

a chain for engaging a well wall and rotating thereagainst to propel the downhole tractor with respect to the well wall, wherein the chain comprises:

a plurality of links, wherein each link comprises at least one tooth formed in an arcuate shape selected based on an arcuate character of the well wall.

18. The downhole tractor of claim 17, wherein the plurality of links contains interconnecting links such that a width of the chain contains no more than two interfacing junctions between connected links.