1. Field of the Invention
The present invention relates to expandable tubulars. More particularly, the invention relates to improved apparatus and methods for expanding tubular strings, including tubulars and the connections therebetween. More particularly still, the invention relates to improved apparatus and methods for expanding tubular strings through the use of expansion tools having optimized, shaped surfaces that reduce axial bending forces and damage to threaded connections.
2. Description of the Related Art
Strings of wellbore tubulars are used to line wellbores and to provide a fluid conduit for the collection of hydrocarbons. Typically, a portion of wellbore is formed by drilling and then a string of tubulars (or “liner” or “casing”), is inserted and cemented into the wellbore to prevent cave-in and to isolate the wellbore from a surrounding formation. Because the wellbore is drilled in sections and each section is cased before continuing to drill, each subsequent section is of a smaller diameter than the one above it, resulting in a telescopic arrangement of casing having an ever-decreasing diameter.
Expanding tubulars in a wellbore involves running a string of tubulars in at a first, smaller diameter and then enlarging their diameter once they are set in place. Downhole expansion has always been appealing as a way to partially overcome the limitations brought about by small diameter tubulars. For example, expanding a downhole tubular even slightly results in an enlarged fluid pathway for hydrocarbons and an enlarged pathway for the passage of a subsequent string of tubulars or tools needed for operations downhole. In another example, expandable tubulars can permit troublesome zones in a wellbore to be sealed off by running a section of tubulars into the wellbore and expanding it against the wellbore walls to isolate a formation. In still another example, expandable production tubing could be inserted into a wellbore at a first diameter and then expanded to permit greater capacity for collecting hydrocarbons.
A typical prior art expansion tool is illustrated in U.S. Pat. No. 5,348,095 and that patent is incorporated by reference herein in its entirety. The '095 patent teaches a tool having a conically shaped first end permitting its insertion into a tubular. The mid portion of the tool has an outer diameter substantially larger than the inner diameter of the tubular to be expanded. Through either fluid or mechanical force or a combination thereof, the tool is forced through the tubular, resulting in an increase in the inner and outer diameters of the tubular.
Other prior art patents illustrate techniques for moving an expansion tool through a string of tubulars. For example, U.S. Pat. No. 6,085,838, incorporated herein by reference, illustrates running a section of casing or liner into a wellbore on a work string that includes a conical expansion tool at its lower end. After the section of liner is located in the wellbore and anchored, the work string and expansion tool are moved upwards due to fluid pressure pumped through the work string and acting upon a lower end of the tool. After expanding the length of tubular, the string and expansion tool are removed, leaving the expanded liner in the wellbore.
When a tubular is expanded by moving an expansion tool through it, a frictional force is developed between the contact surface(s) of the tool and the tubular walls in contact with the tool. A radial expansion force is also created as the tubular walls move directly outwards from the centerline of the tubular. Additionally, there is a force developed along the longitudinal axis of the tubular due to the movement of the expansion tool along its length. This “axial bending” force causes the tubing to bend outwards, or flare as the tool “opens” the tubular to a greater diameter. Of the various forces at work during expansion by an expansion tool, axial bending is the most troublesome due to its progressive nature and its tendency to place an inside wall of a tubular into tension and an outer wall into compression as the cone moves along in the expansion process.
Axial bending stress developed by the type of tool used to produce the graph of
Threaded connections between expandable tubulars are difficult to successfully expand because of the axial bending that takes place as an expansion member moves through the connection. For example, when a pin portion of a connector with outwardly facing threads is connected to a corresponding box portion of the connection having inwardly facing threads, the threads experience opposing forces during expansion. Typically, the outwardly facing threads will be in compression while the inwardly facing threads will be in tension. Thereafter, as the largest diameter portion of a conical expander tool moves through the connection, the forces are reversed, with the outwardly facing threads placed into tension and the inwardly facing threads in compression. The result is often a threaded connection that is loosened due to different forces acting upon the parts during expansion. Another problem relates to “spring back” that can cause a return movement of the relatively thin pin nose. Typically, threaded connections on expandable strings are placed in a wellbore in a “pin up” orientation and then expanded from the bottom upwards towards the surface. In this manner, the pin nose is the last part of the connection to be expanded. In
While the connection of
The foregoing problems with expandable tubulars and in particular, expandable connections between tubulars have been addressed by a number of prior art patents. U.S. Pat. No. 6,622,797 for instance, addresses the problem with an expansion tool having discrete segments along its profile, each segment divided by a smaller, radiused segment and resulting in an increase in diameter of the expansion tool. According to the inventors, the discrete portions create separate, discrete locations of contact between the expansion tool and the inner surface of the tubular, resulting in less friction generation and a more efficiently operating expansion process. In fact, separating the contact points necessarily creates spikes in contact forces between the tool and the tubular which can exacerbate problems associated with axial bending. In another exemplary prior art arrangement shown in U.S. Pat. No. 7,191,841, a fluid pathway is provided in the expansion tool in order to increase or decrease the force needed to move the tool through the tubular. While the forces might be adjustable, the patent drawings illustrate that the tubular walls literally “skip” off the surface of the expansion tool, creating spikes of contact pressure as the tool moves.
There is a need therefore, for an expansion tool that can expand a tubular string in a manner that decreases the likelihood of damage due to forces created during the expansion process. There is a further need for an expansion tool that can reduce contact pressures and spikes in contact pressure between the tool and the tubular or connection being expanded. There is a further need for an expansion tool that has a contact surface that can maintain contact with a tubular or connection wall and thus reduce the effects of axial bending.
An expansion tool for use in a wellbore includes an expansion surface made up of a concave portion, a convex portion and a substantially straight center section therebetween. In one aspect, the center section is formed according to a formula Y=(1.26) (X)−0.13, where X is the wall thickness of a tubular and Y is the length of the center section. In another aspect, the expansion surface includes a first concave portion and a convex portion having an arc length extending the concave portion to a trailing edge of the tool. In another embodiment, the concave and convex portions are radius-shaped and are tangent to each other and substantially equal in size. In one embodiment, the tool includes a nose radius to further ensure a gradual transition of shapes acting upon a tubular string. In one aspect, an optimum radius for the concave and convex radius is determined by providing about 65″ of radius size per each 1″ of tubular wall thickness. The arrangement of the shapes and their relation to each other reduces relatively high and low contact pressures and lessens the effects of axial bending in a tubular or a connection.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
The inventors have discovered through experimentation and finite element analysis (F.E.A.), a computer-based numerical technique for finding solutions, that tubular threaded connections on expandable oilfield casing and the like which are mechanically expanded with an expansion tool exhibit greater damage from axial bending when the contact forces between the tool and the tubular are concentrated in one or two locations along the tool rather than evenly spaced over the length of an expansion surface of the tool. The inventors have also discovered that rapid changes in contact pressure including relatively high spikes of pressure and areas of little or no pressure result in a greater amount or degree of damage from axial bending forces. The result is a need for an expansion tool that will remain in contact with the tubular/connection as much as possible and one that does not contact the tubular with high forces at any one time but rather, distributes the forces over the length of an expansion surface of the tool. The invention disclosed herein is primarily intended to benefit expandable connections between wellbore tubulars. In this specification the term “tubular”, “connection”, and “tubular string” are often used interchangeably and any discussion or illustration of problems or benefits associated with a tubular is equally applicable to a connection between tubulars.
In one embodiment of the invention, an expansion tool is provided having an expansion surface with a first concave portion adjacent a first end of the tool and a second convex portion adjacent the concave portion. The portions are equal in size and arc length, tangent to each other at a point where they meet and include a center section therebetween that is tangent, at each end, to one of the portions. In another embodiment the concave and convex portions are radius-shaped and the tool also includes a nose radius at its leading end having a convex radius shape and a trailing end of the tool includes a tail radius that is essentially an extension of the convex radius. In each case, the alternating shapes that make up the expansion surface of the tool are blended together to minimize abruptness and with it, axial bending of a tubular wall or connection during expansion.
The expansion tool of the present invention, while including a number of different concave and convex shapes along its expansion surface, can include a relatively small overall expansion angle without making the expansion surface so long that friction generated between the tool and the tubular or connection requires an excessive expansion force. For example, by utilizing the shapes disclosed herein, expansion tools can be provided with an average expansion angle of as little as 3 or 4 degrees as opposed to a typical expansion angle of 10 degrees. Because the contact pressures are minimized, the overall force needed to move the tool through a tubular string is not significantly increased even though the tool has a longer expansion surface than prior art conical tools. In one example, a tool having radiused expansion surfaces of 20″ required a maximum expansion force of 90K lbf. when expanding a 5½″ tubular string.
In the embodiment shown, the tool 500 includes a nose radius 200 which is a convex radius commencing at a leading end of the tool and terminating adjacent a concave expansion radius 205. At its second end, the nose radius terminates at a blend point 201 where the tool surface is parallel to the tubular's center line and at a point where the diameter of the tool 500 is intended to be the same diameter as the smallest inside diameter (ID) of a tubular string to be expanded. In some cases, an inside diameter of the tubulars and the threaded connections therebetween will be equal. In those instances, expansion of each will commence at blend point 201. In other instances, the smallest inner diameter in a string might be within a threaded connection. In those cases, point 201 will be designed to contact the ID of the connections and the larger diameter tubulars will be contacted by the tool at a location further along adjacent expansion radius 205. The tool therefore, is designed to contact and commerce expansion at point 201. An exception to the design criteria occurs when an out-of-round tubular or connection is encountered. In that instance, the nose radius 200 will contact and “round out” a tubular that might be oval in shape when initially encountered in a wellbore. Thereafter, the tubular or connection will be round when encountered by point 201 and the expansion radii 205, 220 thereafter.
The tool of
Finite element analysis has shown that an optimum size for the expansion radii exists for each tubular string to be expanded. The size is determined without consideration of the tubular's outside diameter or grade. Rather, the optimum radius is determined by a tubular's wall thickness and the provision of approximately 65″ of radius size per each 1″ of wall thickness. This remains true regardless of the overall diameter of the tubular. The guideline ensures a larger, more gradual expansion radius for a thicker-walled tubular. For example, to determine the optimum expansion radius “R” for a wall thickness of 0.304″ (which is typical of 5.5″ OD wellbore tubulars), the wall thickness “T” is multiplied by 65 (the ratio of expansion to wall thickness, or N) using the calculation: R=T×N. The result is 19.76″. Therefore a radius of about 20″ is preferable for 5.5″ tubular. In another example using a tubular having a 0.582″ wall thickness (which is typical for 11.75″ OD tubulars), the calculation becomes 0.582 “T” multiplied by 65 “N” or 37.83″. Therefore, the preferred radius for 11.75″ tubulars is about 40″. The inventors have determined that while the thickness of a threaded connection is sometimes slightly different than the tubulars in a string, an expansion tool having an optimum radius for a given tubular wall thickness will also be optimum for integral joint connections like the one shown in
In a preferred embodiment, expansion radii 205, 220 are separated by a center section 225 which is straight, tangent to each radius and blends with each radius at either end 203, 204. Center section 225 provides a neutral area of expansion surface after the first concave expansion radius 205 to permit the expansion forces acting upon the tubular, specifically the axial bending forces, to neutralize prior to contact between the tubular and the convex radius 220. By choosing an appropriately sized center section, any contact pressure spikes between the two opposing radii are reduced while the center section does not add so much area to the expansion surface that it creates excess heat and friction during expansion. In one embodiment, relatively small spikes of contact pressure are created at each end of the center section rather than one larger spike at a transition point between two expansion radii.
More particularly, the center section separates the two expansion surfaces to an extent that the tubular shape is not abruptly reversed. Without a center section or with one that is too short, the tubular shape change requirement is instant, causing a severe contact pressure spike between the tubular and the cone. Along with the pressure spikes, area with virtually no contact between the tool and tubular further exaggerate the spikes of pressure on each side of the low pressure point. In fact, the thicker the tubular wall thickness/stiffness, the more resistant the tubular will be to reversing this shape change and the greater the contact pressure spike. Therefore, the center section is dependent upon wall thickness and its length must be increased for thicker wall thicknesses in order to provide more of a separation between the concave and convex expansion surfaces.
Analyses have shown that an optimum center section is one with at least enough length to permit the tubular or connection wall to recover or normalize between contact with the opposed convex and concave expansion surfaces. The inventors have found that the following formula, utilizing wall thickness of a tubular or connection, is usable to determine a minimum center section needed to reduce or eliminate spikes in contact pressure during expansion:
Y=(1.26)(X)−0.13
Where: Y=center section length in inches and; X=pipe wall thickness in inches.
The center section 225 of the shaped cone's expansion surface is especially important when avoiding damage to a connection's engaged threads. Because expanded connections are machined on thin wall tubular to keep expansion force requirements in a reasonable range, there can be relatively few threads engaged in a connection at the outset. The number of engaged threads are important to a connection's mechanical strength and when one or more of the threads is damaged during expansion, those threads cease to contribute to the transfer of applied loads between the male and female connection members. Therefore, when several threads are damaged, the engaged thread body is severely weakened. By maintaining a center section 225 between the opposing radii 205, 220, the change in forces brought about by the different radii is less damaging to the threads.
In addition to avoiding pressure spikes between radii, the center section permits design aspects of the tool to be easily changed. For example, lengthening the center section can permit the amount of radial expansion to be increased while maintaining a relatively small expansion angle. In a tool requiring a fixed expansion surface length, lengthening the center section results in reducing the size of the expansion radii 205, 220 while shortening the center section permits the radii to be enlarged. The ideal design is one that utilizes a center section that is long enough to provide the benefits of a neutral area but short enough to permit the expansion radii to maintain their relatively large and gradual shapes. In one example, a tool with an 8″ expansion curve length has a center section of 0.031″ with corresponding radii size of 39″. Lengthening the center section to 2.0″ results in a reduction of the radii to 36.5″.
It is contemplated that the invention could include expansion radii of different size in some instances. For example, the convex expansion radius 220 could be made larger than the concave radius 205 in order to generate the second half of the expansion more gently for a certain metal seal configuration in an expandable connection. In this case, a center section between the two expansion radii will be especially important for minimizing spikes in contact pressure between the tool and the connection. In another embodiment, particularly useful in tools with longer center sections, a center configuration can be formed from two opposing and opposite radii in order to “spread” out the change in directions as the expansion surfaces are reversed between the concave 205 and convex 220 radii.
Because a tool of the present invention, with its optimized radius shapes results in a larger expansion surface than the prior art 10 degree cones, lubrication may be necessary to minimize heat and expansion force. In other cases, lubrication is necessary due to the material of a tubular. For example, a tubular made of steel with little or no iron, such as stainless steel is much more sensitive to galling or tearing than normal iron tubular grades. Additionally, these tubulars work harden more than normal casing grades. When additional lubrication is desired, the center section is an ideal location for the lubrication ports. In one instance, lubricating ports are drilled so that small openings are present at the surface of the center section allowing well fluids to be pumped between the tool and tubular or threaded connection. Preferably, these openings are formed longitudinally with respect to the centerline of tool and tubular rather than circumferentially, in order to decrease interruptions between the tool and tubular or connections that can cause spikes of contact pressure as they are expanded.
The most efficient port designs for keeping contact pressure spikes minimized are small, slotted openings along the center section length that are longitudinal or parallel with the tubular and tool axis. In one embodiment, the slots are approximately 0.050″ wide to minimize circumferential discontinuity that can create problems a non-uniform expansion surface. Some systems rely upon a passage through the expansion cone to “seal cups” in front of the cone that isolate fluid. For such a system, lubricating holes can be formed between the fluid passageway inside the cone to the center section. In the case of cones that rely solely on force generated by fluid pressure behind the cones, the lubricating ports will require holes drilled from the back of the cone that extend directly to the center section.
As shown in
Comparing the graph of
The actual design of a tool according to the present invention depends first on the wall thickness of the tubulars to be expanded. Using that wall thickness, the radius size is determined in inches using the formula disclosed herein. Thereafter, point 201 (
In addition to the foregoing, the inventors have discovered a number of other advantages to the expansion tool. Expansion force, or that force needed to drive an expansion tool of a larger diameter through a tubular of a smaller diameter, is a product of friction, axial bending, and hoop stress. Friction is developed between the expansion surface of the tool and the tubular wall it contacts. Axial bending, as described herein, is the outward bending of the tubular walls as they are expanded and hoop stress is a circumferential stress as a result of internal expansion pressures. Prior art, 10 degree cones have a relatively small area of expansion surface that enables them to expand a tubular while generating an acceptable amount of expansion force (around 100,000 lbf. for 5½″ tubulars and about 400,000 lbf. for 11¾″ tubulars). In spite of the increased expansion surface areas, the tool of the invention requires no more expansion force than a prior art 10 degree cone due to a reduction in axial bending that compensates for any increase in friction between the expansion surface of the tool and the tubular wall.
Because the tool is necessarily longer than a standard 10 degree tool, the additional length results in improved alignment between the tool and the tubular or connection. With less “wobble” as the tool move axially, the tubular remains straighter than tubing expanded with a shorter, prior art tool. The result is a tubular that is less prone to collapse prematurely due to an unsymmetrical shape when an external pressure is applied. Because expanded tubular is typically much softer than normal grades of casing, it can be more easily damaged. High contact pressures between the tubular or connection and the expansion tool are not only a sign of axial bending but can also be a source of damage to the material of the tubular. Damage like galling, tearing, smearing or other localized yielding can be detrimental to a tubular's materials strength integrity and resistance to corrosion and all can be reduced with an expansion tool that operates in more even manner and develops lower contact pressures. Additionally, because the tool's surfaces reduce the contact pressure during expansion, the tool itself will have a longer usable life with its various surfaces remaining in tolerance longer than a tool subjected to higher contact pressures. Also, because the shaped cone greatly reduces axial bending, flaws in the pipe that occur during its manufacture are less likely to propagate into a crack. Axial bending tends to open flaws that are oriented completely or even partially in the transverse direction (perpendicular to the tubular axis). Therefore, tubular specifications can be relaxed somewhat that will create a lower cost to the operators.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. For example, the tool can be made and used in a variety of ways and still include the advantageous shapes described. The tool could be part of a larger assembly including remotely actuatable liners and hangers and could be made collapsible or of segments whereby the tool assumes its final diameter, including the radiused shapes, after being deployed in a wellbore. Collapsible cones are disclosed in U.S. Pat. No. 6,012,523 and that patent is incorporated herein by reference in its entirety. Additionally, multiple expansion tools or a single tool with additional, larger diameter expansion surfaces along its length can be used to enlarge a tubular in steps, resulting in an overall expansion of up to 30%. Multi-stage passes with prior art conical tools create a compounded amount of damage to a tubular or connection. The tool of the invention, however, produces no such compound damage.