Casing joints, liners, and other oilfield tubulars are often used in drilling, completing, and producing a well. Casing joints, for example, may be placed in a wellbore to stabilize a formation and protect a formation against high wellbore pressures (e.g., wellbore pressures that exceed a formation pressure) that could damage the formation. Casing joints are sections of steel pipe, which may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections are usually designed so that a seal is formed between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomer seal (e.g., an o-ring seal), a thread seal, a metal-to-metal seal formed proximate the connection, or similar seals known in the art.
In FIG. 1, a connection having a metal-to-metal seal at an intermediate position is shown. Such a connection is disclosed as an embodiment of U.S. Pat. No. 6,543,816 issued to Noel. That patent is incorporated herein by reference in its entirety. The connection includes a pin member 101 and a box member 102, each with a single tapered thread, 18 and 19 respectively. The pin thread 18 and the box thread 19 are interrupted at an intermediate position to form corresponding seal surfaces, which form a metal-to-metal seal 20 when a sufficient contact pressure exists therebetween. The metal-to-metal seal 20 shown in FIG. 1 is commonly referred to as a “mid-seal” due to its intermediate position, as opposed to other types of metal-to-metal seals located on the end of the pin member or the box member. To ensure that the desired contact pressure exists to form metal-to-metal seal 20, a positive stop is provided by pin nose 23 on the pin member 101 and shoulder 22 on the box member 102.
One type of thread commonly used to form a thread seal is a wedge thread. In FIG. 2, a connection having a wedge thread is shown. “Wedge threads” are characterized by threads, regardless of a particular thread form, that increase in width (i.e., axial distance between load flanks 225 and 226 and stab flanks 232 and 231) in opposite directions on the pin member 101 and box member 102. The rate at which the threads change in width along the connection is defined by a variable commonly known as a “wedge ratio.” As used herein, “wedge ratio,” although technically not a ratio, refers to the difference between the stab flank lead and the load flank lead, which causes the threads to vary width along the connection. A detailed discussion of wedge ratios is provided in U.S. Pat. No. 6,206,436 issued to Mallis, and assigned to the assignee of the present invention. That patent is incorporated herein by reference in its entirety.
Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference. Continuing with FIG. 2, on the pin member 101, the pin thread crest 222 is narrow towards the distal end of the pin member 101 while the box thread crest 291 is wide. Moving along the axis 105 (from right to left), the pin thread crest 222 widens while the box thread crest 291 narrows. In FIG. 1, the thread surfaces are tapered, meaning that the pin thread 106 increases in diameter from beginning to end while the box thread 107 decreases in diameter in a complimentary manner. Having a thread taper improves the ability to stab the pin member 101 into the box member 102 and distributes stress in the connection.
Generally, thread seals are difficult to achieve with non-wedge threads having broad crests and roots, however, the same thread forms may have thread seals when used for wedge threads. Wedge threads do not have any particular thread form. One example of a suitable thread form is a semi-dovetailed thread form disclosed in U.S. Pat. No. 5,360,239 issued to Klementich, and incorporated herein by reference. Another thread form includes a multi-faceted load flank or stab flank, as disclosed in U.S. Pat. No. 6,722,706 issued to Church, and incorporated herein by reference. Each of the above thread forms is considered to be a “trapped” thread form, meaning that at least a portion of the corresponding load flanks and/or corresponding stab flanks axially overlap. An open (i.e. not trapped) thread form with a generally rectangular shape is disclosed in U.S. Pat. No. 6,578,880 issued to Watts. The above thread forms are examples of thread forms that may be used for embodiments of the invention. Generally, open thread forms such as buttress or stub acme are not suitable for wedge threads because they would impart a large radial force on the box member. A generally square thread form, such as that disclosed by Watts, or a trapped thread form does not impart an outward radial force on the box member. Those having ordinary skill in the art will appreciate that the teachings contained herein are not limited to particular thread forms.
For wedge threads, a thread seal is accomplished by the contact pressure caused by interference over at least a portion of the connection between the pin load flank 226 and the box load flank 225 and between the pin stab flank 232 and the box stab flank 231, which occurs when the connection is made-up. Close proximity or interference between the roots 292 and 221 and crests 222 and 291 completes the thread seal when it occurs over at least a portion of where the flank interference occurs. Generally, higher pressure may be contained with increased interference between the roots and crests (“root/crest interference”) on the pin member 101 and the box member 102 and by increasing flank interference. This particular connection also includes a metal-to-metal seal that is accomplished by contact pressure between corresponding seal surfaces 103 and 104, respectively located on the pin member 101 and box member 102.
Wedge threads typically do not have a positive stop torque shoulder on the connection. For wedge threads that do not have a positive stop torque shoulder, the make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member varies more during make-up for a given torque range to be applied than for connections having a positive stop torque shoulder. As used herein, “make-up” refers to threading a pin member and a box member together. “Selected make-up” refers to threading the pin member and the box member together with a desired amount of torque, or based on a relative position (axial or circumferential) of the pin member with the box member. For wedge threads that are designed to have both flank interference and root/crest interference at a selected make-up, both the flank interference and root/crest interference increase as the connection is made-up (i.e. increase in torque increases flank interference and root/crest interference). For wedge threads that are designed to have root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks and corresponding roots and crests come closer to each other (i.e. clearance decreases or interference increases) during make-up. Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the make-up torque on the connection. Thus, a wedge thread may be able to thread-seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the make-up torque on the connection, however, this also increases stress on the connection during make-up, which could lead to failure during use.
In some well construction operations, it is advantageous to radially plastically expand threaded pipe or casing joints in a drilled (“open”) hole or inside a cased wellbore. Radially plastically expanding a pipe, as used in this application, describes a permanent expansion, or increase, of the inside diameter of a pipe or casing. In a cased wellbore, radially expandable casing can be used to reinforce worn or damaged casing so as to, for example, increase a burst rating of the old casing, thereby preventing premature abandonment of the hole. In open hole sections of the wellbore, the use of radially expandable casing may reduce a required diameter of a drilled hole for a desired final cased hole diameter, and may also reduce a required volume of cement required to fix the casing in wellbore.
An expansion tool is typically used to radially plastically expand a string of casing or tubing disposed inside a wellbore from an initial condition (e.g., from an initial diameter to an expanded condition (e.g., with a larger diameter). One common prior art expansion process, shown in FIG. 3, uses a conically tapered, cold-forming expansion tool (commonly referred to as a “pig”) to expand casing in a wellbore. The expansion tool is generally sealed inside of a pig launcher, which is a belled section attached to a lower end of a casing string that is run into the wellbore. Because the pig launcher must usually pass through the parent casing already installed in the wellbore, the pig launcher has an outer diameter that is less than a “drift diameter” of the parent casing. As used herein, the “drift diameter” is the maximum external diameter that can pass through a casing or tubing string disposed in a well. Generally, the drift diameter is somewhat smaller that the internal diameter of the casing or tubing due to the wellbore not being perfectly straight, or eccentricity or damage to the casing or tubing. Because of this, a tool having exactly the internal diameter of the casing or tubing would be unable to move freely through the casing or tubing.
The casing string is set in place in the hole, usually by hanging-off the casing string from a casing hanger. Then, a working string of drillpipe or tubing is run into the wellbore and attached to the expansion tool (e.g., the working string is generally attached to the leading mandrel). After connecting the drill pipe, the weight of the casing string is supported by the expansion tool. The drill pipe is then used to further lower the casing string to the selected location in the wellbore. The expansion tool includes a tapered section having a taper angle that is generally between 5 degrees and 45 degrees. The expansion tool is generally symmetric about a longitudinal axis thereof. The expansion tool also includes a cylindrical section having a diameter that corresponds to a desired expanded inner diameter of a casing string (not shown) that is followed by a tapered section. The expansion tool may also comprise an axial bore therethrough so that cement and pressurized fluid (e.g., drilling fluid) may be pumped through the working string, through the expansion tool, and into the wellbore.
Cement is pumped through the drill pipe and out of a cement port on the pig. The cement flows between the outside of the casing string to be expanded (hereinafter the “expandable casing string”) and the inside of the wellbore. After the selected amount of cement has been pumped, the cement port is sealed off, typically by a dart designed to seat in the cement port. The dart is usually pumped with drilling fluid through the drill pipe. Continuing to pump drilling fluid pressurizes the pig launcher, which, combined with an axial upward lifting force on the working string, drives the expansion tool 301 forward (i.e. upward toward the surface). As the expansion tool 301 moves forward, the expandable casing string outwardly radially expands to a desired expanded diameter. Expansion continues until the entire expandable casing string has been expanded. In many instances, the expandable casing string will include a length of casing that remains inside the parent casing after expansion. The extra length of casing can be designed to act as a liner hanger for the expanded casing string and to seal between the parent casing and the expanded casing string.
The expansion tool 301 may be started at either the bottom or the top of the expandable casing string depending on the tool design and the application. Radial expansion may be performed at rates of, for example, 25 to 60 feet per minute. Other expansion processes, such as expansion under localized hydrostatic pressure, or “hydroforming,” are known in the art, but are generally not used as much as cold-forming expansion processes. Other expansion tools for cold-forming the casing also exist. Various tools exist for use in cold-forming expansion processes.
While various expansion methods, in particular the tapered expansion tool method, have proven to work quite well on expandable casing strings, the expansion of made-up threaded connections can result in structural sealing problems. Threaded connections that undergo radial plastic expansion have a tendency to exhibit a non-uniform axial elongation and react differently to residual hoop stresses remaining after expansion. Specifically, pin members and box members deform differently during radial expansion. The box member will generally move away from the pin member during radial expansion at locations of high contact stress at make-up, such as seal surfaces for a metal-to-metal seal. This is due in part to the relief, during plastic expansion, of residual stress in the connection that exists from the make-up of the box member with the pin member. This differential displacement phenomenon can result in a loss of preload in axially-engaged seals, making the use of conventional metal-to-metal seals (including, for example, shoulder seals and mid-seals) problematic for plastically radially expanded casing and tubing.
One of the more successful threads for expandable casing applications is the wedge thread. One reason that wedge threads are a suitable for expandable casing applications is that they may not make-up against a radial torque shoulder (i.e. a positive stop), but instead typically make-up by nearly simultaneous contact of thread load flanks and stab flanks. During the expansion process, axial stress in the connection will often cause a radial torque shoulder to fail when the compressive stresses at the shoulder exceed the compressive yield strength of the casing material. The advantages of a wedge thread are independent of the thread form used. When a dovetail-shaped or other closed thread form is used for the wedge thread, the wedge thread will also resist radial forces during and after expansion, which might tend to separate the pin connection from the box connection. An open thread form for the wedge thread may also be used.
Despite the relative success of wedge threads in expandable applications, increased seal reliability of connections following radial expansion is still needed. Designing a sealing arrangement for a connection for the purpose of being radially expanded could provide a more reliable seal by replacing prior art sealing arrangements or providing a redundant sealing arrangement.