In the conventional drilling of an oil and gas well, a series of tubulars, typically strings of casing, liner, and/or screen segments connected together, are sequentially installed in the well bore until the depth of the producing zone of the formation is reached. Standard practice requires that each succeeding string of tubulars placed in the well bore has an outside diameter smaller than the preceding string of tubulars and/or bore hole such that the tubular segments can be passed through the preceding string of tubulars and/or bore hole to their downhole location. The reduction in the diameter of each successive string of tubular segments placed in the well bore results in a significant reduction in the diameter of pipe through which hydrocarbons can be carried to the surface. More importantly, to achieve the desired tubular diameter in the producing zone, the initial bore hole size at the surface must be sufficiently large to allow for a large diameter casing. The large initial bore hole size requires increased drilling time and increased material costs, including increased use of materials such as drilling mud and casing cement.
The technology of expandable tubulars addresses these shortcomings in the conventional casing/liner/screen hanging operations. Through radial expansion of successive strings of tubular segments until the outer wall of those segments contacts the inner wall of the host pipe, it is possible to create a tight fit between the expandable tubulars and the host pipe that holds the tubular segments in place and creates an annular seal. Further, it is possible to achieve a well bore of virtually uniform diameter. The expandable tubulars are expanded by various means known in the art, including, but not limited to, pulling or pushing fixed diameter expansion cones through the tubular, extruding the tubular off of a hydraulically-actuated expansion tool, or rotating an expansion tool while pulling or pushing it through the tubular.
The tubular segments to be expanded are typically coupled together using threaded connections in which the male end, or pin member, of one tubular is threadably connected to the female end, or box member, of an adjacent tubular. Alternatively, the ends of the adjacent tubulars may have a pin member at each end, with the box member being formed by a short coupling threaded onto the pin members. Similarly, a short coupling may be used to connect the pin member of one tubular to the box member of another tubular. Currently, the threaded connections and the tubular segments are expanded downhole during the same operation.
With the standard threaded pipe connections currently in use, problems can arise during and after expansion of the tubular segments at the threaded connection point between segments. First, the sealing ability of the threaded connection is often significantly diminished as a result of the expansion process. The threaded connection area thus becomes a source of potential leaks in the tubular strings. Second, conventional threaded tubular connections are also susceptible to splitting along the length of the box member when the connections are radially expanded. The radial expansion process concentrates the expansion stresses in any thin wall sections present in the box or pin members and can lead to the rupturing or splitting of the thin wall section of the box member. Third, backing off of the threaded connection can occur during the expansion process. Excessive backing off of the threaded connection can significantly decrease the strength or load carrying capability of the threaded connection or, potentially, disengage the connection.
What is needed is a threaded connection for expandable tubulars that maintains its sealing and coupling ability during and after expansion. It is an object of the present invention to provide an apparatus and method allowing for the expansion of a threaded connection between segments of expandable tubulars, while at the same time maintaining the sealing and coupling ability of the threaded connection. Those and other objectives will become apparent to those of skill in the art from a review of the specification below.