1. Field of the Invention
The invention relates generally to devices and methods for conducting liner drilling and subsequent completion of the drilled section by securing the liner into place by anchoring and cementing.
2. Description of the Related Art
In its basic form, a wellbore is drilled using a drill bit that is attached to a drill string fashioned of drill pipe. When the wellbore is drilled to an original desired depth, the drill string and bit are removed from the hole. Then steel casing is inserted into the borehole and cemented in place as a protective tubular sheath to prevent collapse of the borehole wall. The term “casing,” as used herein will refer to those protective sheaths that extend along a portion of the wellbore all the way to the surface. The well can then be drilled to deeper depths in successively smaller diameter intervals below the original depth. These lower intervals are then lined with wellbore liners. As used herein, the term “liner” will refer to those protective sheaths that extend along a portion of the wellbore, but do not extend all the way to the surface.
In addition to traditional drilling using drill strings made up of drill pipe, techniques have been developed recently for casing drilling and liner drilling. In casing drilling, the bottom hole assembly containing the drill bit is threaded to a section of casing and, after drilling, the casing is hung at the top of the wellbore. Liner drilling is a similar concept. In liner drilling, the liner to be cemented in serves as a part of the drilling string while traditional drill pipe usually forms the upper part of the drill string. The bit can be attached to the liner and the liner then rotated within the borehole. Alternatively, a mud motor is attached to liner and the mud motor is used to turn the bit while the liner remains stationary. When liner drilling is completed, the drill pipe portion of the drill string is detached from the liner and withdrawn from the wellbore. The liner portion of the drilling string remains in the borehole, set on the bottom of the hole and is later cemented into place. The bit and mud motor are also left in the hole.
A significant problem with this conventional liner drilling process is that the liner can deform by bending or corkscrewing under its own weight when set down on bottom. This is especially true of very long liners. If the liner is cemented in this condition, it will be permanently deformed and perhaps be unusable for passing large diameter tools through. For this reason, a number of “one-trip” liner drilling arrangements have been developed that incorporate liner hangers into the drilling string on the upper end of the liner so that the liner can be anchored to the pre-existing casing after cementing. An example of a “one-trip” liner drilling system is described in U.S. Pat. No. 5,497,840, issued to Hudson.
A major problem with “one trip” liner drilling systems is their ability to return drill cuttings to the surface of the wellbore. The liner portion of the drill string has a much greater diameter than traditional drill pipe. As a result, the annulus surrounding the liner portion is quite small, leaving little room for pumped down drilling mud and generated cuttings to return to the surface of the well. While this problem is inherent to the process of liner drilling, it is made substantially worse by the presence of any exterior components that extend outwardly into the annulus beyond the diameter of the liner. Thus, externally mounted hangers or packers, that might be used to hang the liner in tension from the casing or liner above could not be run in with the liner during the drilling operation without destroying the ability to drill and remove cuttings effectively during drilling. Thus, there is a need to be able to conduct liner drilling with minimal exterior components to allow annular bypass of returning drilling mud and cuttings.
The present invention addresses the problems of the prior art.