In simultaneous drilling and casing operations, a section of casing follows the drill bit into the borehole. Underreamers located upstream of the drill bit and downstream of the end of the casing are used to enlarge the hole when necessary, such as when an obstruction in the formation is encountered. Such underreamers typically rely on springs or hydraulic pressure to actuate the roller cutters from a retracted, inoperative position to an extended, operating position. When they jam, the underreamers are expensive to remove, with the removal process sometimes requiring the withdrawal of the casing and the associated risk of compromising the integrity of the hole.
Examples of a trailing underreamer used in conjunction with a casing are provided in U.S. Pat. No. 6,419,033 to Hahn et al., U.S. Pat. No. 6,920,944.B2 to Eppink et al., U.S. Pat. No. 6,929,076 to Fanuel et al. and U.S. Pat. No. 7,048,078 to Dewey et al.
It is an object of the present invention to provide an underreamer that does not require springs or hydraulic actuation and that provides an effective means to enlarge a hole into which a casing is to be inserted. More generally, it is an object of the invention to provide a means of enlarging a hole past a pipe inserted into the hole and to a larger diameter than the pipe.
Other objects of the invention will be appreciated by reference to the disclosure and claims that follow.