| 摘要 |
PCT No. PCT/GB93/01011 Sec. 371 Date Feb. 9, 1995 Sec. 102(e) Date Feb. 9, 1995 PCT Filed May 18, 1993 PCT Pub. No. WO93/23652 PCT Pub. Date Nov. 25, 1993.Apparatus for steering a drill bit at the end of a drill string within the borehole to selectively effect drilling along either a curved path or a substantially straight path comprises a first downhole motor assembly coupled to the drill bit and operable to rotate the drill bit to effect drilling, the assembly being arranged to angularly tilt the rotational axis of the drill bit relative to the axis of the section of the borehole being drilled, and a second downhole motor coupled to the first motor assembly to the drill string and operable to rotate the first motor assembly. In addition the apparatus includes a lock sub which is selectively actuable to (i) effect rotation of the drill bit by the first motor assembly while the first motor assembly is maintained at a defined axial orientation so as to cause drilling of the borehole along a curved path in the direction of tilt of the drill bit, or (ii) effect rotation of the drill bit by the first motor assembly while the first motor assembly is rotated by the second motor assembly so as to cause drilling of the borehole along a substantially straight path. Rotation of the drill string as a whole is not required to effect drilling in either mode.
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