A specialty drill rig, with dual top drives for a dual-member drill string, mechanically powers the steerable down-hole rotary-drilling assembly. The down-hole assembly is much like that of a mud motor but without its fluid-driven power section. The rotary bit (usually a tri-cone bit) is continuously rotated by the inner member of the dual-member drill string. The outer drill-string member is used to control steering by orienting the bent housing (also the bearing support for the rotating bit and home for the tracking transmitter) in the proper direction. Straight drilling is accomplished, much as with a mud motor, by slowly rotating the outer pipe. The tracking transmitter is closely positioned behind the bit, providing more timely indication of steering response than possible with most mud motors. The inner drive, however, prohibits use of non-walkover tracking systems.
The inefficiencies of drilling-fluid pumps and down-hole mud motors are not factors with this mechanically driven system. Greater torque can be applied to the bit than with comparable-size mud motors. Only the drilling-fluid flow rate needed to clear cuttings from the hole is pumped. Thus, fluid requirements are substantially lower, and the cleaning system can be much smaller or possibly replaced with a vacuum system. The mechanically driven system is effective for short bores in soft to medium-hard rock, where limited work space may preclude use of a large rig and the pump needed for driving a mud motor. The mechanically driven down-hole system is not as efficient as a mud motor system for large-diameter, long bores in hard rock.
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