| 摘要 |
Joints of drill pipe are provided, formed of inner and outer tubes which are co-extensive and radially spaced apart. The joints are connected to form a drilling string having inner and outer tubular members having drive and bit ends. The drilling string extends from a generally horizontal mast. A bent sub assembly is attached to the bit end of the outer tubular member. The drill bit is attached to the bit end of the inner tubular member by a downhole drive assembly. A separate rotational drive assembly is provided at the mast for each of the tubular members. Means are provided at the mast for axially advancing and retracting the drilling string, so as to apply push and pull forces through the outer tubular member. The drilling mud is brought into the annulus, between the tubular members, at the drive end of the string and is crossed over through a port into the downhole drive assembly immediately adjacent the bit. By having the bent sub assembly attached to the normally nonrotating outer tubular member, the effect of back-torque arising from the bit is avoided and better directional control is achieved. By using the annulus as the supply path for the drilling fluid and keeping the bore of the inner tubular member empty and non-obstructed, a directional survey instrument can be brought within about 6 feet of the drill bit to obtain accurate directional surveys. The assembly is adapted, when drilling directionally, to steer the bit with the stationary outer tubular member and bent sub assembly and to drill with the rotating inner tubular member and attached downhole drive assembly and bit.
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