| 摘要 |
A 2-stage underreamer for use in a subterranean wellbore comprises a unitary body (2) having upper and lower blades (5A, 5B) and (6A, 6B) mounted in slots (3, 4) formed perpendicular to each other in the body. Separate upper and lower piston assemblies are provided for driving the blades (5A, 5B) and (6A, 6B) respectively from a storage configuration in which they are located substantially completely within the slots (3, 4) to a use position as illustrated in the figure. The blades each preferably comprise a steel arm having mounted thereon a base of tungsten nickel cobalt matrix in which diamond and/or tungsten carbide inserts are provided. A signal is given to the surface that the blades are in their extended position by changing the mud.
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