APPARATUS AND METHOD FOR MILLING A WELL CASING

摘要

Apparatus for milling the casing of an oil or gas well comprises a downhole assembly which is run into the well on a rotatable drill string. The assembly comprises a motor (6) the body of which is connected to the drill string and the output shaft of which is connected to a casing mill (9). In use, the drill string is rotated in one rotational direction and the motor (6) is operated to rotate the output shaft thereof in the same rotational direction whereby the speed of rotation of the mill (9) is faster than the rotational speed of the drill string. Because the rotational speed of the milling tool (9) cannot be less than the rotational speed of the output shaft of the motor (6) relative to the body thereof extreme variations in the rotational speed of the milling tool due to winding up of the drill string are avoided. The downhole assembly may incorporate a taper mill (11), stabilizers (10, 8 and 5) and a jet sub (4). The assembly may also incorporate a constant thrust device for producing a substantially constant thrust loading on the milling tool and one or more cement removing tools.