| 摘要 |
An underreamer drill bit assembly including a pilot bit (82) and extendable underreamer arms (88) operatively connected to the pilot bit (82). The underreaming arms (88) have an extended position for underreaming, and a retracted position in which the overall diameter of the underreamer drill bi t assembly is less than the inside diameter of the well casing, permitting the entire bit assembly to be withdrawn through the well casing. In another aspe ct of the invention, the bit assembly is operatively connected to a dual wall pipe assembly (12). A supply of compressed air is conducted through the annulus (24) of the dual wall pipe assembly (12) to a down hole pneumatic hammer (13). Exhaust air from the down hole hammer is directed to the bit assembly for continuous removal of drilling debris through a central evacuation tube (43) of the dual wall pipe assembly. In another aspect of th e invention, a pressurized, incompressible fluid is injected into the annulus between the well casing and the downhole pneumatic hammer.
|
