Reverse circulation drilling system with bit locked underreamer arms

Abstract

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drill bit assembly is less than the inside diameter of the well casing, permitting the entire bit assembly to be withdrawn through the well casing.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

What is claimed is:  
     
         1 . A drill bit assembly for use with a down hole pneumatic hammer, comprising: 
 a driver;    a pilot bit having a shank, a peripheral drilling surface, a lower surface, and a bore extending from the lower surface generally upwardly through the shank;    coupling means rotatably coupling the driver to the pilot bit;    underreamer arms rotatably mounted intermediate the driver and the pilot bit and including underreaming surfaces positionable outside the pilot bit perimeter;    cam surfaces engageable with corresponding surfaces on the underreamer arms for deployment of the underreaming arms into an extended position, for locking the underreaming arms into the extended position, and for retracting the underreamer arms from the extended position by rotational movement between said driver and said pilot bit;    surfaces defining at least one air passageway betweenan outer surface of the driver and the bore in the pilot bit.