Drilling with casing and retrievable drill bit

摘要

Wellbores are formed in the earth with elongated, tubular drillstems which include retrievable bit and drive motor assemblies. The retrievable bit assembly includes a body having locking dogs engageable with cooperating recesses formed in a sub at the bottom of the drillstem. The bit assembly includes radially extendable and retractable arms with cutters thereon for forming the wellbore to a diameter greater than the drillstem, but whereby the arms may be retracted to withdraw the bit assembly through the drillstem with wireline retrieval apparatus or the like. The wellbore may be drilled with a tubular drillstem comprising wellbore casing with a retrievable bit and motor assembly connected to the lower end of the drillstem and retrievable through the drillstem by the wireline retrieval apparatus whereby the casing may be left in the wellbore upon completion of drilling operations.