Patent ID: 8757298

Claim:
A method for drilling boreholes in earth formations, comprising: rotating an outer drill bit with a primary power source about a primary axis of rotation and against an earth formation, the outer drill bit having an outer bit cutting face in cutting engagement with the earth formation and defining an inner bit chamber intersecting said outer bit cutting face and defining an inner bit chamber opening at said outer bit cutting face; with a secondary power source independently rotating an inner drill bit having an inner bit cutting face within said inner bit chamber with said inner bit cutting face located within said inner bit chamber opening and having formation cutting engagement with the formation simultaneously with said rotating said outer drill bit in cutting engagement with the formation; said rotating said inner drill bit being rotating said inner drill bit within said inner bit chamber about an axis of rotation that is laterally offset from said primary axis of rotation and developing an orbital pattern of formation cutting motion of said inner drill bit in cutting engagement with the formation about said primary axis of rotation responsive to rotation of said outer drill bit; and rotating said inner drill bit with said secondary power source independently of rotation of said outer drill bit and simultaneously with rotation of said outer drill bit with said primary power source.