Enlarged bore hole drilling method

The method of drilling enlarged bore holes in earth formation comprising:

• drilling a pilot hole into the earth formation by rotating a pilot bit on a dual pipe string having an outer fluid path defined between inner and outer pipes, while circulating drilling fluid through the bit causing drilling fluid and cuttings to flow from the pilot hole through the space between the pipe string and the bore hole wall;
• halting the circulation of drilling fluid and removing the pipe string and pilot bit from the bore hole;
• replacing the pilot bit with a bore hole enlarging bit having cutters expansible to a gauge in excess of the gauge of the pilot bit;
• moving the enlarging bit on the drill pipe to a selected location in the pilot hole spaced from the bottom of the pilot hole;
• expanding said enlarging bit cutters by supplying drilling fluid through said outer fluid path of said drill pipe to said enlarging bit;
• rotating said drill pipe and enlarging bit to drill an enlarged bore hole toward the bottom of said pilot hole while removing a portion of the drilling fluid and cuttings of earth formation through an inner fluid path defined by said inner pipe;
• ceasing the supply of drilling fluid;
• retracting the enlarging bit cutters;
• and removing said drill pipe and enlarging bit from the enlarged bore hole.