Importance

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Abstract

A method and apparatus for reaming or enlarging a borehole using a bi-center bit with a stability-enhanced design. The cutters on the pilot bit section of the bi-center bit are placed and oriented to generate a lateral force vector longitudinally offset from, but substantially radially aligned with, the much larger lateral force vector generated by the reamer bit section. These two aligned force vectors thus tend to press the bit in the same lateral direction (which moves relative to the borehole sidewall as the bit rotates) along its entire longitudinal extent so that a single circumferential area of the pilot bit section gage rides against the sidewall of the pilot borehole, resulting in a reduced tendency for the bit to cock or tilt with respect to the axis of the borehole. Further, the pilot bit section includes enhanced gage pad area to accommodate this highly-focused lateral loading, particularly that attributable to the dominant force vector generated by the reamer bit section, so that the pilot borehole remains in-gage and round in configuration, providing a consistent longitudinal axis for the reamer bit section to follow.

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First Claim