Drilling Apparatus Including Milling Devices Configured to Rotate at Different Speeds

摘要

In one aspect, an apparatus for forming a borehole is provided that in one embodiment may include a first milling device configured to rotate at a first rotational speed in a first direction, and a second milling device around the first milling device configured to rotate at a second rotational speed in a second direction, wherein the rotational first speed is greater than the second rotational speed. In another aspect, the first and the second milling devices may be configured to rotate in the same direction at different speeds.