Boreholes are formed in subterranean earth formations for a variety of purposes, such as oil and gas extraction and geothermal energy extraction. Such boreholes are typically formed by drilling the borehole into and through subterranean formations using a drill bit, such as an earth-boring rotary drill bit. Different types of earth-boring rotary drill bits are known in the art including, for example, fixed-cutter bits (which are often referred to in the art as “drag” bits), rolling-cutter bits (which are often referred to in the art as “rock” bits), diamond-impregnated bits, and hybrid bits (which may include, for example, both fixed cutters and rolling cutters). The drill bit is rotated and advanced into the subterranean formation. As the drill bit rotates, the cutters or abrasive structures thereof cut, crush, shear, and/or abrade away the formation material to form the borehole (also referred to as a “wellbore”). The drill bit is coupled, either directly or indirectly, to an end of what is referred to in the art as a “drill string,” which comprises a series of elongated tubular segments connected end-to-end and extends into the borehole from the surface of the formation. Often various tools and components, including the drill bit, may be coupled together at the distal end of the drill string at the bottom of the borehole being drilled. This assembly of tools and components is referred to in the art as a “bottom hole assembly” (BHA).
The drill bit may be rotated within the borehole by rotating the drill string from the surface of the formation, or the drill bit may be rotated by coupling the drill bit to a downhole motor, which is also coupled to the drill string and disposed proximate the bottom of the borehole. The downhole motor may comprise, for example, a hydraulic Moineau-type motor having a shaft, to which the drill bit is mounted, that may be caused to rotate by pumping fluid (e.g., drilling mud or fluid) from the surface of the formation down through the center of the drill string, through the hydraulic motor, out from nozzles in the drill bit, and back up to the surface of the formation through the annular space between the outer surface of the drill string and the exposed surface of the formation within the borehole.
In some applications, it may be desirable to enlarge a section of a borehole that has been drilled by a drill bit. Various tools have been developed for enlarging the diameter of a previously drilled borehole. Such tools include so called “reamers,” which comprise a tubular body carrying one or more blades on the lateral sides of the tubular body. The blades have cutting elements or other cutting structures thereon, which remove formation material from the sidewalls of the borehole as the reamer is rotated within the borehole. The reamer may be incorporated in the BHA vertically above a pilot drill bit. In this configuration, as the BHA is advanced into the formation, the pilot drill bit drills the borehole, and the reamer, which follows the pilot drill bit through the formation, reams (i.e., enlarges the diameter of) the borehole previously drilled by the pilot drill bit. Of course, a reamer may be employed anywhere along a drill string as needed or desirable.