US Patent 7182157 Enlarging well bores having tubing therein

An underreamer for forming a cavity within a well bore includes a fluid motor having a first body and a second body arranged about a longitudinal axis. The first body is adapted to rotate about the longitudinal axis in relation to the second body when fluid is passed between the first and second bodies. The fluid motor further defines a longitudinal tubing passage adapted to allow passage of the fluid motor over a tubing string. At least one cutting arm is coupled to rotate with the first body, and radially extendable into engagement with an interior of the well bore to form the cavity.