Extendable arm for formation evaluation measurement-while-drilling tool

摘要

A measurement-while-drilling formation evaluation tool incorporated into a drill collar or sub includes an arm 24 for extending outwardly from the tool and maintaining direct contact between a sensor (22) mounted on the arm, and a borehole wall (18). The arm may be extended outwardly against the formation when drilling fluid is circulated through the centre of the sub and retracted when the mud circulation ceases. Alternatively the arm may extend and retract using an electric actuator. The arm may be deployed intermittently using electric actuating means thereby decreasing drilling interference (caused by the arm) and avoiding damage caused by accidents involving the use of a nuclear source in the sensor.