Chpater 11 - Specialized Downhole Drilling Equipment

Abstract

There are two downhole drilling devices used with air and gas drilling technology: down-the-hole hammer (DTH) and the progressive cavity positive displacement motor (PDM) drilling motor. The DTH can only be used with an air or other gas drilling operation with unstable drilling fluids. The PDM was originally developed for use with incompressible drilling fluids, but operates on unstable foam, aerated, and stable foam drilling fluids. PDMs are nearly always used to drill directional boreholes. There are two basic designs for the DTH. One design utilizes a flow path of the compressed air through a control rod down the center hammer piston and then through the hammer bit. The other design utilizes a flow path in the wall of the hammer housing. This type of hammer design allows for the flow of gas in this small annulus passage and then through the hammer bit. The most commercially successful positive displacement fluid downhole motor has been the progressive displacement “cavity” motor. The driving shaft of these motors is a rigid shaft composed of helical lobe repeating sections and is denoted as the rotor. Downhole pneumatic turbine motors are the only specialized downhole drilling equipment that can be used with dry compressed air (or other drilling gases).