TABLE OF CONTENTS There are two downhole drilling devices used with air and gas drilling technology. These are the down-the-hole hammer (DTH) and the progressive cavity positive displacement motor (PDM) drilling motor. The DTH is unique to air and gas drilling. It can only be used with an air or other gas and with unstable (mist) drilling fluids. The PDM was originally developed for use with incompressible drilling fluids, but will operate on unstable foam, aerated, and stable foam drilling fluids. PDMs that are operated on these drilling fluids have sealed bearing packages and bypasses that are unique to light drilling fluid operations.
There are two basic designs for the DTH. One design utilizes a flow path of the compressed air through a control rod (or feed tube) down the center hammer piston (or through passages in the 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 wall small annulus passage (around the piston) and then through the hammer bit. Some hammers combine these two design concepts. DTHs require rotation of the drill string in order to allow advance of the drill bit in the rock formation. This rotation allows the insert teeth to cover the rock face with rock impacting drill bit inserts and remove the rock cuttings with the exhaust gas from the piston chamber.
Figure 11-1 shows a schematic of a typical control...
