Horizontal directional drilling (HDD) uses a combination of moderate-pressure, low-volume fluid and mechanical cutting to advance most horizontal boreholes.
Using a fluid-jet cutting system that can generate up to 2000 psi, the system is optimized to drill through most soil materials, including compact to dense (firm to hard) sand, silt, and clay formations.
Drilling mud is selected on the basis of soil conditions and is generally a polymer product specifically designed for horizontal applications. These types of mud offer several advantages over bentonite or other high-solids drilling muds:
Where soil materials are too dense to be cut by the fluid-cutting system, Directed Technologies Drilling, Inc. uses downhole mud motors and rotary drilling methods to advance boreholes through gravel formations, coral reefs, even boulders or bedrock, depending upon the lithology of the formation. The mud motor is installed at the end of the drill string on a bent sub (typically 3 degrees). The motor operates from the flow of drilling mud and spins the tri-cone bit. The borehole direction is controlled by orientation of the bent sub. Predictably, mud motors require a considerable mud flow and generally require a larger drill rig and more complex setup.