Mud Rotary
There are several different types of mud rotary drilling. The method DWR employs most often is called direct rotary drilling. Therefore, it is the only method that will be treated here.Direct rotary drilling uses a rotating drill bit at the end of a string of drill pipe. Drilling mud, a water-based drilling fluid that can be composed of all sorts of materials, is pumped into the drill pipe and escapes through the bottom of the bit. Drill cuttings are carried by the mud to the surface, where they settle out of the mud in a settling pit. The mud is then recirculated back into the borehole. Additional mud is added as the well gets deeper and mud is lost to the formation(s) being drilled.
Mud rotary drilling is used primarily in unconsolidated or semi-consolidated formations. Most of North Carolina's Coastal Plain aquifers are found in unconsolidated sands and in limestone formations. Mud rotary excels at this type of drilling. The mud provides effective cuttings removal while stabilizing the borehole walls at the same time.
After the borehole is drilled to the desired depth, the screen and casing for the well are installed in the hole. The mud is partially or fully pumped from the well and borehole. Then, the gravel pack, bentonite plug, and grout seal are emplaced in the well.
Mud rotary drilling, while effective in the Coastal Plain, is often dirty and can sometimes be slow, depending on the formation material. For instance, a tri-cone roller bit that excels at quick drilling in sands, can be your worst nightmare if you then hit a tight clay formation. A drag bit that is great for drilling in clays will get you next to nowhere when drilling in hard limestone. Frequently changing bits is very time consuming, especially as the borehole deepens. A thorough knowledge of the geology of the area in which you are drilling is the best way to optimize your drilling by having the appropriate bits on hand and knowing when you will have to use them.