Many drilling methods are available, and in some cases, any one of them is suitable for installation of a particular well. However, just about every well drilling technique has its limitations, and those confines should be considered at the outset of any well drilling project.
Mud Rotary
Figure 2. Generalized diagram of the direct air rotary drilling method. Compressed air is used to remove drilled cuttings instead of drilling fluid. The compressed air (commonly with injected water, soap, or polymer) is circulated down the center of the drill string to circulate cuttings (blue arrows) from the drill bit back to the land surface.
The direct mud rotary drilling method is one of the most common methods and is suitable for a broad range of well designs and hydrogeologic conditions. There is essentially no practical limit on the depth capabilities of direct mud rotary boreholes. Direct mud rotary drilling rigs can be equipped with a variety of drill bits (tricone button bits, tricone mill tooth bits, drag bits) as appropriate for hard or soft formations.
As shown in Figure 1, drilling fluid is circulated through the mud pits (or above-ground tanks) and the borehole to remove cuttings and stabilize the hole. The drilling fluid is pulled from the pits by a mud pump and delivered up the standpipe to the kelly hose, which conveys the drilling fluid down through the center of the drill pipe to the bit. The drilling fluid then carries the cuttings back to the land surface as it flows up the annulus outside the drill pipe.
Since this drilling method involves keeping the borehole full to the land surface with drilling fluid, the hole can be maintained in a stabilized condition even when unconsolidated or friable formations are encountered. The properties of the drilling fluid must be consistent with the formation being drilled to achieve borehole stability, but the chemical and physical properties of the drilling mud can be monitored and adjusted as needed.
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