Direct mud-rotary drilling is a drilling method in which a fluid is forced down the drill stem, out through the bit, and back up the borehole to remove the cuttings as shown in Figure 9.15.2. The cuttings are removed by settling in a sedimentation tank or pond, and the fluid is circulated back down the drill stem. The drilling fluid can be a liquid, such water or mud (water with special additives, e.g., bentonite and polymers), or it can be gas, such as air or foam (air with additives, e.g., detergents) (Davis, Jehn, and Smith 1991).
Mud-rotary drilling is a flexible and rapid drilling method in all types of geologic materials and depth ranges. The circulating fluid serves to cool and lubricate the bit,
stabilize the borehole, remove the cuttings, and prevent the inflow of formation fluids, thus minimizing cross-contamination of aquifers. In addition, samples can be obtained directly from the circulated fluid when a sample-collecting device is placed in the discharge pipe before the settling tank.
Mud-rotary drilling, however, requires the introduction of some foreign liquids into the aquifer, which can compromise the validity of subsequent monitoring well samples. In addition, contaminants might be circulated with the fluid, and the collection of representative samples is difficult due to the mixing of drill cuttings. Other limitations of mud-rotary drilling include the inability to provide information on the position of the water table and the loss of drilling fluids in fractured materials.
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