Na-bentonite has been used in drilling fluids for oil and gas exploration since the 1920s (Fig. 3). It is also a principal component in fluids used for drilling small-diameter holes for water, mineral exploration, and environmental-monitoring wells. In addition, it is used in the relatively new technique of horizontal directional drilling to place utility cables and pipelines under buildings, streets, highways, and environmentally sensitive areas without the need for costly and disruptive surface trenching (Fig. 4).

These fluids are particularly suitable for drilling applications because of their ability to lubricate the drill pipe and cutting head, and suspend drill cuttings so they can be easily removed from the borehole (Darley and Gray 1988). The bentonite in the fluid also functions to limit loss of the fluid into the soil and rock around the drill hole. In non-water-based drilling and in many emulsion-based fluids, organically modified bentonites offer many of the same properties.

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