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For direct circulation drilling and reverse circulation drilling, the major difference in drilling method is related to the direction of movement of drilling fluid. For direct circulation drilling, the drilling fluid is circulated from the drill stem and then flows up the annulus between the outside of the drill stem and borehole wall. The drilling fluid that carries the drill cuttings flows to the surface and the subsequent settlement pits. Pumps are employed to lift the cuttings free fluid back to the drill stem.
For reverse circulation drilling, the direction of flow of drilling flow is opposite to that of direct circulation drilling. Drilling fluid flows from the annulus between the drill stem and hole wall to the drill stem. The drilling fluid is pumped to an nearby sump pits where cuttings are dropped and settled.
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