Bentonite dispersed in fresh water produces a mud with good cuttings lifting capacity, good drilling rate, and usually adequate filtration control. These bentonite-water muds are commonly used as spud muds for drilling surface hole; however, they are sometimes used for drilling deeper.
Water quality is important in formulating a bentonite-water mud. Chlorides (Cl-) and hardness (Ca++ and Mg++) in the makeup water interfere with the hydration of the bentonite. Calcium ion concentration should not exceed 150 mg/L. If greater than 150 mg/L, it should be treated out with soda ash. Treatment with 0.1 lb of soda ash per barrel of water will remove approximately 100 mg/L of calcium ion. Magnesium hardness, on the other hand, is treated out with sodium hydroxide. At pH of 9.7 magnesium ions will have all been reacted with sodium hydroxide to precipitate Mg(OH)2. Chlorides, however, cannot be treated out of the makeup water. Less than 5000 mg/L chlorides will not seriously hamper hydration of commercial bentonite. When there is more than 20,000 mg/L chlorides, bentonite hydration is essentially prevented. Adding fresh water to reduce the chloride concentration becomes necessary to allow hydration.
In bentonite-water muds, viscosity can be increased by adding more bentonite or adding a bento-nite-extender polymer, or lime or soda ash. The pH is usually maintained in the 8.0 to 9.5 range with caustic soda. Caustic soda flocculates hydrated bentonite; but, this effect can be minimized by slowly adding the caustic soda to the mud while it is being vigorously agitated.
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