Problems with bentonite quality and bentonite-quality-control methods have consistently been observed in the field in recent years. These problems are investigated and proposals are made on how to mitigate them. A number of commercial bentonites have been analyzed and their performances evaluated (1) to determine to what extent quality variations performances evaluated (1) to determine to what extent quality variations occur between commercial bentonites from different manufacturers, (2) to determine the cause(s) of these variations, and (3) to develop a proper quality-control method for bentonite. The results show significant variations in quality between Oil Companies Materials Assn, (OCMA) and API grade bentonite. The main cause of these variations is the different type and amount of extending chemical added. Differences in rheology and fluid loss between bentonite suspensions in fresh water, seawater, and hydrogen-peroxide (H2O2) solutions give a quantitative measure of bentonite extension. Differences in performance before and after hot-rolling do the same.
Commercial bentonite (sodium montmorillonite) is used worldwide as a drilling-fluid additive. Its main functions are to viscosity the mud and to reduce fluid loss to the formation. We have noticed a general decline in bentonite quality over the past years that is reflected not so much in noncompliance with OCMA or API specifications, but rather in an unpredictable performance in a number of applications: (1) bentonite properties change after longterm storage; (2) addition of bentonite to potassium-chloride (KCl)/polymer muds is ineffective in building potassium-chloride (KCl)/polymer muds is ineffective in building viscosity;, (3) flocculation of cement slurries occurs in cases where bentonite is used as an extender; (4) properties of bentonites change significantly from one batch to properties of bentonites change significantly from one batch to another; and (5) poor and/or unpredictable performance of bentonite has been reported for drilling under harsh conditions. As a result of these observations, an investigation mainly started to determine whether significant quality variations occur between commercial bentonites and, if so, to determine their nature, and to develop a proper bentonite quality-control method.