With increasing environmental constraints on the use of mineral oil-base muds, attention has turned towards water-base systems. Recent studies have shown that cationic polymer mud systems exhibit a superior degree of inhibition when compared with other water-base muds. Previous considerations of these mud systems were troubled by high toxicity and operational problems. A newly developed, field-practical, cationic polymer mud system gives excellent shale inhibition without sacrificing environmental acceptability.

Cationic polymers provide more effective shale inhibition by attaching to the negatively charged clay surfaces more strongly than anionic polymers. However, formulating practical drilling fluids using cationic polymers is difficult as they tend to react with bentonite, anionic polymers, and weighting agents causing severe flocculation and precipitation. This can cause problems with product compatibility and high mud weights.

Shale encapsulation using this new system is provided by a high molecular weight polymer while swelling inhibition is achieved by using a low molecular weight polymer and/or inorganic salts. Acute aquatic toxicity tests show this cationic polymer mud system is much more environmentally sensitive compared to those previously used. The product incompatibility problem was overcome so that common additives such as bentonite, anionic polymers, and barite can be added for controlling filter-cake quality, fluid loss, and mud weight, respectively. The stability of the system is maintained using non-ionic polymers. Laboratory studies show that the new cationic polymer mud system is considerably more inhibitive than conventional water-base muds.

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