Dowell Schlumberger Incorporated

Lindsay J Fraser

John L Burba, Iii

The present invention relates to a pseudoplastic fluid composition including water, a mixed metal layered hydroxide dispersed in the water to impart preselected rheological properties to the fluid and a fluid loss additive composition including one or more of a carboxymethyl substituted vegetable starch, a carboxymethylated cellulosic compound or mixtures thereof in an amount effective to inhibit loss of fluid and with the degree of substitution being selected so that the carboxymethyl substituted compound is soluble in the fluid at the concentration of use without materially adversely affecting the rheological properties of the fluid and so that the fluid is resistant to fermentation. The improved composition includes a crosslinked carboxymethylated vegetable starch which has a degree of substitution in the broad range from about 0.3 to about 0.8 and in a narrower range of about 0.4 to about 0.7 and an uncrosslinked carboxymethylated vegetable starch or a carboxymethylated cellulosic which has a degree of substitution in the broad range from about 0.3 to about 0.57 and in a narrower range from about 0.4 to about 0.5. The drilling method includes drilling while circulating the drilling fluid with the improved composition therein.

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Katy, TX, United States of America

Lindsay J Fraser