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Abstract

This invention provides a drilling fluid containing additives composed of copolymers which are obtainable by an industrially favorable process and superior in heat-resisting property. Even though the additives are used in a relatively small amount, such an amount sufficiently prevents changes in the properties of drilling fluid due to various kinds of hard water components being contained in underground water or cement as well as due to sea water, and the additives are able to display the properties of drilling fluid constantly regardless of presence or absence of salts. Drilling fluid additives of the present invention are composed of a water-soluble copolymer (A) having an average molecular weight of 1,000-50,000. The water soluble copolymer (A) is composed of 40-99.5 mole percent of an unsaturated carboxylic acid monomer (I), as shown in the following general formula (I), and 0.5-60 mole percent of an unsaturated (meth)allylether monomer (II), as shown in the general formula (II). ##STR1## In the formula (I), each of A.sup.1 and A.sup.2 independently represents hydrogen, methyl, or --COOX.sup.2 and A.sup.1 and A.sup.2 are not COOX.sup.2 at the same time. A.sup.3 represents hydrogen, methyl, or --CH.sub.2 COOX.sup.3 and, in a case that A.sup.3 is --CH.sub.2 COOX.sup.3, each of A.sup.1 and A.sup.2 independently represents hydrogen or methyl. Each of X.sup.1, X.sup.2, and X.sup.3 independently or at the same time represents hydrogen, monovalent metal, divalent metal, ammonium, or organic amine. In the formula (II), R.sup.1 represents hydrogen or methyl, and Z represents hydroxyl; a sulfonic acid group or its monovalent metal salt, divalent metal salt, ammonium salt, and organic amine salt; a phosphoric (or phosphorous) acid group or its monovalent metal salt, divalent metal salt, ammonium salt, and organic amine salt.

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First Claim