High temperature stable fluid loss control system

Claim 1

A composition capable of imparting fluid loss control to aqueous systems comprising a mixture of: (a) a substantially water insoluble, solid, particulate silicate or alumino-silicate material which contains a major amount of kaolinite, halloysite, montmorillonite or illite or mixtures thereof, or attapulgite or sepiolite and being of a particle size such that at least about 90 percent by weight is capable of passing through a No. 20 U.S. Standard Sieve; in combination with (b) a chemical compound capable of converting to a higher oxidation state under alkaline conditions selected from the group consisting of C 1 -C 5 alkanols, unsubstituted or C 1 -C 3 alkyl, aryl or alkaryl substituted phenols, hydroquinone, tert-butyl catechol catechol, C 1 -C 15 alkanethiol, alkali metal, alkaline earth metal or ammonium salt of sulfite, bisulfite and nitrite, ferrous metal salts of chloride and sulfate and cuprous metal salts of chloride and sulfate; in combination with (c) a reaction product formed between a polymeric material having a weight average molecular weight of at least about 20,000 selected from the group consisting of polyvinyl alcohol and hydroxy C 1 -C 3 alkyl cellulose and at least from about 1 to 200 percent of stoichiometry of a cross-linking agent selected from the group consisting of a compound containing at least one aldehyde group therein or a compound capable of generating in situ at least one aldehyde or an epihalohydrin; the reaction between said polymeric material and said aldehyde carried out in an aqueous acidic medium having a pH of 5.5 or less and the reaction between said polymer and said epihalohydrin being carried out in an aqueous basic medium having a pH of at least about 9.5; wherein the weight ratio of component (a) to (c) is at least from about 0.75:1 to 3:1 and of component (b) to (c) is at least from about 0.2:1 to 1:1.