Aqueous drilling fluid additive, composition and process

Claim 1

In a method of drilling a well into a subterranean formation in which an aqueous drilling fluid containing clayey material is circulated into the well, the steps of forming a filter cake on the wall of the well to decrease loss of fluid from the drilling fluid which comprises: (1) admixing with said drilling fluid an amount of at least about 0.25 pounds per 42 gallon barrel of a terpolymer of: (a) a monovalent alkali metal salt of acrylic acid; (b) an hydroxy alkyl acrylate selected from the class consisting of 2-hydroxyethylacrylate, 2-hydroxybutylacrylate and 2-hydroxypropylacrylate; and (c) acrylamide, a 0.2% by weight aqueous solution of said terpolymer having a viscosity at about 25° C. of from between about 6 centipoises and about 85 centipoises, as determined at a shear rate of 511 reciprocal seconds, the amount of said monovalent alkali metal salt of acrylic acid utilized in said terpolymer being from between about 5 mole percent and about 62 mole percent, the amount of said hydroxy alkyl acrylate utilized in said terpolymer being from between about 2 mole percent and about 40 mole percent, the amount of said acrylamide utilized in said terpolymer being from between about 31 mole percent and about 91 mole percent; and (2) circulating said drilling fluid in said well.