Drilling fluid and method

Claim 1

A viscous aqueous alkaline drilling fluid containing thickeners consisting essentially of the combination of: (a) a solubilized addition copolymer of an ethylenically unsaturated carboxylic acid with at least one other addition polymerizable ethylenically unsaturated monomer, said copolymer having a molecular weight, Mv, of from about 250,000 to 5,000,000 and having an equivalent weight per carboxyl group as defined herein of from over 100 to about 250, and (b) a water soluble macromolecular polysaccharide capable of thickening aqueous systems, said polysaccharide selected from at least one hydroxyethyl cellulose and hydroxypropyl cellulose and having a thickening efficiency such that a solution of 2 percent by weight in distilled water provides a fluid having a Brookfield viscosity at 22° C. of from 3,000 to 200,000 centipoises, the pH of the system being alkaline and from about 7 to 14, the weight ratio of (a) to (b) being from 10 to 90 parts of (a) to from 90 to 10 parts of (b), the quantity of (a) plus (b), in the drilling fluid, being between 0.1 to 5 pounds per barrel of drilling fluid and being such as to effect a Fann viscosity of between 5 and 30 cps. at 600 r.p.m. and 22° C., the fluid components remaining chemically stable under alkaline conditions and being effective in salt brines, and the alkalinity is provided by at least one of sodium carbonate and sodium hydroxide.