Saponified fatty acids as viscosity modifiers for viscoelastic surfactant-gelled fluids

A method for affecting the viscosity of an aqueous fluid gelled with a viscoelastic surfactant (VES) comprising:

• providing an aqueous fluid gelled with at least one amine oxide VES;
• adding to the aqueous fluid, in any order, a fatty acid, an alkaline compound and a water soluble base, where the alkaline compound is selected from the group consisting of alkali metal halide salts, alkaline earth metal halide salts, ammonium salts and combinations thereof, where the fatty acid is selected from the group consisting of those found in plant oils and animal oils, where the alkali or alkaline earth metal halide salt has a cation selected from the group of consisting of Na+, K+, Li+, Cs+, Ca+, Mg+, Ba+, Sr+, and mixtures thereof, and the ammonium salt is selected from the group consisting of ammonium bromide, ammonium iodide, ammonium bicarbonate and ammonium nitrate, and where the water soluble base is selected from the group consisting of alkali metal bases, ammonia, ammonium hydroxide, methylamine, trimethylamine, ethylmethylamine, ethanolamine, triethanolamine, urea, choline, and mixtures thereof;
• and
• forming in situ from about 50 to about 20,000 ppm of a soap reaction product of the fatty acid with the alkaline compound in the presence of the water soluble base;
• where the soap reaction product is present in an amount effective to increase or decrease the viscosity of the gelled aqueous fluid.