US Patent 7345012 Foamed viscoelastic surfactants

A method for increasing the foam stability of foams made with cationic, zwitterionic, and amphoteric viscoelastic surfactant fluid systems by adding an effective amount of an amphiphilic polymeric foam stabilizer containing at least one portion that is a partially hydrolyzed polyvinyl ester or partially hydrolyzed polyacrylate. The foam stabilizer allows foams to be used at temperatures at which the unfoamed fluids would not have stable viscosity, and allows use of lower viscoelastic surfactant concentrations in the liquid phase. Preferred surfactants are betaines and quaternary amines. The fluids are useful in oilfield treatments, for example fracturing and gravel packing.