Patent ID: 7942215

Claim:
A method of drilling into a subterranean formation comprising: drilling a wellbore into a subterranean formation with a drilling fluid comprising water, at least one viscoelastic surfactant (VES) and at least one acid selected from the group consisting of organic acids, mineral acids and combinations thereof, where the VES is present in an amount effective to increase the viscosity of the drilling fluid but for the presence of the at least one acid; and at least partially consuming the at least one acid from the drilling fluid by reaction of the acid with a mineral in the subterranean formation, thereby increasing the pH of the drilling fluid to an extent sufficient to increase the viscosity of the drilling fluid by the action of the VES to inhibit fluid loss into the formation; where the mineral is selected from the group consisting of a carbonate, an evaporite, a salt dome, shale, and combinations thereof.