Embodiments of the Present Invention relate to a reversibly thickenable non-polymeric fluid that has low viscosity in strong acid, gels when the acid concentration is reduced by only a small amount, and is subsequently decomposed by the acid. In particular it relates to an aqueous mixture of zwitterionic surfactants, inorganic acids, and organic acids Most particularly it relates to the use of this fluid as a diverting agent for easily-damaged sandstones, for example prior to matrix acidizing.