Methods of controlling viscosity of acids

A method of treating a subterranean formation penetrated by a wellbore, comprising:

• injecting into said formation a treatment fluid comprising:
• at least one viscoelastic surfactant (VES) at a concentration between about 0.1 percent by weight and about 10 percent by weight;
• a formation-dissolving agent selected from the group consisting of hydrochloric acid, formic acid, acetic acid, lactic acid, glycolic acid, sulfamic acid, malic acid, citric acid, tartaric acid, maleic acid, methylsulfamic acid, chloroacetic acid, aminopolycarboxylic acids, 3-hydroxypropionic acid, polyaminopolycarboxylic acids, salts thereof, and mixtures thereof, and
• a rheology modifier comprising an aliphatic polyglycol ether and a metal salt;
• wherein, after the treatment fluid is injected into said formation, the rheology modifier changes the viscosity of the treatment fluid as a function of an extent of reaction between the formation-dissolving agent and said formation.