Compositions including relatively low reactivity acids and having a pH of from about 2 to about 5, mixed with viscoelastic surfactants (VESs) and internal breakers may serve as fluids, in a non-limiting embodiment as drilling fluids, to open underground hydrocarbon reservoirs with carbonate contents of 10 wt % or above. The fluids initially have low viscosities. After the fluid flows out of the drill bit, the acids react with carbonates in the formation thereby increasing the pH of the fluids causing the VES to gel the fluid at the bottom of the hole and within the formation rock. Even when the subterranean formation contains naturally-occurring fractures, the viscosified fluid will reduce fluid loss into the formation. After drilling through the targeted formation, internal breakers in the viscosified fluids will break down the fluids to permit their removal, and production of the well with very little or no near well bore damage.