Compositions Including Relatively Low Reactivity Acids, Mixed with viscoelastic surfactants (VESs) and internal breakers may serve as drill-in fluids to open underground hydrocarbon reservoirs with carbonate contents of 10 wt % or above. The drill-in fluids have low viscosities in the drilling pipe. After the fluid flows out of the drill bit, the acids react with carbonates in the formation thereby increasing the pH of the drill-in fluids causing the VES to gel the fluid at the bottom of the hole and the downhole annulus between the drilling pipe and the formation rock. The viscosified drill-in fluid will reduce fluid loss and will carry no dissolved drilling debris to the surface. After drilling through the targeted formation, the internal breakers in the viscosified drill-in fluids will break down the fluids to permit their removal, and the well is ready to produce with very little or no near well bore damage.