Viscoelastic surfactant formulations and use in subterranean formations

A well treatment fluid comprising:

• an aqueous fluid;
• an acid;
• a zwitterionic surfactant with a general formula (I):
• where A is a C or SO, E1 is a linear or branched, saturated or unsaturated hydrocarbon chain having 1 to 4 carbon atoms, R1 and R2 are selected from alkyl groups of from 1 to about 4 carbon atoms, and R3 is a linear or branched, saturated or unsaturated alkyl chain having less than 22 carbon atoms or is —(CH2)nNHC(O)—R4, where R4 is a linear or branched, saturated or unsaturated alkyl chain having less than 22 carbon atoms and n ranges from 1 to 6;
• and
• a synergistic co-surfactant containing a primary, secondary, or tertiary amine;
• amido group;
• amine oxide;
• or combinations thereof, wherein the co-surfactant is present in an amount higher than 9.5 wt % and less than 22 wt % based on a total weight of the surfactants,
• wherein the zwitterionic surfactant and synergistic co-surfactant are added to the well treatment fluid as a viscoelastic surfactant formulation further comprising at least one miscible solvent, wherein a viscosity of the viscoelastic surfactant formulation is maintained within a range having a lower limit of more than 85 cP at temperatures ranging from 175° F. to 250° F. wherein the viscosity is measured using a Grace M5600 Viscometer at a shear rate of 100 s−1.