A process for enhancing the productivity of a formation consists of introducing into the formation a viscoelastic fluid which contains at least one surfactant, at least one quaternary amine polyelectrolyte, water, and a non-aqueous solvent. The surfactant forms aggregation structures or vesicles. The fluid, which significantly enhances fluid viscosity and thermal stability, is particularly effective as a diverting fluid to divert an acid treatment package from a high permeability or damaged portion of a formation to a low permeability or undamaged portion of a formation as well as a fracturing fluid. In addition, the fluid is useful for sand control completion.