Wettability alteration is related acidizing damage mechanisms, especially useful in sandstones. In tight formations, acid that is introduced into the formation may be retained by capillary forces. Gas or oil production rates may be severely impaired. Proper selection of surfactant additives is necessary to avoid acid blocking. This is quite often difficult to assess without the benefit of core flow testing with a representative formation core. In most cases, though, proper additive selection will greatly reduce the potential for acid blocking. Acid blocks may be removed by subsequent treatment with an aromatic solvent that contains mutual solvent and surfactant.