Methods comprising identifying an interval of a subterranean formation to be treated, the interval comprising multiple portions, each portion having a different effective permeability; introducing a relative-permeability modifier into the interval; and, introducing a sealant into the interval. The relative-permeability modifier modifies the interval to have a more uniform effective permeability to water-based fluids. The relative-permeability modifier comprises a water-soluble polymer. The more uniform effective permeability of the interval allows for a more uniform treatment of the interval by the sealant than would be allowed without treatment of the interval with the relative-permeability modifier.