Abstract: The permeability of an underground formation near a wellbore can be substantially decreased by the injection of one or more slugs of an aqueous solution containing an in situ forming gel of lignosulfonate and a monomer of acrylamide or acrylic acid, initiated by persulfate or hydroxylamine and a relatively small amount of a metal salt having a polyvalent cation. The gel can be formed in the formation by delaying the mixing of all of the gel components into the aqueous solution until just prior to injection or splitting up the components into two aqueous solutions so that they are not mixed until they are in the formation.