According to one embodiment, a treatment fluid for a well includes: (a) a water-soluble polymer, wherein the water-soluble polymer comprises a polymer of at least one non-acidic ethylenically unsaturated polar monomer; (b) an organic crosslinker comprising amine groups, wherein the organic crosslinker is capable of crosslinking the water-soluble polymer; (c) a Lewis acid; and (d) water; wherein the treatment fluid is a crosslinkable polymer composition. According to another embodiment, a method for blocking the permeability of a portion of a subterranean formation penetrated by a wellbore is provided, the method including the steps of: (a) selecting the portion of the subterranean formation to be treated; (b) selecting estimated treatment conditions, wherein the estimated treatment conditions comprise temperature over a treatment time; (c) forming a treatment fluid that is a crosslinkable polymer composition comprising: (i) a water-soluble polymer, wherein the water-soluble polymer comprises a polymer of at least one non-acidic ethylenically unsaturated polar monomer; (ii) an organic crosslinker comprising amine groups, wherein the organic crosslinker is capable of crosslinking the water-soluble polymer; (iii) a Lewis acid; and (iv) water; (d) selecting the water-soluble polymer, the crosslinker, the Lewis acid, and the water, and the proportions thereof, such that the gelation time of the treatment fluid is at least 1 hour when tested under the estimated treatment conditions; and (e) injecting the treatment fluid through the wellbore into the portion of the subterranean formation.