Abstract: A process for reducing the permeability to water of a thief zone of a porous and permeable subterranean petroleum reservoir by injecting a dispersion of polymeric microparticles in an aqueous fluid down a well and into the thief zone, wherein the polymeric microparticles comprise crosslinked copolymer chains having structural units derived from (i) a water-soluble or water-dispersible monomer with a betaine group, (ii) a water-insoluble monomer, and, (iii) a cross-linking monomer having at least two sites of ethylenic unsaturation, and the polymeric microparticles have a transition temperature above the maximum temperature encountered in the well and at or below the maximum temperature encountered in the thief zone and, and the polymeric microparticles expand in size in the thief zone when they encounter a temperature at or greater than the transition temperature so as to reduce the permeability of the thief zone to water.