Process

A process for producing a suspension of temperature-sensitive water-swellable microgel particles having a solids loading of at least 10% wt/v comprises polymerizing, in a reactor vessel containing a polar solvent and an anionic surfactant, one or more temperature-sensitive monomers, optionally in the presence of one or more non-temperature sensitive co-monomers, with an organic cross-linking agent and with from 0.25% to 2.5% wt/wt (with respect to the total monomer including optional co-monomer) of a free radical initiator; wherein the initiator is fed into the reactor vessel at a steady rate over at least 10% of the period required for the polymerization to reach 90% completion. Also shown is a method of modifying the permeability of water in a subterranean hydrocarbon reservoir.