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The waterflood sweep efficiency can be increased considerably by in-depth placement of a blocking agent. Sodium silicate is one of the few PLONOR chemicals applicable for water control. This paper highlights key results obtained from a research program on qualifying sodium silicate for offshore applications. The main findings of this work can be summarized as follows: Sodium silicate is an alkaline liquid containing nano-size particles with water-like viscosity and good buffer capacity.Sodium silicate should be diluted in brines with a low divalent ion concentration addressing the need of a sufficiently large preflush.Upon reaction, triggered by an activator, sodium silicate forms silicate aggregates and gel which reduce rock permeability.The reaction rate is controlled by the formation temperature and sodium silicate concentrations.The flow behaviour of reacted silicate aggregates is understood by the classical theory on fine particle transport through porous medium, which includes a velocity-dependent deposition rate.Coreflood experiments at high flow rates demonstrated both a good injectivity of non-reacted sodium silicate and dynamic reaction rates similar to bulk reaction rates. At low injection rates, the plugging time decreased and was explained by fines particle transport which increased the deposistion rate of silicate.Interpretations of coreflood experiments unfold the effect of cation exchange. It is concluded that large volumes of sodium silicate can be injected into offshore oil reservoirs. Prior to the injection, a preflush is needed and the silicate is to be diluted in desalinated water. The permeability reduction can be obtained either during dynamic injection or shut-in period. The design parameters involve temperature, velocity and concentration gradients.