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This paper describes results from laboratory experiments and field tests, using basic Na-silicate. The work described has been performed as a part of the programme to qualify chemical systems for large-scale in-depth permeability modification processes in the North Sea.Gelling properties of basic silicate solutions have been measured as a function of pH, silicate concentration and temperature. The gelation times decrease with decreasing pH and increasing concentration. Increasing temperature will also give shorter gelation times. Adsorption of silicate is found to be linearly increasing with increasing silicate concentration.The rock buffer capacity is found to be strongly dependent on mineralogy. As an example, kaolinite has a buffer capacity more than ten times higher than other minerals. Both adsorption and buffer capacity will strongly influence the composition and gelation properties of the silicate solution in an in-depth treatment.In total, three field tests have been performed. The test area was chosen on the basis of Gullfaks reservoir data. From a one well formation test, field information was obtained on silicate adsorption and buffer capacity. The field results confirmed the laboratory data. The other two field pilots were run to obtain information on an optimum injection scheme for injection of silicate solutions into reservoirs with brines with high salinity and hardness. A preflush of fresh water was found to condition the critical reservoir parameters and allow for good injectivity of the basic silicate solution. The potential of water shut-off treatments of high watercut producers was examined in two well tests. The wells produced before treatment at a watercut of 96%. In one of the wells the watercut was reduced to less than 10% at the same fluid production rate and the watercut has been lower than 20% for more than eight months after treatment.