Purpose. Substantiation of technology for creation of a water-blocking zone below an oil reservoir and calculation of the proper composition of a gel-forming compound based on sodium silicate, in order to reduce water cut in production wells. Methodology. The goal of the work was achieved by conducting theoretical and experimental studies on technological processes of water blocking in an oil reservoir, and by identifying patterns of gel formation of sodium silicate and hydration of a micro-cement solution in reservoir conditions on full-scale models. The gel compound included sodium silicate (Na2SiO3, also referred to as liquid glass) and an aluminum salt cross-linker (AS-1). The plugging material mixture of Portland micro-cement and sodium silicate contained calcium oxide, to allow expansion, and a GL-1 reaction inhibitor. The criteria for assessing the creation of a reliable water-blocking zone in an oil reservoir are: the mobility of the aqueous solution of the gel-forming compound during its movement from the wellhead to the bottom of the well, the low permeability of the zone following its creation, and the sufficient strength of the non-shrink micro-cement in the annulus of the well. Findings. A new technology is suggested used to create a water isolation zone is a gel-forming compound based on sodium silicate, which provides a significant reduction of water cut in oil production. It is found that perforation of production string below the oil reservoir at the level of the water-saturated zone followed by injection into a well through perforated channels, the mixture of fresh water and the gel-forming compound prevents water inflow to the bottom of the well. Experiments established that with a gelation time of 2 hours at a temperature of 80 C, the viscosity of the gel is in the range of 1.22.9 Pas, and the density is 10801109 kg/m3. These values for the viscosity of the gel allow transportation from the top of the well to the bottom with the least resistance to motion. Following gelation time, the viscosity increases significantly, and after 3 days the gel viscosity reaches a range of 3.46.7 Pas. The values indicated for the viscosity of the gel are much greater than those of oil. Therefore, the proposed gel-forming compound provides a reliable water shut-off zone at the bottom of an oil reservoir, and prevents the influx of water at the bottom of a well. Originality. The proposed sodium silicate compound allows for the creation of a reliable water shut-off zone and an enhanced grouting material, based on the combination of sodium silicate and micro-cement, which together provide a significant reduction in water cut in wells during oil production. Practical value. A method for studying technological processes of oil reservoir water-blocking has been devised and the rational composition of gel-forming compound and micro-cement grout slurry with an expanding additive and a reaction retarder in reservoir conditions on full-scale models has been established. The application of the research results in oil fields allows reduction of water cut in production wells to 010%, against existing values of 7090%, and an increase in flow rate in producing wells by 2030%.