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A silica gel was applied in a porous gas reservoir, with the purpose of testing mitigation and remediation of CO 2 leakage from geological storage reservoirs. The gel has a high strength and a very low water-like viscosity, that extends its applicability to small pore diameters and low permeability media. The gel was prepared and applied on-site with oilfield equipment. Mixing was upscaled from laboratory- to field-scale, including one unsuccessful attempt. Environmental concerns and additional health and safety requirements were modest as the formulation was composed only of the non-toxic commercial silica-based product Betol K28 T, acetic acid and fresh water. A well was selected and prepared in the Becej natural CO 2 field in Serbia, the well and reservoir were prepared and the gel was placed into a 600 m deep CO 2 - and CH 4 -bearing sandstone layer. The reservoir was selectively sealed to the gas cap through a fast, CO 2 -selective gelation, while the hydraulic pathways of the liquid-filled part of the reservoir remained open. In the regions where no CO 2 was present the gelation reaction was slower and the kinetic was temperature-accelerated. A numerical model was used to simulate the impact of the pre-injection operational procedures and to quantify the impact on the temperature-dependent gelation time. The workflow aligns the needs of a research project with the interests and practical priorities of an operating company. A© 2019 Elsevier Ltd