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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

Injecting CO 2 in to the subsurface for safe storage of CO 2 the pressure propagates far away from the injection point and this can be a potential problem if the overpressure extents to neighbouring subsurface activities or potential leakage pathways. For structural closure trap configurations the CO 2 plume is captured within the local structural closure but the pressure footprint is on a more regional scale. This rise the question on, how large the storage complex needs to be for any individual storage operations and how large an area monitoring activities have to cover. The EC CCS guidance document addresses the issues with statements on competitions between subsurface operations but returns no absolute values. Pressure modelling of CO 2 injection process with state of the art reservoir simulation tools is challenges by use of realistic model boundary conditions in order to model a realistic pressure level. Combined use of models on a site scale and on a regional scale can instruct how boundary conditions are set-up for a site scale model. Pressure management through pressure release wells could be an option to mitigate undesirable over-pressure developments. For local structural closures the pressure release wells can be placed outside the closure hereby mitigate the overpressure without introducing a potential leakage by drilling inside the trap. The paper addresses the issue of selecting model boundary conditions and modelling mitigation of pressure development by use of a large regional model with local structural traps in the Bunter Sandstone Formation in the UK Southern North Sea.