The "all-in-one" carbon storage system involves the co-production of methane and geothermal energy from produced subsurface brines, utilising onsite power generation and carbon capture to run a self-sustaining carbon storage facility. Once the produced brine has been degassed and cooled, CO2 is dissolved into it, and the CO2 saturated brine is reinjected where it sinks due to its relatively higher density, providing secure storage. This study investigates, for the first time, the economic

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... ility and energy balance of such a system within the UK North Sea. We examine the suitability of a depleted hydrocarbon field, which provides access to a saline formation, located in the Inner Moray Firth, Scotland for such a co-production and reinjection facility. We find that operating such a system to produce gas or electricity for sale alone would result in both an energy and economic loss. However, in the full "all-in-one" system scenario, where geothermal energy is used to offset the energy requirements of the capture and injection system, this results in a positive energy balance and a potential revenue stream, whilst also offering additional capacity for storing CO2 from external sources. Whilst the chosen case study site was non-ideal, it demonstrates that reuse of redundant oil & gas infrastructure that would otherwise be decommissioned, using the all-in-one approach, could help to offset a portion of the financial barriers to developing a carbon storage industry in the UK North Sea.