This project will investigate synergies between subsurface energy storage technologies (SST). These technologies have the potential to support the uptake of renewables whilst preserving the energy system's security and reliability. Individually these technologies have to contend with high capital cost and uncertain investment risk profiles. The simple yet cutting-edge idea is to determine how combining these technologies over the operational lifetime of a geological store can lead to net benefits in the form of increased efficiency, reduced costs, and minimised risks. This work will provide evidence-based information on these synergies, when they arise, and how to exploit them. The highly interdisciplinary work is divided into 4 strongly interconnected work packages, which cover the whole span of technology assessment, from technical viability, to economic analysis. Most of the work will be conducted at the Lawrence Berkeley National Laboratory (LBNL) in the USA during the outgoing phase. The return phase will be completed at the University of Edinburgh (UoE) where the benefits of synergies will be integrated into energy network models, and significant industry and public engagement will be undertaken to ensure knowledge transfer. Presenting this novel work to the scientific community (i.e. energy systems, geoenergy) is expected to have a huge impact. The Experienced Researcher (ER), is expected to carry out excellent research and also profit from the state-of-the-art physical and economic modelling experience in SSTs at LBNL, supporting his career goal of an academic career as a University professor. LBNL benefits from the ER's expertise in hydrogen storage and gas network conversion. The UoE will benefit from a novel assessment workflow applicable to its proven expertise in SSTs assessment in the North Sea region of Europe. Naturally, the ER, LBNL and the UoE are strongly committed to the project and delivering a successful outcome.