In the context of energy transition in France, massive energy storage is a key issue for the integration of renewable sources into the energy mix. One of the most promising ideas consists in using fluids to store energy. The Electrolysis–Methanation–Oxy-fuel (EMO) concept is designed to bring a closed-loop solution able to absorb electricity surplus, due in particular to renewable sources integration, and to recover it later, via the transient storage of O2, CO2 and CH4. The EMO addresses two major concerns of the “Power to methane” electric energy storage systems: i) the massive supply of CO2 to feed the methanation and ii) the release of CO2 into the atmosphere after methane combustion. In this concept, the oxygen generated by the electrolysis is used to burn the stored methane produced through combination of hydrogen and CO2, in an oxy-fuel power generator. Due to its relative purity, the emitted CO2 is then easily captured and reused in methane production. The process implies the temporary storage of large amount of fluids (O2, CO2 and CH4). Solution-mined caverns are studied as massive and reversible storage of fluids. The main objectives of the FluidSTORY project are to study the operability, the safety and the integrity of O2 and CO2 storage in salt caverns as well as to investigate the medium to long term (2030-2050) requirements for reaching the energy efficiency and economic profitability of the EMO concept in France. In order to achieve this goal, several electricity production scenarios for 2030-2050 will be developed in a techno-economic task. Economic environment and storage capacity needs for optimal use of EMO technology will be assessed as well as the profitability of the concept. In parallel, availability of storage volumes required by EMO development will be investigated through systematic inventory of the existing salt caverns and geological study of suitable salt formations for building new ones. In order to understand physico-chemical, thermo-dynamical and geochemical phenomena and processes which occur in salt caverns and resulting behavior, a large part of the project is dedicated to address scientific barriers brought by underground storage of O2 and CO2. Two options will be considered: i) each fluid is stored in a separate cavern, or ii) O2 and CO2 are stored together in the same cavern. Theoretical, numerical and experimental works will be carried out on geochemical equilibrium of stored fluids as well as on thermo-dynamical and thermo-mechanical behavior of the cavern. In comparison to the former FluidSTORY proposal submitted in 2014, this new version is enriched with the study of key surface elements and their interactions with the storage caverns. It provides knowledge on the global process and its operational needs. To meet regulatory requirements, the project also includes an analysis of potential risks induced by the storage operation and after cavern abandonment. An operational synthesis of this work, including a guideline for risk management, will be produced to support further development stages of EMO. In order to benefit from industry’s operational experience and to favor future dissemination of the concept, an external advisory board, already including GDF Suez and Air Liquide, is associated to the project. The board will give insight into industrial operability and will help to choose the adequate options at the main steps of the project. The preparation of two PhD theses will feed scientific developments inside the project, one addressing the geochemical behavior of stored fluids, the other one related to the geo-mechanical behavior of the salt caverns.