Gravity effects of fluid storage and withdrawal in a reservoir from 3D forward modelling

The gravity effects of the possible reservoir scenarios after primary exploitation are tested in this work. Starting from the exploitable volume after primary hydrocarbon production of the very small and deep Volve field in the North Sea, we model several scenarios and calculate 3D forward gravity signatures accordingly. Namely, we test water flooding by aquifer rise, carbon dioxide storage, hydrogen storage using different cushion gases, hydrogen storage without cushion gas and hydrogen withdrawal. The differential gravity signature is calculated between two consecutive steps and the results provide detectability thresholds for each scenario. To evaluate effects of reservoir depth on the recovered gravity signatures, we repeat the calculations between 750 and 2750 m depth. Results of the modelling provide reference values for gravity signatures related to fluid storage in the worst-case scenario of a deep and thin (~100 m) reservoir and can provide valid constraints when mass loss estimation is required in leaking reservoirs. When the denser carbon dioxide and water are tested, these always provide detectable gravity signatures (>3 μGal) even at the maximum modelled depth, whilst storage or withdrawal of hydrogen in the modelled depth range, often result in undetectable signatures.