Underground hydrogen storage (UHS) is an effective means to solve large-scale energy storage. The depleted gas reservoirs can be used as the potential targets for UHS due to its huge storage space, good sealing ability, and the existing facilities. CO2 can be injected as the cushion gas to reduce the loss of hydrogen and achieve carbon sequestration. This work proposes a novel analytical method to calculate the hydrogen storage capacity in depleted gas reservoirs using CO2 as cushion gas considering hydrogen storage safety and gas (e.g., CO2, H2, CH4) dissolution in formation water. The multi-components (H2-CO2-CH4-H2O) material balance equations are further developed by considering the edge/bottom water and gas dissolution in water as well as caprock breakthrough and fault instability. The maximum operating pressure of UHS is determined by calculating the caprock-breakthrough pressure and the fault-instability pressure. The proposed method has been applied to evaluate the UHS capacity of a depleted gas reservoir in the Sichuan Basin of China. The maximum pressure threshold of formation is determined to be 42M. The hydrogen storage capacity under different CO2 cushion gas volume conditions is calculated. The study compares with the model without considering dissolution, and the influence of sensitive factors such as temperature and pressure on hydrogen storage capacity is analyzed.
Keywords UHS, CO2, cushion gas, material balance, storage capacity, depleted gas reservoir