Estimation of underground hydrogen storage capacity in depleted gas reservoirs using CO2 as cushion gas

Underground hydrogen storage (UHS) is an effective means to solve large-scale hydrogen energy storage. The depleted gas reservoirs can be used as the potential UHS targets due to its huge storage space, good sealing ability, and the existing facilities. CO2 can be injected as the cushion gas to reduce the hydrogen loss, improve energy storage efficiency and achieve carbon sequestration. This work proposes a novel method to estimate the hydrogen storage capacity in depleted gas reservoirs using CO2 as cushion gas. The multi-components (H2-CO2CH4-H2O) material balance equations are further developed by integrating the edge/bottom water and water invasion, gas (e.g., CO2, H2, CH4) dissolution in formation water as well as caprock breakthrough and fault instability. The maximum UHS operating pressure can be determined by calculating the caprock-breakthrough pressure and the fault-instability pressure. A numerical model is established to validate the proposed UHS capacity model in this work. The impact of dominated factors on the UHS capacity is discussed. The proposed method has been applied to evaluate the UHS capacity of a depleted gas reservoir in the Sichuan Basin of China. Results show that the model validation verifies the accuracy of the proposed model since the average error is only 1.76% between the analytical model developed in this work and the numerical model. The maximum pressure threshold presents the most significant impact on the UHS capacity, followed by CO2 cushion gas volume, formation temperature and water body size. The maximum pressure threshold of formation is determined to be 42 MPa. The hydrogen storage capacity under different CO2 cushion gas injection conditions is calculated. The UHS capacity is increased by 7.76% and 8.61% with dissolution when VCO2_inj is 3000 x 104 m3 and 4000 x 104 m3. The UHS capacity of Well #P4 is 6076 x 104 m3 when VCO2_inj is 4000 x 104 m3 based on the proposed model in this work. This work provides an effective approach to evaluate the hydrogen storage capacity and improve hydrogen storage efficiency by using CO2 as cushion gas considering caprock breakthrough, fault slip and gas dissolution. The established model provides important references for calculating the storage capacity of gas storage facilities in oil and gas reservoirs with edge and bottom water as well as for gas storage in aquifers.