Simulation Study on Injection/Withdrawal Scenarios of Hydrogen-Blended Methane in a Depleted Gas Reservoir

초록

This study presents a quantitative simulation analysis of hydrogen-enriched methane (HENG) storage with nitrogen as the cushion-gas in a depleted gas reservoir by varying three key operational parameters: the injection/withdrawal period, hydrogen blending ratio (5-20%), and injection depth. Ten injection-withdrawal cycles were modeled for each scenario, and performance was evaluated using cycle-averaged and cumulative hydrogen purity, recovery factors, and the mixing zone size. Extending the injection period increased hydrogen purity to 20.00-20.26% and reduced nitrogen to 0.001-0.003%, but recovery decreased from 65.63 to 53.83-41.09% due to enhanced dispersion and residual trapping. The blending ratio was the dominant control: 20% blending yielded 19.9-20.0% purity with nitrogen as low as 0.00-0.03%, whereas 5-10% blending produced lower purity but minimized nitrogen production to 0.97-1.08%. Injection depth affected nitrogen recovery more than purity, increasing from 0.72-1.20% (upper) to 1.46-1.61% (lower), along with thicker mixing zones. Final mixing zone size ranged from 3176 to 5546 blocks, with smaller zones consistently linked to higher purity and lower nitrogen breakthrough. The shut-in period further reduced nitrogen recovery from 6.49 to 1.33% and stabilized mixing behavior. Overall, minimizing late-cycle mixing zone thickness is essential for maintaining HENG storage performance. Although this study provides quantitative insights into HENG operational strategies, the use of a homogeneous grid and simplified fluid properties limits representation of geological heterogeneity and reactive processes. Future work will incorporate heterogeneity and reaction modeling into field-scale simulations to validate and refine these operating strategies for practical deployment.

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