NUMERICAL MODELING OF CONVECTIVE AND MOLECULAR TRANSPORT MECHANISMS OF HYDROGEN IN DEPLETED GAS RESERVOIRS

This study presents a numerical investigation of hydrogen injection and migration in a depleted natural gas reservoir, integrating convective and molecular transport mechanisms. A 2D simulation model based on real geological data was developed using a discretized mesh and realistic operational parameters, including 15 active injection wells each delivering 250,000 Sm³/day. Four main operational scenarios were analyzed: continuous injection over 120 days and three subsequent cycles incorporating post-injection relaxation intervals of 5, 10, and 15 days.
Results show that continuous injection leads to stable reservoir behavior, with progressive pressure buildup and efficient gas accommodation. However, the introduction of relaxation periods reveals improved gas redistribution and saturation homogeneity, especially in the 15-day case, where hydrogen diffusion becomes significantly more uniform across the reservoir volume. Despite minimal differences in global pressure between relaxation scenarios, only the extended relaxation phase ensures optimal pore space utilization and mitigates localized overpressure risks.
These findings emphasize the critical role of tailored injection-relaxation cycles in maximizing hydrogen storage efficiency and maintaining reservoir integrity, contributing to the optimization of underground hydrogen storage strategies in depleted gas formations.