Maximum hydrogen storage in depleted shale reservoirs: A case study on fracture networks and transport mechanisms

Depleted shale gas reservoirs offer significant potential for large-scale H2 storage, thanks to their ample pore space, anaerobic environments, effective sealing conditions, and the ability to reuse existing surface and subsurface infrastructure. This study introduces a comprehensive model that incorporates multiple H2 transport mechanisms, intricate fracture networks, and multi-well horizontal pads. The model is solved semi-analytically using Pedrosa's substitution, Laplace transformation, and Stehfest inversion methods. To account for the interference effects from simultaneous H2 injection across multi-well horizontal pads, the superposition principle is employed. Building on this model, we present a workflow designed for the rapid assessment of hydrogen storage capacity (HSC) in depleted shale gas reservoirs. The Fuling shale gas reservoir in China is used as a case study, revealing a maximum HSC of 1.79 × 108 m3 under a constrained pressure of 40 MPa. The energy storage efficiency of UHS is 1.4 to 10.9 times greater than that of coal, light oil, and natural gas. Storing this amount of H2 can save between 1.7 × 106 and 2.14 × 107 USD compared to the aforementioned traditional energy sources. Additionally, H2 combustion can reduce CO2 emissions by 1.04 × 105 to 6.57 × 105 t and SO2 emissions by 7.72 to 1.74 × 104 t. Sensitivity analyses reveal that at low constrained pressure (LCP), the impact of factors such as the effective diffusion coefficient, storage ratio, and well number on HSC is relatively minor, with variations ranging from 1.6 % to 12.0 %. However, under high constrained pressure (HCP), these factors have a much more pronounced effect, leading to variations that range from 58.3 % up to 9.8 times.