Hydrogen Diffusion in Organic-Rich Porous Media: Implications for Hydrogen Geo-storage

Abstract

Subsurface hydrogen storage for large-scale energy supply to decarbonize a variety of greenhouse gas-emitting activities is gaining momentum worldwide to combat climate change. Hydrogen’s extensive range of flammable concentrations and low minimum ignition energy in air combined with its highly diffusive nature mandate implementation of comprehensive risk management protocols to maintain storage safety. In particular, the role of hydrogen diffusion process (self-diffusion) needs further investigation to understand H2 transport behavior and elude the associated risk of leakage for the long-term safety of the underground hydrogen storage process. Recent molecular simulation studies suggest that, compared to other subsurface storage options, depleted shale reservoirs may offer preferential storage characteristics due to excellent sealing and adsorption capabilities of shale. Data on hydrogen diffusion in organic (kerogen)-rich shale is scarce. Here, we applied molecular dynamics to compute hydrogen self-diffusivity in kerogen systems with different structures at various pressures, ranging from 3 to 41 MPa, and under an isothermal condition of 360 K. Two kerogen types of varying maturity (II-A and II-C) were utilized to create slit nanopores of 0.5 and 2 nm in size. The Knudsen number was calculated based on the mean free path and determined by the density values obtained from simulation to improve diffusion coefficient estimates. Results obtained indicate the K n data in the range from 0.64 to 9.72, demonstrating that the main transport mechanism was transitional in nature. With increasing pressure, diffusivity declined regardless of the slit pore size or kerogen type. H2 diffusion was greater for the 2 nm pore system compared to both the 0.5 nm in II-A (0.004–0.02 cm2/s) and II-C (0.0037 to 0.019 cm2/s). In addition, for a fixed nanopore size, thermal maturity does not seem to impact diffusivity. Finally, simulation results were regressed to delineate a continuous description for diffusivity as a function of pressure. For curve fitting, R2 values were found to be 98 and 92% for 2 nm pore size of II-A and II-C, respectively, while 97 and 79% for 0.5 nm pore size of II-A and II-C, respectively. The results obtained are not only of interest for storage of hydrogen in shale but also for any subsurface hydrogen storage approach where a shale layer acts as a seal to form a trap.

Hydrogen Impact on Cement Integrity during Underground Hydrogen Storage: A Minireview and Future Outlook

“…They also reported that hydrogen adsorption in coals is 5 times less compared to that of CO 2 adsorption at various temperature conditions. Raza et al 73 integrated molecular dynamics simulation to study hydrogen self-diffusivity in shales. They observed that increasing the pressure resulted in reducing the diffusivity, regardless of the pore size or kerogen type.…”

The Future of CO2 Geological Sequestration: An Overview of Mineralization in Aqueous and Supercritical States

“…118 However, the content of Ca 2+ or Mg 2+ varies greatly among them. 119 In the process of geological sequestration, it is crucial to select a suitable site during the process of geological sequestration. 100 Therefore, selecting silicate strata rich in calcium and magnesium ions or sandstone strata with more and smaller silicate mineral particles, would be more conducive to CO 2 mineralization.…”

Hydrogen Adsorption in Porous Geological Materials: A Review

“…For instance, the adsorption isotherms of hydrogen, methane, ethylene, and their binary and ternary mixtures were studied [88]. The diffusion rates of hydrogen, methane, and carbon dioxide in organic kerogen nanopores were investigated [89]. The adsorption of pure H 2 , H 2 mixed with CH 4 , and H 2 mixed with CO 2 in representative kerogen and MMT were studied using grand canonical Monte Carlo (GCMC) [90].…”

“…Comparison of the adsorption of hydrogen and methane in shale showed that the adsorption of both gases increased with increasing pressure [88][89][90][91][92][93]. The adsorption rate gradually decreased with increasing pressure, and the adsorption behavior of methane and hydrogen was Type I Kerogen (organic matter rich in algae and fungal remnants) [88][89][90].…”