Assessment of hydrogen adsorption in high specific surface geomaterials using low-field NMR - Implications for storage and field characterization

Abstract

Natural and produced hydrogen will play a central role in decarbonizing the economy. High specific surface geomaterials can contribute H2 adsorption capacity, affecting both natural systems and storage strategies. Low-field Nuclear Magnetic Resonance NMR provides unique insights into H2 gas adsorption on nanoporous geomaterials. The cumulative NMR amplitude correlates linearly with the hydrogen mass; furthermore, measurements reveal two relaxations: a short-time relaxation for adsorbed H2 gas, and a long-time pressure-dependent relaxation for free H2 gas within pores. NMR measurements and complementary Grand Canonical Monte Carlo simulations show 2.5-to-4 times higher density in adsorbed H2 than bulk H2. The H2 adsorption capacity varies with mineral composition, but in all cases, it increases with specific surface area. Therefore, hydrogen physisorption in nanoporous geomaterials improves storability with respect to single-phase pure gas storage when high specific surface bentonites (at low pressure) or activated carbon (at all pressures) are the adsorbents. While synthetic adsorbents may exhibit higher H2 storage capacities, the low cost of high specific surface geomaterials, the reduced storage buoyancy, and safer controlled release make these natural adsorbents particularly attractive for large-scale storage applications. The insights gained with low-field NMR measurements in the laboratory are directly applicable to the interpretation of down-hole NMR characterization tools in the field.