The impact of humic acid on hydrogen adsorptive capacity of eagle ford shale: Implications for underground hydrogen storage

Abstract

Hydrogen is a clean fuel that is anticipated to play a key role to phase-out fossil fuels and consequently decline the global warming challenges. Large-scale Underground Hydrogen Storage (UHS) is a critical link in hydrogen economy, but it is yet to be successfully achieved at industrial scale, and this is presently a main drawback in the hydrogen economy. Organic-rich shale pores have been suggested as promising geo-storage media for underground storage of hydrogen through the adsorption trapping mechanism. The hydrogen (H2) could be stored as an adsorbed phase on the kerogen surface or clay minerals, thus replacing methane (CH4) on adsorption sites during competitive adsorption. Moreover, methane has been suggested as a promising cushion gas for hydrogen. Experimental measurement of the H2 and CH4 adsorptive capacity of shale is essential for precise description of competitive adsorption behavior between CH4 and H2 in shale, as well as in designing of UHS in shale. Thus, adsorption capacities of methane and hydrogen were measured using a PCTpro adsorption analyzer in this study. Since organic acids are inherently present in geo-storage media, H2 and CH4 adsorption capacities in the fresh shale and shale aged with humic acid (Shale-HA) were compared to evaluate the impacts of adsorbed organic acids on their gas adsorption affinities. Our results showed that at 303 K, the adsorption of hydrogen increased by almost 170 % at 2.5 MPa and 350 % at 4.28 MPa in Shale-HA. The H2 adsorption trends on both shale samples are type IV isotherm adsorption curves that increased steeply with pressure but levelled beyond 3.5 MPa. The low-pressure N2 adsorption-desorption isotherms showed a type II adsorption isotherm. The CH4 adsorption capacity in raw shale was uniformly increased more than that of hydrogen, but there is no significant difference in CH4 adsorption capacity in shale-HA. This work provides fundamental data for hydrogen storage in shale reservoirs, thus facilitates full-scale implementation of hydrogen economy.