Driving Factors for Purity of Withdrawn Hydrogen: A Numerical Study of Underground Hydrogen Storage with Various Cushion Gases

Abstract

The central objective of this study is to improve our current understanding of the hydrodynamic processes arising when hydrogen (H2) is stored in subsurface porous media. In this work, we compare the use of two cushion gases, namely carbon dioxide (CO2) and methane (CH4), for H2storage ina synthetic aquifer. The impacts of viscous instability, gravity segregation, capillary trapping, and CO2 solubility in water on the recovery performance are investigated in detail.In the context of H2 storage, wefocus on both the amount and the purity of the H2that is back produced. A series of very fine-scale numerical simulationswas performed in 2D vertical systems using a fully compositional simulator. A simple three-stage operation strategy (cushion gas injection, H2 injection and H2 production) was designed to trigger the flow behaviour of interest. Based onscaling theory, we analysed the impacts of various mechanisms on the H2 recovery performance, from viscous dominated to gravity dominated flow regimes. Viscous instability and permeability heterogeneity may strongly degrade the purity of the back produced H2. No matter whichgas (CO2 or CH4) is selected as the cushion gas, the less viscous H2 infiltrates the cushion gas, meaning that the displacement does not proceed in a piston-like fashion. In the viscous-dominated scenario, H2 may even bypass the cushion gas of CO2, which subsequently leads to early breakthrough of the cushion gas and thus a dramatic reduction in H2 purity during back production. However, this effect does not arise in the case with CH4 as cushion gas. On the other hand, in the gravity-dominated case, the less dense H2 accumulates above the cushion gas and there is no flow infiltration or bypassing occurring in cases studied here. Therefore, the overall H2recovery performance is much better in the gravity-dominated regime than that in the viscous dominated regime. Finally, we demonstrate that it is important to include the solubility of CO2 when used as cushion gas in aquifer systems. This isbecause CO2 dissolution in water may significantly reduce its gas volume and lead to early water breakthrough during back production.