[Paper Review] Numerical Simulation of the Impact of Different Cushion Gases on Underground Hydrogen Storage in Aquifers Based on an Experimentally-Benchmarked Equation-of-State

Qingqi Zhao, Yuhang Wang+1 more|arXiv (Cornell University)|Jul 18, 2023

CO2 Sequestration and Geologic InteractionsEnvironmental Science3 citations

TL;DR

This study develops a two-phase, three-component reservoir simulator using an experimentally-benchmarked equation of state to investigate how different cushion gases (CO2, CH4, N2) affect hydrogen flow and storage efficiency in aquifers. CO2 as a cushion gas maximizes hydrogen purity due to enhanced aqueous-phase dissolution from density-driven convection, while CH4 and N2 improve hydrogen mobility and production rates.

ABSTRACT

Underground hydrogen storage (UHS) in geological formations is a promising technology for large-scale hydrogen energy storage. Although lessons were learned from similar studies, including geological carbon sequestration and underground gas storage, the unique thermodynamic and physical properties of hydrogen distinguish UHS from the other subsurface storage projects. We developed a two-phase, three components reservoir simulator, which incorporated essential physics based on the fully coupled multi-physics framework of the Delft Advanced Reservoir Simulation (DARSim). Hydrogen rich fingers were observed in the aqueous phase when CO2 was used as the cushion gas, because dissolved CO2 increased brine density, leading to density-driven downward convection which was favorable for hydrogen dissolution into the aqueous phase. The highest purity of produced hydrogen was observed when CO2 was used as the cushion gas, whereas using CH4 and N2 as the cushion gas was favorable for the hydrogen production rate and mobility. This work is the first study that utilizes an EoS based reservoir simulator to investigate hydrogen's flow patterns and interactions with cushion gases in an underground storage system. The developed reservoir simulation tool and research findings from this study will be valuable to support decision making in practical UHS projects.

Motivation & Objective

• To investigate the impact of different cushion gases on hydrogen storage performance in saline aquifers.
• To address the unique thermodynamic and physical challenges of hydrogen compared to other subsurface gases.
• To develop a physics-based reservoir simulator capable of capturing hydrogen-cushion gas interactions.
• To evaluate key performance indicators such as hydrogen purity, production rate, and mobility under varying cushion gas conditions.
• To provide a decision-support tool for real-world underground hydrogen storage projects using experimentally validated equations of state.

Proposed method

• Developed a two-phase, three-component reservoir simulator based on the Delft Advanced Reservoir Simulation (DARSim) framework.
• Incorporated a fully coupled multi-physics model to simulate fluid flow, transport, and thermodynamic behavior.
• Used an experimentally-benchmarked equation of state (EoS) to accurately represent hydrogen and cushion gas phase behavior.
• Simulated hydrogen injection and withdrawal cycles in a representative aquifer model with varying cushion gases (CO2, CH4, N2).
• Tracked hydrogen saturation, phase distribution, and dissolved hydrogen concentrations to assess performance.
• Analyzed flow patterns, including the formation of hydrogen-rich fingers and density-driven convection.

Experimental results

Research questions

• RQ1 How does the choice of cushion gas (CO2, CH4, N2) affect hydrogen purity during withdrawal from underground aquifer storage?
• RQ2 What role does density-driven convection play in hydrogen dissolution and mobility when CO2 is used as a cushion gas?
• RQ3 How do different cushion gases influence the production rate and mobility of stored hydrogen?
• RQ4 What are the dominant flow patterns and phase distribution dynamics during hydrogen injection and extraction?
• RQ5 To what extent does the experimentally-benchmarked equation of state improve simulation accuracy compared to conventional EoS models?

Key findings

• CO2 as a cushion gas led to the highest hydrogen purity due to enhanced dissolution in the aqueous phase driven by density-driven convection.
• Hydrogen-rich fingers formed in the aqueous phase when CO2 was used, indicating significant downward migration and dissolution.
• CH4 and N2 as cushion gases improved hydrogen mobility and production rates, suggesting better recovery efficiency.
• The experimentally-benchmarked equation of state enabled accurate prediction of phase behavior and interfacial interactions.
• The simulation results revealed that CO2-induced brine density increases promoted convective mixing, which increased hydrogen solubility.
• This study is the first to apply an EoS-based reservoir simulator to model hydrogen flow and interactions with cushion gases in underground storage.