Impact of displacement conditions on gas flow and recovery efficiency in hydrogen aquifer storage: Insights from NMR experiments

Abstract

Underground hydrogen storage (UHS) is a promising solution for large-scale and long-duration energy storage. However, pore-scale hydrogen displacement behavior in aquifers remains insufficiently characterized. This study leverages low-field nuclear magnetic resonance (NMR) to investigate multiphase flow dynamics using nitrogen (as a safer analog for hydrogen) and brine in a controlled injection–production experimental setup. We monitored real-time changes in transverse relaxation time (T₂) distributions to semi-quantitatively evaluate the effects of confining pressure, flow rate, and displacement duration on fluid migration and recovery performance. The results show that macropores (T₂ > 70 ms) primarily facilitated initial gas displacement, while small and medium pores (T₂ ≤ 70 ms) became increasingly involved during brine re-imbibition. Despite varying confining pressures (up to 6.5 MPa), high flow rates (0.3 mL/min), and extended displacement periods (up to 10 PV), residual gas saturation remained, indicating that the recovery efficiency could not reach 100 %. The observed recovery behavior with pressure changes underscores the complex interplay between pore structure and displacement dynamics. These results suggest potential limitations in using nitrogen as a direct proxy for hydrogen in UHS, especially in scenarios where diffusion, dissolution, and interfacial effects are significant. A deeper understanding of the underlying mechanisms—such as interfacial properties and pore-scale sweep efficiency—remains necessary and requires support from quantitative measurements. While the current findings offer preliminary insights to guide the optimization of UHS operations through tailored injection parameters, they also underscore the need for more comprehensive studies involving direct hydrogen experimentation.