Understanding Hydrogen Flow in Depleted Natural Gas Reservoirs
Authors: Panja Palash , Edelman Eric , Vega-Ortiz Carlos , Sorkhabi Rasoul , Deo Milind 
Published: August 2024

ABSTRACT:
Large-scale hydrogen storage is crucial for addressing the intermittent nature of renewable energy sources, which vary seasonally and diurnally. For storing energy over days to months, large-scale subsurface storage becomes essential. In this study, the potential of depleted natural gas reservoirs is explored as suitable storage media. This research comprises two main components. Firstly, it investigates the mixing of hydrogen and methane under subsurface conditions to examine the influence of molecular diffusion and gravity. Secondly, it explores flow dynamics during injection, shut-in, and production operations under varying geological parameters. Synthetic reservoir models are developed by changing absolute permeability, and porosity. During an annual cycle, hydrogen is initially injected into the reservoir for 6 months, followed by a 3-month shut-in period. Finally, hydrogen is produced from the reservoir over the course of 3 months. The hydrogen concentration in the produced gas is analyzed to understand the variation over time. Additionally, the cumulative recovery of hydrogen is calculated to measure the total amount of hydrogen successfully extracted from the reservoir over time. By analyzing various scenarios, the study aims to gain a comprehensive understanding of hydrogen behavior within the reservoir.

1. INTRODUCTION
Hydrogen is considered to play a critical role in energy transition to a low-carbon world primarily due to its utilization as a zero-carbon fuel when combusted with oxygen. However, transportation and long-term storage of hydrogen are among the most challenging issues in this potentially significant energy technology. Underground storage presents a viable solution, leveraging existing geological formations to securely store hydrogen on a large scale. Existing literature provides a comprehensive overview of the technical, economic, and environmental considerations associated with underground hydrogen storage. For this study, we investigate depleted natural gas reservoirs as possible storage sites for hydrogen.
The exploration of hydrogen underground storage technology has garnered significant attention in recent years, with a multitude of studies focusing on various methodologies such as depleted reservoirs, aquifer salt caverns, and excavated caverns (Bai et al. 2014; Heinemann et al. 2018; Caglayan et al. 2020; Liu et al. 2020; Epelle et al. 2022; Jahanbani Veshareh et al. 2022; Małachowska et al. 2022; Muhammed et al. 2022; Uliasz-Misiak et al. 2022; Hematpur et al. 2023).
For this study, we investigate depleted natural gas reservoirs as possible storage sites for hydrogen. In other studies, the current challenges, future directions, perspectives and prospects for underground hydrogen storage and the utilization of natural hydrogen resources were discussed (Epelle et al. 2022; Muhammed et al. 2022; Uliasz-Misiak et al. 2022; Hematpur et al. 2023). Despite significant progress in the field, several challenges persist, ranging from technical hurdles to regulatory and policy frameworks.