Evaluating Caprock Integrity During Underground Hydrogen Storage (UHS) in Subsurface Rocks
Authors: Ajibona A. I. , Pandey R. 
Published: August 2024

ABSTRACT:
Hydrogen has been identified as a key component of the clean energy economy value-chain, whose global demand is expected to significantly increase. To meet this demand, a large storage capacity is required, with underground hydrogen storage (UHS) in depleted oil and gas reservoirs projected to provide this capacity. To consider UHS in depleted reservoirs, an effective seal that overlays the reservoir rocks is necessary to prevent loss of hydrogen through leakage. Shale caprocks are recognized as effective seals which have held natural gas in place for geologic time-periods. Limited studies in the past have shown alteration of caprock properties during UHS operations. However, these studies made observations before hydrogen exposure and only one timepoint after extended duration of hydrogen exposure. Work presented here investigates how hydrogen-shale interactions alter the porosity and matrix permeability of the Marcellus shale. Experiments were carried out at 6.9 MPa pressure, which included multiple durations of hydrogen exposure. After each exposure, the porosity and matrix permeability were measured. Unprecedently, results indicated an increase in the porosity and matrix permeability due to continued hydrogen exposure. This change in pore-structure is expected to be physical in nature, driven by possible micro-fracturing at grain boundaries.

1. INTRODUCTION AND BACKGROUND
Amidst the rapid global population growth and urbanization, the rising demand for energy requires a strategic shift towards cleaner energy sources, particularly as the world steers towards achieving sustainable climate goals (Dincer, 2000). Driven by their intrinsically low carbon footprint, renewable sources of energy, such as solar and wind energy are rapidly gaining traction as potential clean energy sources (Levin & Chahine, 2010). Their minimal greenhouse gas emissions offer a significant advantage in addressing the pressing challenge of climate change (Agreement, 2015). However, the inconsistent nature of renewable energy production, driven by weather and geographical factors, creates a critical bottleneck in achieving a sustainable energy future due to the mismatch between supply and demand. This necessitates the development of cost-effective and environmentally friendly energy storage solutions to bridge the gap between excess energy generation and periods of peak demand (Lehtveer et al., 2017). One promising practical solution is to convert the excess energy generated into hydrogen, an efficient and versatile energy carrier that can be stored and utilized on demand. This approach leverages diverse hydrogen production pathways, including thermochemical, electrolytic, biological, and direct solar water splitting, offering flexibility and scalability (Das & Veziroǧlu, 2001; van der Roest et al., 2020; Wang et al., 2012). Despite advancements in hydrogen production technologies, the lack of scalable, long-term storage solutions constitutes a major roadblock (Zhang et al., 2016). Existing methods, encompassing high-pressure tanks, cryogenic storage, and material adsorption, are inherently limited in capacity, and are relatively unsafe (Abdalla et al., 2018; Jain et al., 2013). Underground geological formations, including depleted oil and gas reservoirs, aquifers, and engineered caverns, present the only viable option for accommodating the vast volumes required for grid stabilization and seasonal demand buffering (Tarkowski, 2019). Implementing widespread underground hydrogen storage (UHS) is therefore paramount for unlocking the full potential of renewable energy in transitioning to a sustainable energy future.