Cushion Gas Consideration for Underground Hydrogen Storage: History
Please note this is an old version of this entry, which may differ significantly from the current revision.
Subjects: Energy & Fuels

Due to the increasing world population and environmental considerations, there has been a tremendous interest in alternative energy sources. Hydrogen plays a major role as an energy carrier due to its environmentally benign nature. The combustion of hydrogen releases water vapor while it also has a vast industrial application in aerospace, pharmaceutical, and metallurgical industries. Although promising, hydrogen faces storage challenges. Underground hydrogen storage (UHS) presents a promising method of safely storing hydrogen. The selection of the appropriate cushion gas for UHS is a critical aspect of ensuring the safety, efficiency, and reliability of the storage system. Cushion gas plays a pivotal role in maintaining the necessary pressure within the storage reservoir, thereby enabling consistent injection and withdrawal rates of hydrogen. One of the key functions of the cushion gas is to act as a buffer, ensuring that the storage pressure remains within the desired range despite fluctuations in hydrogen demand or supply. This is achieved by alternately expanding and compressing the cushion gas during the injection and withdrawal cycles, thereby effectively regulating the overall pressure dynamics within the storage facility. Furthermore, the choice of cushion gas can have significant implications on the performance and long-term stability of the UHS system. Factors such as compatibility with hydrogen, cost-effectiveness, availability, and environmental impact must be carefully considered when selecting the most suitable cushion gas. The present study provides a comprehensive review of different types of cushion gases commonly used in UHS, including nitrogen, methane, and carbon dioxide. By examining the advantages, limitations, and practical considerations associated with each option, the study aims to offer valuable insights into optimizing the performance and reliability of UHS systems. Ultimately, the successful implementation of UHS hinges not only on technological innovation but also on strategic decisions regarding cushion gas selection and management. By addressing these challenges proactively, stakeholders can unlock the full potential of hydrogen as a clean and sustainable energy carrier, thereby contributing to the global transition towards a low-carbon future.

  • hydrogen
  • cushion gas
  • reservoir
  • sustainability
  • aquifer
  • natural gas
Fossil fuels have come under increased scrutiny in recent years due to various factors such as high greenhouse gas emissions, fluctuating prices, and a rise in energy needs [1]. As the world’s population continues to rise, these issues are expected to become even more pressing. Considering this, numerous initiatives have been taken to develop alternative sources of energy that are self-sustaining and renewable [1]. Although wind and solar are among the most known renewable energy resources, they have notable limitations. While these options can produce clean usable energy, they are not reliable. Wind and sunlight are not consistently available each day [2]. This creates gaps in energy production, which can strain the ever-increasing demand for energy. On the other hand, Hydrogen is a reliable and clean energy carrier [3]. Hydrogen can be used in industries known to be high GHG emitters such as aviation, chemical manufacturing, and iron and steel production [4]. The combustion of hydrogen mostly releases water vapor as a byproduct, making it a clean energy source with no carbon emissions.
The majority of hydrogen generated today is from the well-established method of steam-methane reforming (SMR), which uses high-temperature steam (700–1000 °C) to produce hydrogen from a methane (CH4) source, such as natural gas [5]. Although promising, SMR releases lots of GHG. Therefore, there have been several research interests in alternative hydrogen production methods from waste materials [6] and electrolysis technology [7][8].
Another issue with hydrogen technology is its storage. The challenges of hydrogen storage are primarily due to its low ambient temperature density, resulting in a low energy-per-unit volume [9]. This necessitates the development of advanced storage methods to achieve higher energy density. Hydrogen can be stored physically as either a gas or a liquid. However, each method has its drawbacks [10]. It should be mentioned that hydrogen could be stored aboveground and underground. Currently, there are several aboveground options for storing hydrogen such as in cryogenic tanks, using a metal–organic framework (MOF), and direct conversion to energy carriers or liquefied hydrogen, all depicted below in Figure 1. However, there are fewer options for storing hydrogen in underground deposits. Regarding aboveground cryogenic storage, it is worth noting that while these tanks can hold both liquid and compressed gaseous hydrogen, there are many requirements to keep the hydrogen viable.
Figure 1. A schematic representation of the above-ground hydrogen storage options: (A) depicts a cryogenic tank, (B) depicts a metal–organic framework (MOF), and (C) depicts direct conversion to energy carriers or liquefied hydrogen.
Ahluwalia and Peng [11] address a crucial aspect of hydrogen storage. Recent strides have been achieved in producing robust composite fiber tanks capable of containing compressed hydrogen (H2) at pressures ranging from 350 to 700 bar. Nevertheless, the aspiration of achieving a system volumetric capacity of 45–60 g/L encounters limitations because, at room temperature, the density of hydrogen is merely 23.5 g/L at 350 bars and 39.5 g/L at 700 bars [11]. An additional requirement of cryogenic tanks is they must be fitted with an in-tank heater as hydrogen has a low critical point of 13.15 bar and −239.96 °C. With these strict requirements in mind, this makes cryogenic tanks challenging both economically and in terms of accessibility.
Metal–organic frameworks (MOFs) are also promising hydrogen storage mediums; however, they are characterized by low volumetric density, slow hydrogen uptake and release, and high cost and scalability issues [1]. In contrast, the direct conversion of hydrogen to its liquid carriers such as ammonia is restricted by cost and environmental impacts. Alternatively, hydrogen could be stored safely and cheaply in various underground deposits.
The top three contenders for underground hydrogen storage (UHS) are salt caverns, aquifers, and depleted hydrocarbon reservoirs [12]. It is worth noting that salt caverns must be excavated and prepared for hydrogen storage, unlike the other storage options, which creates an economic and time disadvantage. On the other hand, aquifers are naturally occurring geological formations that are made up of porous media such as sandstone, limestone, shale, and conglomerates [13]. Surrounding this porous media is nonporous media, also referred to as cap rocks. A drawback of aquifers is the possible reactions of hydrogen with mineral constituents and microbes [13]. Lastly, depleted hydrocarbon reservoirs are described as oil and gas wells in petroleum fields. This option is feasible due to sufficient depth and bottom-hole temperatures that are suitable for thermal energy extraction [13]. However, due to these reservoirs being drilled into various geological formations and media, there is no constant measurement of permeability. This means that different reservoirs will exhibit differing values of gas leakage and loss. The fluctuating value per reservoir creates an economic drawback as the potential for hydrogen gas loss each day could be high [14]. Optimizing and reducing the cost of UHS requires the selection of an effective cushion gas. It should be mentioned that cushion gas in UHS refers to a permanent gas layer used to maintain pressure and ensure the efficient extraction of hydrogen when needed [12][13].
There have been various studies and simulations performed to determine the best cushion gas for UHS such as Iloejesi and Beckinghams [15], who reported the reservoir modeling and performance of carbon dioxide, CO2, as a cushion gas for UHS. Along with CO2, other cushion gases that are under consideration by several researchers include nitrogen (N2), CH4, and residual native gases. Depending on which gas is chosen for an underground storage cavity, possible interactions must be evaluated. When discussing CO2 as a cushion gas, Iloejesi and Beckinghams [15] explain that once injected, CO2 creates conditions often suitable for the dissolution of carbonate and aluminosilicate minerals [15]. However, the possible interactions change with differing storage methods. Porous saline aquifers, for example, involve additional hazards and complications compared to storage in a medium such as caverns including multiphase flow and geochemical reactions. These complexities are not well understood and could potentially impact system operation or efficiency. The interaction between CO2 and different UHS media is not yet fully understood. Further research is needed to determine how CO2 may impact the performance and safety of UHS options such as salt caverns, aquifers, and depleted hydrocarbon reservoirs. Furthermore, the economic and environmental impacts of using different cushion gases in various hydrogen storage media are not yet fully understood. To the authors’ knowledge, there are relatively few studies related to cushion gas considerations for UHS. Table 1 outlines a few reviews in this area, especially those related to UHS. To address the knowledge gaps, the present study provides a comprehensive review of different cushion gases used for hydrogen storage. The study also delves into various mediums for UHS, as well as their advantages and limitations.
Table 1. Summary of previous studies related to underground hydrogen storage.

Study Focus and Key Issues Addressed


  • Provided a comparative discussion on naturally occurring hydrogen.

  • Outline the reaction mechanism of underground hydrogen storage and different geological formations suitable for underground hydrogen storage.


  • Discusses numerous barriers to overcome before the implementation of underground hydrogen storage technology.

  • Reviews institutions that would be of interest for low-cost UHS.

  • Presents an extensive overview of the legal challenges as well as social acceptance of underground hydrogen storage.


  • Discusses different potential cushion gases for natural gas storage.

  • Uses mechanistic numerical simulations to study the influence of the conditions such as operating conditions, rock properties, and molecular diffusion on natural gas storage.


  • An in-depth breakdown of indigenous organic matter in shales and carbonate rocks

  • Discusses non-reservoir ancient sediments containing large amounts of oil.

  • Discusses the distribution of oil, asphalt, and kerogen in varieties of formations


  • The study highlights Underground Hydrogen Storage (UHS) as a viable solution for large-scale, long-term energy storage in a hydrogen-based economy, considering both economic and safety aspects.

  • The paper provides an overview of UHS technology, including geological assessments, technical challenges, recent progress, and future opportunities, with a special focus on UHS initiatives in Australia.


  • This work critically reviews key aspects of underground hydrogen storage (UHS) technology, a crucial element in the transition to renewable energy systems.

  • The article discusses significant scientific and operational challenges regarding the safety and efficiency of large-scale deployment of UHS.


  • This review on Underground Hydrogen Storage (UHS) discusses the role of UHS in energy storage, particularly for hydrogen, and its increased attention due to its large-scale storage efficiency.

  • Outlines the importance of understanding the storage process, including the hydrodynamics of hydrogen, reservoir fluids, and rock systems for safe and efficient monitoring.


  • This review article focuses on the crucial role of underground hydrogen storage (UHS) in the context of global warming and the shift towards renewable energy sources.

  • The article assesses different types of subsurface geological media for hydrogen storage, including natural (depleted oil and gas reservoirs, saline aquifers) and artificial (salt caverns) environments.


  • Comprehensively reviewed different underground hydrogen storage reservoirs.

  • Discussed in depth how different cushion gases benefit or interfere with each reservoir.

  • Meticulously reviewed studies related to the use of different cushion gases for underground hydrogen storage.

This study

This entry is adapted from the peer-reviewed paper 10.3390/encyclopedia4020054


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