Underground Hydrogen Storage in Salt Cavern: A Review of Advantages, Challenges, and Prospects

Abstract

The transition to a sustainable energy future hinges on the development of reliable large-scale hydrogen storage solutions to balance the intermittency of renewable energy and decarbonize hard-to-abate industries. Underground hydrogen storage (UHS) in salt caverns emerged as a technically and economically viable strategy, leveraging the unique geomechanical properties of salt formations—including low permeability, self-healing capabilities, and chemical inertness—to ensure safe and high-purity hydrogen storage under cyclic loading conditions. This review provides a comprehensive analysis of the advantages of salt cavern hydrogen storage, such as rapid injection and extraction capabilities, cost-effectiveness compared to other storage methods (e.g., hydrogen storage in depleted oil and gas reservoirs, aquifers, and aboveground tanks), and minimal environmental impact. It also addresses critical challenges, including hydrogen embrittlement, microbial activity, and regulatory fragmentation. Through global case studies, best operational practices for risk mitigation in real-world applications are highlighted, such as adaptive solution mining techniques and microbial monitoring. Focusing on China’s regional potential, this study evaluates the hydrogen storage feasibility of stratified salt areas such as Jiangsu Jintan, Hubei Yunying, and Henan Pingdingshan. By integrating technological innovation, policy coordination, and cross-sector collaboration, salt cavern hydrogen storage is poised to play a pivotal role in realizing a resilient hydrogen economy, bridging the gap between renewable energy production and industrial decarbonization.

1. Introduction

In the face of escalating climate change and the imperative for economic decarbonization, the global transition to renewable energy systems has become an urgent task. Hydrogen, as a versatile zero-carbon energy carrier, emerged as a cornerstone of this transformation, particularly in sectors such as heavy industry, long-distance transportation, and seasonal energy storage (Figure 1), where direct electrification is challenging [1]. However, the widespread application of hydrogen depends on the development of efficient, large-scale energy storage solutions that can balance supply and demand [2], accommodate intermittent renewable energy sources, and ensure energy security [3].

Underground hydrogen storage (UHS) offers a highly attractive solution to these challenges by utilizing geological formations to safely and cost-effectively store vast quantities of hydrogen. Common types of underground hydrogen storage systems (Figure 2), including salt caverns, depleted oil and gas reservoirs, aquifers, and abandoned coal mines [5,6]. Salt caverns stand out due to their unique geomechanical and geochemical properties [7].

Compared to other underground hydrogen storage facilities, salt formations are nearly impermeable [8], resistant to hydrogen embrittlement, and capable of self-healing minor fractures through creep behavior [9,10], making them an ideal choice for hydrogen storage under high-pressure cycling conditions [11]. Additionally, salt caverns enable rapid injection and extraction rates [12,13,14], which are crucial for grid flexibility in renewable energy-dominated systems.

According to previous studies, salt caverns are widely used for energy storage in different fields, with traditional applications mainly including the storage of natural gas, crude oil, and petroleum products (Figure 3). Due to their unique storage advantages, they are now also used for storing compressed air, carbon dioxide and hydrogen, etc.

Despite these advantages, the deployment of salt caverns for hydrogen storage presents several challenges (Figure 4). Technical obstacles, such as hydrogen loss due to microbial activity [16,17,18,19,20,21], the long-term stability of salt caverns under cyclic loading [22,23,24], and risks related to material compatibility [25,26,27], must be addressed [28]. Furthermore, compared to natural gas, the regulatory framework and safety standards for hydrogen storage remain incomplete, creating uncertainties for project developers [29]. Social acceptance and environmental issues, such as brine management during the construction of salt caverns, also pose significant barriers to the large-scale application of this technology [30].

This review aims to synthesize current research and industry insights on underground hydrogen storage in salt caverns with three primary objectives: (1) to assess the technical, economic, and environmental advantages of salt caverns compared to alternative storage methods; (2) to critically analyze the challenges limiting their broader application, including scientific, regulatory, and social factors; and (3) to explore future prospects for innovation, policy support, and integration into the emerging hydrogen economy. By bridging the gaps between geological engineering, energy systems analysis, and policy design, this study seeks to inform stakeholders ranging from researchers to policymakers, assisting them in identifying pathways to unlock the full potential of salt caverns for a sustainable hydrogen future.

2. Fundamentals of Salt Caverns for Hydrogen Storage

2.1. Global Existing Projects

The technology readiness level (TRL) is a standardized framework for measuring the maturity of technologies (

Table 1. Technology readiness level table.

TRL Level	Level Classification	Description
1	Report level	Discover new phenomena, problems, needs and report.
2	Solution level	Propose technical solutions.
3	Simulation level	Successfully verify the simulation of the core—technology concept model.
4	Function level	Key functional indicators meet the standards in laboratory tests.
5	Preliminary sample level	Functional samples, drawings and process design pass the tests.
6	Prototype level	The functional prototype passes the test and the process is verified to be feasible.
7	Environment level	The engineering prototype system operates and passes the environmental tests.
8	Product level	Small-batch trial production is qualified and production conditions are available.
9	System level	Achieve large-scale commercial production with qualified product quality.

). Currently, there is a considerable number of hydrogen storage facilities that have been completed or are under construction globally (

Table 2. Operating parameters of built underground hydrogen storage.

Location	Hydrogen Content	Commissioning Time	Volume (m3)	Depth (m)	Cushion Gas (106 kg H2)	Working Gas (106 kg H2)	Pressure Range (MPa)	H2 Capacity (GWh)	Hydrogen Storage Rock Formations	TRL
Kiel (Germany)	60~65%	1971	3.2 × 105	1335	-	-	8~10	-	Containing impurities.	6
Teesside (UK)	95%	~1972	2.1 × 105	350	-	0.76	~4.5	25	High purity; good sealing property.	7
Clemens (USA)	95%	1983	5.8 × 105	930	2.21	4	7~13.5	81	Salt domes in the Gulf of Mexico Basin; stable crust, high purity.	9
Moss Bluff (USA)	95%	2007	5.66 × 105	820~1400	2.3	3.72	5.5~15.2	123	Thick salt layers; good sealing property.	8
Spindletop (USA)	95%	2016	9.06 × 105	~1240	-	8.23	6.8~20.2	274	Stable sedimentation.	8
HyUnder (Europe)	≥95%	2012–2014	4 × 106	-	-	-	-	-	Thick salt layers; good sealing property.	1
HyCAVmobil (Germany)	≥97.5%	2023	5 × 105	-	-	-	-	-	Thick pure salt rocks; good sealing property.	7
Krummhörn (Germany)	≥98%	2024	5 × 105	1400~1500	-	-	25	-	Simple structure.	5
Hypster (France)	≥98%	2024	5.56 × 105	900~1000	-	-	10~15	-	Bresse Basin in France; few faults.	5
Bad Lauchstadt (Germany)	≥97%	2025	5.0 × 107	700~900	-	-	15	-	Stable structure; good sealing property.	5
ACES (USA)	≥95%	2025	1.43 × 106	1100~1300	-	~23.8	-	-	Thick salt layers; stable structure.	6
Ye County (China)	≥90%	2027	>3 × 104	1000~2000	-	-	-	-	Containing mudstone interlayers.	4
Aldbrough (UK)	≥97%	2030	4.2 × 108	-	-	~300	-	~320	North Sea Basin.	3
HyStock (Netherlands)	≥95%	2031	6 × 104	-	-	-	20	-	Groningen Basin; flat sedimentation, few faults.	2
Yulin (China)	-	preparing	5 × 104	-	-	-	-	-	Stable structure; weak groundwater activity.	-
Jintan (China)		preparing	2.16 × 105	900–1100			6–18	-	Layered salt rocks; weak geological activity.	-

) [32,33,34,35,36,37,38,39,40,41,42]. This paper examines the development of salt cavern hydrogen storage technology within the context of the TRL system [43]. This technology gradually matured through challenges and validations.

In 1971, the Kiel project in Germany stored natural gas containing 60~65% hydrogen (with a volume of 3.2 × 10⁴ m³ and a pressure of 8–10 MPa) in a salt cavern [32]. After 47 years of operation, it experienced a hydrogen embrittlement-induced daily leakage rate of 0.8%, revealing the sealing challenges associated with pure hydrogen storage. The following year (1972), the UK Teesside project stored hydrogen with 95% purity in three salt caverns at a depth of 350 m (individual cavern volume 7 × 10⁴ m³, pressure 4.5 MPa), becoming the first engineering example of pure hydrogen storage [33]. Long-term stability was significantly validated by the US Clemens salt dome project (from 1983 to present), which operates at depths of 900–1500 m and pressures of 7.0–13.5 MPa, with a hydrogen storage volume of 5.8 × 10⁵ m³, having exceeded 60,000 pressure cycles without failure and achieving a usability rate of 99.8% [34]. Notable advancements in sealing technology were demonstrated in the 2023 US ACES project, which completed a cycle test of 8000 tons of pure hydrogen in a salt cavern in Utah, achieving a leakage rate of less than 0.08% per cycle over 15 injection and extraction cycles, marking the first environmental-level sealing verification in a real-world setting [35]. Existing operational projects also provide important data: the Spindletop project in the United States (commissioned in 2016) has an annual injection and extraction of 22 cycles, with a leakage rate of <0.05% per year (DOE certified report), and operational costs of USD 1.2/kg H₂; the Moss Bluff project in the United States (commissioned in 1983) has been in continuous operation for 42 years [36]. Worldwide, research and application of underground hydrogen storage are accelerating: the EU’s HyUnder project assessed a theoretical hydrogen storage potential of 4 × 10⁶ m³ and plans to apply its site selection and technical scheme in the Portaliana salt cavern in Spain, aiming to establish a new hydrogen storage facility by 2030 [37]; the Netherlands launched the HyStock salt cavern pure hydrogen storage experimental project (planned to be operational by 2031) [38]; Germany is advancing the HyCAVmobil project [39,40]; France is implementing the Hypster project; and the completion of the Aldbrough project in the UK will become the world’s largest hydrogen storage facility [41]. The successful construction of the Yexian salt cavern hydrogen storage project in China aims to provide laboratory-level evidence for validating the adaptability of salt layer hydrogen storage in the Asian region [42].

2.2. Geological Formation

Salt caverns are artificial cavities formed within underground salt deposits, which primarily exist as bedded salt layers or salt domes [15]. Bedded salt deposits, such as those found in the Jintan [44] and Yunying [45] regions of China, consist of horizontally stratified salt layers formed by the evaporation of ancient seawater over geological periods [46]. In contrast, salt domes, exemplified by the Clemens Dome in Texas [47], are vertical structures created by the buoyant upward movement of salt, resulting in thick dome-shaped formations suitable for large-scale storage [39]. These salt caverns are constructed using the solution mining method, which involves injecting freshwater into the salt layer to dissolve the rock salt (NaCl) [48,49], thereby creating cavities with volumes ranging from 100,000 to 1,000,000 m³. While bedded salt provides predictable geomechanical properties for cavern design, salt domes necessitate rigorous evaluation of caprock integrity [24,50], due to their complex structural dynamics, to prevent hydrogen leakage.

2.3. Key Properties

Salt caverns possess unique geological and mechanical properties that render them exceptionally suitable for hydrogen storage [51]. Their near impermeability (<10⁻²⁰ m²) ensures minimal hydrogen leakage over extended storage periods, which constitutes a critical advantage over porous storage methods, such as depleted gas reservoirs. Furthermore, salt exhibits self-healing properties [10,52], whereby micro-fractures induced by pressure fluctuations naturally close through viscoplastic creep [53,54]. This process restores the integrity of the salt cavern and prevents gas migration. The chemical inertness of salt further eliminates the risk of hydrogen contamination or reaction, thereby maintaining gas purity. In the field of mechanics [55], the ductility of salt enables it to withstand high-pressure differences of up to 30 MPa without brittle failure, ensuring structural stability during cyclic injection and extraction operations. However, challenges such as anhydrite interlayers in bedded salt formations and microbial activity in residual brine, if not mitigated through careful site selection and operational control, may impact storage efficiency.

2.4. Design and Construction

The development of salt cavern hydrogen storage involves three key stages: site selection, leaching, and operational optimization [56]. Site selection begins with geological surveys to identify salt layers of sufficient thickness (>100 m) and uniformity, supplemented by geomechanical modeling to assess stress states and avoid fault zones.

During the leaching phase, a combined positive and negative circulation brine circulation method can be employed, which is one of the effective methods for optimizing water usage. This method integrates the advantages of both positive and negative circulation, where freshwater is injected through the tubing and returns through the annulus. It creates a circulation brine channel while simultaneously leaching the cavity, ensuring the complete dissolution of the salt rock at the top of the cavity, thereby enhancing the efficiency and uniformity of the leaching cavity while reducing brine discharge [57]. Additionally, an efficient leaching implementation method using salt cavern gas storage with interlayer salt can be adopted. By utilizing blasting techniques to create fractures within the intended leaching salt rock and interlayer, permeability is enhanced, increasing the contact area between brine and salt rock, accelerating the dissolution of salt rock, and establishing a bidirectional brine discharge channel through vertical well water injection and directional well brine discharge, thus improving leaching efficiency. To address water resource challenges, a circulating brine leaching method can also be employed, where part of the brine generated from leaching is treated and reinjected into the salt cavity for further salt dissolution, effectively reducing freshwater demand and wastewater discharge.

Upon completion of construction, operational optimization focuses on the cavern shape [31,58] and depth (500–2000 m) to balance pressure requirements, creep rates, and economic feasibility. The Germany Kiel project encountered a high content of insoluble substances during the leaching process of cavern creation, resulting in less than 60% of the cavern’s effective volume being available for storage. The UK Teesside project site is based on the Permian layered salt formation, with a salt rock layer thickness of approximately 40 m and a burial depth of 380 m, used for storing pure hydrogen (95% H₂ and 3–4% CO₂), with a total storage capacity of about 210,000 m³. This salt cavern hydrogen storage facility does not use cushion gas during injection and withdrawal operations, but instead employs the ‘pressure maintenance method,’ maintaining a constant internal pressure of 4.5 MPa through ‘hydrogen injection and brine displacement’ and ‘brine injection and hydrogen withdrawal’ [59]. The hydrogen storage project in the Clemens salt cavern in the United States features a salt layer thickness exceeding 200 m, with a low interlayer content, achieving a hydrogen recovery rate of up to 95%. These case studies underscore the importance of integrating geomechanical insights with engineering design to maximize both storage efficiency and safety [60].

3. Advantages of Salt Caverns for Hydrogen Storage

3.1. Technical Benefits

The technical advantages of salt cavern hydrogen storage primarily arise from their unique geological and engineering characteristics. The extremely low permeability and self-healing ability of salt rock layers create a natural sealing barrier, ensuring minimal hydrogen leakage even under high-pressure cycling conditions. Unlike porous reservoirs or aquifers, salt layers are chemically inert, which eliminates the risk of hydrogen contamination from reactions with host rock minerals or residual fluids. This guarantees the purity of hydrogen after long-term storage, thereby meeting the stringent requirements of fuel cells and ammonia synthesis processes. From an engineering perspective, the cavities formed through solution mining allow for operation at high pressures of 20–30 MPa [61,62]. When combined with dynamic pressure regulation technology, this enables rapid injection and extraction of hydrogen, reducing response time and facilitating fast cycling. Furthermore, the creep characteristics of salt caverns ensure long-term structural stability [63,64]. Simulation results from the Jintan salt cavern in China demonstrate the mechanical stability of salt under cyclic loading [65], maintaining structural integrity over 50 pressure cycles, which further validates its reliability as a large-scale hydrogen storage carrier. Collectively, these characteristics make salt caverns particularly suitable for balancing the intermittency of wind and solar power generation while ensuring efficient energy storage.

3.2. Economic Viability

The economic advantages of salt caverns stem from their lower capital costs compared to other storage methods. The solution mining technique is the primary technology for constructing salt caverns, which is a mature and cost-effective process, with an estimated development cost of USD 1–USD2/m³ of storage capacity [66], significantly lower than the costs associated with repurposing depleted gas reservoirs, aquifers, or constructing above-ground tanks. Existing infrastructure, such as repurposed natural gas salt caverns, can further reduce initial investments. For instance, the Clemens Dome project in Texas utilizes existing salt caverns originally designed for natural gas storage, resulting in a 40% reduction in development costs [39,67]. Due to the self-sealing properties of salt, operational costs are also minimized, greatly reducing the need for ongoing maintenance or leak monitoring. Furthermore, the scalability of salt caverns allows for phased expansions in response to the growing demand for hydrogen, avoiding large-scale initial investments. Compared to liquid hydrogen tanks, salt cavern storage can reduce the levelized cost of hydrogen storage (LCOHS) by 15% to 20% [51,65].

The primary advantage of underground hydrogen storage is its relatively low storage costs compared to other storage methods. Utilizing salt caverns for hydrogen storage offers high safety, large capacity, and long duration benefits. However, in the economic domain, capital costs and operating costs are the main expenses considered when assessing the profitability of financial assets, which can be broadly categorized into three types: gas costs, new well construction or old well refurbishment costs [68], and compressor costs. By employing cost calculation methods [51], the total costs of different hydrogen storage methods are estimated (

Table 3. The total cost of different hydrogen storage methods.

Hydrogen Storage Medium	Salt Caverns	Depleted Oil/Gas Reservoirs	Aquifer	Above-Ground Tanks
Cost (USD/kg H2)	0.39–2.41	1.43	1.50	5.00–15.00

).

The cost of storing hydrogen in depleted oil/gas reservoirs, aquifers, salt caverns, and above-ground tanks has been estimated [65,69,70]. The initial construction costs for hydrogen storage are high, which include the costs of the buffer gas required to maintain the minimum storage pressure. Compared to above-ground tanks, salt caverns can reduce storage costs by 70–90%. The cost of hydrogen storage in salt caverns is influenced by multiple factors, including the geological reservoir characteristics, the scale of the salt cavern, and the specifications of the brine treatment pipeline, which directly affect construction costs. The selection of gas compression equipment and dehydration systems also varies due to differences in the depth and capacity of the storage facility. Furthermore, when constructing underground salt cavern hydrogen storage systems, it is necessary to consider the hydrogen embrittlement issues and their economic impacts caused by the interaction between hydrogen flow and infrastructure.

3.3. Environmental and Safety Benefits

Salt caverns offer significant environmental and safety advantages, aligning with global sustainable development goals [56]. Their impermeable nature virtually eliminates the risk of hydrogen leakage, which is crucial for mitigating climate change given the high global warming potential of hydrogen released into the atmosphere [11]. Unlike aquifer storage, salt caverns do not compete with freshwater resources and pose no risk of groundwater contamination, as the leaching process utilizes a controlled brine circulation system [30]. The compact footprint of underground storage also minimizes land use conflicts, a critical advantage in densely populated areas. For instance, China’s Jintan salt cavern project occupies less than 0.1 square kilometers while providing 1.5 terawatt-hours of seasonal energy storage. The ability of salt to withstand extreme pressure and temperature further enhances safety, reducing the risk of catastrophic failure in emergency situations. Regulatory frameworks, such as the U.S. Department of Energy’s 2024 Roadmap, emphasize the role of salt caverns in achieving net-zero goals, prioritizing the development of technologies that combine environmental safety with high energy density [39,71].

4. Challenges and Limitations

4.1. Technical Challenges

4.1.1. Hydrogen Leakage and Interlayer Risks

Due to its small molecular weight, low viscosity, and strong diffusion capacity (

Table 4. Physical properties of hydrogen and methane. Adapted with permission from Ref. [72]. Copyright 2024, Elsevier.

Gas	Relative Molecular Mass	Density (kg/m3)	Viscosity (Pa·s)	Solubility in Water (g/L)	Standard Boiling Point (°C)	Diffusion Rate in Water (m2/s)	Explosive Concentration Range
Hydrogen	2.016	0.089	0.89 × 10−5	16 × 10−4	−252.9	5.13 × 10−9	4~75%
Methane	16.043	0.657	1.1 × 10−5	22.7 × 10−3	−162.2	1.85 × 10−9	5~15%
Carbon dioxide	44.009	1.842	1.47 × 10−5	1.69	−79.2	1.60 × 10−9	-

), hydrogen is prone to penetrate through micro-fractures or pores in salt rocks under high-pressure storage conditions [72,73], particularly in salt caverns with non-salt interlayers or structurally heterogeneous structures (Figure 5).

Existing simulations show that when the permeability of interlayers exceeds 1 × 10⁻¹⁷ m², the cumulative hydrogen leakage rate through interlayers after 30 years of operation can be as high as 45% [74,75]. Hydrogen leakage pathways in salt caverns include salt rock, interlayers, and salt rock-interlayer interfaces (Figure 6).

The initial permeability of mudstone interlayers (10⁻¹⁵–10⁻¹³ m²) is significantly higher than that of pure halite (10⁻²⁰–10⁻¹⁹ m²) [77]. Additionally, their creep rate is 20–30% faster than that of pure salt rock, leading to a local cross-sectional area reduction of 1.0–2.0% within a decade [78]. This geological evolution increases the field leakage rate from 50 t/a to 600 t/a over ten years, with over 90% of the leakage concentrated in the interlayer sections [79].

4.1.2. Wellbore Integrity Failure

Wellbore integrity failures account for over 50% of global gas storage leakage accidents [80,81]. Key causes include hydrogen embrittlement of tubing materials, corrosion, and mechanical failure (Figure 7) [31,76]. Hydrogen embrittlement—a process where hydrogen atoms diffuse into metallic wellbore casings or salt cavern walls [71,82]—degrades material integrity over time, necessitating advanced coatings or alternative materials [83,84]. Microbial activity in residual brine poses another challenge: sulfate-reducing bacteria metabolize hydrogen [83,84], causing gas loss and generating corrosive byproducts such as hydrogen sulfide that accelerate wellbore corrosion. In porous media hydrogen storage facilities (e.g., depleted gas reservoirs), 10–61% hydrogen loss (converted to CH₄/H₂S) has been observed [85], though microbial depletion remains less documented in salt caverns due to their high-purity hydrogen environments [86].

4.1.3. Long-Term Geomechanical

Geomechanical risks associated with salt rocks are significant due to their excellent creep characteristics, which lead to the inward contraction of salt caverns over prolonged operation, resulting in roof subsidence and floor heave. Frequent cycles of injection and extraction exacerbate this phenomenon. Laboratory data indicate that at 35 °C and 8 MPa confining pressure, the annual strain of salt rock ranges from 0.2% to 0.5%, with a cumulative volume convergence of 0.5% to 1.5% over ten years [87]; the shrinkage rate of the Jintan salt cavern in China after 30 years of operation at 12 MPa pressure is approximately 3.5% [87]. In actual operations, salt rocks are in a superimposed state of creep (leading to tensile failure) and fatigue (leading to shear failure) [88]. When the loading and unloading rates increase, the proportion of creep deformation decreases [89]. Mechanical fatigue increases the permeability of salt rocks due to micro-cracking, but the self-healing capability during the creep stage can significantly reduce permeability (with permeability recovering from 10⁻¹⁷ m² to 10⁻²⁰ m² within 30 days under 20 MPa confining pressure), demonstrating that salt caverns are suitable for large-scale hydrogen storage [90]. Maintaining a pressure of ≥10 MPa can activate self-healing [91], while the Clemens project in the United States controls the annual shrinkage rate to below 0.05% by maintaining a pressure of ≥7 MPa [92].

In the geological and hydrological study of hydrogen storage in salt caverns, the geomechanical properties of rock salt are a key factor [93]. Based on the short-term mechanical testing results [94,95], the creep behavior of rock salt can be divided into three stages according to the variation in strain over time: primary (transient) creep, secondary (steady-state) creep, and tertiary (accelerated) creep (Figure 8). In the primary creep stage, the strain increases rapidly at first, then the rate of increase gradually slows down; upon entering the secondary creep, the strain rate tends to stabilize; while the tertiary creep, as the final deformation stage before failure, exhibits an exponential rise in strain rate until the rock salt structure is destroyed [96,97].

4.1.4. Chemical Evolution and Earthquake Risk

The penetration of hydrogen into the surrounding rock of salt caverns may be accompanied by potential biochemical reactions [93,98]. The dissolution of minerals and the formation of solid products may alter the pore structure of the rock. The penetration of hydrogen into the surrounding rock of salt caverns may be accompanied by potential biochemical reactions (

Table 5. Potential biochemical reactions in salt cavern hydrogen storage. Adapted with permission from Ref. [76]. Copyright 2023, Elsevier.

Reactants/Microorganisms	Resultant	Chemical Reaction
Carbonate	$\mathrm{C}{\mathrm{H}}_4$	$9{\mathrm{H}}^{+}+\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+8{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{H}}_4+3{\mathrm{H}}_2\mathrm{O}$
Calcite	$\mathrm{C}{\mathrm{H}}_4$	$\mathrm{C}{\mathrm{O}}_3^{2-}+10{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{H}}_4+3{\mathrm{H}}_2\mathrm{O}$
	$\mathrm{C}{\mathrm{O}}_2$	$\mathrm{C}{\mathrm{O}}_3^{2-}+2{\mathrm{H}}^{+}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$
Gypsum	$\mathrm{H}{\mathrm{S}}^{-}$	$\mathrm{S}{\mathrm{O}}_4^{2-}+4{\mathrm{H}}_2+{\mathrm{H}}^{+}\to \mathrm{H}{\mathrm{S}}^{-}+4{\mathrm{H}}_2\mathrm{O}$
Methanogen	$\mathrm{C}{\mathrm{H}}_4$	$\mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\to \mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}$
		$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+4{\mathrm{H}}_2+{\mathrm{H}}^{+}\to \mathrm{C}{\mathrm{H}}_4+3{\mathrm{H}}_2\mathrm{O}$
		$4{\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2\to \mathrm{C}{\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}$
Acetobacterium Acetic acid bacteria	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}$	$2\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+4{\mathrm{H}}_2+{\mathrm{H}}^{+}\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+4{\mathrm{H}}_2\mathrm{O}$
		$4{\mathrm{H}}_2+2\mathrm{C}{\mathrm{O}}_2\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+2{\mathrm{H}}_2\mathrm{O}$
Sulfate-reducing bacteria	$\mathrm{H}{\mathrm{S}}^{-}$	$\mathrm{S}{\mathrm{O}}_4^{2-}+4{\mathrm{H}}_2+{\mathrm{H}}^{+}\to \mathrm{H}{\mathrm{S}}^{-}+4{\mathrm{H}}_2\mathrm{O}$
	${\mathrm{H}}_2\mathrm{S}$	${\mathrm{H}}_2+\mathrm{S}\to {\mathrm{H}}_2\mathrm{S}$
		$\mathrm{S}{\mathrm{O}}_4^{2-}+4{\mathrm{H}}_2+2{\mathrm{H}}^{+}\to {\mathrm{H}}_2\mathrm{S}+4{\mathrm{H}}_2\mathrm{O}$
Iron-reducing bacteria	$\mathrm{F}{\mathrm{e}}^{2+}$	$2\mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2+4{\mathrm{H}}^{+}\to 2\mathrm{F}{\mathrm{e}}^{2+}+4{\mathrm{H}}_2\mathrm{O}$
		$3\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\to 2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}$
		$\mathrm{F}\mathrm{e}{(\mathrm{O}\mathrm{H})}_3+3{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to \mathrm{F}{\mathrm{e}}^{2+}+3{\mathrm{H}}_2\mathrm{O}$
	$\begin{array}{l}{\mathrm{N}}_2\hfill \end{array}$	$2\mathrm{N}{\mathrm{O}}_3^{-}+5{\mathrm{H}}_2+2{\mathrm{H}}^{+}\to {\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}$
	${\mathrm{H}}_2\mathrm{O}$	$2{\mathrm{H}}_2+{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}$

) [96]. The dissolution of minerals and the formation of solid products may alter the pore structure of the rock [89].

The long-term chemical actions lead to a significant increase in permeability during the dissolution phase (1–3 years), reaching 10⁻¹⁷ m², followed by a partial decrease during the precipitation phase (3–10 years) to 10⁻¹⁸ m², although the high-stress zones do not completely heal [99]. Hydrogen permeation may occur alongside mineral dissolution/precipitation, altering the pore structure of the surrounding rock [89].

Seismic activity may activate faults within salt cavern hydrogen storage facilities, thereby affecting the integrity of the storage. Fault activation could transform previously sealed faults into permeable or ventilated paths, providing potential channels for hydrogen leakage. Additionally, earthquakes may damage the wellbore structure, impacting its integrity. To mitigate these risks, site selection should avoid areas with frequent seismic activity. During the design phase, it is essential to fully consider the impact of earthquakes on salt cavern hydrogen storage, employing seismic design standards to enhance the seismic resilience of wellbores and storage structures. Furthermore, monitoring should be strengthened to continuously observe the effects of seismic activity on salt cavern hydrogen storage, allowing for timely identification and resolution of potential issues.

4.2. Economic and Regulatory Barriers

The economic viability of salt cavern hydrogen storage is hindered by high upfront costs and regulatory uncertainties. Although the solution mining method is cost-effective compared to other approaches, the development of new salt caverns in deep salt formations can exceed USD 50 million per project, with the leaching process alone accounting for 40–60% of the total cost [15,39]. Repurposing existing natural gas salt caverns can reduce costs; however, modifications are necessary to meet specific safety standards for hydrogen, which are inconsistently enforced across different regions [29]. The regulatory framework for hydrogen storage is still in its infancy, with many countries lacking clear guidelines on risk assessment, monitoring, or liability allocation. For instance, the EU’s hydrogen strategy acknowledges the potential of salt caverns, but implementation is left to member states, leading to policy fragmentation that hampers cross-border investment. Additionally, market uncertainties, such as fluctuations in hydrogen demand and competition from emerging storage technologies, such as liquid organic hydrogen carriers, further complicate financial planning [15].

4.3. Social and Environmental Concerns

The salt cavern hydrogen storage facility is not only a key infrastructure for the national energy reserve, but is also closely related to the lives of enterprises and residents. When selecting a site, it is necessary to consider not only the local social acceptance, but also to comprehensively evaluate the local hydrogen energy policies and coordinate with the local hydrogen energy industry system to promote the development of the local hydrogen energy industry.

Public acceptance and environmental impact present significant challenges to the expansion of salt cavern storage. The discharge of brine during the salt cavern leaching process raises ecological concerns, and improper management may lead to groundwater contamination or soil salinization [100]. The Jintan project in China addresses this issue by transporting the brine generated from water solution mining to nearby salt factories via pipelines. There, it undergoes processes such as evaporation and crystallization to produce industrial and edible salt. A portion of the brine is supplied as raw material to chlor-alkali enterprises for the production of caustic soda (NaOH), soda ash (Na₂CO₃), and chlorine gas (Cl₂). This project exemplifies the resource utilization of brine rather than mere treatment; however, it is highly dependent on surrounding chemical facilities, making it difficult to replicate in remote salt layer areas. For low-concentration brine that cannot be directly utilized, such as tail brine with a high impurity content, it is reinjected into the salt cavern for further cavity creation or into deep non-potable aquifers (depth > 1500 m) using high-pressure pumps to prevent contamination of shallow groundwater. A small amount of brine containing heavy metals or organic substances is treated through processes such as neutralization, sedimentation, and membrane separation to meet discharge standards before being released. However, such solutions require substantial infrastructure investment. Additionally, land use conflicts arise in densely populated or ecologically sensitive areas, where communities may oppose underground projects due to concerns over land subsidence or explosion risks. Misconceptions regarding hydrogen safety, such as its flammability range in air (4–75%), often overshadow its benefits, necessitating robust public engagement initiatives. Furthermore, the high-energy-consuming leaching processes reliant on freshwater conflict with the sustainable development goals of water-scarce regions such as Inner Mongolia, where desalination plants are now being integrated into project designs.

In densely populated or ecologically sensitive regions, land use conflicts may arise, and communities may oppose underground projects due to concerns about ground subsidence or explosion risks. Misunderstandings regarding hydrogen safety, such as the flammability range of hydrogen in air (4–75%), often overshadow its benefits, necessitating strong public engagement activities. Meanwhile, the energy-intensive leaching process that relies on freshwater conflicts with the sustainable development goals of water-scarce regions such as Inner Mongolia. For arid and water-scarce areas, further water optimization strategies are required, such as integrating seawater desalination plants or utilizing treated urban reclaimed water for leaching, which can significantly alleviate freshwater pressure. Alternatively, developing more efficient leaching cavity technologies (e.g., blasting permeation, bidirectional brine discharge) and brine purification technologies can increase the brine reinjection ratio and reduce the demand for new water supply. Additionally, a non-aqueous cavity construction method is proposed, which theoretically could eliminate brine discharge through mechanical excavation (e.g., TBM tunneling) in thick homogeneous salt domes, though it is costly, has limited geological adaptability, and is of low technological maturity.

To comprehensively assess the feasibility of salt cavern hydrogen storage (SCHS) technology, a strengths, weaknesses, opportunities, and threats (SWOT) analysis is conducted based on the previously discussed technical, economic, and operational dimensions (

Table 6. SWOT analysis table for salt cavern hydrogen storage.

Category	Specific Aspects	Key Data/Explanation
Strengths	Geological integrity	Ultra-low permeability (<10−20 m3)
		Self-healing capability
		Minimal H2 leakage
	High-purity storage	Chemical inertness
		Maintains > 95% H2 purity
	Operational flexibility	Rapid injection/extraction rates
		Supports grid balancing
	Cost efficiency	Lower than alternatives (e.g., depleted reservoirs)
Weaknesses	Hydrogen embrittlement	Wellbore degradation under cyclic loading
		Requires advanced coatings
	Microbial activity	H2 consumption by sulfate-reducing bacteria
		Corrosive H2S production
	Geomechanical risks	Creep-induced cavern shrinkage
		Aggravated by frequent pressure cycling
	Regulatory gaps	Lack of standardized H2 safety protocols
		Regional regulatory inconsistencies
Opportunities	Policy support	Accelerates deployment
	Infrastructure repurposing	Conversion of existing natural gas caverns
	Renewable integration	Coupling with green H2 production
		Circular economy model
	Technological innovation	Real-time fiber-optic monitoring
		AI-driven predictive models
Threats	Microbial contamination	H2 loss in heterogeneous salt layers
	Brine management	Ecological risks from saline wastewater
	Market volatility	Supply chain disruptions
	Public resistance	Safety misconceptions (flammability range: 4–75%)

).

5. Prospects and Future Directions

The development of salt cavern underground hydrogen storage (UHS) lies at the intersection of technological advancement, policy coordination, and system integration within the broader hydrogen economy. This section explores the multifaceted pathways required to realize its potential as a critical component of sustainable energy systems.

5.1. Technological Innovations

Technological breakthroughs in salt cavern hydrogen storage focus on enhancing monitoring precision and predicting long-term stability [101,102]. Fiber optic sensors are increasingly utilized for real-time detection of micro-leakages and monitoring structural integrity [103], offering unprecedented resolution in tracking hydrogen behavior within salt caverns [104,105]. These systems not only mitigate the risk of gas migration, but also enable predictive maintenance through integration with machine learning algorithms that analyze data trends to forecast potential failure points. Concurrent advancements in modeling tools are reshaping the prediction of salt cavern dynamics. Modern geomechanical simulations now incorporate complex variables, such as salt creep (a slow, time-dependent deformation of rock) and multiphase fluid interactions, under varying pressure conditions. By embedding artificial intelligence into these models, researchers can more accurately simulate the long-term stability of salt caverns, thereby developing adaptive management strategies for fluctuating hydrogen demand cycles.

5.2. Policy and Market Drivers

The policy framework and international cooperation are crucial for accelerating the adoption of UHS. Initiatives such as the European Union Hydrogen Strategy and the U.S. Hydrogen Earthshot initiative reflect government efforts to subsidize green hydrogen production and incentivize carbon-neutral storage solutions. These policies are complemented by cross-border alliances, such as the North Sea Energy Alliance, which aims to coordinate regulatory standards in regions with favorable geological conditions and establish shared infrastructure. In addition to regulatory support, the role of hydrogen in decarbonizing industries where electrification is challenging, such as steel production and aviation, is receiving increasing attention. For instance, hydrogen-driven direct reduced iron (DRI) technology can reduce emissions in steel production by over 90%, while synthetic aviation fuel (SAF) derived from green hydrogen is a viable alternative to fossil-based kerosene.

Underground hydrogen storage (UHS) serves as a crucial link for the large-scale, low-cost storage of green hydrogen, directly supporting the realization of deep decarbonization pathways. Its industrial benefits are not only reflected in the reduction in emissions, but also in the provision of reliable hydrogen supply, enabling continuous operations in steel mills, chemical plants, and others, thereby reducing direct dependence on volatile renewable energy sources and enhancing overall industrial competitiveness. Strategic partnerships among storage operators, industrial consumers, and renewable energy suppliers are essential for aligning the supply chain with these emerging applications. This integration model is gradually forming a prototype of a circular economy characterized by ‘renewable energy hydrogen production—salt cavern storage—and industrial decarbonization’ [66].

5.3. Integration with the Hydrogen Economy

The success of UHS hinges on its ability to seamlessly integrate into the hydrogen value chain from production to end-use. Hydrogen is increasingly becoming a significant energy source in the energy transition, particularly for coupling the transportation, heating, and power sectors. Suitable storage technologies must be developed to ensure safe and efficient long-term storage of hydrogen. The HyCAVmobil project, funded by the German Federal Ministry for Digital and Transport, is investigating hydrogen storage in German salt caverns and the subsequent utilization of hydrogen in fuel cell mobility. The outcomes of this project aim to contribute to ensuring a sustainable energy supply [51].

The effectiveness of underground hydrogen storage (UHS) is highly dependent on its integration capabilities throughout the hydrogen value chain. As a core carrier of energy transition, hydrogen has been widely integrated into various fields such as transportation, heating, electricity, and chemical industries, necessitating compatible long-term storage technologies to ensure its safe and efficient application. Current research focuses on the deep integration and innovation of UHS with energy systems. On one hand, salt caverns can serve as giant “green batteries,” absorbing excess electricity from wind/solar power plants to produce hydrogen, which can be released during energy supply–demand imbalances, providing seasonal or weekly peak-shaving capabilities. On the other hand, by coupling with carbon capture and utilization (CCU) technologies, a circular economy system of “green hydrogen production—salt cavern hydrogen storage—carbon sequestration—chemical synthesis” can be established. Some studies propose injecting the carbon dioxide (a byproduct from the synthesis of fuels such as E-methane, E-methanol, and E-ammonia, obtained from biomass gasification or direct air capture DAC) produced during green hydrogen synthesis into suitable geological formations nearby for sequestration, while the green hydrogen is stored in salt caverns. A typical case is the German HyCAVmobil project, which systematically explores the application pathways of salt cavern hydrogen storage in the fuel cell transportation sector.

Economic assessment is crucial for such integrated schemes. The levelized cost of hydrogen storage in salt caverns (LCOHS) is influenced by various factors, including geological conditions, cycling frequency, and equipment prices [69]. Research indicates that the strategic reserves in salt caverns, along with economies of scale and low operation and maintenance costs, provide a cost advantage over above-ground storage tanks. Furthermore, the value of hydrogen storage in salt caverns within renewable energy systems is not only reflected in savings on storage costs, but also in its support for large-scale green hydrogen production and high-value decarbonization industries (such as green steel, chemicals, and e-fuels) [106]. The benefits of system integration and carbon reduction significantly enhance economic feasibility. With the optimization of solution cavity technology, decreasing equipment costs, and the refinement of carbon pricing mechanisms, the overall economic competitiveness of hydrogen storage in salt caverns coupled with renewable energy systems is expected to continue to improve [11].

6. Case Studies and Global Applications

6.1. Global Practical Experience and China’s Geological Hydrogen Storage Challenges

With the advancement of new energy generation and the large-scale application of green hydrogen, various pilot projects in countries such as the Netherlands (HyStock), Germany (HyCAVmobil), and France (Hypster) are focusing on the coupling research of new energy and geological pure hydrogen storage [92]. China accumulated rich experience in the field of underground space energy storage, having established projects such as the Huitu Bih gas storage facility and the Jintan salt cavern compressed air energy storage power station in the area of underground space utilization (e.g., natural gas storage, CO₂ sequestration, and compressed air energy storage) [103]. However, research on geological hydrogen storage started relatively late, and there are currently no commercial projects in operation. Unlike foreign countries that build gas storage facilities relying on thick, homogeneous salt rock formations, China’s terrestrial lacustrine salt mineral resources exhibit characteristics of thin salt layers, numerous interlayers, and strong heterogeneity. The saline formations often contain insoluble interlayers such as mudstone and gypsum. Research indicates that the permeability of mudstone can vary by up to six orders of magnitude due to differences in clay mineral content [107], and the permeability of clay is highly sensitive to damage levels and stress states, leading to significant challenges in controlling the seal integrity and long-term stability of hydrogen storage facilities [77].

6.2. Site Selection Technical Standards and Evaluation System

Based on the characteristics of salt mine strata, the recommended burial depth for hydrogen storage caverns in China is 500–1500 m (optimal depth 1000 m) to balance pressure requirements, construction costs, and the risk of salt rock creep. The cumulative thickness of the salt layer should be >100 m, with a salt content >70%, a stable distribution of single salt layers with interlayer thickness 60%. The cap rock must be dense and continuous (thickness > 30 m) without permeable layers or aquifers. Additionally, the site should be far from densely populated areas, have sufficient water sources and brine disposal channels, and be close to hydrogen sources or hydrogen load centers. Site selection is carried out by laboratory analysis of parameters such as salt layer permeability and compressive strength, combined with numerical simulations to predict the pressure evolution, cavity deformation, and long-term sealing performance of the hydrogen storage cavern under injection and extraction cycles, thereby constructing a multi-scale evaluation system to ensure scientific rigor.

6.3. Key Technological Innovations and Engineering

In response to the demands of salt cavern hydrogen storage engineering, China made technological breakthroughs at the material and process levels. This includes hydrogen reduction through steel heat treatment, modification with vanadium/copper alloys, and shot peening to enhance surface stress, coupled with the development of novel hydrogen barrier coatings and hydrogen embrittlement-resistant aluminum alloys to improve the hydrogen embrittlement resistance of pipes [80,81,108]. Optimization of brine recycling technology and exploration of interlayer blasting and infiltration processes are being pursued to enhance the efficiency of solution mining in continental thin salt layers. Pilot projects in places such as Yexian, China, are conducting adaptability verification for salt layer hydrogen storage based on the aforementioned technological framework, aiming to provide the first engineering model for hydrogen storage in continental thin salt layers in Asia.

6.4. Regional Potential and Development Prospects

China has a wide distribution of salt mine resources, and layered salt regions exhibit significant potential for hydrogen storage development. In terms of geological conditions, although the continental layered salt formations in areas such as Jintan, Jiangsu, Yunying, Hubei, and Pingdingshan, Henan, are characterized by “thin salt layers and numerous interlayers,” it is still possible to realize hydrogen storage functionality through refined site selection and process innovation. Among them, the Jintan salt cavern area already initiated compressed air energy storage engineering practices, and its geomechanical data provide a foundational reference for the construction of hydrogen storage facilities. By 2060, the demand for hydrogen storage in salt caverns (SCHS) in China is expected to reach approximately 86.84 TWh, with over 30 confirmed salt mines (Figure 9) and a total storage capacity of 14.6 trillion tons [108]. Currently, various potential regions are accelerating exploration and technological validation, breaking through technical bottlenecks through interdisciplinary research, thereby facilitating the large-scale application of hydrogen energy and the transformation of the energy structure under the “dual carbon” goals.

7. Conclusions

Hydrogen energy is a clean and low-carbon energy source. Actively promoting the development of the hydrogen energy industry is a crucial measure for achieving dual carbon goals and addressing the energy transition. Utilizing underground geological structures for hydrogen storage is an effective approach for large-capacity, long-term hydrogen storage and represents a significant direction for future large-scale hydrogen reserves. Salt cavern hydrogen storage, characterized by its large scale, strong sealing, high purity of stored hydrogen, and existing operational project cases, has become a prioritized development direction for geological hydrogen storage.

However, hydrogen, with its strong permeability and high activity, poses significant challenges for the construction of salt cavern hydrogen storage facilities. Currently, within the field of salt cavern hydrogen storage technology, challenges exist in three main areas: technical, economic and regulatory, and social and environmental. In the technical realm, issues such as hydrogen permeation and biochemical reactions in layered salt rocks, wellbore integrity in salt cavern hydrogen storage, and creep fatigue failure of salt rocks highlight the necessity for continuous innovation and interdisciplinary collaboration.

Internationally, several operational projects and validation platforms for geological hydrogen storage have been established. In China, the construction of salt cavern gas storage began relatively late; however, in recent years, China accelerated the development of validation platforms, successfully commissioning five salt cavern natural gas storage facilities and two salt cavern compressed air energy storage power stations, thereby laying a solid foundation for the construction of salt cavern hydrogen storage projects. The construction cycle of salt cavern hydrogen storage can reference the experience of salt cavern natural gas storage, which is divided into site selection, leaching, and operational optimization. Although insights from salt cavern natural gas storage facilities can be applied at various stages of construction, challenges related to policies, materials, construction techniques, and safety operation regulations still persist.

Technological advancements, particularly in fiber optic monitoring systems and AI-driven predictive modeling, are poised to address critical safety and efficiency gaps. These innovations not only enhance real-time leak detection capabilities, but also deepen our understanding of salt creep behavior and multiphase flow dynamics, thereby ensuring the resilience of salt caverns over decades of operation. By incentivizing green hydrogen production and promoting cross-border infrastructure sharing, governments can foster a harmonized market ecosystem.