Underground Hydrogen Storage: A Comprehensive Review of Technologies, Geological Formations, and Future Prospects

Abstract

Hydrogen (H₂) is becoming a meaningful way to store energy for long-term use and support thorough decarbonization in systems that use renewable energy. Underground hydrogen storage (UHS) has strategic benefits over above-ground systems because it can hold large volumes, is contained by geology, and is cheap to operate in cycles. This review compares four key geological formations for underground hydrogen storage (UHS): salt caverns, lined rock caverns, depleted hydrocarbon reservoirs, and saline aquifers. Each system is evaluated based on storage mechanisms, efficiency, safety, technological maturity, and economic feasibility. This review also introduces a unified cross-media evaluation framework, a TRL-risk matrix, a technology development roadmap, and novel insights into AI-based monitoring, offering prescriptive guidance for large-scale UHS implementation. Salt caverns have high injectivity, maintain their purity, and undergo 6 to 12 cycles per year at pressures of 60 to 180 bar; however, they are only found in certain places. Lined rock caverns can be built anywhere, but sealing and economic issues make them difficult to use. Depleted hydrocarbon reservoirs with TWh-scale capacity and already built infrastructure. Saline aquifers, on the other hand, have the most potential in the world but need enhanced management of microbiological responses and cushion gas optimization. A synthesis of current studies highlights key research gaps in cyclic geomechanics, hydrogen–rock–microbe interactions, and liner performance for high-pressure storage. The review concludes with techno-economic and safety considerations and identifies future directions for deploying geological UHS as a critical component of a net-zero hydrogen economy.

1. Introduction

Fossil fuel emissions are a significant contributor to climate change, posing a major threat to the Earth. The quest for sustainable and clean energy solutions has led to a growing interest in hydrogen (H₂) as a versatile and environmentally friendly energy carrier. As an essential alternative to existing fossil fuels, H₂ can help address urgent environmental concerns, mitigate greenhouse gas emissions, and make the energy landscape more sustainable.

Hydrogen is central to the global energy transition due to its potential to decarbonize sectors such as power, transportation, and industry [1]. Hydrogen has the potential to decarbonize various sectors, including power generation, transportation, chemicals, and heating, due to its clean, versatile, and energy-dense characteristics. Hydrogen does not release carbon dioxide when it is not used, and it can be made from renewable sources [2]. It allows a net-zero carbon economy. Hydrogen must be produced and supplied at a low cost, stored, and integrated into large-scale energy systems to meet the demands of supply and demand [3]. Underground hydrogen storage (UHS) is a key technology in the new H₂ economy. It is meant to fix the time difference between when renewable energy is made and when it is needed. As the world moves toward cleaner energy, being able to securely, efficiently, and sustainably store vast amounts of H₂ becomes a key part of deep decarbonization.

Underground Hydrogen Storage (UHS) is the best option for large-scale, long-term storage among the current options compared to High-pressure tanks [4], cryogenic storage [5], metal hydrides [6], and adsorption materials [7], which are all examples of surface-based solutions. UHS uses natural and artificial subsurface geological formations to store vast amounts of H₂ at high pressure. This is based on decades of experience with underground natural gas storage. Its strategic advantages include a bigger storage capacity (gigawatt to terawatt hours), geological containment for safety, and a lower long-term energy cost per unit [8,9].

This article examines the present and future of UHS by analyzing the primary geological formations utilized for storage, including salt caverns, lined rock caverns, abandoned mines, depleted hydrocarbon reserves, and saline aquifers. Each formation has its own set of challenges and problems that need to be solved. In the US and Europe, salt caverns made via solution mining are already well-known for storing H₂ because they are very tight, stable, and can cycle quickly [10]. However, they are limited by their location and have a moderate amount of storage space. Lined rock caverns (LRCs) and abandoned mines are other options when salt formations are not present. They are more flexible in terms of location, but they are also more expensive to build and raise issues about the integrity of the materials used [11]. Depleted oil and gas reserves are very promising because they have been shown to hold geological confinement and have existing infrastructure in place [12]. This makes them good places to store H₂ seasonally and strategically. Saline aquifers are the most common and widely spread choice. They have a lot of potential, but they are also the least understood and least explored right now due to geological uncertainty and recovery issues [13,14].

The future use of UHS is closely tied to advancements in H₂ generation technology, the expansion of renewable energy, shifts in the energy market, and new regulations. UHS will help balance energy systems, reduce the need to curtail renewables, enable sector coupling, and enhance energy security as countries implement H₂ roadmaps and develop H₂ infrastructure, including electrolysis hubs, pipeline networks, and storage corridors. However, to make this happen, people from various fields, including geology, reservoir engineering, materials science, microbiology, policy, and economics, need to collaborate.

Although numerous studies have reviewed UHS, most focus on either specific geological systems or isolated aspects such as microbiology, geomechanics, or hydrodynamics. Existing reviews do not provide an integrated comparison across all storage formations using a combined set of criteria for capacity, cyclic performance, geomechanical integrity, microbiological risks, and techno-economics.

Numerous review articles have addressed underground hydrogen storage from specific disciplinary or geological perspectives, such as geomechanics in salt caverns [12], microbial behavior in depleted reservoirs [1], or aquifer hydrodynamics [15,16]. Some studies focus exclusively on a single storage type [17], while others provide broad overviews without applying a uniform framework for comparison [3,8]. In contrast, this review presents a more comprehensive and structured approach. It applies a unified evaluation framework (as shown in Table 6) to systematically compare the four primary UHS types across functional, technical, economic, and environmental criteria. It also introduces an integrated technology readiness and risk matrix that combines Technology Readiness Levels (TRLs) with geotechnical and operational risk assessments. Additionally, the review outlines a phased development roadmap for UHS deployment that aligns with international energy transition targets. Notably, it also provides a forward-looking analysis of how artificial intelligence can be integrated into UHS planning and operations, an area that has been largely overlooked in existing literature.

This study also highlights problems that still need to be addressed, including cyclic stresses, hydrogen reactivity, and cushion-gas optimization. This sets research objectives for the large-scale use of UHS in new hydrogen economies.

Table 1. Evaluation criteria for UHS technologies.

Criteria	Description
Geological Integrity	Caprock quality, seal strength, permeability
Storage Capacity & Scale	Storage volume (GWh–TWh), scalability potential
Cyclic Performance	Frequency of injection/withdrawal cycles
Hydrogen Purity Retention	Effect of storage on hydrogen quality
Technology Maturity (TRL)	Level of commercial or pilot readiness
Economic Feasibility	Capital and operating costs, LCOHS
Environmental Impact	Risks such as leakage, brine disposal, and microbial effects

presents a standardized evaluation framework used to compare each UHS method.

2. Underground Hydrogen Storage: Classification and Principles

Underground hydrogen storage (UHS) may mitigate the disparity between supply and demand caused by the variability of renewable energy sources. The UHS method also makes storage safer than standard above-ground solutions since it reduces the amount of oxygen that stored H₂ can come into contact with, like in aquifers [1,15].

There has been a lot of recent research on UHS in depleted gas and oil reserves, aquifers, and man-made underground spaces, such as salt and rock caverns. These geological formations have certain good traits, such as (i) being gas-tight enough, (ii) having walls that are much thicker than those of regular storage tanks, and (iii) being very far underground, which makes them safer.

Storing H₂ in porous medium, like aquifers or depleted reservoirs, requires the right geological structures, which are different from those needed for storing natural gas. These structures consist of well-confined porous and permeable formations encircled by impermeable cap rock or seals to store H₂ with minimal losses safely [16]. Even though the frequency of gas extraction varies, with H₂ storage happening more often, most of what we know about storing H₂ in subsurface systems comes from storing natural gas (NGS).

Table 2. Comparison of four underground hydrogen storage methods by function, characteristics, and challenges.

Storage Type	Function	Characteristics	Problems
Salt Caverns	Artificial salt caverns for pure H2 storage	High purity Rapid cycling Good seal	Salt basins High cost Water use
Lined Rock Caverns	Use old mine voids for H2 storage	Existing voids Low cost Local to mines	Leakage risk Structure weak Gas mixing
Depleted Reservoirs	Reuse oil/gas reservoirs for bulk H2 storage	Existing wells Known geology Large volume	Contamination Old wells leak H2S risk
Saline Aquifers	Natural porous rock with water for H2 storage	Large capacity Widely available Lower cost	Seal uncertain Microbial H2S H2 solubility

compares four underground hydrogen storage types by function, characteristics, and problems. Salt caverns provide pure, fast-cycled storage but are costly and location-limited. Depleted fields reuse infrastructure but risk contamination. Aquifers offer large, widespread capacity yet face sealing and microbial issues. Abandoned mines are low-cost but prone to leakage and instability.

Before evaluating each type of UHS, the table below (

Table 3. Technology Readiness and Deployment Status of UHS Methods.

Storage Type	TRL (2025)	Example Projects	Deployment Status
Salt Caverns	8–9	ACES (USA), Teesside (UK)	Commercial
Lined Rock Caverns	4–6	HyBRIT (Sweden)	Pilot
Depleted Reservoirs	6–7	Projects in the UK, Austria, The Netherlands	Field Trials
Saline Aquifers	2–4	HyUsPRe (EU), Germany simulations	Conceptual/Lab Stage

) summarizes their current technology readiness, ongoing projects, and level of implementation.

2.1. Salt Caverns

Salt caverns are optimal for extensive, long-term storage to equilibrate fluctuating supply and demand as decarbonization and the implementation of renewable energy accelerate. Salt caverns leverage the near-zero permeability, self-healing characteristics, and chemical inertness of rock salt, in contrast to surface H₂ storage tanks or porous formations like aquifers or depleted oil and gas reservoirs [17].

Salt caverns are currently the most mature and commercially deployed UHS method, particularly suited for frequent cycling and high-purity hydrogen storage. Salt caverns have excellent storage and scalability. Over 100,000 to over a million cubic meters can be stored in caves at depths of 500 to 2000 m. Extremely high-pressure H₂ storage, usually between 20 and 30 megapascals (MPa), is made possible by these depths and volumes [18]. An example of a project that has demonstrated long-term dependability is the Clemens Dome in the United States, which has achieved an H₂ usability rate of 99.8% and over 60,000 pressure cycles without failure [17]. Salt caverns function like large “energy batteries,” storing excess renewable energy during peak times and releasing H₂ when demand increases, or output decreases [19,20].

Table 4. Key characteristics, advantages, and limitations of salt caverns for UHS.

Aspect	Description	Advantages	Limitations
Geological Integrity	Very low permeability (<1 × 10−20 m2), self-healing capability.	Excellent seal, minimal leakage risk, high safety.	Limited to regions with suitable salt geology (salt domes, thick beds) [18]
Capacity & Scale	Depth: 500–2000 m; Volume: up to 1,000,000 m3 per cavern.	High energy density (up to 300 kWhe/m3), suitable for GWh-scale storage.	Total resource limited by salt deposit extent and thickness [18].
Cyclic Performance	Operating pressure: 60–180 bar; capable of 6–12 complete cycles/year.	Rapid injection/withdrawal, ideal for short-term (daily/weekly) grid balancing.	Cavern creep and fatigue under frequent pressure cycling [21].
Hydrogen Purity	Minimal interaction with rock matrix; purity retention > 99.8%.	Retrieved H2 requires little to no purification, reducing operational cost.	Brine residuals during construction may cause minor contamination [21].
Economic Indicators	CAPEX: $1–2/m3 cavern volume; LCOHS: $0.39–2.41/kg H2.	Most cost-effective UHS option compared to surface storage (80–90% cheaper).	High upfront capital; water-intensive construction; brine disposal cost [22].
Environmental Impact	Operational leakage <0.05%/year; small surface footprint.	Low lifecycle emissions; safe long-term containment.	Significant water use and brine production during leaching phase [23].

presents the key characteristics, advantages, and limitations of using salt caverns for underground hydrogen storage (UHS). It covers aspects such as geological integrity, storage capacity, performance, hydrogen purity, economic factors, and environmental impact. The table highlights why salt caverns are a viable and efficient option for large-scale hydrogen storage, while also outlining their geographic and operational constraints.

Salt cavern storage has also been demonstrated to be cost-effective, particularly in terms of capital expenditure (CAPEX) and the levelized cost of H₂ storage (LCOHS). Since solution mining, a well-established and affordable building technology, costs $1–2 per cubic meter, salt caverns are better for the economy [24]. Hydrogen storage in salt caverns is 80–90% cheaper than above-ground tanks. Salt cavern UHS costs $0.39–$2.41 per kg of H₂, while surface tanks cost $5.00–$15.00 [17,25]. The geological integrity of salt reduces leakage and monitoring costs, cutting operational costs. The U.S. Clemens project cut development costs by 40% by merging H₂ storage with natural gas salt cavern infrastructure [26]. This kind of cost-effectiveness makes it more likely that salt cavern systems can be used on a large scale, especially in areas with salt domes or bedded salt formations.

Salt caves have proven more effective than other types of storage media in changing conditions. For instance, the U.S. Advanced Clean Energy Storage (ACES) project in Utah completed 15 successful injection/extraction cycles with a leakage rate of less than 0.08% per cycle, which is the industry standard [27,28]. The pressure maintenance methods are performed for the German Teesside projects instead of cushion gas [29]. Fiber-optic sensor technologies are also being integrated into wellbores to monitor the structure in real-time and detect leaks early, adding layers of redundancy and diagnostic knowledge [30].

Salt caverns offer superior operational flexibility, with rapid injection and withdrawal capabilities ideal for balancing short-term energy fluctuations [31]. In contrast, the injection and production rates are limited in the aquifers and depleted oil and gas fields because of discrepancies in rock permeability and fluid pressure. Salt caverns serve dual functions for long-term seasonal storage and short-term grid stability [32]. Additionally, because salt caverns maintain H₂’s purity during storage, there is no need for complex and expensive purification methods when it is retrieved. This makes operations easier and reduces the cost of downstream processing [27].

Salt cavern storage represents the most technologically mature UHS option, benefiting from decades of operational heritage in natural gas and town gas storage. Proven applications include the pioneering Clemens Dome facility in the USA, which has successfully undergone over 60,000 pressure cycles with 99.8% hydrogen recovery, the large-scale Advanced Clean Energy Storage (ACES) project in Utah targeting 1.5 TWh of green hydrogen storage, and active European pilots such as HyCAVmobil in Germany and the Teesside project in the UK, which integrate storage into regional hydrogen hubs. Economically, salt caverns offer a highly competitive levelized cost of hydrogen storage (LCOHS), driven by capital expenditures for cavern leaching and well completion, operational costs for compression and monitoring, and significant cost-saving potential through retrofitting existing natural gas cavern infrastructure, reducing development costs by up to 40%. Nevertheless, key challenges remain, including geographical limitations to regions with suitable salt deposits, long-term stability concerns related to cavern creep and volume loss, sustainable management of water use and brine disposal, especially in arid areas, and the need to adapt existing natural gas storage regulations to address hydrogen’s higher diffusivity and embrittlement risks.

Solution mining requires a significant amount of water, which poses a sustainability issue in arid regions. The long-term carbon reductions and strategic energy security these systems provide may make them valuable. Improved water management and safe brine disposal may inspire innovative salt cavern designs and operations [33].

Salt caverns are a cleaner choice for storing things than surface tanks or fossil-based storage options from a broader environmental perspective. Salt caverns are a suitable choice for energy infrastructure sensitive to climate change because they leak only small amounts of water, less than 0.05% per year in some U.S. projects. The U.S. Department of Energy’s 2024 Hydrogen Strategy and the European Union’s REPowerEU plan both highlight the importance of underground storage for the growth of the H₂ ecosystem [27]. The essay, on the other hand, discusses several technological and regulatory problems.

In salt caves with layers of mudstone or gypsum, permeability goes up by several orders of magnitude, which makes leaks worse. If nothing is done, rocks having a permeability of more than 1 × 10⁻¹⁷ m² can lose 45% of their H₂ over 30 years [34]. Over time, salt rocks slowly change shape, which can cause the cavern to shrink and the structure to become less stable. Some salt caverns lose 1–3.5% of their volume during the course of 30 years [35]. There are rules for storing natural gas, but H₂ needs its own risk assessment and material compatibility criteria because it is lighter, more diffusive, and chemically active [36]. Highlighting successful programs like ACES in the U.S. and HyCAVmobil in Germany can help change how people perceive them and promote policy advocacy [37].

Table 5. Distinct aspects of salt caverns for H₂ storage.

Aspect	Description	References
Formation Process	Created through solution mining in salt domes or layered salt deposits.	[3]
Impermeability	The salt surrounding the cavern exhibits high impermeability to H2.	[38]
Construction Method	Constructed by injecting water from the surface, eliminating the need for sinking shafts and tunneling.	[39]
Cycle Flexibility	Allows for 6–12 storage cycles per year, accommodating short-term fluctuations in demand or supply.	[40]
Preferred for UHS	Salt caverns are favored for UHS due to their stable chemical characteristics and substantial capacities.	[41]
Current Projects	Large-scale projects are underway in the UK, USA, Germany, and Poland.	[13]
Industrial Utilization	Stored H2 is used in industrial plants, aviation, and petrochemical facilities.	[12]
Depth and Size	Depths up to 2000 m, heights of 300–500 m, and diameters of 50–100 m.	[42]
Storage Capacity	A single cavern can store up to approximately 1,000,000 cubic meters of H2.	[43]
Density Challenges	H2’s low density (0.08988 g/L) presents storage challenges, requiring compression to enhance density.	[44]
Operating Pressure	Caverns operate at 60 and 180 bar pressures, achieving energy densities of up to 300 kWhel/m3.	[44]
Microbial Activity	Low risk is associated with microbial activity, but site-specific studies and monitoring are essential.	[45]
Impact of Brine	High salinity and brine content can reduce microbial diversity, but they require proper disposal or utilization.	[45]

outlines the distinct technical and operational aspects of using salt caverns for H₂ storage. It highlights key features such as their formation, impermeability, storage capacity, and cycle flexibility, along with current global projects and industrial applications. The table also notes specific challenges like hydrogen’s low density and the need for compression, while emphasizing the overall suitability of salt caverns for large-scale underground hydrogen storage.

2.2. Lined Rock Caverns and Abandoned Mines

As part of the global effort to create scalable, safe, and cost-effective storage systems that can balance the intermittency of renewable energy sources and ensure a stable H₂ supply for industry and transport, rock caverns and abandoned mines are emerging as viable underground storage sites (UHS) [46]. Both methods are based on the larger idea of subterranean energy storage, which has worked well for natural gas, oil, and compressed air. However, they need to be carefully re-evaluated for H₂, which is a small, diffusive, and reactive molecule. Table 6 presents a comparative analysis of Lined Rock Caverns (LRCs) and Abandoned Mines for underground hydrogen storage (UHS). It examines key aspects, such as definition, advantages, challenges, safety, cyclic capability, environmental risks, and technology readiness.

Table 6. Comparative analysis of Lined Rock Caverns and Abandoned Mines for UHS [38,39,46,47].

Aspect	Lined Rock Caverns (LRCs)	Abandoned Mines
Definition	Purpose-built excavations in competent rock, sealed with a multi-layer liner.	Pre-existing, often irregular voids from mining activities, requiring significant retrofit.
Primary Advantage	Site selection flexibility independent of specific geology; can be sized as needed.	Low initial excavation cost; potential for rapid deployment using existing voids.
Key Challenges	Very high construction cost ($10–20/m3); hydrogen embrittlement of liners; long-term sealing integrity.	Poor and unknown geotechnical stability; complex sealing needs; potential groundwater contamination and legacy pollutants.
Safety & Integrity	Dependent on liner performance and rock mass quality. Risk of progressive micro-leakage.	Inherently high risk due to unknown fractures, connections to the surface, and potential for collapse.
Cyclic Capability	Moderate. Limited by liner material fatigue and thermal-mechanical stresses.	Very low. Unsuitable for frequent pressure cycling due to structural instability.
Environmental Risk	Controlled during construction. Risk linked to liner failure.	High. Risks include acid mine drainage, groundwater chemistry alteration, and surface subsidence.
Technology Readiness	Pilot stage (e.g., HyBRIT). Proven for compressed air (CAES), not yet for H2.	Conceptual/Pre-feasibility stage. Considered high-risk and not recommended for large-scale H2.

Table 6. Comparative analysis of Lined Rock Caverns and Abandoned Mines for UHS [38,39,46,47].

Aspect	Lined Rock Caverns (LRCs)	Abandoned Mines
Definition	Purpose-built excavations in competent rock, sealed with a multi-layer liner.	Pre-existing, often irregular voids from mining activities, requiring significant retrofit.
Primary Advantage	Site selection flexibility independent of specific geology; can be sized as needed.	Low initial excavation cost; potential for rapid deployment using existing voids.
Key Challenges	Very high construction cost ($10–20/m3); hydrogen embrittlement of liners; long-term sealing integrity.	Poor and unknown geotechnical stability; complex sealing needs; potential groundwater contamination and legacy pollutants.
Safety & Integrity	Dependent on liner performance and rock mass quality. Risk of progressive micro-leakage.	Inherently high risk due to unknown fractures, connections to the surface, and potential for collapse.
Cyclic Capability	Moderate. Limited by liner material fatigue and thermal-mechanical stresses.	Very low. Unsuitable for frequent pressure cycling due to structural instability.
Environmental Risk	Controlled during construction. Risk linked to liner failure.	High. Risks include acid mine drainage, groundwater chemistry alteration, and surface subsidence.
Technology Readiness	Pilot stage (e.g., HyBRIT). Proven for compressed air (CAES), not yet for H2.	Conceptual/Pre-feasibility stage. Considered high-risk and not recommended for large-scale H2.

On the other hand, abandoned mines are human-made voids left over from prior mining activities. They are often uneven, damaged, and connected to groundwater systems. A thorough analysis of these two storage options, capacity, safety, ease of use, and environmental impact, indicates that engineered rock caverns can be technically viable for H₂ storage to some extent. Still, they remain limited by sealing and cost constraints [48,49]. On the other hand, abandoned mines, despite their apparent economic benefits and existing void volume, have ongoing structural, environmental, and operational risks that make them unsuitable for safe, large-scale H₂ containment.

When it comes to system capacity, rock caves can store a modest to high amount of H₂, depending on geology, excavation design, and pressurization methods. Compressed air or oil storage systems in Scandinavia, for example, can hold between 50,000 and 1,000,000 cubic meters. This means they can store tens to hundreds of gigawatt-hours of energy at pressures between 5 and 15 MPa [50]. The main benefit of rock caverns is that they can be designed in many different ways. However, rock caverns are not very good at storing H₂ because they require artificial sealing systems, such as concrete or polymer linings, to prevent gas from leaking through the common microfractures in most crystalline rocks [51,52]. So, even if a rock cavern may have a large theoretical volumetric capacity, its practical H₂ capacity, which is the amount that can be safely stored for long periods of time without losing a lot of it, is lower. This limitation is exacerbated in cyclic storage systems, where fluctuations in pressure and temperature cause sealing materials to wear out and microcracks to spread. So, even though rock caverns could theoretically hold a lot of H₂, their actual storage efficiency depends not on the size of the cavity, but on how well their sealing methods work and how well they are maintained over time [53,54].

In contrast, abandoned mines have capacity potential that is not always clear and can change [55]. The energy density of stored H₂, which is based on both volume and pressure, is still much lower in mines than it is in designed rock caverns or salt formations. Also, the irregular shape of mines makes it hard for pressure to be evenly distributed throughout injection and withdrawal cycles. This creates stress-concentration zones that further reduce usable capacity. Large abandoned mines could be lined with concrete or polymer to make them more stable and better at keeping water out. But these kinds of changes are too expensive and challenging to implement due to the complex access routes and broken infrastructure. Even though abandoned mines may have a lot of space, their actual storage capacity is much smaller. Also, the technical requirements for storing pressurized H₂ make it unsuitable for long-term or high-density storage [56].

Safety concerns create a more significant distinction between rock caverns and abandoned mines. In designed rock caverns, safety primarily comes from structural integrity, geostatic confinement, and engineered barriers, rather than from natural impermeability [57]. Hydrogen embrittlement is a well-known problem in steels, cements, and some polymer liners. It is a significant problem because H₂ molecules can enter the microstructures of metals or cementitious materials, weakening mechanical connections over time. Studies in laboratories, notably those from German H₂ storage research programs, show that keeping material permeability below 10⁻¹⁹ m² is essential for technical tightness [58]. However, in the real world, rock-cement systems often exceed this level after thermal and mechanical cycling.

Additionally, crystalline rocks do not self-heal like halite formations in salt caverns do. Once a fracture starts, it tends to spread with repeated changes in tension. As a result, the sealing efficiency slowly decreases, and the likelihood of H₂ leakage increases over time [59].

Nevertheless, abandoned mines are inherently dangerous places to store H₂. The primary safety threats come from geological degradation, unregulated paths to the surface, and leftover pollutants or reactive elements from previous mining operations. Most abandoned mines have groundwater systems running through them. Because H₂ can catch fire easily (4–75% in air) and requires very little energy to ignite, even small leaks could pose a serious safety risk. The possibility of H₂-oxygen explosions in mine voids with leftover air or oxygenated groundwater should not be ignored, especially in shallow or inadequately ventilated mines [60,61].

The fact that purpose-built rock caverns are easier to utilize and work with makes them even better than repurposed abandoned mines. But the exorbitant cost of excavation and lining, which can range from USD 10 to 20 per cubic meter of storage space, makes them less practical than salt caverns. Also, rock caverns are relatively stable, but they do not have the self-sealing characteristic of salt; therefore, liners and wellheads need to be checked and maintained regularly [62,63]. However, rock caves can be good places to store H₂ for the medium to long term if they are built for seasonal or strategic reserves rather than for daily cycling [64].

On the other hand, it is tough to safely or efficiently run abandoned mines [39,65]. Even though reusing existing voids is a good idea since it lowers the cost of initial excavation, the total cost of sealing, monitoring, and risk reduction is often more than building new, purpose-built storage facilities [66]. So, from a practical and economic perspective, abandoned mines are not suitable for use because they are challenging to operate, perform poorly, and incur high long-term maintenance costs [67]. This means they should not be part of a national H₂ infrastructure unless they are used under exact, regulated pilot conditions [68].

Table 7. Distinct aspects of rock caverns and abandoned mines.

Aspect	Description	References
Formation Process	Excavation of caverns in igneous or metamorphic rocks, followed by steel or plastic liner encapsulation. Unused mines with large underground spaces can also be repurposed for H2 storage.	[64]
Construction Method	Mining and installing impermeable concrete liners involve significant expenses. Liners must withstand slight elastic or plastic deformations. When caverns are at shallow depths, water curtains may be required to store gases with low vapor pressure; at sufficient depths, hydrostatic pressure prevents fluid migration through fractures.	[65]
Operational Flexibility	Capable of operating effectively under elevated pressure levels, accommodating multiple storage cycles throughout the year, making them suitable for peak-load demands. Storage facilities can be filled and drained dynamically at the same rate as H2 production.	[43]
Current Applications and Projects	Utilized for natural gas storage in Sweden. The HyBRIT project in Sweden is developing a pilot facility for H2 storage with a 100 cubic meter capacity, aiming to produce ‘green steel.’ Plans are underway for larger-scale facilities capable of storing up to 100,000–120,000 cubic meters of H2 gas, equivalent to 100 GWh of electricity, supporting significant sponge iron production for 3–4 days.	[66]
Lined Rock Cavern (LRC) Technology	LRCs feature interconnected layers, including a steel/polymer liner, a concrete liner, and a drainage system. The surrounding rock mass bears the load from high gas pressure, while the lining ensures gas-tight conditions and minimizes pressure on the lined rock cavern. LRCs offer greater versatility and site selection flexibility than traditional underground gas storage methods.	[67]
Drainage System	Implemented to counteract groundwater pressure during construction and operation, especially when storage pressure is lower than the natural hydrostatic pressure. Ensures safe storage by preventing water infiltration and maintaining structural integrity.	[68]

demonstrates the distinct features of rock caverns and abandoned mines for hydrogen storage, focusing on their formation, construction, and operational characteristics. It describes the engineering requirements such as liners, drainage systems, and pressure management, as well as their current applications, including the HyBRIT pilot project in Sweden. Table 7 also highlights the advantages of Lined Rock Cavern (LRC) technology in terms of flexibility, pressure handling, and suitability for dynamic hydrogen storage.

2.3. Depleted Reservoirs

The repurposing of end-of-life oil and gas fields for hydrogen storage, leveraging existing wells, data, and infrastructure for low capital expenditure and a levelized cost of storage around $1.23/kg H₂, demonstrates high technical readiness supported by pilot projects in locations like the UK, Austria, and the USA, drawing an analogy to natural gas storage. However, this approach faces specific challenges, including hydrogen interactions with brine and residual hydrocarbons, microbial consumption, gravity override and mixing issues, and concerns over caprock integrity due to hydrogen’s smaller molecule size, all of which are critical considerations outlined alongside storage capacity and infrastructure advantages in its characteristic profile [69].

Hydrocarbon reservoirs containing oil or gas are geological formations that have undergone multiple stages of diagenesis, including source rock development, migration, and the passage of time, to function as storage for hydrocarbons. Typically, these reservoirs are covered by an impermeable layer, often aquifer-supported from the bottom or edges. When a gas field nears the end of its productive phase, it is commonly converted into a gas storage facility. The depletion of a gas reservoir, either already depleted or nearing depletion, is characterized by low pressure and high-water saturation in the region once occupied by gas, owing to aquifer water displacement. Consequently, the gas saturation behind the waterfront varies from minimal, corresponding to residual gas saturation near the initial gas/water contact, to maximal, representing gas saturation near the gas/water contact [70]. A depleted gas reservoir can be conceptualized as an aquifer (geological trap) segment with minimal water in the pores, primarily occupied by trapped gas [71].

Table 8. Key characteristics, advantages, and limitations of depleted reservoirs for UHS.

Aspect	Description	Advantages	Limitations
Geological Integrity	Proven caprock seal over geological/industrial time.	High containment confidence; extensive historical data.	H2’s smaller molecule/diffusivity may challenge seal [46].
Capacity & Scale	Very large pore volumes; GWh to multi-TWh scale.	Largest single-site storage potential; seasonal storage.	Effective capacity reduced by residual fluids & cushion gas [72].
Cyclic Performance	Rates constrained by reservoir permeability.	Suitable for seasonal/strategic cycles (weeks/months).	Low deliverability; prone to viscous fingering/gravity override [72].
Hydrogen Purity	High risk of mixing with residual CH4 and microbial conversion.	---	Major purity loss concern; may require downstream separation [73].
Economic Indicators	Low CAPEX (existing infra); LCOHS: $1.23/kg H2.	Fastest deployment path; leverages “sunk-cost” assets.	Costs for well re-completion, monitoring, gas separation [73].
Environmental Impact	Reuses industrial footprint; potential for coupled CCS.	Minimizes new disturbance; dual decarbonization service.	Risk of geochemical reactions & mobilizing metals/microbes [74].

presents the key characteristics, benefits, and challenges of using depleted reservoirs for underground hydrogen storage (UHS). It highlights their large storage capacity, proven geological integrity, and cost advantages due to existing infrastructure.

Pore volume, permeability, residual fluid saturation, and cushion-working gas interactions determine subsurface H₂ storage capacity in depleted reservoirs. Both studies show that depleted gas and oil fields are the best large-scale H₂ storage solutions, with GWh to TWh capacities, depleted gas [8]. The study found that cushion gas accounts for 25–80% of reservoir capacity, with CO₂ serving as both a pressure-support medium and a long-term sequestered phase.

In depleted oil reservoirs, multiphase H₂, brine, and residual oil interactions affect capacity considerably. Hydrogen pore space is reduced by residual oil and water saturations, and hysteresis effects between drainage and imbibition cycles can trap gas in pore throats. Volumetric inefficiencies are mitigated by huge, well-characterized storage capacities and pre-existing infrastructure in many oilfields. After caprock and well integrity testing, many mature oilfields might safely sustain multi-TWh capacities, according to their global reservoir study [75].

Hydrogen’s small molecular size poses containment challenges but facilitates faster pressure equalization during injection and withdrawal.

Underground hydrogen storage safety includes wellbore integrity, caprock sealing, geomechanical stability, and geochemical or microbiological reaction prevention. Both authors agree that depleted reservoirs, which have safely held hydrocarbons over geological timeframes, allow secure confinement if H₂’s peculiar features are handled [64].

The primary safety problem in gas reservoirs is gas mixing and gravity override, where buoyant H₂ migrates upward over denser cushion gases, expanding the interfacial area and incurring purity losses. Williams et al. observed that horizontally widespread formations with thin vertical thickness increase mixing. The study also found that natural heterogeneity, like poor vertical permeability layers, reduces gravity override. To avoid up-coning and cross-flow, homogenous formations need operational rate control [76].

Safety is less important than the integrity of the caprock. Both studies stress the importance of keeping an eye on capillary entrance pressure and fracture thresholds to keep H₂ from leaking through micro-fractures or clay interlayers because its molecular radius is only 0.12 nm. Dilshan et al. [77] used the Young–Laplace equation for capillary sealing to quantify the stable height of an H₂ column beneath the caprock based on wettability and interfacial tension. Pore throat residual trapping makes it harder to recover, but it also provides a safety buffer. The evaluation found that depleted oil reservoir safety assurance requires hydrodynamic modeling, geochemical analysis, and microbiological risk assessment.

Ease of use refers to the H₂ storage system installation, operation, and cycling. Both studies underline that exhausted hydrocarbon resources include wells, pipelines, and geological data from previous production periods, making them highly operationally convenient. Building new salt caverns or aquifers is much more expensive than using this infrastructure.

As the cushion-gas ratio increased, withdrawal efficiency improved, and pressure–volume relationships were predictable and stable. They also showed that injection-well design and rate schedules can balance dispersion and gravity segregation to improve H₂ extraction rates.

Understanding fluid–fluid and fluid–rock interactions simplify oil reservoir operations. The scientists noted that H₂ storage’s cyclic nature stresses the caprock; therefore, real-time seismic and pressure studies are needed to verify mechanical stability during charge–discharge operations.

Hydrogen manufacturing plants are near oilfields, making deployment easier. Integrating H₂ storage into energy infrastructure might simplify logistics and save transportation costs for many refineries and petrochemical plants near mature oilfields. Bibliographic analysis also showed that most research and pilot programs are in regions with well-mapped depleted reservoirs (USA, China, UK, Saudi Arabia, Germany, and Australia), indicating an increasing global preparedness to repurpose hydrocarbon assets for clean-energy storage.

The environmental impact of underground H₂ storage includes lifetime emissions, land use, subsurface ecosystem disturbance, and carbon sequestration. Both studies recommend drained reservoirs because they reuse geological formations instead of building new surface infrastructure, reducing ecological footprints.

Williams et al.’s innovative use of CO₂ as a cushion gas combines H₂ energy storage with carbon capture and storage (CCS) [78]. One geological site can be used as a dual-use decarbonization system, temporarily storing H₂ for renewable intermittency and permanently storing CO₂ in the reservoir’s bottom strata (depleted gas). The simulations show that this configuration improves H₂ storage efficiency and reduces greenhouse gas emissions by immobilizing large amounts of CO₂. If structural and caprock seals are intact, the CO₂ leakage risk is small as it remains immobile below the H₂ zone.

Chemical and microbiological processes complicate the environmental safety of oil-bearing formations. Redox changes caused by H₂ injection can dissolve or precipitate minerals, releasing trace metals or changing groundwater chemistry. Most reactions are gradual, and site selection (favoring reservoirs with inert lithologies such as quartzose sandstones) reduces long-term dangers. Hydrogen conversion to methane or H₂ sulfide by microbes may cause local pollution. Reduce these processes by sterilizing, controlling injection temperatures, or excluding CO₂ when not necessary.

From a systems perspective, both studies see UHS as crucial to large-scale renewable integration. Excess wind or solar electricity can be stored as H₂ and released during demand peaks to reduce the energy system’s carbon footprint. Ali et al. [79] add that reusing exhausted fields reduces surface disturbance, avoids drilling footprints, and uses existing monitoring wells for environmental surveillance. Standardized subsurface H₂ regulations are needed to ensure environmental compliance and minimize unforeseen interactions with neighboring aquifers.

2.4. Aquifers

Hydrogen is an integral part of decarbonized energy networks since it can be used in many ways and has a high energy density per mass. For large-scale H₂ energy storage, we need safe, cheap, and eco-friendly ways to store it. Salt caverns, exhausted hydrocarbon deposits, and saline aquifers are suitable for the environment and cost-effective, especially when there are not many alternative formations around. Saline aquifers are huge, have a lot of pore volume, and have been studied for decades for use in natural gas storage and carbon capture and storage (CCS) projects. They can provide geological, hydrodynamic, and operational information for H₂ storage.

Aquifers are made of porous, permeable sedimentary rocks soaked with saline water and covered by impermeable layers. Hydrogen injection displaces brine and fills pore spaces as free gas. Storage behavior depends on H₂–brine displacement, capillary trapping, dissolution, and buoyant migration. Hydrogen has lower density, viscosity, and brine solubility than natural gas, which affects its subsurface flow and recovery efficiency. In aquifer models, H₂ recovery factors reach 78%, with a potential global energy efficiency of 40% (electricity-to-H₂-to-electricity).

Big aquifers can store enough for the nation. For example, Canada’s Keg River Aquifer can store 3.24 Mt H₂ (~2.3 EJ), meeting its national grid’s seasonal energy needs. Aquifers are the most accessible and environmentally friendly choice for large-scale H₂ storage since they are widespread and generally near renewable power sites [80].

Salt caves are far smaller than saline aquifers, which may store several times more. For grid-scale and seasonal applications, their pore volume, geological stability, and area allow multi-terawatt-hour energy storage. Formation depth, porosity, permeability, and caprock integrity affect storage. Sedimentary basins worldwide, including North America, Europe, and Asia, include thick aquifers appropriate for this use [81].

Recoverable H₂ makes about 20–50% of the injected gas, while the rest maintains reservoir pressure as cushion gas. Aquifers may need 80% more cushion gas than depleted reservoirs, requiring a significant but reasonable capital expenditure. Injection pressures, caprock thresholds, and migration paths must be determined by numerical modeling and site characterization.

Table 9. Key characteristics, advantages, and limitations of saline aquifers for UHS.

Aspect	Description	Advantages	Limitations
Geological Integrity	Requires proven regional seal; pristine structures.	Theoretically high containment security; no legacy wells.	Highest geological uncertainty; seal quality unknown [80].
Capacity & Scale	Largest global potential; multi-TWh scale.	Abundant, widespread resource; continental-scale storage.	Low recovery factor (20–50%); very high cushion gas need (up to 80%) [80].
Cyclic Performance	Injection/withdrawal is slow, governed by Darcy flow.	Suitable for very long-term (seasonal/annual) storage.	Very low deliverability; unsuitable for frequent cycling [70].
Hydrogen Purity	Significant dissolution into brine; geochemical/bio reactions.	---	High risk of purity loss and hydrogen loss [70].
Economic Indicators	CAPEX for new wells/characterization; LCOHS: ~$1.29/kg H2.	Low intrinsic cost of pore space.	Very high upfront characterization costs; large locked cushion gas capital [81].
Environmental Impact	No hydrocarbon legacy; deep target zones.	Minimal surface footprint; no operational brine production.	Risk of altering deep biosphere; potential for induced seismicity [81].

shows the key characteristics, advantages, and limitations of using saline aquifers for underground hydrogen storage (UHS). It emphasizes their vast global potential and minimal surface impact but highlights major challenges such as low hydrogen recovery, high cushion gas requirements, and uncertain geological sealing.

With vast global potential and proximity to renewable hubs, saline aquifers offer scalable, long-duration hydrogen storage, despite challenges in recovery and site uncertainty.

Safety is key to aquifer storage. The best aquifers are tectonically stable, far away from fault zones and folded strata, and have enough sealing pressure to keep gas during injection and withdrawal. Evidence from past natural gas and CO₂ storage operations shows that well-designed aquifer systems are safe.

Integrated system design, geological screening, material engineering, operational management, and robust monitoring supported by transparent regulations and stakeholder engagement determine safety performance.

After site characterization, saline aquifers are easier to deploy than other large-scale storage solutions. Aquifers use pore spaces, unlike salt caves, which need costly excavation. They can be built alongside renewable energy and H₂ hubs using existing gas infrastructure.

Compressed H₂ is injected into the aquifer using wells, generating a storage plume like natural gas. A portion is permanently trapped as cushion gas; the rest is seasonal working gas. Reservoir management, optimizing pressure gradients, minimizing viscous fingering and gas channeling, and regulating contamination, determines system performance.

However, aquifer operations require complex modeling, control, and site-specific tweaking. Complex modeling and pilot testing are needed to account for fluid-rock interactions, multiphase flow dynamics, and heterogeneity. These procedures are feasible under regulated settings, according to laboratory research and field analogs like town-gas storage in Germany and France.

Integration potential makes aquifer storage easy to integrate with electrolysis plants, renewable energy sites, and H₂ transport networks for seamless conversion and dispatch. The small surface footprint reduces environmental impact.

The greenest subterranean H₂ storage method is a saline aquifer. It does not displace freshwater or dispose of brine like some salt caverns. Aquifers are deep below potable water zones, so maintaining sealing integrity reduces groundwater contamination risk.

Since H₂ is non-toxic, modest leaks offer little ecological risk, but its flammability requires containment. Indirect impacts from injection pressures may include subsurface microbial changes, mineral changes, or modest seismic reactions, recognized from CO₂ storage analogs, as shown in Figure 1. Comprehensive risk evaluations and environmental monitoring are required for public acceptance, say the authors.

Large-scale H₂ storage helps decarbonize the electricity system, promoting environmental sustainability. Aquifer storage buffers intermittent renewables, lowering fossil-fuel peaker plants and ensuring clean power. Lifecycle analyses show that H₂ storage in aquifers has a much lower environmental impact than batteries or compressed air.

Public perception is a key social and environmental aspect. Transparency regarding safety, environmental preservation, and community benefits builds trust. Participatory governance—involving local stakeholders in site selection and oversight—can reduce resistance and speed project rollout.

Economically, aquifer storage is competitive. The average levelized cost of H₂ is $1.29 US/kg, second only to depleted oil and gas reservoirs ($1.23 US/kg) and far lower than salt or hard-rock caverns ($1.61–2.77 US/kg). Aquifers. Modest operational expenditures for compression energy, maintenance, and monitoring follow capital-intensive site search and construction. As H₂ prices fall, cushion gas expenses, the key capital lock-in, diminish.

Government, industry, and academia should collaborate to improve research, pilot projects, regulatory standards, and social acceptance, according to the study. Scaling aquifer storage internationally and integrating it into H₂ supply networks requires continuous monitoring, adaptive management, and legal clarity.

Geological abundance, environmental responsibility, and technical scalability combine in saline aquifer H₂ storage. Its system capacity outperforms existing storage technologies, allowing national and continental seasonal energy balancing. Aquifers are essential to the net-zero transition due to their high pore volumes, widespread availability, and proximity to renewable sources.

Mature engineering and regulatory procedures make safety challenging but manageable. To reduce the risk of CO₂ and natural gas storage, you can use site-specific geological characterization, sealing integrity-based caprock selection, corrosion-resistant materials, and extensive monitoring networks based on decades of expertise. Design, operational optimization, and surveillance can keep an eye on chemical reactions, H₂ embrittlement, and leaks, as well as microbial consumption. The safety of H₂ depends on the surface and geological systems.

Using current skills and infrastructure simplifies utilization. Mature drilling, compression, and pipeline technology need only minor H₂ adaptation. Flexible operational cycles allow seasonal and short-term storage and fast grid stabilization response times. Regional H₂ hubs with renewable electricity and industrial customers are possible with modular aquifer storage.

Sustainability and low environmental impact are supported by aquifer storage. The deep geological framework keeps H₂ separate from biospheric processes and lowers environmental stress by stopping brine discharge and surface disturbance. More usage of renewable energy sources indirectly lowers greenhouse gas emissions, air pollution, and the use of resources. Keeping an eye on the environment makes sure that it stays safe for a long time and for the ecology.

Saline aquifers are the backbone of the global H₂ economy because they combine capacity, safety, utility, and care for the environment. Even if there are rules, high cushion gas costs, and gaps in subsurface microbiology, research and pilot projects are filling them. Aquifer H₂ storage offers a technologically and ecologically sound means to store energy securely, large-scale, and sustainably as nations decarbonize.

The study concluded that saline aquifers are the most fantastic geological medium for long-term H₂ storage since they can hold a lot of H₂, are safe for the environment, are easy to work with, and are cheap. If they are studied, regulated, and used by the community, their use could speed up the world’s move to a carbon-neutral energy system.

The storage concept is based on the use of deep, porous, saline-saturated rock formations as the subsurface medium for hydrogen storage, where gas is injected and withdrawn through wells in a manner analogous to other underground storage technologies. These formations are characterized by a vast global storage potential; however, they require a large volume of cushion gas to maintain pressure and deliverability, and they are associated with significant geological uncertainty due to heterogeneity in rock properties. In terms of technological maturity, this option has the lowest technology readiness level, with development largely confined to numerical simulations and laboratory-scale experiments, such as those conducted under the H2020 “HyUsPRe” project, while knowledge from CO₂ and natural gas storage provides useful analogies. Economically, saline aquifer storage benefits from relatively low infrastructure capital expenditure, but the high cost of cushion gas results in substantial capital being locked up, leading to a levelized cost of hydrogen storage of approximately USD 1.29 per kilogram of hydrogen, which is highly site-specific. Key challenges include maximizing the hydrogen recovery factor, managing the complex fluid dynamics of a highly mobile gas, reliably predicting geochemical and microbial interactions within the reservoir, and ensuring robust site characterization and selection.

Accurate assessment of injection potential and assurance of the safety and efficiency of H₂ storage depend on evaluating the technological challenges connected to large-scale H₂ storage in saline aquifers. Essential factors in aquifer storage include the footprint of injected H₂ underground and its retrieval during cyclic injection/recovery processes. Several elements determine effective storage operations: geological formation characteristics, capacity, fluid and hydrogeological qualities, geological interferences, operational conditions, regulatory frameworks, and societal support.

Table 10. Synopsis of review articles regarding UHS.

Topic	Spotlight	References
A review on underground hydrogen storage: Insight into geological sites, influencing factors and future outlook	Hydrodynamics Wettability Capillary pressure Diffusion Solubility Adsorption Chemical reactions	[16]
Perspectives and prospects of underground hydrogen storage and natural hydrogen	H2 production Formation mechanisms Geological locations Serpentinization reactions	[86]
Storage integrity during underground hydrogen storage in depleted gas reservoirs	Depleted gas reservoirs Storage Integrity Geochemical reactions Microbial activities H2 cycling	[1]
Theoretical and Technological Challenges of Deep Underground Energy Storage in China	Energy reserves Deep underground energy storage Bedded rock salt Salt cavern Rock mechanics	[87]
Underground hydrogen storage to balance seasonal variations in energy demand: Impact of healthy configuration on storage performance in deep saline aquifers	Saline aquifer Bunter sandstone formation Well optimization	[88]
Comprehensive review of geomechanics of underground hydrogen storage in depleted reservoirs and salt caverns	Underground gas storage Cyclic injection and production Fault reactivation Caprock and wellbore integrity Experiments and modeling Leakage	[12]
Underground hydrogen storage: A UK perspective	H2 strategy Salt caverns Saline aquifers Depleted oil and gas reservoirs	[3]
Mechanical behavior of a new segmented lining for underground rock caverns with high internal pressure	Segmented lining Retractable support Underground cavern Mechanical behavior Stress path	[89]
Hydrogen-Based Energy Systems: Current Technology Development Status, Opportunities and Challenges	Environment Large-scale energy storage Storage costs Storage facilities	[90]
Allowable Pillar Width for Salt Cavern Gas Storage Based on Triangular Well Layout: A Case Study in China	Rock salt Pillar width Triangular well pattern Stability analysis Numerical simulation	[91]
Hydrogen storage capacity of salt caverns and deep aquifers versus demand for hydrogen storage: A case study of Poland	Energy storage Deep aquifers Salt caverns H2 storage needs	[92]
A review of analog case studies relevant to large-scale underground hydrogen storage	Town gas storage Helium storage North Sea gas reservoirs	[93]

illustrates numerous reviews accessible to the public regarding UHS, spanning various perspectives such as geological analysis [82,83], microbial behavior [1,8], and detailed examinations of hydrodynamics [8], geomechanics [84,85], geochemical interactions [42], and well integrity [8]. Despite the comprehensive coverage provided by these reviews, a focused analysis of depleted gas reservoirs and lined rock caverns remains absent.

3. Challenges and Opportunities

H₂ emerges as a strong contender for replacing traditional fossil fuels as the dominant energy carrier in the foreseeable future. Among the several ways energy can be stored nowadays, subterranean geological formations are the most practical. This paper repeatedly recommends four basic underground geological storage media types, deficient reservoirs, saline aquifers, and salt caverns, in great detail. Many agree that UHS guarantees a sustainable energy future for the rising world population. The increasing number of scientific papers on UHS supports the good agreement among scientists. Still, the change from Underground Gas Storage (UGS) to UHS offers many difficulties that must be resolved. Geological, technical, financial, and political elements affect whether UHS is adopted worldwide. Emphasizing operating sites and technical challenges, including hydrodynamics, geomechanics, geochemistry, and microbiology, this analysis provides a platform for guiding research activities in UHS. By defining the general extent of this review, it aims to simplify the following projects in the field of research.

Table 11. Operational envelopes and key performance indicators for UHS technologies.

Criterion	Salt Caverns	Lined Rock Caverns	Depleted Reservoirs	Saline Aquifers
Typical Depth (m)	500–2000 [64]	100–500 [11]	1000–3500 [80]	1000–3000 [72]
Operating Pressure (bar)	60–180	50–150	100–300	100–250
Working Gas Recovery (%)	95–99	85–95	50–80	20–50
Cushion Gas Req. (% total)	20–40	20–40	25–50	50–80
Dominant Loss Mechanism	Diffusion through salt.	Permeation through the liner.	Mixing, dissolution, microbial.	Dissolution, residual trapping.
Critical Material Issue	Wellhead/casing embrittlement	Liner embrittlement & fatigue.	Wellbore cement & casing.	Wellbore materials.
TRL	8–9 (Commercial)	4–6 (Pilot)	6–7 (Pilot/Feasibility)	2–4 (Conceptual/Lab)

compares the operational envelopes and key performance indicators of various underground hydrogen storage (UHS) technologies, including salt caverns, lined rock caverns, depleted reservoirs, and saline aquifers. It covers factors such as depth, operating pressure, gas recovery rates, cushion gas requirements, loss mechanisms, material challenges, and technology readiness levels (TRL).

Due to their identified reservoir properties and characteristics, depleted reservoirs and saline aquifers exhibit larger storage capacities than salt caverns. Consequently, salt caverns are deemed unsuitable for UHS in regions with denser populations that necessitate significantly higher energy production [1].

Among the four geological formations under consideration, depleted reservoirs emerge as the most dependable option for UHS, owing to their track record of successfully accommodating natural gas for both seasonal and long-term storage purposes [11]. However, when cyclical storage is prioritized, especially in areas with low populations, salt caves show more viability. Their lowered need for cushion gas and improved capacity for repeated injection and withdrawal cycles help to explain this [3].

Conducting a thorough risk assessment for a particular UHS operation requires careful evaluation of the frequency and degree of every possible risk factor. A holistic review should encompass aspects such as temperature and pressure fluctuations, the risk of leakage, contamination, and flammability throughout the operational phases [1]. Additionally, among the various methodologies proposed in the literature for risk assessment, the reliability criteria maintenance approach emerged as the most effective for storing H_2v in salt caverns [94].

When considering potential locations for UHS, assessing them using data analogous to natural gas storage is essential. The overall criteria for site selection should encompass geological, economic, and technical factors [95]. Safety considerations in site selection for UHS primarily revolve around reservoir lithology, the exploration stage for salt caverns, and the exploration stage and volume for depleted reservoirs and aquifers [13].

Sandstone formations should be given priority when storing H₂ in depleted reservoirs and aquifers since their better permeability improves H₂ storage efficiency. Furthermore, in cases of extended injection times, formations with strong water-wet properties are better [96]. It is also important to avoid storing H₂ at depths greater than or equal to 3700 m, as below this threshold, natural upward migration of H₂ through the caprock may occur. Strictly dipping structures with stratified barriers can mitigate issues like viscous fingering [13].

Efficient storage of H₂ heavily relies on strategic placement and management of injection and withdrawal wells. Multiple shallow wells below the caprock for extraction and injection are advised while optimizing rates to prevent issues like viscous fingering and lateral gas migration [86]. This approach not only enhances H₂ recovery but also minimizes contamination and consumption by microbial and geochemical processes underground. Implementing successive injection-withdrawal cycles further boosts H₂ recovery rates. Figure 2 illustrates the comparison of economic competitions, H₂ purity retention, cycling capability, technology maturity, and geographical availability for all UHS types.

When constructing salt caverns for H₂ storage, it is preferable to target salt domes rather than bedded salt formations. The latter tend to contain interlayers that facilitate H₂ migration during operational cycles. Maintaining brine temperature throughout construction is essential to avoid changes in cavern structure resulting from thermal stresses [97]. Furthermore, setting project goals ahead of time is critical since the cavern shape directly affects deliverability.

During operation, continual pressure and temperature monitoring is imperative to prevent thermal or tensile stresses that could deform the lined rock cavern or lead to fractures over time. Maintaining internal cavern pressure within 24–80% of the rock’s overburden pressure provides a safety buffer. Operating at higher temperatures or lower pressures is discouraged to mitigate risks [98].

Injection of H₂ can trigger geochemical reactions in the reservoir, altering its mineralogical and petrophysical properties over successive cycles. To mitigate capacity reduction, storing H₂ in quartz-rich sandstone reservoirs is recommended from a geochemical perspective [99,100]. Furthermore, microbial activity caused by increasing subsurface H₂ concentration in UHS operations can provide economic issues. It is advisable to spot low CO₂ concentration depleted pools. Additionally, surrounding reservoir rocks should have high concentrations of reactive iron-bearing minerals and low concentrations of SO₄⁻² and CO₃⁻² bearing minerals to minimize unwanted microbial activity [96].

On the other hand, a creative solution meant to cut storage costs and improve the volumetric efficiency of H₂ uses solid carriers inside reasonably priced storage tanks. Utilizing metal hydrides or specialized nanostructures, H₂ gas molecules can be stored in tanks with lower pressure requirements. However, such storage devices need low temperatures for H₂ absorption and heat for gas molecule release. Therefore, cost-effectiveness depends on thoroughly investigating and optimizing the thermodynamic exchange of these carrier systems [101,102].

Underground features include mines, salt caverns, oil and gas reservoirs, and aquifers, are often used in comparison to UHS, reflecting practices in seasonal increases in natural gas output. Establishing a comprehensive H₂ storage system demands tackling many problems, including development costs, contamination hazards, natural losses, and geographical limits. Five locations offer substantial UHS, including four salt caverns in Texas and one in Teesside, England [3,80]. However, the cushion gas remaining in geological features represents a significant expense akin to other large storage vessels. Experience with natural gas storage suggests that this cushion typically amounts to 15% of storage capacity. Furthermore, the effects of pressure cycling on rock formations are not entirely known since the rock mass acting as the storage vessel might not be homogeneous, causing unanticipated forms and unwanted chemical reactions [73,103].

4. Future Research Directions and Implementation Pathways

Based on the thorough examination in Section 2 and Section 3, we have found a number of important research gaps and problems with implementation. This part gives clear, doable steps for future work, going beyond just a survey of the literature to give an organized plan for improving underground hydrogen storage (UHS) technology.

4.1. Priority Research Themes

The underlying science of hydrogen-rock-microbe interactions constitutes a principal research frontier. Although prior research has individually analyzed geochemical, microbiological, and geomechanical processes, their integrated dynamics within subsurface hydrogen systems are still not fully comprehended. Future studies should focus on combined laboratory tests that keep an eye on mineral reactions, microbial metabolism, and mechanical responses all at once when hydrogen is loaded in cycles. These types of tests should concentrate on the real-time measurement of hydrogen consumption rates across various lithologies and the evaluation of material deterioration under typical reservoir circumstances. At the same time, more complex numerical modeling is needed to build next-generation simulators that can combine multi-phase reactive transport with microbial kinetics and geomechanical responses. These tools will be very helpful for figuring out how hydrogen will escape and making storage work better in different types of rocks.

Another big problem is figuring out how to make materials that work with hydrogen. Carbon steel well casings, Portland cement, and elastomeric seals are all examples of conventional oil and gas materials that can become brittle and permeable when exposed to hydrogen. To make and test materials that can withstand hydrogen, including high-entropy alloys for building wells, polymer-modified cements with lower permeability, and composite liner systems that can heal themselves for rock caverns, we need a dedicated research project. Alongside these efforts, standardized testing methodologies and material qualification databases must be created expressly for UHS applications. These will be the basis for trustworthy infrastructure design.

Cushion gas optimization needs to be given strategic attention since it has a big effect on the economy, especially in aquifer and depleted reservoir systems. Research should carefully assess alternative cushion gases, such as CO₂ and N₂, and formulate effective management systems that provide partial recovery or usage of cushion gas resources. Advanced reservoir simulation for operational optimization can help reduce cushion needs even further while still keeping the system’s ability to deliver. Techno-economic evaluations that weigh the price of cushion gas against the need for storage efficiency and hydrogen purity must also be done.

4.2. Technology Development Roadmap

To provide a structured pathway for the implementation of underground hydrogen storage (UHS),

Table 12. Roadmap for UHS development and integration (2025–2040).

Phase	Years	Focus Areas
I	2025–2030	Small-scale pilots, cushion gas R&D, geochemical & microbial studies.
II	2030–2035	Commercial testing, regulatory frameworks, sensor integration.
III	2035–2040	Full-scale hubs, AI-based optimization, grid integration.

outlines a phased technology roadmap covering the period from 2025 to 2040, based on current TRLs and projected research priorities.

The following discussion expands on these roadmap phases and identifies the key actions needed in each. A phased strategy for technology development, aligned with the Technology Readiness Levels mentioned above, is suggested to progressively advance UHS deployment. The first phase, which will last from 2025 to 2030, should focus on testing all types of storage on a small scale. This involves enhancing cycling methods and ensuring they integrate seamlessly with renewable energy systems for salt caverns. To prove that containment and recovery work in real-life situations, depleted reservoirs need 3 to 5 large-scale pilot projects with full monitoring. Saline aquifers need their first specialized hydrogen storage pilots in properly defined formations. Lined rock caverns, on the other hand, need to finish long-term material testing and show that they work reliably over hundreds of cycles.

The next phase of commercial testing (2030–2040) should set up standard design and operation rules for each type of storage, use digital twin technologies to optimize in real time, and build integrated monitoring networks that use fiber optics, chemical tracers, and geophysical methods. The last phase of grid-scale integration (2040+) will be about building regional hydrogen storage hubs that combine different types of storage, connecting these hubs to hydrogen transportation networks and applications that link different sectors, and using AI to manage the energy system for the best performance.

4.3. Policy and Regulatory Framework Development

Along with technical progress, a policy and regulatory framework that helps must be put in place. International standards for hydrogen purity requirements that are consistent, maximum permissible leakage rates that are variable for each kind of storage, and processes for well integrity that are particular to hydrogen service are all very important. Regulatory measures should be founded on risk-based concepts that take into account the differing risk profiles of cavern-based and porous media storage, the geology of the site, the size of the activity, and how close it is to people. To deal with the disadvantages of being the first to move, we also need economic incentive mechanisms. These could include risk-sharing arrangements for pilot projects, carbon credit systems that recognize the grid-balancing value of hydrogen storage, and faster permitting processes for repurposing existing hydrocarbon infrastructure.

4.4. Knowledge Gaps Requiring Immediate Attention

There are a number of information gaps that need to be looked into right away. Long-term integrity studies with more than 10,000 pressure cycles are necessary to comprehend the cumulative impacts on storage formations and infrastructure. To go from understanding hydrogen behavior in the lab to understanding it in the real world, we need rigorous upscaling research. It is necessary to create detailed protocols for characterizing the environmental baseline for pre-injection site assessment. Also, it is important to look into the best ways to get people involved and be open about UHS projects to build social license and public support.

It will need a coordinated, interdisciplinary effort to turn UHS from a potential idea into a key part of the hydrogen economy. Working together strategically through international consortia that bring together experts from business, academia, and government will speed up progress. Following open science concepts, such as sharing data and models amongst projects, can stop people from doing the same thing again and over again and help everyone learn more. An adaptive management style that changes technological techniques based on what works in the field will make sure that things keep getting better. Lastly, using holistic systems thinking to include UHS in larger energy system planning will make it most useful for integrating renewable energy and deeply reducing carbon emissions. By paying close attention to these research goals and implementation paths, underground hydrogen storage can play an important role in moving us toward a future with sustainable energy.

4.5. Integration of Artificial Intelligence and Digital Technologies

The operational intricacy and multifaceted characteristics of subterranean hydrogen storage pose considerable difficulties for traditional modeling and control systems. AI and ML have the power to change the way UHS works, make it safer, and allow for predictive management of all sorts of storage. AI algorithms can optimize the injection and withdrawal cycles for salt caverns by looking at real-time data on cavern pressure, temperature, and surface deformation. This helps to reduce volume loss caused by creep and extend the life of the cavern. In depleted reservoirs and saline aquifers, where fluid dynamics involve complex multiphase interactions with geochemical and microbial components, ML-powered coupled-process digital twins can predict hydrogen migration, purity loss, and microbial activity with greater accuracy than traditional reservoir simulators. These models can use data from fiber-optic sensors spread out over a large area, downhole fluid samples, and surface monitoring arrays to send out early warnings of dangers to containment or performance loss, as illustrated in Figure 3. AI-powered anomaly detection systems may constantly look at data from strain gauges, gas composition sensors, and groundwater monitors in lined rock caverns to find early symptoms of liner fatigue or seal failure.

5. Concluding Remarks

This research examined the technical, economic, and environmental feasibility of storing hydrogen underground in salt caverns, lined rock caverns, depleted hydrocarbon reservoirs, and saline aquifers. Based on storage capacity, injectivity, geological integrity, cyclic deliverability, and long-term sustainability, each geological formation has its own pros and cons.

Salt caverns are the best choice for short-term, frequent storage because they offer the highest cycle efficiency, the fastest discharge rates, and the lowest risk of contamination. Their main problems are that they are hard to find and that solution mining uses a lot of water. Lined rock caverns are a good option when salt formations are not available, but sealing performance, hydrogen embrittlement, and high construction costs still make them hard to use on a large scale.

Depleted reservoirs and saline aquifers are the best places to store significant amounts of water for a long time. Depleted oil and gas fields offer proven containment, mature data availability, and existing infrastructure, making it easy to implement at low cost. Saline aquifers have the most promise worldwide, as they have abundant pore space and are found throughout the world. However, we need to manage better how H₂, rock, and microbes interact, optimize cushion gas, and develop new ways to monitor them.

Addressing geomechanical risks, microbial activity, material compatibility, and cushion gas strategies is essential for advancing UHS to commercial scale. Geomechanical characterization under cyclic stresses, quantification and mitigation of microbial consumption and geochemical reactions, liner and wellbore material development to avoid embrittlement, and integrated techno-economic assessment of cushion gas management, monitoring strategies, and environmental protection are priorities. These improvements make UHS a key part of large-scale renewable energy storage, thereby stabilizing the grid and helping the world move toward a hydrogen economy with no net carbon emissions.