Four Geological Formation Types and Their Technical Tradeoffs

Underground hydrogen storage relies on four principal geological formation types: salt caverns, depleted oil and gas reservoirs, saline aquifers, and engineered rock caverns or mine voids—each presenting distinct tradeoffs in sealing integrity, geographic availability, capital cost, and compatibility with high-pressure hydrogen. Salt caverns are identified across multiple literature sources as the most technically mature option, owing to their low permeability, chemical inertness relative to hydrogen, and proven use in natural gas storage. Depleted oil and gas reservoirs offer the advantage of pre-existing well infrastructure and detailed subsurface characterisation, reducing exploration costs. Saline aquifers are geographically widespread but require extensive assessment of caprock integrity before deployment.

Engineered hard-rock caverns and abandoned mine voids represent an emerging fourth category, positioned as urban-compatible alternatives where natural geological options are limited or geographically inaccessible. Key technical challenges common to all formation types include: hydrogen leakage through caprocks, microbial methanogenesis (the conversion of H₂ to CH₄ by subsurface microbes), geomechanical stability under cyclic pressure loading, and compressor system design for injection and withdrawal cycles. As noted by WIPO, the global energy transition is generating significant patent activity in energy storage technologies, of which UHS represents one of the most capital-intensive and strategically significant segments.

How UHS Innovation Matured: From 2018 Feasibility Studies to 2026 System Integration

UHS innovation followed a clear maturation arc spanning approximately 2018 to 2026, moving from foundational risk characterisation through development-phase publication clusters to a patent filing surge dominated by Chinese assignees, and arriving in 2025–2026 at system-level integration architectures. This progression mirrors the broader pattern that the IEA has documented for maturing clean energy technologies: early academic feasibility work precedes commercial IP filings by approximately three to five years.

The pre-2020 foundational period produced the scientific groundwork: hydrogeochemical risk modelling in depleted gas fields (2018) and techno-economic feasibility of offshore wind–hydrogen–underground storage integration (2020). Between 2020 and 2022, a dense cluster of publications appeared covering benchmark simulation methodologies for UHS operations, compressor system boundary conditions for cavern storage, optimum geological storage depth analysis—peaking at approximately 1,100 m for structural trapping—and case studies in Spain, Germany, Southern Ontario, and Italy.

The 2022–2024 patent filing surge saw Chinese assignees dominate, covering shared UHS control systems, pressure-differential porous reservoir storage, abandoned mine void fuel cell systems, and solar-powered subsurface hydrogen production. Saudi Arabian Oil Company filed internationally in both WO and US jurisdictions on hydrogen-enriched gas storage in subsurface formations—specifically addressing simultaneous injection and withdrawal capability, a feature absent from legacy natural gas storage infrastructure.

“The integration of UHS with renewable energy production systems—electrolysis, storage, and power recovery—is the defining architectural direction for 2026–2030.”

The most recent signals from 2025–2026 include engineered underground-scale hydrogen repository structures, hydrogen blending into underground natural gas storage systems, liquid hydrogen underground repositories with advanced thermal insulation, and solid oxide electrolysis cell (SOEC)-coupled high-temperature solid-state hydrogen storage systems. These represent a decisive transition toward system integration and multi-energy coupling—no longer treating underground storage as a standalone component, but as one node in a complete renewable-hydrogen-power value chain.

Four Patent Clusters Defining the UHS Technology Landscape

The retrieved patent dataset resolves into four distinct technology clusters, each addressing a different layer of the underground hydrogen storage problem stack—from geological media selection through engineered structures to operational architecture and full system integration.

Cluster 1: Salt Cavern and Geological Porous Media Storage

The dominant approach in the literature involves injecting compressed hydrogen into natural geological formations. Salt caverns offer stable pressure containment due to the creep behaviour of halite, while porous media—saline aquifers and depleted reservoirs—rely on structural trapping via caprocks. Research published in 2022 across multiple studies established that the optimal storage depth for structural trapping is approximately 1,100 m, and that microbial methanogenesis and sulfate reduction are key scientific challenges requiring geochemical modelling. Case studies in Spain, Germany, Southern Ontario, and Italy form the evidence base for near-term deployment.

Cluster 2: Engineered Underground Cavern and Rock Formation Systems

Chinese patent filers have concentrated on constructing purpose-built underground storage structures within rock formations for contexts where natural geological options—salt domes, depleted fields—are unavailable or geographically distant from demand centres. These systems typically incorporate sealing metal liner layers, reinforced concrete shaft structures, water drainage mechanisms, pressure balancing layers, and anti-seepage membranes. Abandoned mine voids represent a lower-cost variant of this approach, repurposing existing underground infrastructure. Key assignees include Zhongtai (Suzhou) Hydrogen Energy Technology Co., Ltd., PowerChina East China Engineering Corporation, and China Energy Engineering Group New Energy Co., Ltd.

Cluster 3: Subsurface Formation Storage with Simultaneous Injection/Withdrawal

A technically differentiated cluster addresses the operational inflexibility of existing underground gas storage—specifically, the inability to simultaneously inject and withdraw hydrogen from different zones of the same field. Saudi Arabian Oil Company has filed patents in both WO and US jurisdictions covering this capability for depleted oil fields, deep saline formations, and other porous subsurface formations. The retrieved filings also address hydrogen purification post-extraction using pressure swing adsorption (PSA) and temperature swing adsorption (TSA) units, with purified hydrogen then used to fuel gas turbines for electricity generation. This dual-mode architecture is described as a capability not offered by legacy natural gas storage infrastructure.

Cluster 4: Integrated Renewable-Hydrogen-Underground Storage Systems

A growing patent cluster couples electrolysis-based hydrogen production—using wind, solar, or off-peak grid electricity—directly with underground storage as a complete energy system. These inventions address the seasonal energy storage gap by integrating hydrogen production, compression, underground injection, withdrawal, and power regeneration into unified system architectures. Notable sub-variants include hydrogen blending into underground natural gas storage systems, solar-powered subsurface electrolysis in depleted reservoirs, pressure-differential porous reservoir systems for dual-use storage and grid balancing, and wind-solar-hydrogen underground integrated energy systems. PLA Army Engineering University’s 2023 patent specifically targets urban underground space to reduce surface land requirements and improve resistance to natural disasters—a uniquely urban constraint not addressed by geological formation approaches.

Geographic and Assignee Landscape: China’s Dominance and Saudi Arabia’s IP Strategy

China dominates by volume in the retrieved patent dataset, with Chinese assignees collectively outnumbering all non-Chinese assignees by a substantial margin. International PCT (WO) filings and United States (US) filings are present but fewer, led primarily by Saudi Arabian Oil Company. European research institutions appear predominantly in the literature domain—as academic publications—rather than in patent filings, a pattern consistent with EPO data showing European academic institutions lagging commercial entities in energy storage patent output.

The breadth of Chinese filings—spanning rock caverns, mine voids, liquid hydrogen repositories, blended gas storage, and urban underground systems—signals coordinated industrial policy driving rapid domestic UHS deployment, rather than purely exploratory R&D. Key Chinese assignees include CNPC, PowerChina East China Engineering Corporation, State Grid Hubei Electric Power Co., Ltd., PetroChina Shenzhen New Energy Research Institute Co., Ltd., Chengdu University of Technology, Shengneng Energy (Zhejiang) Co., Ltd., and China Metallurgical Wuhan Survey Engineering Technology Co., Ltd., among others.

Saudi Arabian Oil Company stands out as the only major oil and gas company with multiple distinct UHS-specific filings in this dataset—targeting both WO and US jurisdictions on subsurface formation storage and simultaneous injection/withdrawal architecture. Notably, non-Chinese international filing expansion by Chinese UHS assignees has been limited to date, though this may shift as Chinese projects seek export markets. Individual and academic filers also appear: Meleghegyi, András (Hungary, WO) filed on groundwater-based electrolysis and hydrogen storage, and Indian Institute of Technology Bombay (IN) filed on metal hydride vehicular storage—adjacent to rather than directly within the UHS domain.

Application Domains: Seasonal Storage, Energy Security, and Blending

Underground hydrogen storage serves three principal application domains in the retrieved dataset: grid-scale seasonal energy storage, national energy security and strategic reserves, and power-to-gas hydrogen blending in existing gas networks. The dominant application is seasonal energy balancing—storing surplus renewable electricity as hydrogen during periods of low demand and withdrawing it during peak demand periods.

Literature from Germany, Ireland, Australia, and the UK frames UHS specifically as a multi-day to multi-month storage mechanism that no battery technology can replicate at equivalent scale, as also acknowledged in energy storage frameworks published by IRENA. The Irish Sea offshore wind-to-underground storage concept demonstrates discounted payback periods of 7.8 to 20.3 years depending on offtake period, signalling approaching commercial viability for integrated offshore wind projects.

National energy security applications are documented across Italy (EU Hystories project), South Korea, China, Canada (Ontario), and Australia. South Korea’s analysis specifically highlights UHS as advantageous for storing both domestically produced and imported hydrogen. China’s literature stresses UHS as a mechanism to absorb curtailed wind and solar electricity and build strategic hydrogen reserves—a framing consistent with China’s dual policy objectives of grid stabilisation and energy independence.

The power-to-gas and hydrogen blending sub-cluster represents the lowest-barrier near-term deployment pathway: hydrogen is mixed with natural gas at predefined ratios before injection into existing underground gas storage infrastructure, avoiding the need to commission entirely new geological formations. PetroChina Shenzhen New Energy Research Institute Co., Ltd.’s 2025 filing introduces a dynamic operating conditions adjustment sub-system that optimises hydrogen concentration in real time based on grid conditions and downstream demand—enabling a transitional pathway that can be deployed within existing operational frameworks.

A niche application domain addresses depleted tight (ultra-low permeability) oil and gas reservoirs. Chengdu University of Technology has filed two closely related patents (2023 and 2025) on this approach, which leverages existing well infrastructure and detailed reservoir characterisation data. Key risks in shallow tight reservoirs include microbial methanogenesis and H₂ diffusion losses—the same biological and physical mechanisms that constrain porous media storage more broadly.

Frontier Directions and Strategic Implications for 2026–2030

Based on filings dated 2024–2026 in the retrieved dataset, six frontier directions are shaping the UHS technology landscape for the next planning horizon—each addressing a distinct constraint in the current deployment stack.

Engineered scalable underground repository structures for non-geological contexts are a clear 2025 frontier signal. China Metallurgical Wuhan Survey Engineering Technology Co., Ltd. filed two related patents covering standardised, scalable underground hydrogen repository structures designed to function across diverse geologies without requiring specific geological prerequisites—directly addressing the constraint that salt caverns and depleted fields are geographically concentrated away from major hydrogen demand centres in eastern China and South and East Asia more broadly.

Liquid hydrogen underground repositories represent a high-density storage direction. Zhongshan Advanced Cryogenic Technology Research Institute has filed a series of patents from 2022 to 2025 on underground liquid hydrogen repositories, leveraging the stable, constant-temperature underground environment to reduce the boil-off losses that afflict surface liquid hydrogen tanks. The volumetric energy density of liquid hydrogen—845 times that of gaseous hydrogen at ambient conditions—makes this approach attractive for scenarios where storage footprint is constrained.

SOEC-coupled high-temperature solid-state hydrogen production-storage integration is the most technically ambitious frontier direction. Xi’an Thermal Power Research Institute Co., Ltd. (affiliated with China Energy Engineering Group) filed two patents in 2025 on integrating solid oxide electrolysis cells (SOEC) with high-temperature solid-state hydrogen storage, targeting conversion efficiencies exceeding 100% (electricity-to-hydrogen) by using waste heat recovery. While not underground storage per se, this system-level integration feeds directly into underground storage infrastructure.

Groundwater-based underground hydrogen storage is an unconventional emerging approach. Individual inventor Meleghegyi, András filed a 2025 WO patent on a system that electrolyzes groundwater in situ and stores the produced hydrogen within sealed cartridge assemblies submerged below the groundwater table—targeting locations without suitable geological formations but with accessible groundwater.

From a strategic standpoint, microbial activity—methanogenesis and sulfate reduction—is identified as the most underaddressed technical risk in porous formation storage. IP strategists should monitor filings around biocide injection, microbial monitoring systems, and reservoir conditioning methods, which are currently sparse in the dataset, suggesting a white space for both research and IP development. Non-Chinese operators should also monitor Chinese assignees’ international filing expansion, which has been limited to date but may grow as Chinese projects seek export markets. The full-cycle integration of UHS with renewable energy production systems—electrolysis, compression, underground injection, withdrawal, and power recovery—is described as the defining architectural direction for 2026–2030, with growing evidence that levelised cost of storage in underground hydrogen is competitive with alternative long-duration technologies at the 2030 horizon.