Modeling fidelity upon competitive adsorption for underground hydrogen storage in depleted shale reservoirs: A minireview

With the continuous advancement of global carbon neutrality goals, hydrogen, as a clean and efficient energy carrier, is increasingly recognized as a critical enabler for energy transition and emission reduction. However, mismatches between hydrogen supply and demand may arise due to seasonal fluctuations and intermittent renewable generation, underscoring the essential role of effective storage. Depleted shale gas reservoirs, due to their substantial available storage capacity and unique geological characteristics, are regarded as promising candidates for large-scale underground hydrogen storage (UHS). Despite this potential, a comprehensive understanding of UHS in such reservoirs remains limited. To address this gap, this review provides a systematic synthesis of recent advances in geochemical and biochemical reactions, surface and interfacial phenomena, adsorption–diffusion processes, and reservoir-scale simulations relevant to UHS in depleted shale gas reservoirs. Drawing on these interdisciplinary insights, the technical, safety, and economic challenges associated with UHS in depleted shale gas reservoirs are examined. Finally, potential future research directions in areas such as experimental studies, numerical simulations, and supporting technologies are highlighted. This review delivers the first systematic perspective on H₂-brine-shale interactions and multi-field coupling processes in UHS within depleted shale gas reservoirs, offering essential guidance for advancing future research and enabling practical implementation.

The various types of microorganisms encountered in subsurface reservoirs are identified, and their consequences for underground hydrogen storage and carbon dioxide storage are described. In underground hydrogen storage reservoirs, most microorganism activities have negative consequences (pore clogging, reduction in permeability, corrosion, change in composition and loss of stored gas). In underground carbon dioxide storage most of those negative consequences also apply, but transforming some of the stored CO2 into methane by the biomethanation process can be beneficial and potentially exploited. Although laboratory studies and simplified-system simulations have qualitatively explained the microorganism processes involved at the pore scale in underground hydrogen storage and underground carbon dioxide storage reservoirs, it is difficult to model these processes quantitatively at the reservoir scale. The simplifying assumptions, scales and dimensions of the majority of bioreactive transport models fail to take adequate account of reservoir heterogeneities, biofilm development complexities, periodic fluctuations in fluid-flow and nutrient supply. These limitations mean that most of the existing bioreactive transport models are unable to reliably quantify changes in gas composition, gas loss, permeability, or the degree of corrosion likely to occur across heterogeneous underground hydrogen storage or underground carbon dioxide storage reservoirs. However, several opportunities exist to improve field-scale bioreactive transport model performance for underground hydrogen storage and underground carbon dioxide storage in the coming years. These include building on the knowledge gained from the existing simplified models by incorporating new modelling techniques and more detailed reservoir scale information. Exploiting DNA sequencing offers the capability to better characterize the properties of reservoir microorganism communities. Physics-informed machine learning techniques could be tailored to provide efficient surrogate models for simulations of heterogeneous reservoirs. Such improvements should lead to simulation models capable of accommodating more complex reservoir-scale assumptions. Document Type: Review Cited as: Wood, D. Multiple microorganism influences and interactions limit the reliability of underground hydrogen and carbon storage simulation models at the reservoir scale. Sustainable Earth Resources Communications, 2025, 1(2): 53-68. https://doi.org/10.46690/serc.2025.02.04

Underground hydrogen storage: Characteristics and prospects

The demand for hydrogen is growing. The IEA 2021 Hydrogen report showed that global hydrogen demand reached 94 Mt in 2021, a 5% increase in demand from 2020. Hydrogen demand is expected to reach 180 Mt by 2030. This increasing demand would require storage at scale. Of existing and potential hydrogen storage technologies, underground hydrogen storage in porous media is being considered for large-scale hydrogen storage based on successes with underground gas storage. However, there are no detailed site selection criteria for underground hydrogen storage in porous media. The objective of this study is to showcase the key geological and reservoir engineering parameters that affect underground hydrogen storage and demonstrate how petrophysical data could help in screening sites, site characterization, and hydrogen plume monitoring. We used numerical simulation modeling of a synthetic reservoir to create a base-case model representative of the hydrodynamic conditions relevant to underground hydrogen storage in porous media. We carried out a two-step sensitivity analysis. In the first step, we determined the key parameters impacting the storage and flow of hydrogen in porous media. In the second stage, we examined in detail the extent further ranges of those key parameters had on hydrogen storage potential. The findings of the two-step sensitivity analysis resulted in the development of preliminary site selection criteria. The study showed that the reservoir depth or current pressure, the reservoir dip, and the flow capacity were the top three factors impacting the optimal withdrawal of hydrogen. These highly sensitive parameters also indicate the need to reduce the uncertainty associated with these parameters when selecting potential sites for hydrogen storage in porous media. When the site selection criteria were applied to depleted fields in Northern California, we were able to see how uncertainties in geological and reservoir parameters can change a site’s ranking for potential hydrogen storage. This study quantifies uncertainties in data and identifies where and how petrophysical measurements could reduce the uncertainty associated with the key parameters relevant to underground hydrogen storage, selecting optimal sites for hydrogen storage, and tracing hydrogen leaks during the monitoring phase.

Global attention has shifted back to hydrogen, the ultimate green alternative energy, due to the intensifying extreme climate and greenhouse impacts. It is now a promising energy carrier for large-scale underground hydrogen storage, which may benefit the peak-shaving of electric grids and the supply from other sustainable energies. The prospects of underground hydrogen storage are bright due to the abundance of subsurface formations such as depleted oil and gas reservoirs, saline aquifers, and salt caverns and the traditional energy industry's extensive underground procedures and management knowledge. However, hydrogen's unique thermodynamic and chemical properties pose challenges as well, requiring further research and modeling to provide a firm basis and validate engineering practice. Some typical concerns, like hydrogen embrittlement, have long plagued industry. The intricate and difficult-to-reach nature of the underground makes fundamental mechanics, mathematical modeling, and numerical simulation essential tools for further advancement. Integration of experiments and simulations at the multi-scale (molecular, pore, and Darcy) provides data support and solution validation for the pursuit of comprehensive descriptions and accurate predictions of hydrogen storage and extraction processes. Given this, our paper reviews the thermodynamic properties of hydrogen, focuses on the phase equilibrium processes at different scales in underground hydrogen storage, and introduces the related mathematical models and simulation methods. Based on recent research progress and comparing four typical trapping mechanisms in carbon capture, utilization and storage, we highlight some unique hydrogen-induced effects: (1) compositional grading; (2) competitive adsorption; (3) hydrogen-induced rock alteration, in conjunction with phase equilibrium issues, discuss their interplay mechanisms, offering constructive insights for further in-depth research. Document Type: Original article Cited as: Feng, X., Liu, J., Shi, J., Hu, P., Zhang, T., Sun, S. Phase equilibrium, thermodynamics, hydrogen-induced effects and the interplay mechanisms in underground hydrogen storage. Computational Energy Science, 2024, 1(1): 46-64. https://doi.org/10.46690/compes.2024.01.05

<p>Underground storage of hydrogen (H2) could be an alternative or important supplement to energy storage. However, there is still lack of knowledge about fundamental biogeochemical aspects of underground H2 storage. The BMBF-funded project H2_ReacT investigates fundamental petrophysical, geochemical and biogeochemical aspects of underground H2 storage. The work presented here addresses the microbial consumption of H2 and the involved microorganisms at potential underground storage sites.</p><p>Microbial reactions that consume H2 are still a major uncertainty factor for underground H2 storage. Microbial life is widespread in the crust of the earth and geological formations suitable for underground H2 storage often contain a deep subsurface biosphere. Thus, an underground H2 storage site needs to be seen as a habitat for microorganisms. Microbial activity at the H2 storage site might affect the stored H2 as well as the integrity of the storage site itself. A specific interest is to gain information about microbial activity that might result in a loss of stored hydrogen as well as the production of unwanted metabolic products e.g. H2S. The importance of specific conditions with relevance for underground hydrogen storage i.e. elevated pressure, high temperature and rock material, will be addressed.</p><p>Preliminary results showed the consumption of H2 by indigenous microorganisms from a porous rock reservoir fluid. Hydrogen was consumed at different temperature and pressure conditions relevant for underground H2 storage. Here, hydrogen consumption rates were strongly influenced by temperature and pressure. Currently effects of several geochemical parameters on microbial H2 consumption are studied in more detail. Furthermore, molecular biological approaches are used to identify the involved microorganisms.</p>

Role of geochemical reactions on caprock integrity during underground hydrogen storage

Summary Underground hydrogen storage (UHS) has the potential to balance fluctuating sustainable energy generation and energy demand by offering large-scale seasonal energy storage. Depleted natural gas fields or underground gas storage fields are attractive for UHS as they might allow for cost-efficient hydrogen storage. The amount of cushion gas required and the purity of the backproduced hydrogen are important cost factors in UHS. This study focuses on the role of molecular diffusion within the reservoir during UHS. Although previous research has investigated various topics of UHS such as microbial activity, UHS operations, and gas mixing, the effects of diffusion within the reservoir have not been studied in detail. To evaluate the composition of the gas produced during UHS, numerical simulation was used here. The hydrogen recovery factor and methane-to-hydrogen production ratio for cases with and without diffusive mass flux were compared. A sensitivity analysis was carried out to identify important factors for UHS, including permeability contrast, vertical-to-horizontal permeability ratio, reservoir heterogeneity, binary diffusion coefficient, and pressure-dependent diffusion. Additionally, the effect of numerical dispersion on the results was evaluated. The simulations demonstrate that diffusion plays an important role in hydrogen storage in depleted gas reservoirs or underground gas storage fields. Ignoring molecular diffusion can lead to the overestimation of the hydrogen recovery factor by up to 9% during the first production cycle and underestimation of the onset of methane contamination by half of the back production cycle. For UHS operations, both the composition and amount of hydrogen are important to design facilities and determine the economics of UHS, and hence diffusion should be evaluated in UHS simulation studies.

The objective of this study is to showcase the key geological and reservoir engineering parameters that influence underground hydrogen storage, demonstrate the value of some petrophysical data, and show how hydrogen storage differs between depleted gas fields and saline aquifers for reservoir and geomechanical modeling. We utilized numerical simulation modeling to create a base-case model of a synthetic reservoir that accurately represented the hydrodynamic conditions relevant to underground hydrogen storage in porous media. A two-step sensitivity analysis was then conducted. Firstly, we identified the critical parameters that significantly influence the storage and flow of hydrogen in porous media. Subsequently, we analyzed the geomechanical impact of underground hydrogen storage. In addition, we compared the behavior of hydrogen storage to natural gas storage. The study showed that the reservoir depth or current pressure, the reservoir dip, and the flow capacity were the top three factors impacting the optimal withdrawal of hydrogen. The study also revealed that rock displacement and stress changes were important to be monitored, while changes in strain were not significant. If it is assumed that injection occurs in a critically stressed rock, hydrogen injection and withdrawal in saline aquifers could result in more incidence of microseismicity compared to hydrogen storage in depleted fields or even gas storage in depleted fields. This study quantifies uncertainties in data and pinpoints areas where petrophysical measurements could minimize the uncertainty associated with critical parameters relevant to underground hydrogen storage. It also identifies gaps in measurements for hydrogen storage in porous media. These parameters with large uncertainty are crucial for selecting optimal sites for hydrogen storage and detecting subsurface integrity issues when monitoring for underground hydrogen storage in porous media.

In response to the surging global demand for clean energy solutions and sustainability, hydrogen is increasingly recognized as a key player in the transition towards a low-carbon future, necessitating efficient storage and transportation methods. The utilization of natural geological formations for underground storage solutions is gaining prominence, ensuring continuous energy supply and enhancing safety measures. However, this approach presents challenges in understanding gas-rock interactions. To bridge the gap, this study proposes a data-driven strategy for contact angle prediction using machine learning techniques. The research leverages a comprehensive dataset compiled from diverse literature sources, comprising 1045 rows and over 5200 data points. Input features such as pressure, injection rate, temperature, salinity, rock type, and substrate were incorporated. Various artificial intelligence algorithms, including Support Vector Machine (SVM), k-Nearest Neighbors (KNN), Feedforward Deep Neural Network (FNN) and Recurrent Deep Neural Network (RNN), were employed to predict contact angle, with the FNN algorithm demonstrating superior performance accuracy compared to others. The strengths of the FNN algorithm lie in its ability to model nonlinear relationships, scalability to large datasets, robustness to noisy inputs, generalization to unseen data, parallelizable training processes, and architectural flexibility. Results show that the FNN algorithm demonstrates higher accuracy (RMSE = 0.9640) than other algorithms (RMSE RNN = 1.7452, RMSE SVM = 1.8228, RMSE KNN = 1.0582), indicating its efficacy in predicting the contact angle testing subset within the context of underground hydrogen storage. The findings of this research highlight a low-cost and reliable approach with high accuracy for estimating contact angle of water–hydrogen–rock system. This technique also helps determine the contribution and influence of independent factors, aiding in the interpretation of absorption tendencies and the ease of hydrogen gas flow through the porous rock space during underground hydrogen storage. • Hydrogen emerges as a leading clean energy solution, vital for a low-carbon future. • Underground storage methods ensure continuous energy supply and safety. • Machine learning tackles challenges in optimizing storage efficiency and reducing environmental risks. • Data-driven approach predicts gas-rock interactions for underground hydrogen storage. • FNN algorithm excels in accuracy, scalability, and robustness for contact angle prediction.

The successful commercialisation of underground hydrogen storage (UHS) is contingent upon technological readiness and social acceptance. A lack of social acceptance, inadequate policies/regulations, an unreliable business case, and environmental uncertainty have the potential to delay or prevent UHS commercialisation, even in cases where it is ready. The technologies utilised for underground hydrogen and carbon dioxide storage are analogous. The differences lie in the types of gases stored and the purpose of their storage. It is anticipated that the challenges related to public acceptance will be analogous in both cases. An assessment was made of the possibility of transferring experiences related to the social acceptance of CO2 sequestration to UHS based on an analysis of relevant articles from indexed journals. The analysis enabled the identification of elements that can be used and incorporated into the social acceptance of UHS. A framework was identified that supports the assessment and implementation of factors determining social acceptance, ranging from conception to demonstration to implementation. These factors include education, communication, stakeholder involvement, risk assessment, policy and regulation, public trust, benefits, research and demonstration programmes, and social embedding. Implementing these measures has the potential to increase acceptance and facilitate faster implementation of this technology.

Underground hydrogen (H 2 ) storage in salt caverns, aquifers, and depleted gas reservoirs could become a central component of Europe’s transition to a decarbonised energy system. However, subsurface microbial activity may compromise the technical and economic viability by reducing hydrogen purity and volume through consumption and by-product formation. This study presents a multidimensional assessment of microbial risks in underground hydrogen storage, integrating geological classification, expected microbial processes, and techno-economic consequences. Salt caverns, depleted gas fields, and aquifers are compared in terms of microbial activity potential and associated hydrogen losses. The technical analysis is complemented by a review of the policy, economic, and technological context, applying both PESTLE and SWOT frameworks. Results highlight substantial differences in microbial risk and storage performance, with hydrogen losses ranging from <0.1% per year in salt caverns to ∼30% per year in high-risk aquifers. Such losses can increase the levelised cost of storage (LCOS) up to duplicate it, depending on the scale of purification required. This paper provides a basis for understanding the impact of microbial activity on performance, cost, and feasibility of underground hydrogen storage. It demonstrates that microbial risk classification should be integrated early in the design process and considered in techno-economic evaluations. The findings also highlight regulatory and systemic gaps that must be addressed to support robust policy and investment frameworks for large-scale deployment.

<p>Underground hydrogen storage (UHS) in porous media has been proposed as an effective and sustainable energy storage method to balance renewable energy supply and seasonal demand. To determine the potential for and conduct realistic risk assessments of the UHS technology, learnings from more mature underground fluid storage technologies, such as underground storage of natural gas (UGS) or CO<sub>2</sub> (UCS), can be used. Here we discuss the caveats related to the use of these technologies as analogues to UHS and highlight current knowledge gaps that need to be addressed in future research to make UHS a secure and efficient technology.</p><p>Abiotic and biotic reactions between the rock and the fluids, often not considered in UCS and UGS operations, play an important role in UHS and can change the chemical environment in the reservoir dramatically. The mineralogy of the reservoir and cap rocks, as well as the in-situ pore fluid chemistry, is of vital importance and the characterisation efforts should not be limited to the reservoir quality.</p><p>The risk assessment of UHS operation may follow similar production cycles as in UGS, but there are important lessons to be learnt from UCS. UCS aims to store injected gas permanently and different CO<sub>2</sub> trapping mechanisms are contributing to storage security. Residual trapping, which locks parts of the CO<sub>2</sub> within the pore space, may reduce the commercial profitability in UHS, but can assist to mitigate potential leakage of hydrogen. The dissolution of hydrogen in the pore water will likely play a minor role in UHS compared to UCS, while the precipitation of minerals containing hydrogen during UHS has not yet been appropriately investigated.</p><p>The main storage process in gas storage is the accumulation of buoyant fluid underneath a low-permeability cap rock in a three-dimensional trap. Storage sites are determined by different parameters: UGS is mainly used in depleted gas fields (hence sites with proven gas storage security), while UCS sites are usually located deeper than 800m for efficiency reasons, under conditions at which CO<sub>2</sub> is present as a high-density supercritical phase. None of these restrictions are a pivotal for UHS and a new set of constrains should be formulated specifically designed to the properties of hydrogen. These must involve:</p><ul><li>The unique properties of hydrogen (high diffusivity and low density and, thus, high buoyancy) require potential storage sites to have well-understood cap rocks with minimal diffusion and capillary leakage risk.</li> <li>A reservoir architecture and heterogeneity that guarantees economically sensible injection and withdrawal rates by choosing sites, which minimise the isolation of hydrogen from the main plume during UHS operations.</li> <li>Site monitoring protocols will also need to be re-evaluated for different scales, as well as for the dynamic properties of hydrogen, such as low density and fluid mobility.</li> </ul><p>It is certain that leakage along abandoned wells, the main risk for leakage in UCS and UGS, will also pose a risk to the containment of injected hydrogen. Therefore, hydrogen storage site locations require a comprehensive investigation into abandoned and operational (deep) petroleum and (shallow) water exploration and production wells.</p>

For large scale seasonal storage, Underground Hydrogen Storage (UHS) can be used to balance fluctuating sustainable energy generation and energy demand. Similar to underground natural gas storage, depleted gas fields potentially allow for cost-efficient hydrogen storage. One of the major cost factors in UHS is the amount of cushion gas required and the purity of the hydrogen produced during the production cycle. The hydrocarbon gas remaining in the reservoir can be used as cushion gas to significantly reduce UHS costs. To evaluate the composition of the gas produced during the production cycle of UHS, numerical simulation was applied. One of the important processes in UHS is molecular diffusion within the reservoir. The hydrogen recovery factor and methane to hydrogen production ratio were compared for cases with and without diffusive mass flux. Furthermore, a sensitivity analysis was carried out to identify important factors for UHS. The following parameters were investigated: permeability contrast, vertical to horizontal permeability ratio, reservoir heterogeneity, binary diffusion coefficient, and pressure dependent diffusion. In addition, the effects of numerical dispersion on the results were evaluated and are discussed. The results of numerical simulation show the importance of diffusion on hydrogen storage in depleted gas reservoirs. Molecular diffusion plays a major role in case of heterogeneous reservoirs and large permeability contrasts. In low permeability zones, the diffusive mass transport becomes dominant over advective flux. Hydrogen propagates further into the low permeable layers of the reservoir when diffusion effects are considered compared with the cases neglecting diffusion. Similar effects are observed during the production cycle. Hydrocarbon gas in low permeability zones becomes more mobile due to diffusive transport. Thus, a larger amount of methane is back-produced with hydrogen for the cases when diffusion is simulated. It is shown that if molecular diffusion is ignored, the hydrogen recovery factor can be overestimated by up to 9% during the first production cycle and the onset of methane contamination can be underestimated by half of the back production cycle. Simulating pressure dependent diffusion might be important for specific configurations and should be covered in a sensitivity. The results show that molecular diffusion within the reservoir has an impact on the onset of methane contamination when hydrocarbon gas is used as cushion gas in UHS. Also, the total amount of hydrogen produced is overestimated. For UHS operations, both, the composition and amount of hydrogen is important to design facilities and to determine the economics of UHS and hence diffusion should be evaluated in UHS simulation studies.

Accurately assessing the adsorption and diffusion behaviors of H2, CH4, and their mixtures are essential for estimating underground hydrogen storage (UHS). This understanding is critical for the safe and efficient storage of H2 in depleted shale gas reservoirs. Although H2 adsorption in kerogen has been extensively studied, adsorption-induced swelling remains unexplored in UHS. In this study, we investigate adsorption mechanisms using Lagrangian and Eulerian approaches and analyze diffusion in kerogen through molecular simulations. Our results reveal that in the presence of cushion gases like CH4, which exhibit stronger adsorption than H2, neglecting kerogen deformation can lead to an underestimation of storage capacity by approximately 40%. Furthermore, increasing pressure makes H2 adsorption behavior deviate from the consistent swelling trend that is observed with CH4, with kerogen either swelling or contracting depending on the pore size. Simulations also predict that H2 self-diffusion coefficient in porous kerogen is 1 order of magnitude higher than CH4. These findings highlight the importance of incorporating kerogen flexibility into the modeling of UHS involving multiple gas species to improve the accuracy and safety of H2 storage operations in shale reservoirs.