Expectations for energy storage are high but large-scale underground hydrogen storage in porous media (UHSP) remains largely untested. This article identifies and discusses the scientific challenges of hydrogen storage in porous media for safe and efficient large-scale energy storage to enable a global hydrogen economy. To facilitate hydrogen supply on the scales required for a zero-carbon future, it must be stored in porous geological formations, such as saline aquifers and depleted hydrocarbon reservoirs. Large-scale UHSP offers the much-needed capacity to balance inter-seasonal discrepancies between demand and supply, decouple energy generation from demand and decarbonise heating and transport, supporting decarbonisation of the entire energy system. Despite the vast opportunity provided by UHSP, the maturity is considered low and as such UHSP is associated with several uncertainties and challenges. Here, the safety and economic impacts triggered by poorly understood key processes are identified, such as the formation of corrosive hydrogen sulfide gas, hydrogen loss due to the activity of microbes or permeability changes due to geochemical interactions impacting on the predictability of hydrogen flow through porous media. The wide range of scientific challenges facing UHSP are outlined to improve procedures and workflows for the hydrogen storage cycle, from site selection to storage site operation. Multidisciplinary research, including reservoir engineering, chemistry, geology and microbiology, more complex than required for CH₄ or CO₂ storage is required in order to implement the safe, efficient and much needed large-scale commercial deployment of UHSP.

Carbon capture and storage (CCS) can help nations meet their Paris CO₂ reduction commitments cost-effectively. However, lack of confidence in geologic CO₂ storage security remains a barrier to CCS implementation. Here we present a numerical program that calculates CO₂ storage security and leakage to the atmosphere over 10,000 years. This combines quantitative estimates of geological subsurface CO₂ retention, and of surface CO₂ leakage. We calculate that realistically well-regulated storage in regions with moderate well densities has a 50% probability that leakage remains below 0.0008% per year, with over 98% of the injected CO₂ retained in the subsurface over 10,000 years. An unrealistic scenario, where CO₂ storage is inadequately regulated, estimates that more than 78% will be retained over 10,000 years. Our modelling results suggest that geological storage of CO₂ can be a secure climate change mitigation option, but we note that long-term behaviour of CO₂ in the subsurface remains a key uncertainty.

Large-scale underground storage of hydrogen gas is expected to play a key role in the energy transition and in near future renewable energy systems. Despite this potential, experience in underground hydrogen storage remains limited. This work critically reviews the most important elements of this crucial technology, including hydrogen properties and their significance for subsurface operations, sources for hydrogen and historical hydrogen storage operations, to set the state of the art. The cyclical nature of hydrogen storage operations will produce pressure and stress changes within the reservoir that could affect the integrity of the well, the reservoir, the caprock and the entire subsurface storage complex. To minimize geomechanical leakage risks and optimize the storage operation it is crucial to understand the pressure and stress history of the storage site, to optimize well locations to manage pressure and to identify the reservoir-specific cushion gas to working gas ratio. Finally, we outline the major scientific and operational challenges required to ensure the safe and efficient deployment of underground hydrogen storage at a large scale.

The first Spanish Technological Development plant for CO2 storage is currently under development in Hontomín (Spain), in a fractured carbonate reservoir. The subsurface 3D geological structures of the Hontomín site were interpreted using well-log and 3D seismic reflection data. A shallow low velocity zone affects the wave propagation and decreases the coherency of the underlying seismic reflections, deteriorating the quality of the seismic data, and thus preventing a straightforward seismic interpretation. In order to provide a fully constrained model, a geologically supervised interpretation was carried out. In particular, a conceptual geological model was derived from an exhaustive well-logging analysis. This conceptual model was then improved throughout a detailed seismic facies analysis on selected seismic sections crossing the seismic wells and in consistency with the regional geology, leading to the interpretation of the entire 3D seismic volume. This procedure allowed characterizing nine main geological levels and four main fault sets. Thus, the stratigraphic sequence of the area and the geometries of the subsurface structures were defined. The resulting depth-converted 3D geological model allowed us to estimate a maximum CO2 storage capacity of 5.85 Mt. This work provides a 3D geological model of the Hontomín subsurface, which is a challenging case study of CO2 storage in a complex fractured carbonate reservoir.Jaume Almera agreement (ALM-09-027: Characterization, Development and Validation of Seismic Techniques applied to CO2 Geological Storage Sites) and by the European Union through the Technology Demonstration Plant of Compostilla OXYCFB300 Project (European Energy Programme for Recovery). Additional support has been provided by Spanish Ministry of Education Science (CSD2006-00041), Generalitat de Catalunya (2009SGR006) and CSIC JAE-Doc postdoctoral research contract (E.S.).Peer reviewe

The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide‐angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high‐resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at ∼40–41 km depth in the northern part of the High Atlas. The P‐wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling efforts.