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.

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.

The first Spanish Technological Development plant for CO 2 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-3 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 few 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 CO 2 storage capacity of 5.85 Mt. This work provides a 3D geological model of the Hontomín subsurface, which is a challenging case study of CO 2 storage in a complex fractured carbonate reservoir.

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, highresolution 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.