Geoenergy - Volume 1, Issue 1, 2023

A quantitative seismic interpretation of the Gassum Formation at the onshore aquifer gas storage near the Danish town of Stenlille is presented with its implications for exploiting the gas storage facility as a potential CO2 demonstration site. Our objective was to better outline the reservoir heterogeneity of the Gassum Formation based on a 3D seismic volume and 20 wells available from the Stenlille gas storage facility. We derived new absolute and relative P-impedance inversion products, which are useful for delineating lithological distributions of thin sandstone reservoirs and shaly beds within the Gassum Formation. Our results broadly agree with previously published seismic interpretations, and build upon these by the identification of deviations in parts of the Gassum interval that should be considered in any subsequent development of a static reservoir model. Hence, this work is an important contribution to the planning of drilling new CO2 injection wells as part of the development and management of the Stenlille CO2 storage demonstration site. Nevertheless, we also recommend a modern seismic reprocessing of the 3D seismic data combined with newly acquired 2D data from the Stenlille structure as input into further quantitative seismic interpretation studies to refine reservoir characterization and reduce associated uncertainties.

Supplementary material: video is available at https://doi.org/10.6084/m9.figshare.c.6662413

Various approaches are used at the preliminary stage (before flow modelling studies) to evaluate the resources of saline aquifers for storing CO2. The two approaches used most commonly are the volumetric ‘open aquifer’ and ‘closed aquifer’ approaches, which provide the end members for the resources of a given aquifer store. We propose a novel approach based on extending the analytical aquifer methods, which have been routinely used for decades in the oil and gas industry, to CO2 saline aquifer storage. The CO2 storage resource is derived from the pore volume liberated by pressurization to accommodate CO2 in the store, which is the sum of: (i) the volume of the store obtained by compression; and (ii) the volume of water outflow towards the regional aquifer. It enables the CO2 storage resources and storage efficiencies for pressure-limited stores (meaning when the resource is not even more limited by CO2 migration issues) to be estimated. Formalism is presented, as well as implications and limitations of the method. With this method, storage efficiencies are directly connected to closed store efficiencies that are corrected for the water outflux and pressure dissipation from the CO2 store into the regional aquifer, reconciling the ‘closed’ and ‘open’ approaches.

We present an application to the Havnsø saline aquifer structure in order to highlight the consequences of the CO2 storage resource evaluation and also of appraisal and development phases.

A regional assessment of the potential for geothermal heat production, power generation and also heat storage requires an evaluation of the heat in place and its recharge. Both are controlled by the flow properties and natural fluid flow through the reservoir at the present day, which must be thoroughly analysed based on the understanding and modelling of the architecture of deep reservoirs. In the Upper Rhine Graben, well known for its vast geothermal potential, a characterization of the structural organization at the basin scale and an understanding of mass and heat transfer are, therefore, useful for correctly estimating the technical and economic potential for geothermal energy. The distribution of the resources can be quantified based on the basin analysis approach. This is classically used for hydrocarbon resources and is applied in this study to predict the geothermal fluid-flow paths. The role of faults and their hydraulic regime on temperature field heterogeneities is investigated. Several scenarios with or without rift basin internal faults and various transmissivity ranges along these faults are analysed and compared. The measured temperature field is used for the calibration of the different model scenarios results. The scenario with the most significant role for internal faults as fluid-flow pathways in the western Upper Rhine Graben is the one that provides the most accurate reproduction of the temperature field. Such output highlights the importance of quantifying the hydraulic behaviour of faults and associated fracture networks in space and time.

Supplementary material: dataset containing geometry, facies distribution and main results of scenario 4 is available at https://doi.org/10.6084/m9.figshare.c.6628506

Utilizing existing temperature and structural geology information around Granite Springs Valley, Nevada, we build 3D stochastic temperature models with the aims of evaluating the 3D uncertainty of temperature and choosing between candidate exploration well locations. The data used to support the modelling are measured temperatures and structural proxies from 3D geologic modelling (distance to fault, distance to fault intersections and terminations, Coulomb stress change and dilation tendency), the latter considered ‘secondary’ data. Two stochastic geostatistical techniques are explored for incorporating the structural proxies: cosimulation and local varying mean.

With both the cosimulation and local varying mean methods, many equally-likely temperature models (i.e. realizations) are produced, from which temperature probability profiles are calculated at candidate well locations. To aid in choosing between the candidate locations, two quantities summarize the temperature probabilities: V prior and entropy. V prior quantifies the likelihood for economic temperatures at each candidate location, whereas entropy identifies where new information has the most potential to reduce uncertainty.

In general, the cosimulation realizations have smoother spatial structure, and extrapolate high temperatures at candidate locations that are located along the direction of the longest spatial correlation, which are down dip from existing temperature logs. The smooth realizations result in tight temperature probability profiles that are easier to interpret, but they have unrealistic temperature reversals in some locations because of the dipping ellipsoid shape created and that the cosimulation technique does not enforce a conductive geothermal gradient as a baseline (i.e. linearly increasing temperature with depth). The local varying mean results produce realizations with more realistic geothermal gradients, with temperatures increasing downward since a depth-temperature relationship is included. However, because they have much noisier spatial nature compared to cosimulation, it is harder to interpret the temperature probability profiles. The different local varying mean results allow the geologist to determine which proxy (e.g. dilation v. distance to fault termination) should be used given the specific geothermal system. In general, V prior from local varying mean results identify locations that are close to high values for the structural proxies: areas with higher probabilities for higher temperatures. The entropy results identify where uncertainty is greatest and therefore new drilling information could be most useful. Though these techniques provide useful information, even when applied to areas of sparse data, our comparison of these two techniques demonstrates the need for new geothermal geostatistics techniques that combine the advantages of these two methods and that are tailored to the spatial uncertainty issues inherent in geothermal exploration.

The Southern North Sea Basins of the United Kingdom were renowned for their hydrocarbon resources and exploited extensively from the 1960s to the 1990s. The Permian Leman Sandstone in particular formed an excellent reservoir due to its extensive clean aeolian sediments and was subsequently explored for decades, resulting in a wealth of subsurface data that are now widely accessible. The strata of the Leman Sandstone comprises mixed continental deposits from aeolian, fluvial and lacustrine environments which interfinger with the saline lake deposits of the Silverpit Formation. With the potential reassessment of depleted gas reservoirs in the North Sea for use as sequestration targets for captured carbon dioxide, there is significant renewed interest in the reservoir geology of the Leman Sandstone. A regional study of the sedimentology and petrophysical properties of the Leman Sandstone and Silverpit formations within quadrants 43, 44, 48 and 49 of the Southern North Sea has been conducted. Multiple interactions between the depositional environments are observed, resulting in a complex interplay between aeolian and lacustrine transgressive/regressive events, and migration and expansion/contraction of the fluvial system. In wireline petrophysical data, each depositional environment, along with their transitional environments, form relatively distinct clusters that can be used as a predictive tool for reservoir interpretation in the absence of core, despite extensive sediment recycling between environments.

The North Alpine Foreland Basin in SE Germany is a post-mature petroleum basin and today Germany's most prolific deep geothermal energy play. Drilling of deep wells is often challenged by the complex pore pressure distribution, which has been studied in the past, but quality and reliability of individual pore pressure measurements and indicators have so far been barely addressed. This is particularly critical, since most datasets originate from old hydrocarbon wells and often display limited availability and poor quality. This paper analyses pore pressure measurements and indicators from 315 deep hydrocarbon and geothermal wells. The dataset covers pressure measurements, drilling mud weights, caliper logs, drilling events and gas readings. A large number of pressure measurements are exposed to uncertainties, resulting predominantly from incomplete pressure build-ups. In addition, investigation of drilling mud weights combined with wellbore instabilities, gas readings and pore pressure-related drilling problems suggest that many wells were subject to underbalanced drilling and mud weight alone is not a reliable pore pressure indicator. The study provides a recommendation for pre-drill pore pressure prediction based on the investigated datasets, which also presents a reference case for other post-mature petroleum basins transitioning to new industries, such as deep geothermal.

Thematic collection: This article is part of the Earth as a thermal battery: future directions in subsurface thermal energy storage systems collection available at: https://www.lyellcollection.org/topic/collections/thermal-energy

Repurposing depleted oil and gas reservoirs for secondary storage may play an important role in the transition to low-carbon energy. The integrity of the cap rocks overlying the reservoirs is an important factor for gas storage and needs to be understood prior to repurposing. In some cases, old cap-rock cores collected during exploration and development of oil and gas fields may be available for characterization using modern techniques but after being stored for decades these cores are likely to have experienced many changes in moisture, which can lead to physical changes. A comparative study of samples taken from old, unpreserved mudstone core and samples from a recently acquired and preserved core taken from the same formation shows that the mineralogy, porosity and permeability results are relatively similar between the two cores. The differences in the porosity measurements between the old and new core samples are primarily due to natural variations in grain size, rather than the preservation status of the cores. Geomechanical data, however, show significant and non-systematic differences between the old samples and the new samples, suggesting that old core samples are not suitable for geomechanical characterization. In the absence of new, well-preserved core, old core samples may provide suitable porosity, permeability and mineralogical data, whereas the old, unpreserved core is unlikely to provide reliable geomechanical data.

Supplementary material: Individual BIB-SEM porosity measurements for each sample are available at https://doi.org/10.6084/m9.figshare.c.6725765

Thematic collection: This article is part of the Fault and top seals 2022 collection available at: https://www.lyellcollection.org/topic/collections/fault-and-top-seals-2022

Increasing the proportion of electricity from renewable energy sources is critical for the reduction of CO2 emissions required to meet the commitments of the Paris Agreement. However, intermittency in energy generation due to the daily and seasonal variability of wind and solar radiation is a major challenge. Addressing this issue requires the development of large-scale energy storage systems. Low-carbon hydrogen production and storage in geological reservoirs offers a potential seasonal energy storage solution. Here, we provide the first assessment of the underground storage capacity for hydrogen in Argentina and identify sites where future clusters of low-carbon hydrogen production could be co-located with suitable geological storage sites. We outline a production-based methodology that couples historical hydrocarbon production with remaining reserves to provide a reliable estimation of the amount of hydrogen that can be stored in each reservoir. Assuming a cushion gas requirement of 50%, our results show that a combined storage capacity of 2860 TWh of hydrogen exists. This far exceeds the requirements to meet the seasonal electricity demand for residential consumption in the country. The results of our sensitivity analysis show that the two most sensitive variables are temperature and compressibility, suggesting that high-pressure shallow reservoirs are most suitable for underground hydrogen storage. We additionally propose locations for two H2 production and storage hubs that would permit the integration of storage sites with renewable developments and natural gas reserves that could be combined with carbon capture and storage to produce low-carbon hydrogen.

Thematic collection: This article is part of the Hydrogen as a future energy source collection available at: https://www.lyellcollection.org/topic/collections/hydrogen

The High Temperature Effects on Bentonite Buffers (HotBENT) Experiment, conducted at the Grimsel Test Site, aims to obtain monitoring and sampling data from an in situ large-scale and long-term test of an engineered barrier system (EBS) consisting primarily of a bentonite buffer exposed to high temperatures (up to 200°C at the heater surface). The in situ data complemented by laboratory testing and modelling will enhance existing databases and strengthen the understanding of buffer performance at such high temperatures. A robust assessment of the effect of such higher thermal loading on the most relevant safety functions and the resulting requirements on buffer performance would allow further optimization in the design, realization and operation of geological repositories and the management of interim storage facilities. This optimization would lead to reduced repository footprints (reduced number and/or spacing of disposal canisters and buffer dimensions) or new strategies for canister loading and, hence, requirements for cooling periods. HotBENT will contribute to assessing the effects of this higher thermal loading under realistic gradients and conditions, which cannot be obtained in the laboratory. It will allow an assessment of mineralogical, physical and chemical processes, such as smectite–illite transformation or cementation processes, which could impact the swelling, thermal and hydraulic properties and, hence, the key safety-relevant properties of the bentonite buffer. HotBENT was constructed in the period 2019–21 and has been operating at target temperatures since May 2022. The objectives, design and implementation of HotBENT, as well as the early transient phase of heating and hydration, are summarized in this paper.

Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Finland has a significant role in the European Union cobalt supply, with the largest known cobalt resources and only cobalt-producing mines in Europe. In addition, Finland is a globally significant producer of refined cobalt. However, Finland, along with the rest of Europe, is strongly dependent on cobalt imports. Recycled cobalt covers a small fraction of the country's demand, as well as that of the rest of Europe. Most of the cobalt deposits in Finland can be grouped into five genetic types (± subtypes). The orthomagmatic and Outokumpu types have been among the most important sources of cobalt in Finland. The Talvivaara type hosts the largest known cobalt resource in Europe, mostly in the Terrafame Sotkamo deposit. The most common cobalt minerals in the Finnish deposits are sulfides, and to a lesser extent sulfarsenides. These are concentrated by froth flotation, and the concentrates treated in Finnish cobalt refineries. An exception to this is the Terrafame Sotkamo mine, where bio-heap leaching technology is utilized for metal recovery, and metalliferous liquid is processed to battery-grade chemicals. Assessment of undiscovered mineral resources, prospectivity modelling, the exploration history and recent discoveries highlight the exploration potential for several types of Co-enriched mineral deposits in Finland.

Thematic collection: This article is part of the energy-critical metals for a low carbon transition collection available at: https://www.lyellcollection.org/topic/collections/critical-metals

The Tellus stream sediment and deep soil geochemistry data sets for Northern Ireland were used to locate four types of critical metals anomalies in granite bedrocks of the Mourne Mountains. A curvi-linear array of Nb, REE, Th and U soil anomalies across the eastern Mourne Mountains correlated with late-stage and eutectic temperature minerals in the roof zone of the most peralkaline F- and volatile-rich granite body, remobilized on micron to millimetre scales. Li, Be, B, As, Sn, Mn3+ and Ce4+ partitioned into pockets of late-stage heterogeneously distributed F-rich silicic residual melts and relatively oxidizing halide-rich magmatic fluids, resulting in drusy mineral and hydrothermal assemblages. Isolated soil anomalies correlated with amorphous Mn3+- and Ce4+-rich masses infilling drusy cavities, which resulted from short-distance percolation of small volumes of late-stage magmatic fluids. A significant As plume in stream sediments emanated from a greisen that hosted multiple critical and base metals including Sn, from reactions between large volumes of magmatic As + halide-rich fluids and mafic silicate + diverse accessory minerals on the metre- to kilometre-scale along geological structures. Diverse, small-scale REE anomalies in the soil data along structural features in the western Mournes correlate with vein mineralization resulting from episodic migration of hydrous fluids of variable composition, probably with a much smaller magmatic component than elsewhere. The regional geochemical dataset proved useful to develop a multi-stage model for enrichment of critical metals in the Mourne Mountains granites, which is analogous to the petrogenesis of some of the igneous-hosted economic deposits of critical metals.

Thematic collection: This article is part of the energy-critical metals for a low carbon transition collection available at: https://www.lyellcollection.org/topic/collections/critical-metals