Description: A previous geological and petrophysical model of the fluvio-deltaic Book Cliffs outcrops contained eight lithotypes, within each of which a number of lithologies were grouped. While this model was an adequate representation of the overall depositional architecture, for reservoir-geological purposes the potential reservoir and non-reservoir lithologies needed to be separated. Here, a new and more detailed geological model is presented in which more differentiation is put on the potential reservoir lithologies. This new model contains 12 lithologies with layers down to 1 m in thickness. Assuming a burial depth of 3 km and an average clay content, representative rock physical properties are assigned to lithologies based on published data. After the model thickness has been stretched by a factor of 4 in order to represent a more realistic reservoir, a full-waveform forward seismic response is modelled. These data are used as inputs into an iterative elastic wave-equation-based inversion scheme, with the goal to retrieve the rock properties and layer geometries. The results of this conceptual study show that sandstone units in the shoreface and distributary channels, which are potential reservoirs, are successfully identified. The recovery of medium parameters has a high resolution because the non-linear relationship between rock properties and the seismic data has been exploited.

Description: The kinetics of dolomite dissolution have been experimentally investigated under subsurface conditions characteristic of the Rotliegend gas fields in the NE of The Netherlands. Experiments were performed in closed, stirred, batch reactors at far from equilibrium conditions, with dolomite powders of different grain sizes. The experiments were repeated for all grain sizes at 25°C and an experiment was also conducted in deionized water. The rates were derived from the measured concentrations of Mg 2+ or Ca 2+ released from dolomite dissolution and were normalized by the surface area of the minerals at each time interval. Regression of the rates with the pH resulted in the kinetic rate constants of: log k 1 = –8.16 ± 0.06 at 25°C and log k 2 = –7.61 ± 0.05 at 100°C (300 – 350 μm), log k 3 = –7.88 ± 0.20 at 25°C, log k 4 = –7.45 at 100°C (75 – 100 μm), log k 5 = –6.62 ± 0.50 at 25°C and log k 6 = –5.96 ± 1 at 100°C (20 – 25 μm). The results obtained in this study indicate that in an acidic regime the dissolution of dolomite in brine is a factor of 2 faster than in deionized water. It was also shown that the dissolution rates, when normalized by surface area, increase with decreasing grain size.

Description: In order to accurately model the subsurface of a new salt cavern gas storage facility at the Jemgum site in the Lower Saxony Basin (NW Germany), modern 3D pre-stack depth-migrated seismic data were utilized during an early planning stage. Gas storage projects are often regarded purely as engineering projects. However, the involvement of geoscientists provided an opportunity to further add operational value to the project by applying state-of-the-art geological modelling techniques. As part of the site characterization, a new generic workflow was developed to facilitate a more time-efficient creation of a static, multi- z -valued salt interface, essential for modelling of any complex salt structure. Uncertainties related to the reconstruction of salt interface meshes were analysed accordingly and used to quantitatively determine spatial uncertainties of the final static depth model. Furthermore, seismic-attribute images were created which enabled the detection of a mid-Miocene fluvial system evident at shallow depths of just 100–300 m below mean sea level. The study demonstrates that seismic data, targeting Rotliegend gas sands at depths of more than 4000 m, can be very beneficial to better understand the near-surface hydrogeological processes for gas storage.

Description: Detection of the chemical constituents of hydrocarbons in the hydrocarbon-bearing fluid inclusions in diagenetic mineral cements, secondary fractures and overgrowths could be a useful indicator of the nature of oil in a basin. Microscope-based Raman spectroscopy is a non-destructive, optical vibrational spectroscopic technique that can precisely isolate and analyse hydrocarbon fluid inclusions (HCFIs). The main challenge with Raman spectral studies on natural HCFIs is the common presence of fluorescence emission from minerals and aromatic compounds in HCFIs leading to the masking of Raman signals. The present study is a demonstration of how best the Raman signals from natural hydrocarbon-bearing fluid inclusions could be detected using an excitation wavelength of 785 nm with suitable optical parameters and with special wafer preparation techniques to negate the background fluorescence. Using the laser Raman technique we were able to detect peaks corresponding to cyclohexane (786 and 3245 cm –1 ), benzene and bromobenzene (606, 1010, 1310, 1486 and 1580 cm –1 ), carbon monoxide (2143 cm –1 ), nitrogen (2331 cm –1 ), ethylene (1296 cm –1 ), sulphur oxide (524 cm –1 ), carbonyl sulphide (2055 cm –1 ), hydrogen sulphide in liquid form (2580 cm –1 ) along with the presence of a broad peak of liquid water at 3100–3500 cm –1 , peaks of calcium carbonate (710, 854 cm –1 ) and calcium sulphate (1135 cm –1 ). The study samples were specially prepared with fluorescence-quenching dyes added with a resin-hardener mixture to eliminate background fluorescence. Nine fluid-inclusion assemblages in minerals like quartz, feldspar and calcite from the RV-1 well of the Ratnagiri Block, Mumbai Offshore Basin, India were investigated.

Description: Despite the current easing in demand for increased oil production linked to the global downturn in crude prices, energy demand continuously increases and the long-term demand will require maximizing the productivity of reservoirs and a search into the exploitation of new resources in increasingly challenging environments. In this study, we present the results from the monitoring of the very first multistage stimulation experiment at a shale gas reservoir in Saudi Arabia, presenting an analysis of the microseismicity induced during the treatment. Our aim was to analyse microseismic events to better understand fracture growth and the role of pre-existing fractures in these reservoirs. Microseismic (MS) event monitoring is used to track the creation of fractures during and after the stimulation, and therefore to evaluate the effect of the reservoir stimulation. The monitoring includes a downhole array of 12 3C-sensors that were deployed in a vertical well with a 30.5 m level spacing. A total of 415 MS events were located and analysed, with the results outlining induced fractures extending consistently with an average azimuth of N335° E, normal to the horizontal section of the treatment well. This implies that there are no changes in the local stress direction along the treatment well either in situ or induced along the treatment. There are significant changes in total length and aspect ratio (length/width) of the fractures induced in the different stages. These variations could be attributed to in situ fracturing, local rock heterogeneity or the influence of the treatment parameters. In general, early and late stages of stimulation show the longest fracture networks, with events induced further away from the initiation point. We found no immediate relationship between treatment parameters (peak pressure and pumping rates) and fracture extension. Sensitivity analysis using Monte Carlo simulation methods shows a higher location uncertainty for events located at the early stages, thus limiting the interpretation from monitored seismicity in the early stages. An analysis of magnitude distribution with distance shows a decrease in sensitivity of one degree of magnitude for every 375 m, and a maximum viewing distance of approximately 700 m for the current set-up. The low number of located events does not provide a complete enough dataset for a robust analysis of changes in b -value (slope in linear part of magnitude distribution) during the treatment: however, magnitude distributions, corrected for array sensitivity, provide a useful variable for the validation of geomechanical models currently being developed for the reservoir.

Description: This thematic set emerged out of the 3rd EAGE Sustainable Earth Science Conference, held in the picturesque town of Celle in Lower Saxony, Germany, on 13 – 15 October 2015. This conference series was established to develop and promote an emerging group of applied geoscience topics focused on technologies for sustainable use of the subsurface to serve the energy transition; and the focus so far has been mainly on geothermal energy, CO 2 storage and energy storage. Inspired by the research emerging from this conference, the conveners proposed a thematic set on the common ground between long-term geological storage of CO 2 and the shorter-term cyclic storage of energy. The resulting set of papers is very rewarding, since they capture examples of all these topics and illustrate the complementary nature of subsurface studies on the storage of energy and CO 2 . We have arranged the set to start with two review papers, followed by three papers on energy storage and then a further three on CO 2 storage.

Description: Human society needs to achieve a low-carbon energy mix this century. To achieve this, we need: (a) an appreciation of the value of Earth's atmosphere; and (b) a sustainable approach for low-carbon energy. For sustainable developments, three pillars need to work together: the environment, social equity and economics. To address the societal aspects of the low-carbon energy transition, we need to appreciate that our future depends on protecting the Earth's atmosphere. By reviewing the discovery of the greenhouse gas effect over the last 200 years, we establish the essential motivation for changing human behaviour with regard to energy use. From this basis, we consider the challenge of how to achieve this energy transition or, more specifically, how to overcome the dissonances related to societal acceptance, economic hurdles and lack of progress with deployment of low-carbon energy options. The last decade has seen a significant growth in the renewable energy and natural gas sectors: however, CCS has made limited progress. This has to change if the human population is to significantly reduce greenhouse gas emissions. In order to accelerate reductions in global CO 2 emissions, all low-carbon energy options must be deployed at an increasing rate in the coming decades.

Description: Compressed air energy storage (CAES) in porous formations is considered as one option for large-scale energy storage to compensate for fluctuations from renewable energy production. To analyse the feasibility of such a CAES application and the deliverability of an underground porous formation, a hypothetical CAES scenario using an anticline structure is investigated. Two daily extraction cycles of 6 h each are assumed, complementing high solar energy production around noon. A gas turbine producing 321 MW of power with a minimum inlet pressure of 43 bar at 417 kg s –1 air is assumed. Simulation results show that using six wells the 20 m-thick storage formation with a permeability of 1000 mD can support the required 6 h continuous power output of 321 MW, even reaching 8 h maximally. For the first 30 min, maximum power output is higher, at 458 MW, continuously dropping afterwards. A sensitivity analysis shows that the number of wells required does not linearly decrease with increasing permeability of the storage formation due to well inference during air extraction. For each additional well, the continuous power output increases by 4.8 h and the maximum power output within the first 30 min by 76 MW.

Description: The injection or extraction of fluids in the subsurface for energy purposes (e.g. geothermal exploitation, CO 2 storage or geological energy storage) requires both the operation efficiency and the associated environmental risks to be assessed and controlled. Even though scientific and technological progress allows more accurate 3D modelling of the subsurface, we still do not have a thorough understanding of coupled underground hydromechanical processes. Indeed, the injection or production of fluids interacting with existing geological features can still result in unintended and unexpected ‘harmful’ consequences. This review aims to propose a unified strategy ranging from an understanding of the hydromechanical factors at the origin of the induced seismicity to seismic risk evaluation expressed in terms of ground-motion effects. The challenge is to utilize mechanical modelling to anticipate the evolution of seismicity; how the population perceives this is also an important factor to be taken into account in this risk evaluation. While mechanical modelling may include some degree of uncertainty, probabilistic analysis is capable of providing a quantitative estimation of the risk incurred and feedback to the exploitation strategy.

Description: Prediction of the reservoir quality of oolitic carbonates is one of the main challenges for petroleum geology due to the inherited heterogeneity in pore systems. This study investigated and characterized the sedimentological, stratigraphic, morphological and granulometric data of oolitic outcrop strata of the Khartam Member of the Khuff Formation in Saudi Arabia, including the giant hydrocarbon reservoirs (Khuff reservoirs), and the results have implications for exploration and development. Linking ooid morphology to sequence stratigraphy revealed that the distribution of the oolitic beds and their pore systems are mainly controlled by stratigraphic position, morphology (controlled by primary mineralogy) and grain size. The systematic distribution of ooids with distinct original mineral compositions allows for the prediction of the porosity distribution and occurrence within analogous reservoirs. The ooids in the studied interval were aragonitic (lower interval), bimineralic (middle interval) and aragonitic (upper interval). Linking ooid grain size with depositional environments indicated that the ooid grain size increases with water energy and follows the upwards-shallowing pattern of the studied succession. These results can be used to improve the understanding and prediction of the reservoir quality of oolitic carbonates.