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: Depleted gas reservoirs are potential sites for CO 2 storage; therefore, it is important to evaluate their storage capacity. Historically, there have been difficulties in identifying the reservoir drive mechanism of gas reservoirs using traditional P / z plots, having direct impacts for the estimation of the original gas in place (OGIP) and dependent parameters for both theoretical and effective CO 2 storage capacity estimation. Cole plots have previously provided an alternative method of characterization, being derived from the gas material balance equation. We use production data to evaluate the reservoir drive mechanism in four depleted gas reservoirs (Hewett Lower Bunter, Hewett Upper Bunter, and North and South Morecambe) on the UK Continental Shelf. Cole plots suggest that the North Morecambe and Hewett Upper Bunter reservoirs experience moderate water drive. Accounting for cumulative water influx into these reservoirs, the OGIP decreases by up to 20% compared with estimates from P / z plots. The revised OGIP values increase recovery factors within these reservoirs; hence, geometrically based theoretical storage capacity estimates for the North Morecambe and Hewett Upper Bunter reservoirs increase by 4 and 30%, respectively. Material balance approaches yield more conservative estimates. Effective storage capacity estimates are between 64 and 86% of theoretical estimates within the depletion drive reservoirs, and are 53 – 79% within the water drive reservoirs. Supplementary material: A more detailed description of the aquifer modelling is available at https://doi.org/10.6084/m9.figshare.c.3803770.v1

Description: Large-scale energy storage in the geological subsurface (e.g. by storing hydrogen gas) may help to mitigate effects of a fluctuating energy production arising from the extensive use of renewable energy sources. The applicability of hydrogen (H 2 ) storage in a porous sandstone formation is investigated by defining a usage scenario and a subsequent numerical simulation of a storage operation at an existing anticlinal structure in the North German Basin. A facies modelling approach is used to obtain 25 heterogeneous and realistic parameter sets. The storage operation consists of the initial filling with nitrogen used as cushion gas, the initial filling with H 2 , and six withdrawal periods with successive refilling and shut-in periods. It is found that, on average, the storage can sustain a continuous power output of 245 MW for 1 week when using five storage wells, while peak performance can be as high as 363 MW, indicating that the storage is mainly limited by the achievable extraction rates. The median of the maximum pressure perturbation caused by this storage is around 3 bars and can be observed at a distance of 5 km from the wells.

Description: Total operated the Lacq CO 2 capture and storage demonstration pilot between January 2010 and March 2013. The injector well, RSE-1, was plugged and permanently abandoned in April and May 2015. As part of a risk assessment study, the hypothetical presence of micro-annuli in the outer cements of the well was studied, consisting of a 2.2 km-long pathway through two cement sheaths. A model was set up to couple the overlying Lasseube aquifer, the well cements and the Mano reservoir. The model concluded that no CO 2 can flow upwards into the Lasseube aquifer as the CO 2 storage reservoir is strongly depleted. The volumes of aquifer water that may flow downwards into the reservoir are very limited, with simulations suggesting rates of less than 0.01 m 3 /day. The cumulative volume of water that could flow down to the reservoir by the end of 2200 would be 0.0006% of the aquifer volume in the worse case. Both computed flow rates and volumetric hydrogeological consequences were therefore found to be of very small amplitude. This study concluded on the lack of risks related to a hypothetical loss of cement bonds with the shaly formations all along the cement sheaths of the 95/8 and 7 inch casings.

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: 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.