Hydraulic fracturing in unconventional gas reservoirs: risks in the geological system, part 2

Hydraulic fracturing is a method used for the production of unconventional gas resources. Huge amounts of so-called fracturing fluid (10,000–20,000 m³) are injected into a gas reservoir to create fractures in solid rock formations, upon which mobilised methane fills the pore space and the fracturing fluid is withdrawn. Hydraulic fracturing may pose a threat to groundwater resources if fracturing fluid or brine can migrate through fault zones into shallow aquifers. Diffuse methane emissions from the gas reservoir may not only contaminate shallow groundwater aquifers, but also escape into the atmosphere where methane acts as a greenhouse gas. The working group “Risks in the Geological System” as part of ExxonMobil’s hydrofracking dialogue and information dissemination processes was tasked with the assessment of possible hazards posed by migrating fluids as a result of hydraulic fracturing activities. In this work, several flow paths for fracturing fluid, brine and methane are identified and scenarios are set up to qualitatively estimate under what circumstances these fluids would leak into shallower layers. The parametrisation for potential hydraulic fracturing sites in North Rhine-Westphalia and Lower Saxony (both in Germany) is derived from literature using upper and lower bounds of hydraulic parameters. The results show that a significant fluid migration is only possible if a combination of several conservative assumptions is met by a scenario.     

Relations between denudation,glaciation, and sediment deposition: implications from the Plio‐Pleistocene Central Alps

Despite abundant data on the early evolution of the Central Alps, the latest stage exhumation history, potentially related to relief formation, is still poorly constrained. We aim for a better understanding of the relation between glaciation, erosion and sediment deposition. Addressing both topics, we analysed late Pliocene to recent deposits from the Upper Rhine Graben and two modern river sands by apatite fission‐track and (U‐Th‐Sm)/He thermochronology. From the observed age patterns we extracted the sediment provenance and paleo‐erosion history of the Alpine‐derived detritus. Due to their pollen and fossil record, the Rhine Graben deposits also provide information on climatic evolution, so that the erosion history can be related to glacial evolution during the Plio‐Pleistocene. Our data show that Rhine Graben deposits were derived from Variscan basement, Hegau volcanics, Swiss Molasse Basin, and the Central Alps. The relations between glaciation, Alpine erosion, and thermochronological age signals in sedimentary rocks are more complex than assumed. The first Alpine glaciation during the early Pleistocene did not disturb the long‐term exhumational equilibrium of the Alps. Recent findings indicate that main Alpine glaciation occurred at ca. 1 Ma. If true, then main Alpine glaciation was coeval with an apparent decrease of hinterland erosion rates, contrary to the expected trend. We suggest that glaciers effectively sealed the landscape, thus reducing the surface exposed to erosion and shifting the area of main erosion north toward the Molasse basin, causing sediment recycling. At around 0.4 Ma, erosion rates increased again, which seems to be a delayed response to main glaciation. The present‐day erosion regime seems to be dominated by mass‐wasting processes. Generally, glacial erosion rates did not exceed the pre‐glacial long‐term erosion rates of the Central Alps.     

A methodology for dealing with regional change in integrated water resources management

The paper presents a methodology on how to consistently deal with the future change and management options in integrated water resources management (IWRM). It is based on a conceptual framework with a five step procedure for the formulation and analysis of a so-called ‘parameterised regional futures’. Developing and testing the approach for IWRM is realised for the upper part of the Western Bug River catchment (Ukraine). Special attention is paid to scenarios of change covering climate and land use. The future regional climate is downscaled with the model CCLM. Land cover is projected after retrospective change detection and the derivation of prospective algorithms. Parameters of the interrelations between land use and the water cycle are tackled through using the concept of the model PWF-LU. The methodology is currently being tested to analyse the impacts of mid-term regional change and management options on the water cycle of the catchment.     

The regional pressure impact of CO<Subscript>2</Subscript> storage: a showcase study from the North German Basin

A regional scale, showcase saline aquifer CO₂ storage model from the North German Basin is presented, predicting the regional pressure impact of a small industrial scale CO₂ storage operation on its surroundings. The intention of the model is to bridge the gap between generic and site-specific, studying the role of fluid flow boundary conditions and petrophysical parameters typically found in the North German Basin. The numerical simulation has been carried out using two different numerical simulators, whose results matched well. The most important system parameters proved to be the model’s hydrological boundary conditions, rock compressibility, and permeability. In open boundary aquifers, injection-induced overpressures dissipate back to hydrostatic level within a few years. If a geological flow barrier is present on at least one side of the aquifer, pressure dissipation is seriously retarded. In fully closed compartments, overpressures can never fully dissipate, but equilibrate to a compartment-wide remnant overpressure. At greater distances to the injection well, maximum fluid pressures are in the range of a few bar only, and reached several years to decades after the end of the actual injection period. This is important in terms of long-term safety and monitoring considerations. Regional pressure increase impacts the storage capacities of neighbouring sites within hydraulically connected units. It can be concluded that storage capacities may be seriously over- or underestimated when the focus is on a single individual storage site. It is thus necessary to assess the joint storage capacities and pressure limitations of potential sites within the same hydraulic unit.     

Modelling biofilm growth in the presence of carbon dioxide and water flow in the subsurface

The concentration of greenhouse gases – particularly carbon dioxide (CO₂) – in the atmosphere has been on the rise in the past decades. One of the methods which have been proposed to help reduce anthropogenic CO₂ emissions is the capture of CO₂from large, stationary point sources and storage in deep geological formations. The caprock is an impermeable geological layer which prevents the leakage of stored CO₂, and its integrity is of utmost importance for storage security. Due to the high pressure build-up during injection, the caprock in the vicinity of the well is particularly at risk of fracturing. Biofilms could be used as biobarriers which help prevent the leakage of CO₂ through the caprock in injection well vicinity by blocking leakage pathways. The biofilm could also protect well cement from corrosion by CO₂-rich brine.     

Can pyrolysis-GC/MS be used to estimate the degree of thermal alteration of black carbon?

Very little is known about the macromolecular properties of biomass combustion residues referred to as black carbon (BC). Pyrolysis-gas chromatography–mass spectrometry (Py-GC/MS) was performed on: (i) peat from Spain at 400–1200 °C to investigate the effect of charring on pyrolysis fingerprint and (ii) natural charcoal from Laos in order to link molecular information to published chemical and reactivity parameters. Confirming earlier Py-GC/MS studies, the BC in the artificially charred peat and the natural charcoal produced predominantly benzene, toluene, C₂-benzenes, PAHs and benzonitriles. Furthermore, some charcoal samples produced significant amounts of phenols, methoxyphenols, carbohydrate markers, n-alkanes and n-alkenes upon pyrolysis, reflecting non-charred and weakly charred biomass. A series of pyrolysis product ratios related to the degree of dealkylation of the pyrolysis products (benzene/toluene, naphthalene/C₁-naphthalenes, C₁-naphthalenes/C₂-naphthalenes, benzofuran/C₁-benzofurans and benzonitrile/C₁-benzonitrile) increased with increasing artificial charring (peat) and, for the natural charcoal, these ratios were in accordance with established chemical and reactivity parameters related to charring intensity from other methods: proportion of aromatic C obtained from solid state ¹³C nuclear magnetic resonance spectroscopy (NMR), the proportion of charred material as estimated from NMR in conjunction with a molecular mixing model (NMR–MMM) and the resistance to acid dichromate oxidation. The alkyl side chains of aromatic pyrolysis products are probably inherited from short chain aliphatic C chains that cross link the predominantly aromatic building blocks of BC, and these linkages seem to disappear with increasing charring intensity. Thus, the degree of thermal alteration of BC can be discerned from the pyrolysis fragmentation pattern.     

Clay-mineral and grain-size distributions in surface sediments of the White Sea (Arctic Ocean): indicators of sediment sources and transport processes

In this study, the grain-size and clay-mineral compositions of 73 surface sediment samples collected in a variety of environmental settings in the White Sea are presented to characterize recent sedimentation processes, reconstruct transport pathways, and identify potential source areas of the terrigenous components. Areas >100 m deep are invariably characterized by silty clay, whereas areas <100 m deep exhibit more heterogeneous grain-size compositions plausibly explained by coastal erosion and (re-)distribution mechanisms, particularly tidal currents. The dominance of sand in the estuarine areas of the Onega and Dvina rivers as well as toward the Gorlo Strait connecting the White Sea with the Barents Sea is attributed to increased current speeds. Illite and smectite are the dominant clay minerals in recent sediments of the southwestern and eastern White Sea sectors, respectively. Their distribution patterns largely depend on the geology of the source areas, and mirror surface circulation patterns, especially in Dvina Bay. Smectite is a key clay mineral in White Sea surface sediments, as it reveals the dominating influence of the Northern Dvina’s runoff on sedimentation and water circulation throughout the basin. In comparison to other Eurasian shelf seas, the White Sea is characterized by a greater diversity of clay-mineral assemblages, which range from illite- to smectite-dominated sectors containing variable amounts of chlorite and kaolinite.     

Shock experiments in support of the Lithopanspermia theory: The influence of host rock composition,temperature, and shock pressure on the survival rate of endolithic and epilithic microorganisms

Abstract– Shock recovery experiments were performed with an explosive set‐up in which three types of microorganisms embedded in various types of host rocks were exposed to strong shock waves with pressure pulse lengths of lower than 0.5 μs: spores of the bacterium Bacillus subtilis, Xanthoria elegans lichens, and cells of the cyanobacterium Chroococcidiopsis sp. 029. In these experiments, three fundamental parameters were systematically varied (1) shock pressures ranging from 5 to 50 GPa, (2) preshock ambient temperature of 293, 233 and 193 K, and (3) the type of host rock, including nonporous igneous rocks (gabbro and dunite as analogs for the Martian shergottites and chassignites, respectively), porous sandstone, rock salt (halite), and a clay‐rich mineral mixture as porous analogs for dry and water‐saturated Martian regolith. The results show that the three parameters have a strong influence on the survival rates of the microorganisms. The most favorable conditions for the impact ejection from Mars for microorganisms would be (1) low porosity host rocks, (2) pressures <10–20 GPa, and (3) low ambient temperature of target rocks during impact. All tested microorganisms were capable of surviving to a certain extent impact ejection in different geological materials under distinct conditions.     

Concurrent tectonic and climatic changes recorded in upper Tortonian sediments from the Eastern Mediterranean

The upper Tortonian Metochia marls on the island of Gavdos provide an ideal geological archive to trace variations in Aegean sediment supply as well as changes in the North African monsoon system. A fuzzy-cluster analysis on the multiproxy geochemical and rock magnetic dataset of the astronomically tuned sedimentary succession shows a dramatic shift in the dominance of 'Aegean tectonic' clusters to 'North African climate' clusters. The tectonic signature, traced by the starvation of the Cretan sediment, now enables to date the late Tortonian basin foundering on Crete, related to the tectonic break-up of the Aegean landmass, at c.  8.2 Ma. The synchronous decrease in the North African climate proxies is interpreted to indicate a change in the depositional conditions of the sink rather than a climatic change in the African source. This illustrates that interpretations of climate proxies require a multiproxy approach which also assesses possible contributions of regional tectonism.     

Bayesian updating via bootstrap filtering combined with data-driven polynomial chaos expansions: methodology and application to history matching for carbon dioxide storage in geological formations

Model calibration and history matching are important techniques to adapt simulation tools to real-world systems. When prediction uncertainty needs to be quantified, one has to use the respective statistical counterparts, e.g., Bayesian updating of model parameters and data assimilation. For complex and large-scale systems, however, even single forward deterministic simulations may require parallel high-performance computing. This often makes accurate brute-force and nonlinear statistical approaches infeasible. We propose an advanced framework for parameter inference or history matching based on the arbitrary polynomial chaos expansion (aPC) and strict Bayesian principles. Our framework consists of two main steps. In step 1, the original model is projected onto a mathematically optimal response surface via the aPC technique. The resulting response surface can be viewed as a reduced (surrogate) model. It captures the model’s dependence on all parameters relevant for history matching at high-order accuracy. Step 2 consists of matching the reduced model from step 1 to observation data via bootstrap filtering. Bootstrap filtering is a fully nonlinear and Bayesian statistical approach to the inverse problem in history matching. It allows to quantify post-calibration parameter and prediction uncertainty and is more accurate than ensemble Kalman filtering or linearized methods. Through this combination, we obtain a statistical method for history matching that is accurate, yet has a computational speed that is more than sufficient to be developed towards real-time application. We motivate and demonstrate our method on the problem of CO₂ storage in geological formations, using a low-parametric homogeneous 3D benchmark problem. In a synthetic case study, we update the parameters of a CO₂/brine multiphase model on monitored pressure data during CO₂ injection.