Mining Safety and Sustainability I

Safety and sustainability are becoming ever bigger challenges for the mining industry with the increasing depth of mining. It is of great significance to reduce the disaster risk of mining accidents, enhance the safety of mining operations, and improve the efficiency and sustainability of development of mineral resource. This book provides a platform to present new research and recent advances in the safety and sustainability of mining. More specifically, Mining Safety and Sustainability presents recent theoretical and experimental studies with a focus on safety mining, green mining, intelligent mining and mines, sustainable development, risk management of mines, ecological restoration of mines, mining methods and technologies, and damage monitoring and prediction. It will be further helpful to provide theoretical support and technical support for guiding the normative, green, safe, and sustainable development of the mining industry

Experimental and numerical modelling investigations into coal mine rockbursts and gas outbursts

Rockbursts and gas outbursts are a longstanding hazard in underground coal mining due to their sudden occurrences and high consequences. These hazards are becoming prominent due to the increase in mining depth, difficult mining conditions, and adverse gas pressure conditions. Several researchers have proposed different theories, mechanisms, and indices to determine the rockbursts and gas outbursts liability but most of them focus on only some aspects of the complex engineering system for the ease to represent them using partial differential equations. They have often ignored the dynamics of changing mining environment, coal seam heterogeneity and stochastic variations in the rock properties. Most of the indices proposed were empirical and their suitability to different mining conditions is largely debated. To overcome the limitations of previous theories, mechanisms and indices, a probabilistic risk assessment framework was developed in this research to mathematically represent the complex engineering phenomena of rockbursts and gas outbursts for a heterogeneous coal seam. An innovative object-based non-conditional simulation approach was used to distribute lithological heterogeneity occurring in the coal seam to respect their geological origin. The dynamically changing mining conditions during a longwall top coal caving mining (LTCC) was extracted from a coupled numerical model to provide statistically sufficient data for probabilistic analysis. The complex interdependencies among several parameters, their stochastic variations and uncertainty were realistically implemented in the GoldSim software, and 100,000 equally likely scenarios were simulated using the Monte Carlo method to determine the probability of rockbursts and gas outbursts. The results obtained from the probabilistic risk assessment analysis incorporate the variations occurring due to lithological heterogeneity and give a probability for the occurrence of rockbursts, coal and gas outbursts, and safe mining conditions. The framework realistically represents the complex mining environment, is resilient and results are reliable. The framework is generic and can be suitably modified to be used in different underground mining scenarios, overcoming the limitations of earlier empirical indices used.Open Acces

An experimental and numerical investigation into hydraulic fracture propagation in naturally fractured shale gas reservoirs

Despite the large amount of research conducted to date, the performance of hydraulic fractures in naturally fractured structures and their effect on hydrocarbon production is still not well understood. To this end, this research aimed at developing a better understanding of natural fracture and hydraulic fracture interaction in shale formations by two- and three-dimensional discrete element modelling (DEM) approaches, results of which were compared against the large-scale hydraulic fracturing experiments. The samples used in the experimental research were collected from the Hope Cement Works shale quarry in Derbyshire, UK. The mineralogy as well as the mechanical, elastic, and flow properties of samples were obtained through several laboratory sample characterisation tests. The subsequent true-triaxial hydrofracturing experiments with acoustic measurements were performed on one homogeneous and one naturally fractured 0.3 × 0.3 m × 0.3 m rock samples, which reflected the temporal information on hydraulic fracture initiations. For further identification of the location and geometry of hydraulic and natural fractures, computed tomography (CT) and seismic velocity tomography analyses were conducted. The preliminary two-dimensional discrete element modelling research results were obtained using discrete fracture network (DFN) approach in two-dimensional Particle Flow Code (PFC2D). The two-dimensional model results provided a fundamental understanding of the effects of certain parameters, in particular the angle of approach, differential stress, mechanical properties as well as ubiquity and randomness of natural fractures on fracture interaction mechanisms. However, in view of the limitations of two-dimensional representation of both the laboratory and field scale applications, three-dimensional discrete element models were developed using XSite, results of which were first compared against the findings of true triaxial hydrofracturing experiments, and then extended through a parametric research. The effects of mechanical properties of natural fractures and operational parameters on fracture interaction mechanisms were then analysed using 3D XSite models. A curved shape hydraulic fracture, which propagated perpendicular to the minimum horizontal stress direction (x) in the homogeneous sample model, agreed well with the CT scan analysis and seismic wave velocity tomography results from the laboratory experiments. Similarly, the natural fracture and hydraulic fracture interaction observed in the second heterogenous/fractured sample, particularly the arrest by the main natural fracture and the subsequent crossing with offset mechanisms, were captured well by the developed 3D numerical models. Both experimental, and the parametric two- and three-dimensional particle- and lattice-based discrete element modelling research have demonstrated that the hydraulic fracture propagation in homogeneous rock with no weakness planes/natural fractures is mainly controlled by the differential stress, as it is growth is perpendicular to the minimum horizontal stress with no observed branching/diversion. The presence of natural fractures, on the other hand, introduced the additional effects of mechanical properties of natural fractures on observed interaction mechanisms in such a way that the stronger natural fractures are found to be favouring the crossing mechanism. Importantly, the ubiquity and randomness of natural fractures, which increased the complexity in hydraulic fracture growth significantly, have shown that the hydraulic fracture almost always propagates along the nearest natural fracture plane as the least resistant and shortest path, instead of being controlled by the differential stress. These findings, indeed, emphasised the dominating role of natural fractures and their dispersion within the reservoir on hydraulic fracture propagation and subsequent fracture interaction mechanisms. Regarding the operational parameters, lower flow rate and low viscosity fluids are found to be leading to arrest mechanism with increased dilation of natural fractures, while higher flow rate and high viscosity fluids resulted in direct crossing mechanisms with observable increase in total stimulated areas.Open Acces

Geomechanical characterisation of organic-rich shale properties using small scale experiments and homogenisation methods

PhD ThesisShale, or mudstone, is the most common sedimentary rock. It is a heterogeneous, multimineralic natural composite consisting of clay mineral aggregates, organic matter, and variable quantities of minerals such as quartz, calcite, and feldspar. Determination of the mechanical response of shales through experimental procedures is a challenge due to their heterogeneity and the practical difficulties of retrieving good-quality core samples. Therefore, in recent years extensive research has been directed towards developing alternative approaches for the mechanical characterisation of shale rocks. In this study, a nanoscale mechanical mapping technique called PeakForce QNM R has been combined with imaging and chemical analysis in order to investigate the mechanical response of each constituent of the shale microstructure. Isotropic elastic behaviour was observed for silt inclusions while a highly anisotropic response was found in the clay matrix. Organic matters with different levels of thermal maturity were investigated and the elastic moduli were determined. These information are essential and useful in order to predict or understand the macroscopic mechanical response of shale rocks. Indentation testing was then carried out in order to scale up the nano-mechanical measurements. This test allows for generating data related to the mechanical behaviour of shale rocks from shale cuttings. Shale samples with a range of mechanical behaviour, from soft to hard, and mineralogical compositions were used in these tests. Issues related to indentation testing such as loading and unloading rate, tip shape and creep behaviour were studied. The capabilities and limitations of this test applied to shale rock were further clarified. Aside from these experimental studies, the Micromechanical modelling (rock physics), a mathematical description of composite-like material, was theoretically and practically studied as an alternative approach for predicting the elastic response of shale rocks. The limitations and the ranges of applicability of the micromechanical formulations were evaluated using direct numerical modelling of shale microstructure. Suitable formulations for homogenisation of shale composite structure were determined. Finally, the data obtained in the nano-scale experiments, as input data, and the results of indentation testing, as the validation III data sets, were adopted for these mathematical formulations. In the last step, numerical modelling of indentation test was undertaken to back-calculate the plastic response of shale samples using the load-displacement curves obtained from this test. The recently developed Material Point Method has been implemented to simulate the large deformation that can occur when pressing the indenter into the shale surface. The nonuniqueness problem of the indentation curve for pressure-sensitive materials was addressed using two different indenter geometries. Inverse analysis was conducted simultaneously until a set of parameters was found matching both experimental curves

Coupled hydraulic-mechanical-chemical processes in porous and fractured rocks

Gekoppelte hydraulische, mechanische und chemische Prozesse sind bei zahlreichen Anwendungen im Untergrund zu berücksichtigen und entscheidend für die Analyse des Potenzials, der Sicherheit und Risiken sowie der zukünftigen Entwicklung eines Geosystems. Einige solcher Untergrundnutzungen wie die Endlagerung radioaktiver Abfälle, die Speicherung von Energieträgern oder die Geothermie haben darüber hinaus einen hohen Stellenwert bei der Energiewende im Rahmen der weltweiten Klimaschutzaktivitäten. Diese Arbeit fokussiert sich auf die Entwicklung von methodisch-analytischen Ansätzen und Werkzeugen, welche zur Charakterisierung von gekoppelten Prozessen in porösen und geklüfteten Gesteinen herangezogen werden können. Durch Betrachtung zweier unterschiedlicher Lithologien sowie Beobachtungsmaßstäben vom µm- bis in den m-Bereich trägt diese Arbeit zu einem besseren Verständnis gekoppelter Prozesse in Reservoir- und Wirtsgesteinen bei. Der erste Teil der Arbeit befasst sich mit hydraulisch-mechanischen (HM) Prozessen und Eigenschaften in geklüfteten Gesteinen. Klüfte stellen wichtige Fließpfade im Gestein dar, sind jedoch auch hochsensibel gegenüber mechanischen Einflüssen. In der ersten Studie wird ein systematischer Methodenvergleich für eine nicht-invasive Quantifizierung von hydraulischen Öffnungsweiten präsentiert. Drei verschiedene Messinstrumente, ein tragbarer Luftpermeameter, eine Mikroskopkamera und ein 3D-Laserscanner, werden an einer natürlichen Einzelkluft in einem bekannten deutschen Reservoiranalogon (Flechtinger Sandstein) angewandt und bewertet. Dieses Fallbeispiel zeigt, dass der Luftpermeameter die robustesten Ergebnisse liefert, die größte Eindringtiefe besitzt und erhebliche Vorteile in Bezug auf Mobilität, Zeitaufwand und Datenauswertung bietet. Bei den Ansätzen zur optischen Kluftcharakterisierung erfolgt die Bestimmung der hydraulischen Öffnungsweite indirekt mittels verschiedener Modellannahmen auf Basis der mechanischen Öffnungsweite und der Kluftrauigkeit. Dabei ergeben sich Abweichungen von bis zu 27 % (Mikroskopkamera) und bis zu 260 % (3D-Laserscanner) verglichen mit den Ergebnissen des Luftpermeameters. Aufbauend auf der Einzelkluftanalyse wird die präsentierte Methodik auf den Feldmaßstab transferiert sowie für die Anwendung an einer Wirtsgesteinsformation für die nukleare Endlagerung optimiert. In der zweiten Studie wird eine HM Charakterisierung einer Auflockerungszone im Opalinuston des Felslabors Mont Terri in der Schweiz vorgenommen. Die Analyse des diskreten Kluftnetzwerks in der untersuchten EZ-B Nische, bestehend aus tektonischen und künstlichen Diskontinuitäten, zeigt, dass die offenliegende Auflockerungszone durch hydraulische Öffnungsweiten von bis zu 112 µm gekennzeichnet ist. Durch fortschreitende Austrocknung der Tunnelwände über einen Zeitraum von etwa 15 Jahren wurden Selbstheilungsprozesse größtenteils unterbunden. Auch die mithilfe von Nadelpenetrometertests vor Ort ermittelten physikalisch-mechanischen Gesteinsparameter verdeutlichen die Sensitivität des Opalinustons gegenüber der Tunnelbelüftung und bilden eine negative Korrelation von Gesteinsfestigkeit bzw. -steifigkeit und Wassergehalt aufgrund eines ausgeprägten hydromechanischen Kopplungsverhaltens ab. Der zweite Teil der Arbeit konzentriert sich auf hydraulisch-chemische (HC) Prozesse im porösen Medium und adressiert den reaktiven Transport in einem Reservoirgestein. Insbesondere durch Lösungs- oder Fällungsreaktionen hervorgerufene Porositäts- und Permeabilitätsänderungen können die Reservoireigenschaften signifikant verändern. Die dritte Studie untersucht die Auflösung von Calcit-Zement im Flechtinger Sandstein sowie die Übertragbarkeit der experimentellen Calcitlösungsraten von der µm-mm-Skala (Mineraloberfläche) auf die cm-Skala (Kernproben). Anhand von Durchflussexperimenten an vier Sandsteinkernen wird die Bandbreite der Lösungsraten auf der cm-Skala für unterschiedliche Reaktionszeiträume sowie variierende hydraulische Randbedingungen ermittelt. Dem gegenübergestellt werden zeitlich und räumlich aufgelöste Calcitlösungsraten auf der µm-mm-Skala aus Oberflächenanalysen mittels vertikal scannender Interferometrie. Auf Grundlage segmentierter Röntgen-Mikrocomputertomografie Scans der Kernproben wird ein geometrischer Ansatz etabliert, um die fluidzugängliche Oberfläche des heterogen verteilten Calcitzements im niedrigpermeablen und komplexen Sandstein abzuschätzen. Auf Grundlage dieses eingeführten Oberflächenparameters wird die Rateninformation der µm-mm-Skala auf die Kernskala übertragen, wobei die Abweichungen zwischen den aufskalierten und den gemessenen Lösungsraten für alle untersuchten Proben weniger als eine Größenordnung betragen. Im Rahmen dieser Arbeit werden verschiedene Messmethoden und Herangehensweisen identifiziert, entwickelt, optimiert und validiert, die zur Beschreibung und Interpretation von gekoppelten hydraulisch-mechanisch-chemischen Prozessen und zur Bestimmung damit zusammenhängender Schlüsselparameter in geklüfteten und porösen Gesteinen genutzt werden können. Der Untersuchungsansatz und die erzielten Ergebnisse bilden daher eine wertvolle Grundlage für die Vorhersage des Verhaltens natürlicher Systeme auf höheren Längen- und Zeitskalen

Integrated Geomechanical Characterization of Anisotropic Gas Shales: Field Appraisal, Laboratory Testing, Viscoelastic Modelling,and Hydraulic Fracture Simulation

This research provides a multiscale geomechanical characterization workflow for ultra-tight and anisotropic Goldwyer gas shales by integrating field appraisal, laboratory deformation and ultrasonic testing, viscoelastic modelling, and hydraulic fracture simulation. The outcome of this work addresses few of the practical challenges in unconventional reservoirs including but not limited to (i) microstructure & compositional control on rock mechanical properties, (ii) robust estimation of elastic anisotropy, (iii) viscous stress relaxation to predict the least principal stress Shmin at depth from creep, (iv) influence of specific surface area on creep, and (v) impact of stress layering on hydraulic fracturing design