Geomechanically coupled modelling of fluid flow partitioning in fractured porous media.

Naturally fractured reservoirs are characterised with complex hydro-mechanical dynamics. In these reservoirs, hydrocarbons can be stored and produced from the rock matrix, the fracture network, or both. Normally the fracture network is depleted much faster than the matrix blocks due to its increased hydraulic conductivity; consequently, the recovery factor is low for these reservoirs. Additionally, the in-situ stress profile changes with reservoir depletion and affects fluid flow dynamics of the fractured reservoir. Therefore, dynamic characterisation of fractured reservoirs is considered a challenging task, responsible for inefficient exploitation of their reserves. This dissertation focuses on characterising matrix-fracture fluid flow partitioning subjected to variable overburden stress loading. Understanding of the matrix-fracture hydro-mechanical interaction would assist in developing optimum production plans to maximize recovery from fractured reservoirs. Initially, three different fracture implementation techniques - (1) simulating fracture as an equivalent porous medium; (2) implementing it as a sub-dimensional feature within the porous matrix; and (3) considering fracture domain as an open channel - were evaluated using a set of published laboratory core flooding data. The best fracture simulation approach was identified to be fracture implementation as an open channel interacting with matrix block. This approach takes into consideration the coupling of Darcy flow equation in the matrix domain to Navier-Stokes flow formulation in the fracture. The efficiency of this fracture simulation approach was significantly enhanced when coupled further with poro-elasticity physics and stress dependent permeability. In the next step, the coupled open channel fracture simulation approach was applied to perform a sensitivity analysis on the effect of all parameters of the governing equations on fracture and matrix flow. The results of this analysis were statistically analysed, with specific attention to the analytical formulation of the governing equations, to develop coupled empirical flow models for fracture and matrix. These empirical models incorporate both flow physics of matrix and fracture, as well as mechanical loading impacts. An analysed multiphase flow scenario demonstrated the compatibility of the coupled simulation approach with multiphase flow investigations in fractured porous media. A novel core flooding set-up, capable of separated fracture and matrix flow measurement, was designed and built to enable laboratory evaluation of the developed empirical models. This set-up enabled monitoring of pressure front within matrix and fracture, taking the advantages of several differential pressure transducers along the core plug length. Variation of the matrix and fracture flow in response to different stress loading scenarios was investigated in the laboratory. Furthermore, laboratory validation indicated that the matrix flow model is capable of predicting laboratory measurements with an acceptable accuracy; however, the fracture flow model seemed to need more improvement. Probable factors that could have caused inaccuracy in the fracture flow model were discussed and actions for improving it were recommended as an extension to this research. Application of the empirical models in fractured porous medium characterisation simulations reduces the coupling-related numerical complexities. The coupled empirical models can predict flow dynamics of fractured reservoirs under various stress regimes. They demand much less computational effort and, as they incorporate geometrical factors, they can be up-scaled conveniently. In terms of production planning for fractured reservoirs, the empirical models can assist engineers to manage matrix and fracture production efficiently based on overburden stress variations

The Dynamics of Coarse Sediment Transfer in an Upland Bedrock River

Bedrock channels in UK upland environments have received relatively little attention despite their importance within upland river systems and their influence on controlling the conveyance of sediment downstream. This thesis aims to quantify and model the transfer of coarse sediment through Trout Beck, an upland bedrock reach in the North Pennines, UK. The transport of coarse sediment has been quantified through field monitoring of the sediment characteristics, repeat magnetic tracer surveys and in situ bed load impact sensors. This was carried out in conjunction with surveys of channel morphology, using terrestrial laser scanning and repeat dGPS surveys and continuous flow monitoring. This has enabled sediment transport dynamics to be related to the hydraulic conditions throughout the reach. Differences between channel types have been conceptualised using the continuum of the ‘fluvial trinities’. This model demonstrates that the interaction of sediment and channel morphology is partly disconnected in bedrock channels. Conversely, in partially alluvial and alluvial channels there are important feedbacks between sediment stored locally in the channel, channel form and sediment transport. It has been shown that bedrock, partially alluvial and alluvial sections of the river channel have a considerable and varied influence on conveyance of sediment through these types of reaches. Sediment storage defines the partially alluvial and alluvial sections of the channel, with very little sediment storage in bedrock reaches, except in hydraulically sheltered sites. More efficient sediment transfer through bedrock channels is the result of the local hydraulics. The low resistance to flow and stable channel boundaries cause little sediment storage and a downstream conveyance of the full grain-size distribution during periods when flow is competent and sediment is supplied from external sources. The detailed morphological survey has provided the necessary boundary conditions, along with the flow data, to apply a one-dimensional hydraulic model (HEC-RAS) of the bedrock channel. The modelling results have quantified the hydraulic regime of the channel. Furthermore, using local shear stress as a proxy for sediment transport, sediment transport potential for the dominant grain-size distribution of the reach (16-256 mm) has been assessed for different locations in the channel. There are significant differences in the critical threshold of shear stress for sediment transport down reach. Sediment which is transported through the bedrock reach will be deposited and stored, in the partially alluvial and alluvial sections of the reach, at the same flow conditions. As the flow magnitude increases above the critical threshold, the sediment transport potential increases throughout the whole channel until the conditions in the whole reach have the potential to transport sediment. The sediment transport potential in the bedrock sections of the channel is always greater than in the partially alluvial and alluvial sections of the channel. By combining the field and modeling approaches an improved understanding of the flow thresholds and spatial variations in sediment transport, in an upland bedrock channel, has been achieved

Geomorphic Landform Design as an Alternative for Conventional Valley Fill Surface Mine Reclamation: Assessing Conceptual Design, Groundwater Modeling, and Contaminant Desorption

This research aimed to evaluate the potential of applying geomorphic landform design (GLD) principles to valley fill reclamation, specifically in southern West Virginia, central Appalachia, USA. When constructing reclaimed landforms, GLD aims to mimic the geomorphology of reference landforms that are stable and in erosive and hydrologic equilibrium. Challenges with the technique have been identified related to use in central Appalachia. Reference landform design values vary by location and need to be quantified at a local scale for site-specific design. Due to the steep slopes of existing valleys, constructing engineered landforms that naturally blend in with the surrounding environment may not ensure stability. Less steep, more stable slopes of geomorphic landforms could create greater stream disturbance to maintain fill volumes. Potential benefits of GLD with respect to groundwater movement and contaminant desorption have also not been quantified. This research presents three major objectives to assess geomorphic landform design in central Appalachia: 1) define the geomorphic characteristics of mature landform reference sites in southern West Virginia; 2) quantify the issues associated with implementing geomorphic reclamation on a field scale at an existing valley fill; and, 3) compare models of groundwater movement and desorption of selenium in reclamation alternatives for a southern WV surface mine. Geomorphic properties of drainage length and drainage density for mature landforms in central Appalachia were 408 ft and 62 ft/ac, respectively. Slopes were steep (\u3e20%), aspects were well distributed in all directions, vegetation was predominately dense core forest, and ephemeral channel heads developed where erosive surface processes created concentrated flow and sediment transport. Potential issues associated with implementing GLD in central Appalachia with respect to landform stability, stable channel mitigation, and mass balance were confirmed. No geomorphic design was able to satisfy all three criteria when the permitted area of impact was maintained. Expanding the area of impact beyond permit boundaries promoted more success in meeting design criteria, but did not comply with reclamation regulations governing excess spoil placement and constructed hillslopes. A quantitative comparison of the groundwater movement and selenium desorption between alternative reclamation designs confirmed potential benefits to geomorphic reclamation. Selenium desorption was reduced by 23-39% in geomorphic fills and was attributed to improved groundwater movement. Geomorphic reclaimed landforms exhibited 23-45% lower infiltration volumes, 12-63% lower groundwater discharge volumes, and approximately 50% shorter groundwater residence times. These findings will be used to provide recommendations to government agencies and the surface mining industry on the practicality of implementing geomorphic reclamation as an alternative to conventional valley fill reclamation in central Appalachia

Biogeomorphology of Bedrock Fluvial Systems: Example from Shawnee Run, Kentucky, USA

The dynamic interactions between fluvial processes and vegetation vary in different environments and are uncertain in bedrock settings. Bedrock streams are much less studied than alluvial in all aspects, and in many respects act in qualitatively different ways. This research seeks to fill this lacuna by studying bedrock streams from a biogeomorphic perspective. The first part of this research aims to identify the impacts of woody vegetation that may be common to fluvial systems and rocky hillslopes in general, or that may be unique to bedrock channels. A review of the existing literature on biogeomorphology — mostly fluvial and rocky hillslope environments — was carried out, and field examples of biogeomorphic impacts (BGIs) associated with fluvial systems of six various bedrock environments were then examined to complement the review. This research shows that bedrock streams exhibit both shared and highly concentrated BGIs in relation to alluvial streams and bedrock hillslope environments. It shows that while no BGIs associated with bedrock streams are unique to the environment, the bioprotective function related to root-banks (when the root itself creates the stream bank) and the processes related to bioweathering and erosion are rarely addressed in alluvial fluvial literature, despite their importance in bedrock fluvial environments. The second part of the dissertation is largely grounded upon the important BGIs associated with bedrock fluvial environments identified in the first part. Drawing from ecological lexicon, this part introduces some biogeomorphic concepts, most importantly biogeomorphic keystone species and equivalents, with respect to different biotic impacts on surface processes and forms. Later, it explores these concepts by examining the general vs. species-specific BGIs of trees on a limestone bedrock-controlled stream, Shawnee Run, in central Kentucky. Results suggest that Platanus occidentalis plays a keystone role by promoting development of biogeomorphic pools in the study area. Further, some species play equivalent roles with respect to surface processes and landforms by promoting development of avulsion-associated islands and can be recognized as biogeomorphic equivalents. Finally, this dissertation also examines the relative importance of systematic up-to downstream vs. local scale variation explaining channel morphology and biogeomorphological phenomena in Shawnee Run. Results show that local scale variation − primarily attributable to the local scale structural controls, incision status and edaphic variation − largely explains channel morphology and vegetation patterns. These patterns may therefore be common in bedrock rivers strongly influenced by geological controls

Flow and transport in fractured geothermal reservoirs on different scales: Linking experiments and numerical models

Die Erdwärme stellt eine wichtige erneuerbare Energiequelle der Zukunft dar, um den Grundbedarf der Menschen an Wärme und Strom zu decken und die Abhängigkeit von fossilen Brennstoffen wie Erdöl und Kohle zu verringern. Die Internationale Energiebehörde schätzt, dass bis zum Jahr 2050 3,5% der weltweiten Energieversorgung durch Geothermie erfolgen können. Die Vorteile der Geothermie liegen dabei in der guten bedarfsabhängigen Regulierbarkeit sowie der uneingeschränkten weltweiten Verfügbarkeit bei gleichzeitig geringem Flächenbedarf. Darüber hinaus ist die Geothermie als eine der wenigen erneuerbaren Energien vollständig grundlastfähig und damit unabhängig von stark wechselnden Umwelteinflüssen, wie Windstärke oder Sonneneinstrahlung. Die größte Herausforderung bei der Geothermie liegt in der Erschließung von Niederenthalpie-Lagerstätten, die in Tiefen von einigen Kilometern liegen. Eine Möglichkeit hierzu stellt die Technologie des Enhanced Geothermal Systems (EGS) dar, die geringdurchlässige Gesteinsschichten eines Reservoirs wirtschaftlich nutzbar macht. Bei EGS werden durch hydraulische Stimulation bestehende natürliche Kluftsysteme erweitert und neue Klüfte geschaffen und so ein effektiver Wärmeaustausch zwischen dem geklüfteten Reservoirgestein und zirkulierenden Fluiden ermöglicht. Bisher gibt es allerdings nur wenige Pilotanlagen, wie z.B. in Soultz-sous-Forêts, Frankreich. Der Nachteil dieser Technologie ist, dass die so entstandenen Klüfte nur einen sehr kleinen Teil des Reservoirvolumens darstellen und sich alle an der Fluidzirkulation beteiligten natürlichen und induzierten Prozesse auf engstem Raum abspielen. Das grundlegende Verständnis der hochlokalisierten physikalischen Prozesse und Wechselwirkungen stellt somit den Schlüsselfaktor für einen erfolgreichen, umweltverträglichen und sicheren Betrieb von EGS dar. Ein besonderes Augenmerk muss auf die gegenseitigen Wechselwirkungen zwischen der Kluft und dem zirkulierenden Fluid sowie dem damit verbundenen Transport von Wärme und gelösten Stoffen gelegt werden. Die Kluftöffnung wird oft vereinfacht als der Abstand zwischen zwei parallelen Platten dargestellt. In Wirklichkeit bestehen die Verbindungen zwischen zwei Bohrungen jedoch aus einem kleinräumigen Netzwerk einzelner Klüfte, die wiederum ein stark veränderliches inneres Porenvolumen aufweisen. Die vorliegende Arbeit trägt zu einem besseren Verständnis der Entstehung und geometrischen Beschaffenheit von bevorzugten Fluidwegsamkeiten in geklüfteten Reservoiren sowie der damit verbundenen Transportprozesse bei. Das übergeordnete Ziel der einzelnen Studien ist eine Verknüpfung experimenteller Untersuchungen mit numerischen Modellen, um die relevanten, teilweise skalenabhängigen physikalischen Prozesse in Klüften zu identifizieren und quantifizieren. In den ersten beiden Studien (Kapitel 4 und 5) werden eine Vielzahl von stochastisch einzigartigen granitähnlichen Kluftgeometrien erstellt. Anschließend werden numerische Modelle entwickelt, um die präferentiellen Fluidpfade und deren Eigenschaften im Klufhohlraum unter geothermie-typischen Strömungsbedingungen und unter Verwendung der komplexen Navier-Stokes-Gleichungen zu quantifizieren. Das Ziel der ersten Studie ist die Quantifizierung von räumlichen Unterschieden zwischen den dreidimensionalen und den vereinfachten zweidimensionalen Kluftmodellen. Ein Vergleich zwischen äquivalenten Modellierungen mittels der Navier-Stokes-Gleichungen und dem lokalen kubischen Gesetz erlaubt eine Vorhersage über die Gültigkeit dieser Vereinfachungen. In Abhängigkeit von Fließund Scherrichtung sowie dem angelegten Druckgradienten bilden sich in allen Klüften Kanäle aus, die einen großen Teil des Volumenstroms umfassen, während im Rest der Kluft nur geringe Anteile an Fluidbewegung zu beobachten sind. Innerhalb dieser Kanäle zeigen beide Fließgesetze eine gute Übereinstimmung sowohl für rein laminare als auch turbulente Strömungen (mit Reynolds-Zahlen deutlich über 1). Außerhalb von Kanälen ergibt sich unabhängig vom Fließregime für die zweidimensionale Vereinfachung eine deutliche Überschätzung des zu erwartenden Volumenstroms. In der zweiten Studie werden die einzelnen Kanäle innerhalb der dreidimensionalen Kluft hinsichtlich ihrer Geometrie sowie Transporteigenschaften quantifiziert. Die Ergebnisse zeigen eine starke Anisotropie hinsichtlich der Fließ- und Scherrichtung. Obwohl eine senkrechte Ausrichtung von Strömung und Scherung zu einem deutlich verbesserten Durchfluss führt, haben die gut ausgebildeten und geraden Kanäle nur eine begrenzte Kontaktfläche mit dem umgebenden Gestein und behindern somit einen effizienten Wärmeaustausch. Anders ist dies bei einer parallelen Ausrichtung von Scherung und Strömung. In diesem Fall sind die Kanäle deutlich weniger ausgeprägt und haben zudem einen stark verlängerten absoluten Fließweg und damit verbundene höhere Kontaktfläche. Die dritte Studie (Kapitel 6) umfasst die Verknüpfung von Triaxialexperimenten, durchgeführt an zwei Sandsteinenderivaten mit steigenden Temperaturund Druckbedingungen, mit numerischen Modellen. Ziel ist eine Vorhersage der hydraulischen und mechanischen Gesteinseigenschaften eines potentiellen Reservoirgesteins. Die Ergebnisse zeigen eine poroelastische Kompaktion des Gesteins sowie anschließende nichtlineare Deformation, welche beide mit numerischen Modellen vorhergesagt werden können. Das Drucker-Prager-Kriterium ermöglicht die Bewertung der kritischen Scherspannung unter Berücksichtigung der drei Hauptspannungen. Die Studie zeigt, dass kleinstskalige Veränderungen, wie die mineralogische Zusammensetzung, zwar die Materialeigenschaften des Gesteins beeinflussen, numerische und analytische Modelle dessen Verhalten dennoch beschreiben können. In der vierten und fünften Studie (Kapitel 7 und 8) werden die kleinskalig gewonnen Erkenntnisse sowie weiterführende Felduntersuchungen dazu genutzt, um ein Modell des großräumigen Strömungsregimes im geklüfteten Reservoir von Soultz-sous-Forêts zu entwickeln. In der vierten Studie wird ein Strukturmodell des Soultz-Reservoirs entwickelt und das Strömungsregime entlang von Klüften zwischen den einzelnen Bohrungen mittels numerischer Modelle bestimmt. Durch die Verknüpfung mit den experimentellen Daten mehrerer Zirkulations- sowie Tracerversuche kann das Strömungsregime in bohrlochfernen Bereichen des Reservoirs quantifiziert werden. Darüber hinaus kann eine geologische Struktur identifiziert werden, die die Bohrungen GPK3 und GPK4 zwar hydraulisch separiert, allerdings störungsparallel eine Anbindung an das Fließregime des Oberrheingrabens herstellt. In der fünften Studie wird auf Grundlage des zuvor entwickelten hydraulischen Modells die Sensitivität der Produktionstemperatur hinsichtlich verschiedener operativer Rahmenbedingungen (Injektionstemperatur und Fließraten) untersucht

Kentucky Water Resources Research Institute Annual Technical Report FY 2016

The 2016 Annual Technical Report for Kentucky consolidates reporting requirements for the Section 104(b) base grant award into a single document that includes: 1) a synopsis of each student research enhancement project that was conducted during the period, 2) citations for related publications, reports, and presentations, 3) a description of information transfer activities, 4) a summary of student support during the period, and 5) notable awards and achievements. No funds were requested for general program administration activities. However, travel funds were provided to support the participation of the director and associate director in the annual meeting of the National Institutes for Water Resources in Washington, DC from February 27 - March 1, 2017

Bridge Scour : Basic Mechanisms and Predictive Formulas

This report aims at presenting basic knowledge on bridge scour and the processes governing its evolution as well as summarizing the most common formulas used to calculate scour depth at bridges. Design procedures concerning bridge scour in several different countries are also discussed, including United States, Australia, and the United Kingdom. The situation in Sweden with regard to bridge scour is briefly reviewed and several case studies are presented where marked scour holes have been detected at bridges. Two cases of bridge failures in Sweden are included where local scour was the main reason for the collapse.Bridge scour is typically separated into pier, abutment, and contraction scour, where each mechanism is controlled by different physics and governing parameters. Each type of bridge scour is discussed separately in the report with sections on basic mechanisms, governing parameters, common predictive formulas, and concluding remarks.The report also includes a brief summary on the expected influence of climate change on bridge scour. Larger and more intense rainfalls in the future imply larger flows in the rivers with increased bridge scour as a result.The report deals only with scour induced by bridges; other types of scour, such as general scour due to longitudinal transport gradients in the river, scour related to secondary flows in river bends, or scour downstream hard bottom, are not discussed. Most of the formulas included to estimate bridge scour are valid for friction material and only a few examples are given that are applicable to cohesive sediment, mainly related to recommended design procedures from different countries. Also, the objective of bridge scour analysis is often to estimate the maximum scour depth, occurring at equilibrium conditions under a certain flow, implying that most of the formulas are valid for such conditions