Sonja Philipp | Georg-August-Universität Göttingen (original) (raw)

Papers by Sonja Philipp

Research paper thumbnail of Strukturgeologische Studien als Beitrag zum Erfolg tiefengeothermischer Projekte

Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unteri... more Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unterirdische Wärmetauscher erzeugt. Zur Wärme-und Stromerzeugung wird dann wiederholt Wasser in den Untergrund verpresst, welches erhitzt und wieder gefördert wird. Dafür werden im Allgemeinen Systeme aus Injektions-und Förderbohrungen (‚Dubletten') von 2-5 km Tiefe verwendet, um die erforderlichen Temperaturen zu erreichen. Der kritische Parameter für die wirtschaftliche Nutzbarkeit geothermischer Reservoire (‚Erfolg') ist jedoch eine nötige hohe Permeabilität. In den meisten Reservoiren müssen zu niedrige natürliche Permeabilitäten -oder zu kleine Wärmeaustauschflächen -durch die Öffnung bzw. Scherung vorhandener Brüche oder die Erzeugung künstlicher hydraulischer Brüche erhöht werden (‚Reservoirstimulation'). Um Stimulationen erfolgreich durchzuführen, müssen dabei das vorhandene Bruchsystem und das gegenwärtige Spannungsfeld möglichst genau bekannt sein. Dafür sind strukturgeologische Studien von besonderer Bedeutung (vgl. Philipp et al. 2005).

Research paper thumbnail of Strukturgeologische Studien als Beitrag zum Erfolg tiefengeothermischer Projekte

Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unteri... more Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unterirdische Wärmetauscher erzeugt. Zur Wärme-und Stromerzeugung wird dann wiederholt Wasser in den Untergrund verpresst, welches erhitzt und wieder gefördert wird. Dafür werden im Allgemeinen Systeme aus Injektions-und Förderbohrungen (‚Dubletten ') von 2–5km Tiefe verwendet, um die erforderlichen Temperaturen zu erreichen. Der kritische Parameter für die wirtschaftliche Nutzbarkeit geothermischer Reservoire (‚ ...

Research paper thumbnail of Volcanoes Behave as Composite Materials: Implications for Modeling Magma Chambers, Dikes, and Surface Deformation

Agu Fall Meeting Abstracts, Dec 1, 2005

By definition, composite volcanoes are composed of numerous alternating material units or layers ... more By definition, composite volcanoes are composed of numerous alternating material units or layers such as lavas, sediments, and pyroclastics. Commonly, these layers have widely different mechanical properties. In particular, some lava flows and welded pyroclastic flows may be stiff (with a high Young's modulus), whereas others, such as non-welded pyroclastic units and sediments, may be soft (with a low Young's modulus). As a consequence, even if the loading (tectonic stress, magmatic pressure, or displacement) is uniform, the stresses within the composite volcano will vary widely. In this sense, the behavior of composite volcanoes is similar to that of general composite materials. The deformation of the surface of a volcano during an unrest period results from stresses generated by processes and parameters such as fluid pressure in a geothermal field or a magma chamber, a regional tectonic event, and a dike injection. Here we present new numerical models on mechanics of magma chambers and dikes, and the associated surface deformation of composite volcanoes. The models show that the surface deformation during magma-chamber inflation and deflation depends much on the chamber geometry, the loading conditions, and the mechanical properties of the rock units that constitute the volcano. The models also indicate that the surface deformation induced by a propagating dike depends much on the mechanical properties of the layers between the dike tip and the surface. In particular, the numerical results show that soft layers and weak contacts between layers may suppress the dike-induced tensile stresses and the associated surface deformation. Many dikes may therefore become injected and arrested at shallow depths in a volcano while giving rise to little or no surface deformation. Traditional analytical surface-deformation models such as a point source (Mogi model) for a magma-chamber pressure change and a dislocation for a dike normally assume the volcano to behave as a homogeneous, isotropic half space. The present numerical results, combined with field studies, indicate that such analytical models may yield results that have little similarity with the actual structure being modeled.

Research paper thumbnail of Magma chambers, dyke injections, and surface deformation in composite volcanoes

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Research paper thumbnail of Applicability of failure criteria and empirical relations of mechanical rock properties from outcrop analogue samples for wellbore stability analyses

Agu Fall Meeting Abstracts, Dec 1, 2013

Knowledge of failure criteria, Young's modulus and uniaxial and tensile strengths, are important ... more Knowledge of failure criteria, Young's modulus and uniaxial and tensile strengths, are important to avoid borehole instabilities and adapt the drilling plan on rock mechanical conditions. By this means, a considerable reduction of the total drilling costs can be achieved. This is desirable to enlarge the profit margin of geothermal projects which is rather small compared with hydrocarbon projects. Because core material is rare we aim at predicting in situ rock properties from outcrop analogue samples which are easy and cheap to provide. The comparability of properties determined from analogue samples with samples from depths is analysed by performing conventional triaxial tests, uniaxial compressive strength tests and Brazilian tests of both quarry and equivalent core samples. Equivalent means that the quarry sample is of the same stratigraphic age and of comparable sedimentary facies and composition as the associated core sample. We determined the parameters uniaxial compressive strength (UCS), Young's modulus, and tensile strength for 35 rock samples from quarries and 14 equivalent core samples from the North German Basin. A subgroup of these samples, consisting of one volcanic rock sample, three sandstone and three carbonate samples, was used for triaxial tests. In all cases, comparability of core samples with quarry samples is evaluated using thin section analyses. For UCS versus Young's modulus and tensile strengths, linear- and non-linear regression analyses were performed. We repeat regression separately for clastic rock samples or carbonate rock samples only as well as for quarry samples or core samples only. Empirical relations have high statistical significance and properties of core samples lie within 90% prediction bands of developed regression functions of quarry samples. With triaxial tests we determined linearized Mohr-Coulomb failure criteria, expressed in both principal stresses and shear and normal stresses, for quarry samples. Comparison with samples from larger depths shows that it is possible to apply the obtained principal stress failure criteria on clastic and volcanic rocks, but less so for carbonates. Carbonate core samples have higher strengths and develop larger angles between fault normal and main principal stress than quarry samples. This considerably reduces the residuals between quarry failure criteria and core test results. Therefore, it is advised to use failure criteria, expressed in shear and normal stresses, for prediction of core sample failure conditions. We conclude that it is possible to apply failure criteria on samples from depth if the comparability, especially textural comparability and similar porosities, of chosen outcrop analogues samples is ensured. Applicability of empirical relations of UCS with Young's modulus and tensile strength to rocks at depths is expected. Presented results may help predict mechanical properties for in situ rocks, and thus develop suitable geomechanical models for the adaptation of the drilling strategy on rock mechanical conditions. The authors appreciate the support of 'Niedersächsisches Ministerium für Wissenschaft und Kultur' and 'Baker Hughes' within the gebo research project (http: www.gebo-nds.de).

Research paper thumbnail of Mechanical rock properties, fracture propagation and permeability development in deep geothermal reservoirs

Deep geothermal reservoirs are rock units at depths greater than 400 m from which the internal he... more Deep geothermal reservoirs are rock units at depths greater than 400 m from which the internal heat can be extracted using water as a transport means in an economically efficient manner. In many geothermal reservoirs, fluid flow is largely, and may be almost entirely, controlled by the permeability of the fracture network. No flow, however, takes place along a particular fracture network unless the fractures are interconnected. For fluid flow to occur from one site to another there must be at least one interconnected cluster of fractures that links these sites, that is, the percolation threshold must be reached. In "hydrothermal systems", only the natural fracture system (extension and shear fractures) creates the rock or reservoir permeability that commonly exceeds the matrix permeability by far; in "petrothermal systems", by contrast, interconnected fracture systems are formed by creating hydraulic fractures and massive hydraulic stimulation of the existing fracture system in the host rock. Propagation (or termination, that is, arrest) of both natural extension and shear fractures as well as man-made hydraulic fractures is mainly controlled by the mechanical rock properties, particularly rock toughness, stiffness and strengths, of the host rock. Most reservoir rocks are heterogeneous and anisotropic, in particular they are layered. For many layered rocks, the mechanical properties, particularly their Young's moduli (stiffnesses), change between layers, that is, the rocks are mechanically layered. Mechanical layering may coincide with changes in grain size, mineral content, fracture frequencies, or facies. For example, in sedimentary rocks, stiff limestone or sandstone layers commonly alternate with soft shale layers. In geothermal reservoirs fracture termination is important because non-stratabound fractures, that is, fractures not affected by layering, are more likely to form an interconnected fracture network than stratabound fractures, confined to single rock layers. Thus, to minimise exploration risks and for effective stimulation, the geometry of the fracture system and the mechanical properties of the host rock must be known. Here we present first results of structural geological field studies of fracture systems in outcrop analogues studies of rocks that could be used to host man-made geothermal reservoirs in sedimentary rocks in the North German Basin. As examples, we show data from different lithologies, including Buntsandstein (Lower Triassic), a sandstone-shale succession and Muschelkalk (Middle Triassic), a limestone-marl succession. We analyse natural fracture systems and the effects of rock heterogeneities, particularly stiffness variations between layers (mechanical layering) on the propagation of natural fractures. Important fracture parameters include attitude, aperture and interconnectivity to fracture systems. The field studies are supplemented by laboratory measurements of the above mentioned rock mechanical properties. Our field studies indicate that many fractures become arrested at layer contacts, particularly at contacts between layers with contrasting mechanical properties. Measurements of thousands of fractures indicate that even very thin layers (mm to cm-scale thicknesses) of shale or marl may be responsible for the arrest of many fractures. Our results suggest that the propagation and aperture variation of fractures are important parameters in the permeability development of deep geothermal reservoirs. These studies provide a basis for models of fracture networks and fluid transport in future man-made reservoirs. We conclude that the likely permeability of a man-made geothermal reservoir can be inferred from field data from outcrop analogues, laboratory measurements, and numerical models.

Research paper thumbnail of The significance of fractures in Europe

Research paper thumbnail of Why are volcanic eruptions so rare? (in German)

Research paper thumbnail of Strukturgeologische Untersuchungen und numerische Modellierungen zur KlUftung im Muschelkalk des Kraichgaus, Baden-Wurttemberg

Geotectonic Research

ABSTRACT

Research paper thumbnail of Prognose von Bruchsystemen und Permeabilität im Muschelkalk des Oberrheingrabens: Ergebnisse von Geländestudien und numerischen Modellierungen

Kurzfassung Hier präsentieren wir Ergebnisse aus einer Fallstudie zur Prognose von Bruchsystemen ... more Kurzfassung Hier präsentieren wir Ergebnisse aus einer Fallstudie zur Prognose von Bruchsystemen und Permeabilität des Muschelkalks im Oberrheingraben. Geländestudien und Lineament-Auswertungen im östlich angrenzenden Kraichgau zeigen, dass neben zahlreichen Störungen meist zwei deutlich bevorzugte Kluftscharen auftreten, deren Orientierungen in verschiedenen Aufschlüssen sowie zwischen verschiedenen Schichten stark variieren. Geringmächtige (mm-bis cm-Maßstab) Schichten von Mergelstein reichen aus, um viele Klüfte zu stoppen, jedoch sind rund 50% aller Klüfte nicht-schichtgebunden und können so mehrere Schichten hydraulisch miteinander vernetzen. Einzelne Kluftscharen weisen dabei höhere effektive Permeabilitäten auf als andere Richtungen. Die gleichen Kluftscharen zeigen auffällig häufig Hinweise auf Paläofluidtransport (Mineralisation bzw. Verkarstung). Abschätzungen des möglichen Volumenstroms für den Oberen Muschelkalk weisen noch große Unsicherheiten bezüglich der Kluftöffnung...

Research paper thumbnail of How facies and diagenesis affect fracturing of limestone beds and reservoir permeability in limestone–marl alternations

Marine and Petroleum Geology

Facies and diagenetic heterogeneities in carbonate reservoir rocks affect both, fracture distribu... more Facies and diagenetic heterogeneities in carbonate reservoir rocks affect both, fracture distribution and fracture permeability. Many studies focussed on fracture patterns in limestone–marl alternations, as e.g. fluid flow models, are based on laterally continuous beds. Here we examine 4010 fractures in multiple layers of limestone–marl alternations using a modified scan-line method. The studied successions belong to the Blue Lias Formation (Hettangian–Sinemurian), exposed on the coast of the Bristol Channel, United Kingdom. We combine methods of sedimentology and structural geology with rock physics to gain a better understanding of the role of facies, diagenesis and petrophysical properties (tensile and compressive strength, hardness, porosity) on the distribution of fractures (fracture orientation, density, spacing and height). Fracture distribution varies significantly despite similar bed thicknesses, indicating that planar bedding planes (i.e. well-bedded limestones, WBL) and b...

Research paper thumbnail of Conditions and threshold for magma transfer in the layered upper crust: Insights from experimental models

[1] Magma transfer, i.e., dike propagation, is partly controlled by Young's modulus (elastici... more [1] Magma transfer, i.e., dike propagation, is partly controlled by Young's modulus (elasticity) contrasts (ratio upper layer to lower layer modulus) in the host rock. Here we try to better constrain the elasticity contrasts controlling the propagation velocity of dikes and their arrest. We simulate dike propagation in layered elastic media with different elasticity contrasts. Salted gelatin and water represent host rock and magma, respectively. For common density ratios between magma and host rock (~1.1), velocity variations are observed and a critical threshold in the elasticity contrast between layers results in the Young's modulus ratio of 2.1 ± 0.6. Naturally occurring elasticity contrasts can be much higher than this experimental threshold, suggesting that dike arrest due to heterogeneous elastic host rock properties is more frequent than expected. Examples of recently deflected or stalled dikes inside volcanoes and the common presence of high-velocity bodies below vol...

Research paper thumbnail of Architecture, fracture system, mechanical properties and permeability structure of a fault zone in Lower Triassic sandstone, Upper Rhine Graben

Tectonophysics, 2015

ABSTRACT Close to the western Upper Rhine Graben Main Fault, Alsace, a NNE-SSW-striking fault zon... more ABSTRACT Close to the western Upper Rhine Graben Main Fault, Alsace, a NNE-SSW-striking fault zone, crosscutting porous, thick bedded Lower Triassic Bunter sandstone was investigated in detail, including its architecture, discontinuity system, mechanical rock properties and implications on its permeability structure and fault zone type. Field observations indicate a three-part fault zone structure including core-, transition- and damage zone. The at least 14 m thick fault core is composed of various slip surfaces and deformation bands, which encompass fractured host rock lenses. When connected, host rock lenses may transport fluids through the core zone. Adjacent transition zones are highly fractured in R1-orientation, show folded beds and contain P1-oriented deformation bands. R1 and P1- fractures are synthetic shear fractures and project with an acute angle (10-20°) towards the fault plane. Only in the damage zone, fault-parallel striking fractures occur. Here, increasing fracture apertures and connectivity may increase the permeability towards the fault core. Mechanical rock properties from 12 rock samples (Young’s modulus, uniaxial compressive strength, tensile strength) measured in all the parts of the fault zone, show highest values within the transition zone. In-situ measurements of rebound-hardnesses with a Schmidt-Hammer and analytical approaches, however, indicate that effective Young’s moduli are two to sixteen times lower than the Young’s moduli of intact rock. Values clearly decrease towards the fault core, even in the transition zone and are in average lower than effective Young’s moduli in the damage zone. Although many fault zones in sandstone are sealing structures these field study show, that fa

Research paper thumbnail of Why are eruptions so rare-2013

Research paper thumbnail of Quantification of rock heterogeneities by structural geological field studies combined with laboratory analyses

Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are ... more Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are typical in sedimentary successions. The knowledge of in-situ mechanical rock properties is crucial for a better understanding of processes such as fracturing and fluid transport in fractured reservoirs. To estimate in situ rock properties at different depths it is important to understand how rocks from outcrops differ from rocks at depth, for example due to alteration and removal of the overburden load. We aim at quantifying these properties by performing structural geological field studies in outcrop analogues combined with laboratory analyses of outcrop samples and drill-cores. The field studies focus on 1) fault zone infrastructure and 2) host rock fracture systems in two different study areas with different lithologies, the North German and the Bristol Channel Basin. We analyse quantitatively the dimension, geometry, persistence and connectivity of fracture systems. The field studies...

Research paper thumbnail of Heterogeneities of mechanical properties in potential geothermal reservoir rocks of the North German Basin

Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are ... more Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are typical phenomena in sedimentary basins such as the North German Basin. To be able to model reservoir stimulation in layered stratifications and to better adapt the drilling strategy to the rock mechanical conditions it is important to have knowledge about the effects of heterogeneous rock properties on fracture propagation and fault zone infrastructure for typical sedimentary reservoir rocks in the North German Basin. Therefore we aim at quantifying these properties by performing structural geological field studies in outcrop analogues combined with laboratory analyses. The field studies in Rotliegend sandstones (Lower Permian), the sandstones of the Middle Bunter (Lower Triassic) and the sandstones of the Upper Keuper (Upper Triassic) focus on 1) host rock fracture systems and 2) fault zone infrastructure. We analyse quantitatively the dimension, geometry, persistence and connectivity ...

Research paper thumbnail of Fracture propagation and fluid transport in palaeogeothermal fields and man-made reservoirs in limestone

Geothermal reservoirs are rock units from which the internal heat can be extracted using water as... more Geothermal reservoirs are rock units from which the internal heat can be extracted using water as a transport means in an economically efficient manner. In geothermal reservoirs in limestone (and similar in other rocks with low matrix permeability), fluid flow is largely, and may be almost entirely, controlled by the permeability of the fracture network. No flow, however, takes place along a particular fracture network unless the fractures are interconnected. For fluid flow to occur from one site to another there must be at least one interconnected cluster of fractures that links these sites (the percolation threshold must be reached). In order to generate permeability in man-made reservoirs, interconnected fracture systems are formed either by creating hydraulic fractures or by massive hydraulic stimulation of the existing fracture system in the host rock. For effective stimulation, the geometry of the fracture system and the mechanical properties of the host rock (particularly roc...

Research paper thumbnail of Empirical relations of rock properties of outcrop and core samples from the Northwest German Basin for geothermal drilling

Information about geomechanical and physical rock properties, particularly uniaxial compressive s... more Information about geomechanical and physical rock properties, particularly uniaxial compressive strength (UCS), are needed for geomechanical model development and updating with logging-while-drilling methods to minimise costs and risks of the drilling process. The following parameters with importance at different stages of geothermal exploitation and drilling are presented for typical sedimentary and volcanic rocks of the Northwest German Basin (NWGB): physical (P wave velocities, porosity, and bulk and grain density) and geomechanical parameters (UCS, static Young’s modulus, destruction work and indirect tensile strength both perpendicular and parallel to bedding) for 35 rock samples from quarries and 14 core samples of sandstones and carbonate rocks. With regression analyses (linear- and non-linear) empirical relations are developed to predict UCS values from all other parameters. Analyses focus on sedimentary rocks and were repeated separately for clastic rock samples or carbonat...

Research paper thumbnail of Strukturgeologische Gelandestudien in Aufschlussanalogen und Permeabilitatsentwicklung in geothermischen Reservoiren

Geotectonic Research, 2008

ABSTRACT Anhand zweier Fallstudien in geothermischen Reservoirgesteinen Deutschlands (Buntsandste... more ABSTRACT Anhand zweier Fallstudien in geothermischen Reservoirgesteinen Deutschlands (Buntsandstein, Muschelkalk) zeigen wir, wie strukturgeologische Gellindestudien in Aufschlussanalogen helfen konnen, die Permeabilitatsentwicklung potentieller Geothermiereservoirezu verstehen und damit den Erfolg tiefengeothermischerProjekte zu erhOhen.

Research paper thumbnail of Fault zone characteristics, fracture systems and permeability implications of Middle Triassic Muschelkalk in Southwest Germany

Journal of Structural Geology, 2015

ABSTRACT Fault zone structure and lithology affect permeability of Triassic Muschelkalk limestone... more ABSTRACT Fault zone structure and lithology affect permeability of Triassic Muschelkalk limestone-marl-alternations in Southwest Germany, a region characterized by a complex tectonic history. Field studies of eight fault zones provide insights into fracture system parameters (orientation, density, aperture, connectivity, vertical extension) within fault zone units (fault core, damage zone). Results show decreasing fracture lengths with distances to the fault cores in well-developed damage zones. Fracture connectivity at fracture tips is enhanced in proximity to the slip surfaces, particularly caused by shorter fractures. Different mechanical properties of limestone and marl layers obviously affect fracture propagation and thus fracture system connectivity and permeability. Fracture apertures are largest parallel and subparallel to fault zones and prominent regional structures (e.g., Upper Rhine Graben) leading to enhanced fracture-induced permeabilities. Mineralized fractures and mineralizations in fault cores indicate past fluid flow. Permeability is increased by the development of hydraulically active pathways across several beds (non-stratabound fractures) to a higher degree than by the formation of fractures interconnected at fracture tips. We conclude that there is an increase of interconnected fractures and fracture densities in proximity to the fault cores. This is particularly clear in more homogenous rocks. The results help to better understand permeability in Muschelkalk rocks.

Research paper thumbnail of Strukturgeologische Studien als Beitrag zum Erfolg tiefengeothermischer Projekte

Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unteri... more Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unterirdische Wärmetauscher erzeugt. Zur Wärme-und Stromerzeugung wird dann wiederholt Wasser in den Untergrund verpresst, welches erhitzt und wieder gefördert wird. Dafür werden im Allgemeinen Systeme aus Injektions-und Förderbohrungen (‚Dubletten') von 2-5 km Tiefe verwendet, um die erforderlichen Temperaturen zu erreichen. Der kritische Parameter für die wirtschaftliche Nutzbarkeit geothermischer Reservoire (‚Erfolg') ist jedoch eine nötige hohe Permeabilität. In den meisten Reservoiren müssen zu niedrige natürliche Permeabilitäten -oder zu kleine Wärmeaustauschflächen -durch die Öffnung bzw. Scherung vorhandener Brüche oder die Erzeugung künstlicher hydraulischer Brüche erhöht werden (‚Reservoirstimulation'). Um Stimulationen erfolgreich durchzuführen, müssen dabei das vorhandene Bruchsystem und das gegenwärtige Spannungsfeld möglichst genau bekannt sein. Dafür sind strukturgeologische Studien von besonderer Bedeutung (vgl. Philipp et al. 2005).

Research paper thumbnail of Strukturgeologische Studien als Beitrag zum Erfolg tiefengeothermischer Projekte

Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unteri... more Bei der tiefen Geothermie werden zur Schaffung eines künstlichen geothermischen Reservoirs unterirdische Wärmetauscher erzeugt. Zur Wärme-und Stromerzeugung wird dann wiederholt Wasser in den Untergrund verpresst, welches erhitzt und wieder gefördert wird. Dafür werden im Allgemeinen Systeme aus Injektions-und Förderbohrungen (‚Dubletten ') von 2–5km Tiefe verwendet, um die erforderlichen Temperaturen zu erreichen. Der kritische Parameter für die wirtschaftliche Nutzbarkeit geothermischer Reservoire (‚ ...

Research paper thumbnail of Volcanoes Behave as Composite Materials: Implications for Modeling Magma Chambers, Dikes, and Surface Deformation

Agu Fall Meeting Abstracts, Dec 1, 2005

By definition, composite volcanoes are composed of numerous alternating material units or layers ... more By definition, composite volcanoes are composed of numerous alternating material units or layers such as lavas, sediments, and pyroclastics. Commonly, these layers have widely different mechanical properties. In particular, some lava flows and welded pyroclastic flows may be stiff (with a high Young's modulus), whereas others, such as non-welded pyroclastic units and sediments, may be soft (with a low Young's modulus). As a consequence, even if the loading (tectonic stress, magmatic pressure, or displacement) is uniform, the stresses within the composite volcano will vary widely. In this sense, the behavior of composite volcanoes is similar to that of general composite materials. The deformation of the surface of a volcano during an unrest period results from stresses generated by processes and parameters such as fluid pressure in a geothermal field or a magma chamber, a regional tectonic event, and a dike injection. Here we present new numerical models on mechanics of magma chambers and dikes, and the associated surface deformation of composite volcanoes. The models show that the surface deformation during magma-chamber inflation and deflation depends much on the chamber geometry, the loading conditions, and the mechanical properties of the rock units that constitute the volcano. The models also indicate that the surface deformation induced by a propagating dike depends much on the mechanical properties of the layers between the dike tip and the surface. In particular, the numerical results show that soft layers and weak contacts between layers may suppress the dike-induced tensile stresses and the associated surface deformation. Many dikes may therefore become injected and arrested at shallow depths in a volcano while giving rise to little or no surface deformation. Traditional analytical surface-deformation models such as a point source (Mogi model) for a magma-chamber pressure change and a dislocation for a dike normally assume the volcano to behave as a homogeneous, isotropic half space. The present numerical results, combined with field studies, indicate that such analytical models may yield results that have little similarity with the actual structure being modeled.

Research paper thumbnail of Magma chambers, dyke injections, and surface deformation in composite volcanoes

The user has requested enhancement of the downloaded file.

Research paper thumbnail of Applicability of failure criteria and empirical relations of mechanical rock properties from outcrop analogue samples for wellbore stability analyses

Agu Fall Meeting Abstracts, Dec 1, 2013

Knowledge of failure criteria, Young's modulus and uniaxial and tensile strengths, are important ... more Knowledge of failure criteria, Young's modulus and uniaxial and tensile strengths, are important to avoid borehole instabilities and adapt the drilling plan on rock mechanical conditions. By this means, a considerable reduction of the total drilling costs can be achieved. This is desirable to enlarge the profit margin of geothermal projects which is rather small compared with hydrocarbon projects. Because core material is rare we aim at predicting in situ rock properties from outcrop analogue samples which are easy and cheap to provide. The comparability of properties determined from analogue samples with samples from depths is analysed by performing conventional triaxial tests, uniaxial compressive strength tests and Brazilian tests of both quarry and equivalent core samples. Equivalent means that the quarry sample is of the same stratigraphic age and of comparable sedimentary facies and composition as the associated core sample. We determined the parameters uniaxial compressive strength (UCS), Young's modulus, and tensile strength for 35 rock samples from quarries and 14 equivalent core samples from the North German Basin. A subgroup of these samples, consisting of one volcanic rock sample, three sandstone and three carbonate samples, was used for triaxial tests. In all cases, comparability of core samples with quarry samples is evaluated using thin section analyses. For UCS versus Young's modulus and tensile strengths, linear- and non-linear regression analyses were performed. We repeat regression separately for clastic rock samples or carbonate rock samples only as well as for quarry samples or core samples only. Empirical relations have high statistical significance and properties of core samples lie within 90% prediction bands of developed regression functions of quarry samples. With triaxial tests we determined linearized Mohr-Coulomb failure criteria, expressed in both principal stresses and shear and normal stresses, for quarry samples. Comparison with samples from larger depths shows that it is possible to apply the obtained principal stress failure criteria on clastic and volcanic rocks, but less so for carbonates. Carbonate core samples have higher strengths and develop larger angles between fault normal and main principal stress than quarry samples. This considerably reduces the residuals between quarry failure criteria and core test results. Therefore, it is advised to use failure criteria, expressed in shear and normal stresses, for prediction of core sample failure conditions. We conclude that it is possible to apply failure criteria on samples from depth if the comparability, especially textural comparability and similar porosities, of chosen outcrop analogues samples is ensured. Applicability of empirical relations of UCS with Young's modulus and tensile strength to rocks at depths is expected. Presented results may help predict mechanical properties for in situ rocks, and thus develop suitable geomechanical models for the adaptation of the drilling strategy on rock mechanical conditions. The authors appreciate the support of 'Niedersächsisches Ministerium für Wissenschaft und Kultur' and 'Baker Hughes' within the gebo research project (http: www.gebo-nds.de).

Research paper thumbnail of Mechanical rock properties, fracture propagation and permeability development in deep geothermal reservoirs

Deep geothermal reservoirs are rock units at depths greater than 400 m from which the internal he... more Deep geothermal reservoirs are rock units at depths greater than 400 m from which the internal heat can be extracted using water as a transport means in an economically efficient manner. In many geothermal reservoirs, fluid flow is largely, and may be almost entirely, controlled by the permeability of the fracture network. No flow, however, takes place along a particular fracture network unless the fractures are interconnected. For fluid flow to occur from one site to another there must be at least one interconnected cluster of fractures that links these sites, that is, the percolation threshold must be reached. In "hydrothermal systems", only the natural fracture system (extension and shear fractures) creates the rock or reservoir permeability that commonly exceeds the matrix permeability by far; in "petrothermal systems", by contrast, interconnected fracture systems are formed by creating hydraulic fractures and massive hydraulic stimulation of the existing fracture system in the host rock. Propagation (or termination, that is, arrest) of both natural extension and shear fractures as well as man-made hydraulic fractures is mainly controlled by the mechanical rock properties, particularly rock toughness, stiffness and strengths, of the host rock. Most reservoir rocks are heterogeneous and anisotropic, in particular they are layered. For many layered rocks, the mechanical properties, particularly their Young's moduli (stiffnesses), change between layers, that is, the rocks are mechanically layered. Mechanical layering may coincide with changes in grain size, mineral content, fracture frequencies, or facies. For example, in sedimentary rocks, stiff limestone or sandstone layers commonly alternate with soft shale layers. In geothermal reservoirs fracture termination is important because non-stratabound fractures, that is, fractures not affected by layering, are more likely to form an interconnected fracture network than stratabound fractures, confined to single rock layers. Thus, to minimise exploration risks and for effective stimulation, the geometry of the fracture system and the mechanical properties of the host rock must be known. Here we present first results of structural geological field studies of fracture systems in outcrop analogues studies of rocks that could be used to host man-made geothermal reservoirs in sedimentary rocks in the North German Basin. As examples, we show data from different lithologies, including Buntsandstein (Lower Triassic), a sandstone-shale succession and Muschelkalk (Middle Triassic), a limestone-marl succession. We analyse natural fracture systems and the effects of rock heterogeneities, particularly stiffness variations between layers (mechanical layering) on the propagation of natural fractures. Important fracture parameters include attitude, aperture and interconnectivity to fracture systems. The field studies are supplemented by laboratory measurements of the above mentioned rock mechanical properties. Our field studies indicate that many fractures become arrested at layer contacts, particularly at contacts between layers with contrasting mechanical properties. Measurements of thousands of fractures indicate that even very thin layers (mm to cm-scale thicknesses) of shale or marl may be responsible for the arrest of many fractures. Our results suggest that the propagation and aperture variation of fractures are important parameters in the permeability development of deep geothermal reservoirs. These studies provide a basis for models of fracture networks and fluid transport in future man-made reservoirs. We conclude that the likely permeability of a man-made geothermal reservoir can be inferred from field data from outcrop analogues, laboratory measurements, and numerical models.

Research paper thumbnail of The significance of fractures in Europe

Research paper thumbnail of Why are volcanic eruptions so rare? (in German)

Research paper thumbnail of Strukturgeologische Untersuchungen und numerische Modellierungen zur KlUftung im Muschelkalk des Kraichgaus, Baden-Wurttemberg

Geotectonic Research

ABSTRACT

Research paper thumbnail of Prognose von Bruchsystemen und Permeabilität im Muschelkalk des Oberrheingrabens: Ergebnisse von Geländestudien und numerischen Modellierungen

Kurzfassung Hier präsentieren wir Ergebnisse aus einer Fallstudie zur Prognose von Bruchsystemen ... more Kurzfassung Hier präsentieren wir Ergebnisse aus einer Fallstudie zur Prognose von Bruchsystemen und Permeabilität des Muschelkalks im Oberrheingraben. Geländestudien und Lineament-Auswertungen im östlich angrenzenden Kraichgau zeigen, dass neben zahlreichen Störungen meist zwei deutlich bevorzugte Kluftscharen auftreten, deren Orientierungen in verschiedenen Aufschlüssen sowie zwischen verschiedenen Schichten stark variieren. Geringmächtige (mm-bis cm-Maßstab) Schichten von Mergelstein reichen aus, um viele Klüfte zu stoppen, jedoch sind rund 50% aller Klüfte nicht-schichtgebunden und können so mehrere Schichten hydraulisch miteinander vernetzen. Einzelne Kluftscharen weisen dabei höhere effektive Permeabilitäten auf als andere Richtungen. Die gleichen Kluftscharen zeigen auffällig häufig Hinweise auf Paläofluidtransport (Mineralisation bzw. Verkarstung). Abschätzungen des möglichen Volumenstroms für den Oberen Muschelkalk weisen noch große Unsicherheiten bezüglich der Kluftöffnung...

Research paper thumbnail of How facies and diagenesis affect fracturing of limestone beds and reservoir permeability in limestone–marl alternations

Marine and Petroleum Geology

Facies and diagenetic heterogeneities in carbonate reservoir rocks affect both, fracture distribu... more Facies and diagenetic heterogeneities in carbonate reservoir rocks affect both, fracture distribution and fracture permeability. Many studies focussed on fracture patterns in limestone–marl alternations, as e.g. fluid flow models, are based on laterally continuous beds. Here we examine 4010 fractures in multiple layers of limestone–marl alternations using a modified scan-line method. The studied successions belong to the Blue Lias Formation (Hettangian–Sinemurian), exposed on the coast of the Bristol Channel, United Kingdom. We combine methods of sedimentology and structural geology with rock physics to gain a better understanding of the role of facies, diagenesis and petrophysical properties (tensile and compressive strength, hardness, porosity) on the distribution of fractures (fracture orientation, density, spacing and height). Fracture distribution varies significantly despite similar bed thicknesses, indicating that planar bedding planes (i.e. well-bedded limestones, WBL) and b...

Research paper thumbnail of Conditions and threshold for magma transfer in the layered upper crust: Insights from experimental models

[1] Magma transfer, i.e., dike propagation, is partly controlled by Young's modulus (elastici... more [1] Magma transfer, i.e., dike propagation, is partly controlled by Young's modulus (elasticity) contrasts (ratio upper layer to lower layer modulus) in the host rock. Here we try to better constrain the elasticity contrasts controlling the propagation velocity of dikes and their arrest. We simulate dike propagation in layered elastic media with different elasticity contrasts. Salted gelatin and water represent host rock and magma, respectively. For common density ratios between magma and host rock (~1.1), velocity variations are observed and a critical threshold in the elasticity contrast between layers results in the Young's modulus ratio of 2.1 ± 0.6. Naturally occurring elasticity contrasts can be much higher than this experimental threshold, suggesting that dike arrest due to heterogeneous elastic host rock properties is more frequent than expected. Examples of recently deflected or stalled dikes inside volcanoes and the common presence of high-velocity bodies below vol...

Research paper thumbnail of Architecture, fracture system, mechanical properties and permeability structure of a fault zone in Lower Triassic sandstone, Upper Rhine Graben

Tectonophysics, 2015

ABSTRACT Close to the western Upper Rhine Graben Main Fault, Alsace, a NNE-SSW-striking fault zon... more ABSTRACT Close to the western Upper Rhine Graben Main Fault, Alsace, a NNE-SSW-striking fault zone, crosscutting porous, thick bedded Lower Triassic Bunter sandstone was investigated in detail, including its architecture, discontinuity system, mechanical rock properties and implications on its permeability structure and fault zone type. Field observations indicate a three-part fault zone structure including core-, transition- and damage zone. The at least 14 m thick fault core is composed of various slip surfaces and deformation bands, which encompass fractured host rock lenses. When connected, host rock lenses may transport fluids through the core zone. Adjacent transition zones are highly fractured in R1-orientation, show folded beds and contain P1-oriented deformation bands. R1 and P1- fractures are synthetic shear fractures and project with an acute angle (10-20°) towards the fault plane. Only in the damage zone, fault-parallel striking fractures occur. Here, increasing fracture apertures and connectivity may increase the permeability towards the fault core. Mechanical rock properties from 12 rock samples (Young’s modulus, uniaxial compressive strength, tensile strength) measured in all the parts of the fault zone, show highest values within the transition zone. In-situ measurements of rebound-hardnesses with a Schmidt-Hammer and analytical approaches, however, indicate that effective Young’s moduli are two to sixteen times lower than the Young’s moduli of intact rock. Values clearly decrease towards the fault core, even in the transition zone and are in average lower than effective Young’s moduli in the damage zone. Although many fault zones in sandstone are sealing structures these field study show, that fa

Research paper thumbnail of Why are eruptions so rare-2013

Research paper thumbnail of Quantification of rock heterogeneities by structural geological field studies combined with laboratory analyses

Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are ... more Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are typical in sedimentary successions. The knowledge of in-situ mechanical rock properties is crucial for a better understanding of processes such as fracturing and fluid transport in fractured reservoirs. To estimate in situ rock properties at different depths it is important to understand how rocks from outcrops differ from rocks at depth, for example due to alteration and removal of the overburden load. We aim at quantifying these properties by performing structural geological field studies in outcrop analogues combined with laboratory analyses of outcrop samples and drill-cores. The field studies focus on 1) fault zone infrastructure and 2) host rock fracture systems in two different study areas with different lithologies, the North German and the Bristol Channel Basin. We analyse quantitatively the dimension, geometry, persistence and connectivity of fracture systems. The field studies...

Research paper thumbnail of Heterogeneities of mechanical properties in potential geothermal reservoir rocks of the North German Basin

Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are ... more Heterogeneous rock properties in terms of layering and complex infrastructure of fault zones are typical phenomena in sedimentary basins such as the North German Basin. To be able to model reservoir stimulation in layered stratifications and to better adapt the drilling strategy to the rock mechanical conditions it is important to have knowledge about the effects of heterogeneous rock properties on fracture propagation and fault zone infrastructure for typical sedimentary reservoir rocks in the North German Basin. Therefore we aim at quantifying these properties by performing structural geological field studies in outcrop analogues combined with laboratory analyses. The field studies in Rotliegend sandstones (Lower Permian), the sandstones of the Middle Bunter (Lower Triassic) and the sandstones of the Upper Keuper (Upper Triassic) focus on 1) host rock fracture systems and 2) fault zone infrastructure. We analyse quantitatively the dimension, geometry, persistence and connectivity ...

Research paper thumbnail of Fracture propagation and fluid transport in palaeogeothermal fields and man-made reservoirs in limestone

Geothermal reservoirs are rock units from which the internal heat can be extracted using water as... more Geothermal reservoirs are rock units from which the internal heat can be extracted using water as a transport means in an economically efficient manner. In geothermal reservoirs in limestone (and similar in other rocks with low matrix permeability), fluid flow is largely, and may be almost entirely, controlled by the permeability of the fracture network. No flow, however, takes place along a particular fracture network unless the fractures are interconnected. For fluid flow to occur from one site to another there must be at least one interconnected cluster of fractures that links these sites (the percolation threshold must be reached). In order to generate permeability in man-made reservoirs, interconnected fracture systems are formed either by creating hydraulic fractures or by massive hydraulic stimulation of the existing fracture system in the host rock. For effective stimulation, the geometry of the fracture system and the mechanical properties of the host rock (particularly roc...

Research paper thumbnail of Empirical relations of rock properties of outcrop and core samples from the Northwest German Basin for geothermal drilling

Information about geomechanical and physical rock properties, particularly uniaxial compressive s... more Information about geomechanical and physical rock properties, particularly uniaxial compressive strength (UCS), are needed for geomechanical model development and updating with logging-while-drilling methods to minimise costs and risks of the drilling process. The following parameters with importance at different stages of geothermal exploitation and drilling are presented for typical sedimentary and volcanic rocks of the Northwest German Basin (NWGB): physical (P wave velocities, porosity, and bulk and grain density) and geomechanical parameters (UCS, static Young’s modulus, destruction work and indirect tensile strength both perpendicular and parallel to bedding) for 35 rock samples from quarries and 14 core samples of sandstones and carbonate rocks. With regression analyses (linear- and non-linear) empirical relations are developed to predict UCS values from all other parameters. Analyses focus on sedimentary rocks and were repeated separately for clastic rock samples or carbonat...

Research paper thumbnail of Strukturgeologische Gelandestudien in Aufschlussanalogen und Permeabilitatsentwicklung in geothermischen Reservoiren

Geotectonic Research, 2008

ABSTRACT Anhand zweier Fallstudien in geothermischen Reservoirgesteinen Deutschlands (Buntsandste... more ABSTRACT Anhand zweier Fallstudien in geothermischen Reservoirgesteinen Deutschlands (Buntsandstein, Muschelkalk) zeigen wir, wie strukturgeologische Gellindestudien in Aufschlussanalogen helfen konnen, die Permeabilitatsentwicklung potentieller Geothermiereservoirezu verstehen und damit den Erfolg tiefengeothermischerProjekte zu erhOhen.

Research paper thumbnail of Fault zone characteristics, fracture systems and permeability implications of Middle Triassic Muschelkalk in Southwest Germany

Journal of Structural Geology, 2015

ABSTRACT Fault zone structure and lithology affect permeability of Triassic Muschelkalk limestone... more ABSTRACT Fault zone structure and lithology affect permeability of Triassic Muschelkalk limestone-marl-alternations in Southwest Germany, a region characterized by a complex tectonic history. Field studies of eight fault zones provide insights into fracture system parameters (orientation, density, aperture, connectivity, vertical extension) within fault zone units (fault core, damage zone). Results show decreasing fracture lengths with distances to the fault cores in well-developed damage zones. Fracture connectivity at fracture tips is enhanced in proximity to the slip surfaces, particularly caused by shorter fractures. Different mechanical properties of limestone and marl layers obviously affect fracture propagation and thus fracture system connectivity and permeability. Fracture apertures are largest parallel and subparallel to fault zones and prominent regional structures (e.g., Upper Rhine Graben) leading to enhanced fracture-induced permeabilities. Mineralized fractures and mineralizations in fault cores indicate past fluid flow. Permeability is increased by the development of hydraulically active pathways across several beds (non-stratabound fractures) to a higher degree than by the formation of fractures interconnected at fracture tips. We conclude that there is an increase of interconnected fractures and fracture densities in proximity to the fault cores. This is particularly clear in more homogenous rocks. The results help to better understand permeability in Muschelkalk rocks.