TY - JOUR A1 - Wang, Lei A1 - Rybacki, Erik A1 - Bonnelye, Audrey A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Experimental investigation on static and dynamic bulk moduli of dry and fluid-saturated porous sandstones JF - Rock mechanics and rock engineering N2 - Knowledge of pressure-dependent static and dynamic moduli of porous reservoir rocks is of key importance for evaluating geological setting of a reservoir in geo-energy applications. We examined experimentally the evolution of static and dynamic bulk moduli for porous Bentheim sandstone with increasing confining pressure up to about 190 MPa under dry and water-saturated conditions. The static bulk moduli (K-s) were estimated from stress-volumetric strain curves while dynamic bulk moduli (K-d) were derived from the changes in ultrasonic P- and S- wave velocities (similar to 1 MHz) along different traces, which were monitored simultaneously during the entire deformation. In conjunction with published data of other porous sandstones (Berea, Navajo and Weber sandstones), our results reveal that the ratio between dynamic and static bulk moduli (K-d/K-s) reduces rapidly from about 1.5 - 2.0 at ambient pressure to about 1.1 at high pressure under dry conditions and from about 2.0 - 4.0 to about 1.5 under water-saturated conditions, respectively. We interpret such a pressure-dependent reduction by closure of narrow (compliant) cracks, highlighting thatK(d)/K(s)is positively correlated with the amount of narrow cracks. Above the crack closure pressure, where equant (stiff) pores dominate the void space,K-d/K(s)is almost constant. The enhanced difference between dynamic and static bulk moduli under water saturation compared to dry conditions is possibly caused by high pore pressure that is locally maintained if measured using high-frequency ultrasonic wave velocities. In our experiments, the pressure dependence of dynamic bulk modulus of water-saturated Bentheim sandstone at effective pressures above 5 MPa can be roughly predicted by both the effective medium theory (Mori-Tanaka scheme) and the squirt-flow model. Static bulk moduli are found to be more sensitive to narrow cracks than dynamic bulk moduli for porous sandstones under dry and water-saturated conditions. KW - Porous sandstone KW - Static bulk modulus KW - Dynamic bulk modulus KW - Narrow KW - (compliant) cracks KW - Equant (stiff) pores Y1 - 2020 U6 - https://doi.org/10.1007/s00603-020-02248-3 SN - 0723-2632 SN - 1434-453X VL - 54 IS - 1 SP - 129 EP - 148 PB - Springer CY - Wien ER - TY - JOUR A1 - Nardini, Livia A1 - Rybacki, Erik A1 - Krause, Michael A1 - Morales, Luiz F.G. A1 - Dresen, Georg T1 - Control of the geometric arrangement of material heterogeneities on strain localization at the brittle-to-ductile transition in experimentally deformed carbonate rocks JF - Journal of Structural Geology N2 - Triaxial high temperature (900 °C) deformation experiments were conducted at constant strain rate in a Paterson-type deformation apparatus on cylinders of Carrara marble with two right or left stepping, non-overlapping weak inclusions of Solnhofen limestone, oriented at 45° to the cylinders’ longitudinal axes. Applying different values of confinement (30, 50, 100 and 300 MPa) we induced various amounts of brittle deformation in the marble matrix and investigated the effect of brittle precursors on the initiation and development of heterogeneity-induced high temperature shear zones. Viscosity contrast between the matrix and the inclusions induces local stress concentration at the tips of these latter. The initial arrangement of the inclusions results in either an overpressured (contractional) or underpressured (extensional) domain in the step-over region of the sample. At low confinement (30 and 50 MPa) abundant brittle deformation is observed, but the spatial distribution of microfractures is dependent on the kinematics of the step-over region: microcracks occur either along the shearing plane between inclusions (in extensional bridge samples), or broadly distributed outside the step-over region (contractional bridge samples). Accordingly, ductile deformation localizes along the inclusions plane in the extensional bridge samples as opposed to distributing over large areas of the matrix in the contractional bridge samples. If microcracking is suppressed (high confinement), strain is accommodated by viscous creep and strain progressively de-localizes in extensional bridge samples. Our experiments demonstrate that brittle precursors enhance the degree of localization in the ductile deformation regime, but only if the interaction of pre-existing heterogeneities induces an extensional mean stress regime in between. KW - High-temperature shear zones KW - Triaxial deformation KW - Brittle precursors KW - Strain localization Carrara marble Y1 - 2019 U6 - https://doi.org/10.1016/j.jsg.2020.104038 SN - 0191-8141 VL - 135 PB - Pergamon Press CY - Oxford ; Frankfurt, M. ER - TY - JOUR A1 - Wang, Lei A1 - Dresen, Georg A1 - Rybacki, Erik A1 - Bonnelye, Audrey A1 - Bohnhoff, Marco T1 - Pressure-dependent bulk compressibility of a porous granular material modeled by improved contact mechanics and micromechanical approaches BT - effects of surface roughness of grains JF - Acta materialia N2 - The change of the mechanical properties of granular materials with pressure is an important topic associated with many industrial applications. In this paper we investigate the influence of hydrostatic pressure (P-e) on the effective bulk compressibility (C-eff) of a granular material by applying two modified theoretical approaches that are based on contact mechanics and micromechanics, respectively. For a granular material composed of rough grains, an extended contact model is developed to elucidate the effect of roughness of grain surfaces on bulk compressibility. At relatively low pressures, the model predicts that the decrease of bulk compressibility with pressure may be described by a power law with an exponent of -1/2 (i.e., C-eff proportional to P-e(1/2) ), but deviates at intermediate pressures. At elevated pressures beyond full contact, bulk compressibility remains almost unchanged, which may be roughly evaluated by continuum contact mechanics. As an alternative explanation of pressure-dependent bulk compressibility, we suggest a micromechanical model that accounts for effects of different types of pore space present in granular materials. Narrow and compliant inter-granular cracks are approximated by three-dimensional oblate spheroidal cracks with rough surfaces, whereas the equant and stiff pores surrounded by three and four neighboring grains are modeled as tubular pores with cross sections of three and four cusp-like corners, respectively. In this model, bulk compressibility is strongly reduced with increasing pressure by progressive closure of rough-walled cracks. At pressures exceeding crack closure pressure, deformation of the remaining equant pores is largely insensitive to pressure, with almost no further change in bulk compressibility. To validate these models, we performed hydrostatic compression tests on Bentheim sandstone (a granular rock consisting of quartz with high porosity) under a wide range of pressure. The relation between observed microstructures and measured pressure-dependent bulk compressibility is well explained by both suggested models. KW - Bulk compressibility KW - Granular materials KW - Roughness KW - Micromechanical model KW - Contact model Y1 - 2020 U6 - https://doi.org/10.1016/j.actamat.2020.01.063 SN - 1359-6454 SN - 1873-2453 VL - 188 SP - 259 EP - 272 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Wang, Lei A1 - Kwiatek, Grzegorz A1 - Rybacki, Erik A1 - Bonnelye, Audrey A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Laboratory study on fluid-induced fault slip behavior: the role of fluid pressurization rate JF - Geophysical research letters : GRL N2 - Understanding the physical mechanisms governing fluid-induced fault slip is important for improved mitigation of seismic risks associated with large-scale fluid injection. We conducted fluid-induced fault slip experiments in the laboratory on critically stressed saw-cut sandstone samples with high permeability using different fluid pressurization rates. Our experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Slow stick-slip episodes (peak slip velocity < 4 mu m/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity < 0.4 mu m/s mainly occurs in response to slow fluid injection rate. Fluid-induced fault slip may remain mechanically stable for loading stiffness larger than fault stiffness. Independent of fault slip mode, we observed dynamic frictional weakening of the artificial fault at elevated pore pressure. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field-scale fluid injection projects.
Plain Language Summary Human-induced earthquakes from field-scale fluid injection projects including enhanced geothermal system and deep wastewater injection have been documented worldwide. Although it is clear that fluid pressure plays a crucial role in triggering fault slip, the physical mechanism behind induced seismicity still remains poorly understood. We performed laboratory tests, and here we present two fluid-induced slip experiments conducted on permeable Bentheim sandstone samples crosscut by a fault that is critically stressed. Fault slip is then triggered by pumping the water from the bottom end of the sample at different fluid injection rates. Our results show that fault slip is controlled by fluid pressure increase rate rather than by the absolute magnitude of fluid pressure. In contrast to episodes of relatively rapid but stable sliding events caused by a fast fluid injection rate, fault creep is observed during slow fluid injection. Strong weakening of the dynamic friction coefficient of the experimental fault is observed at elevated pore pressure, independent of fault slip mode. These results may provide a better understanding of the complex behavior of fluid-induced fault slip on the field scale. KW - fault slip KW - fluid injection KW - induced seismicity KW - fluid pressurization KW - rate KW - stick-slip KW - fault creep Y1 - 2020 U6 - https://doi.org/10.1029/2019GL086627 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 6 PB - Wiley CY - Hoboken, NJ ER - TY - JOUR A1 - Nardini, Livia A1 - Rybacki, Erik A1 - Döhmann, Maximilian J.E.A. A1 - Morales, Luiz F.G. A1 - Brune, Sascha A1 - Dresen, Georg T1 - High-temperature shear zone formation in Carrara marble BT - The effect of loading conditions JF - Tectonophysics N2 - Rock deformation at depths in the Earth’s crust is often localized in high temperature shear zones occurring at different scales in a variety of lithologies. The presence of material heterogeneities is known to trigger shear zone development, but the mechanisms controlling initiation and evolution of localization are not fully understood. To investigate the effect of loading conditions on shear zone nucleation along heterogeneities, we performed torsion experiments under constant twist rate (CTR) and constant torque (CT) conditions in a Paterson-type deformation apparatus. The sample assemblage consisted of cylindrical Carrara marble specimens containing a thin plate of Solnhofen limestone perpendicular to the cylinder’s longitudinal axis. Under experimental conditions (900 °C, 400 MPa confining pressure), samples were plastically deformed and limestone is about 9 times weaker than marble, acting as a weak inclusion in a strong matrix. CTR experiments were performed at maximum bulk shear strain rates of ~ 2*10-4s-1, yielding peak shear stresses of ~ 20 MPa. CT tests were conducted at shear stresses of ~ 20 MPa resulting in bulk shear strain rates of 1-4*10-4s-1. Experiments were terminated at maximum bulk shear strains of ~ 0.3 and 1.0.Strain was localized within the Carrara marble in front of the inclusion in an area of strongly deformed grains and intense grain size reduction. Locally, evidences for coexisting brittle deformation are also observed regardless of the imposed loading conditions. The local shear strain at the inclusion tipis up to 30 times higher than the strain in the adjacent host rock, rapidly dropping to 5times higher at larger distance from the inclusion. At both investigated bulk strains, the evolution of microstructural and textural parameters is independent of loading conditions. Ourresults suggest that loading conditions do not significantly affect material heterogeneity-induced strain localization during its nucleation and transient stages. KW - Shear zones KW - localization KW - marble KW - torsion KW - loading conditions Y1 - 2018 U6 - https://doi.org/10.1016/j.tecto.2018.10.022 SN - 0040-1951 VL - 749 SP - 120 EP - 139 PB - Elsevier CY - Amsterdam [u.a.] ER - TY - JOUR A1 - Wang, Lei A1 - Kwiatek, Grzegorz A1 - Rybacki, Erik A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Injection-induced seismic moment release and laboratory fault slip BT - implications for fluid-induced seismicity JF - Geophysical research letters N2 - Understanding the relation between injection-induced seismic moment release and operational parameters is crucial for early identification of possible seismic hazards associated with fluid-injection projects. We conducted laboratory fluid-injection experiments on permeable sandstone samples containing a critically stressed fault at different fluid pressurization rates. The observed fluid-induced fault deformation is dominantly aseismic. Fluid-induced stick-slip and fault creep reveal that total seismic moment release of acoustic emission (AE) events is related to total injected volume, independent of respective fault slip behavior. Seismic moment release rate of AE scales with measured fault slip velocity. For injection-induced fault slip in a homogeneous pressurized region, released moment shows a linear scaling with injected volume for stable slip (steady slip and fault creep), while we find a cubic relation for dynamic slip. Our results highlight that monitoring evolution of seismic moment release with injected volume in some cases may assist in discriminating between stable slip and unstable runaway ruptures. KW - induced seismicity KW - seismic moment release KW - fluid injection KW - stick slip KW - fault creep KW - acoustic emission Y1 - 2020 U6 - https://doi.org/10.1029/2020GL089576 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 22 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Döhmann, Maximilian J.E.A. A1 - Brune, Sascha A1 - Nardini, Livia A1 - Rybacki, Erik A1 - Dresen, Georg T1 - Strain Localization and Weakening Processes in Viscously Deforming Rocks BT - Numerical Modeling Based on Laboratory Torsion Experiments JF - Journal of geophysical research : JGR N2 - Localization processes in the viscous lower crust generate ductile shear zones over a broad range of scales affecting long‐term lithosphere deformation and the mechanical response of faults during the seismic cycle. Here we use centimeter‐scale numerical models in order to gain detailed insight into the processes involved in strain localization and rheological weakening in viscously deforming rocks. Our 2‐D Cartesian models are benchmarked to high‐temperature and high‐pressure torsion experiments on Carrara marble samples containing a single weak Solnhofen limestone inclusion. The models successfully reproduce bulk stress‐strain transients and final strain distributions observed in the experiments by applying a simple, first‐order softening law that mimics rheological weakening. We find that local stress concentrations forming at the inclusion tips initiate strain localization inside the host matrix. At the tip of the propagating shear zone, weakening occurs within a process zone, which expands with time from the inclusion tips toward the matrix. Rheological weakening is a precondition for shear zone localization, and the width of this shear zone is found to be controlled by the degree of softening. Introducing a second softening step at elevated strain, a high strain layer develops inside the localized shear zone, analogous to the formation of ultramylonite bands in mylonites. These results elucidate the transient evolution of stress and strain rate during inception and maturation of ductile shear zones. KW - dislocation creep KW - torsion KW - 2-D numerical model KW - rheological weakening KW - two phase aggregates KW - strain localization Y1 - 2018 U6 - https://doi.org/10.1029/2018JB016917 SN - 0148-0227 SN - 2169-9356 VL - 124 IS - 1 SP - 1120 EP - 1137 PB - Union CY - Washington, DC ER - TY - JOUR A1 - Schuster, Valerian A1 - Rybacki, Erik A1 - Bonnelye, Audrey A1 - Herrmann, Johannes A1 - Schleicher, Anja Maria A1 - Dresen, Georg T1 - Experimental deformation of opalinus clay at elevated temperature and pressure conditions BT - Mechanical properties and the influence of rock fabric JF - Rock mechanics and rock engineering N2 - The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (p(c)) of 50-100 MPa, temperatures (T) between 25 and 200 degrees C and strain rates (epsilon) (over dot ) of 1 x-10(-3)-5 x-10(-6) -s(-1). Using a Paterson-type deformation apparatus, samples oriented at 0 degrees, 45 degrees and 90 degrees to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed p(c)-T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45 degrees and 90 degrees to bedding at p(c) = 100 MPa, strain is localized in discrete shear zones. Compressive strength (sigma(max)) increases with increasing pc, resulting in an internal friction coefficient of approximate to 0.31 for samples deformed at 45 degrees and 90 degrees to bedding, and approximate to 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and epsilon(over dot )do not significantly affect deformation behavior of dried samples. However, sigma(max) and Young's modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength sigma(max) and Young's modulus E at similar deformation conditions. Strength and Young's modulus of samples deformed 90 degrees and 45 degrees to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior. KW - Clay rock KW - Sandy facies of Opalinus Clay KW - Triaxial deformation experiments KW - Microstructural deformation mechanisms KW - Pressure-temperature and strain rate-dependent mechanical behaviour KW - Anisotropy Y1 - 2021 U6 - https://doi.org/10.1007/s00603-021-02474-3 SN - 0723-2632 SN - 1434-453X VL - 54 SP - 4009 EP - 4039 PB - Springer CY - Wien ER -