TY - JOUR A1 - Schuster, Valerian A1 - Rybacki, Erik A1 - Bonnelye, Audrey A1 - Dresen, Georg T1 - Authors’ Reply to the Discussion by Crisci et al. (2021): 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 KW - opalinus clay KW - testing procedure KW - pore pressure generation KW - strain rate KW - drying-induced micro cracks Y1 - 2021 U6 - https://doi.org/10.1007/s00603-021-02675-w SN - 0723-2632 SN - 1434-453X VL - 55 SP - 467 EP - 469 PB - Springer CY - Wien 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 - 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 -