Strain Partitioning and Frictional Behavior of Opalinus Clay During Fault Reactivation

The Opalinus Clay (OPA) formation is considered a suitable host rock candidate for nuclear waste storage. However, the sealing integrity and long-term safety of OPA are potentially compromised by pre-existing natural or artificially induced faults. Therefore, characterizing the mechanical behavior and microscale deformation mechanisms of faults and the surrounding rock is relevant for predicting repository damage evolution. In this study, we performed triaxial tests using saw-cut samples of the shaly and sandy facies of OPA to investigate the influence of pressure and mineral composition on the deformation behavior during fault reactivation. Dried samples were hydrostatically pre-compacted at 50 MPa and then deformed at constant strain rate, drained conditions and confining pressures (pc) of 5–35 MPa. Mechanical data from triaxial tests was complemented by local strain measurements to determine the relative contribution of bulk deformation and fault slip, as well as by acoustic emission (AE) monitoring, and elastic P-wave velocity measurements using ultrasonic transmissions. With increasing pc, we observe a transition from brittle deformation behavior with highly localized fault slip to semi-brittle behavior characterized by non-linear strain hardening with increasing delocalization of deformation. We find that brittle localization behavior is limited by pc at which fault strength exceeds matrix yield strength. AEs were only detected in tests performed on sandy facies samples, and activity decreased with increasing pc. Microstructural analysis of deformed samples revealed a positive correlation between increasing pc and gouge layer thickness. This goes along with a change from brittle fragmentation and frictional sliding to the development of shear zones with a higher contribution of cataclastic and granular flow. Friction coefficient at fault reactivation is only slightly higher for the sandy (µ ~ 0.48) compared to the shaly facies (µ ~ 0.4). Slide-hold-slide tests performed after ~ 6 mm axial shortening suggest stable creeping and long-term weakness of faults at the applied conditions. Our results demonstrate that the mode of fault reactivation highly depends on the present stress field and burial history

Authors’ Reply to the Discussion by Crisci et al. (2021) on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions Mechanical Properties and the Influence of Rock Fabric”

This is a repond to the comments raised in Crisci´s et al. paper “Discussion on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions ¬ Mechanical Properties and the Influence of Rock Fabric” (2021). We are pleased to use the opportunity to clarify issues related to testing procedures and interpretation in more detail.Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347Helmholtz-Gemeinschaft http://dx.doi.org/10.13039/501100001656Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217

Authors’ Reply to the Discussion by Crisci et al. (2021) on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions Mechanical Properties and the Influence of Rock Fabric”

Introduction!#!Neck dissection (ND) is a surgical procedure addressing cervical lymph nodes and metastases in patients with oral squamous cell carcinoma (OSCC). The aim of this study was to analyze clinical decisions regarding indications and variations of ND in Oral and Maxillofacial Surgery (OMFS) in Germany.!##!Material and methods!#!A nationwide survey of the German Association of Oral and Maxillofacial Surgery was performed using dynamic online questionnaires including 38 questions. Data about oncological centers, case numbers, and staging procedures were collected. Relevant aspects, such as inclusion of level IIb and levels IV and V to ND, uni- vs. bilateral ND, and the influence of extra-nodal extension (ENE) of metastases on extension of ND were evaluated.!##!Results!#!Eighty-four OMFS of university and non-university hospitals participated in the study (responding rate 21.4%). Sixty-six (78.57%) stated to work at certified cancer centers and 53.57% of the hospitals treated between 50 and 100 OSCC cases per year. CT and/or MRI of the head and neck was performed in most of the staging procedures. Level IIb was included by 71 (93.42%) of the participants in selective ND. Levels IV and V were included by 53 (69.74%) in node-positive neck. In solitary ipsilateral metastases (ENE-), 49 participants (62.82%) stated to perform exclusively an ipsilateral ND and 40 (51.95%) stated to perform only an ipsilateral ND in ENE+.!##!Conclusion!#!This study demonstrated a high rate of certified cancer centers in Germany showing differences regarding staging procedures, indications, and extension of ND, especially in increasingly complex cases.!##!Clinical relevance!#!Clinical decisions regarding ND are dependent on case-individual aspects and must be decided individually

Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)

We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures, pc, at up to 50 MPa, temperatures, T, between 25 and 100 °C, and differential stress, σ, acting perpendicular to the fracture surface of up to 45 MPa. Fracture transmissivity decreases non-linearly and irreversibly by about an order of magnitude with increasing confining pressure and differential stress, with a slightly stronger influence of pc than of σ. Increasing temperature reduces fracture transmissivity only at high confining pressures when the fracture aperture is already low. An increase in the fracture surface roughness by about three times yields an initial fracture transmissivity of almost one order of magnitude higher. Fractures with similar surface roughness display the highest initial transmissivity within slate, graywacke, quartzite and granite samples, whereas the transmissivity in claystone and granitic gouge material is up to several orders of magnitude lower. The reduction in transmissivity with increasing stress at room temperature varies with composition and uniaxial strength, where the deduction is lowest for rocks with a high fraction of strong minerals and associated high brittleness and strength. Microstructural investigations suggest that the reduction is induced by the compaction of the matrix and crushing of strong asperities. Our results suggest that for a given surface roughness, the fracture transmissivity of slate as an example of a target reservoir for unconventional EGS, is comparable to that of other hard rocks, e.g., granite, whereas highly altered and/or clay-bearing rocks display poor potential for extracting geothermal energy from discrete fractures

Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)

We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures, pc, at up to 50 MPa, temperatures, T, between 25 and 100 °C, and differential stress, σ, acting perpendicular to the fracture surface of up to 45 MPa. Fracture transmissivity decreases non-linearly and irreversibly by about an order of magnitude with increasing confining pressure and differential stress, with a slightly stronger influence of pc than of σ. Increasing temperature reduces fracture transmissivity only at high confining pressures when the fracture aperture is already low. An increase in the fracture surface roughness by about three times yields an initial fracture transmissivity of almost one order of magnitude higher. Fractures with similar surface roughness display the highest initial transmissivity within slate, graywacke, quartzite and granite samples, whereas the transmissivity in claystone and granitic gouge material is up to several orders of magnitude lower. The reduction in transmissivity with increasing stress at room temperature varies with composition and uniaxial strength, where the deduction is lowest for rocks with a high fraction of strong minerals and associated high brittleness and strength. Microstructural investigations suggest that the reduction is induced by the compaction of the matrix and crushing of strong asperities. Our results suggest that for a given surface roughness, the fracture transmissivity of slate as an example of a target reservoir for unconventional EGS, is comparable to that of other hard rocks, e.g., granite, whereas highly altered and/or clay-bearing rocks display poor potential for extracting geothermal energy from discrete fractures

Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric

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 (pc) of 50–100 MPa, temperatures (T) between 25 and 200 °C and strain rates (ε˙) of 1 × 10–3–5 × 10–6 s−1. Using a Paterson-type deformation apparatus, samples oriented at 0°, 45° and 90° 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 pc–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° and 90° to bedding at pc = 100 MPa, strain is localized in discrete shear zones. Compressive strength (σmax) increases with increasing pc, resulting in an internal friction coefficient of ≈ 0.31 for samples deformed at 45° and 90° to bedding, and ≈ 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and ε˙ do not significantly affect deformation behavior of dried samples. However, σmax 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 σmax and Young’s modulus E at similar deformation conditions. Strength and Young’s modulus of samples deformed 90° and 45° to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347Helmholtz-Gemeinschaft http://dx.doi.org/10.13039/501100001656Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217