ALBERT

Description: 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.

Description: 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 μm/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity 〈 0.4 μm/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.

Description: To understand the physical mechanisms governing fluid-induced seismicity at field-scale fluid injection projects, 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. The data archived here acts as supplementary material to Wang et al. (2020; https://doi.org/10.1029/2019GL086627). Experiments were conducted at room temperature using a servo-hydraulic tri-axial deformation apparatus (MTS) equipped with a pore pressure system (Quizix pumps) at Experimental Rock Deformation Laboratory, GFZ. To investigate the correlation between fault slip and fluid pressure, we applied two different fluid injection schemes (hereafter tests “SC1” and “SC2”, respectively). ‘TestSC1’ refers to the fluid-induced fault slip experiment performed at fluid pressurization rate of 2 MPa/min while ‘TestSC2’ indicates the fluid-induced fault slip experiment performed at fluid pressurization rate of 0.5 MPa/min. The other boundary conditions for both experiments are similar. In addition, to simultaneously record acoustic emission (AE) events induced by artificial fault slip, 16 piezoelectric transducers (PZTs, resonance frequency ~1 MHz) contained in brass cases were directly mounted to the surface of samples, ensuring full azimuthal coverage for AE events. AE waveforms were amplified first by 40 dB using preamplifiers equipped with 100‐kHz high‐pass filters and then recorded at a sampling rate of 10 MHz with 16‐bit amplitude resolution. Each experiment lasted for about 4 hours. Throughout the experiment, mechanical data (measured by MTS) and hydraulic data (measured by Quizix pump) were all synchronously monitored with a sampling rate of 10 Hz whereas acoustic emission data were recorded with a sampling rate of 10 MHz. All results shown are recorded as a function of experimental time. The data are provided in tab-separated ASCII-Format (.txt). 2020-002_Wang-et-al_TestSC1.zip and 2020-002_Wang-et-al_TestSC2.zip are composed of 7 txt files and 8 txt files, respectively, as described below in Table 1. The first column represents time in second and the subsequent columns are indicated by the corresponding header at the first row. The second row indicates the unit for each column data. The raw data was processed with MATLAB. The algorithms we implemented include the moving average method, statistical regression and our developed MATLAB-based codes.

Description: 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 (Pe) on the effective bulk compressibility (Ceff) 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., Ceff ∝Pe−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.

Description: 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 (Ks) were estimated from stress–volumetric strain curves while dynamic bulk moduli (Kd) were derived from the changes in ultrasonic P- and S- wave velocities (~ 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 (Kd/Ks) 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 that Kd/Ks is positively correlated with the amount of narrow cracks. Above the crack closure pressure, where equant (stiff) pores dominate the void space, Kd/Ks 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.

Description: The temporal evolution of gouge compaction determines fluid transfer and rock rupture dynamics. Thus, studies on the time‐dependent creep compaction processes of shale materials may elucidate the chemo‐mechanical behavior of shallow clay‐rich zones. We investigated this problem by combining creep experiments conducted in triaxial compression under upper crustal conditions with modeled pressure solution processes in Tournemire shale. The shale samples were deformed parallel and perpendicular to the bedding at low (10 MPa, 26°C, this study) and high (80 MPa, 26°C, published by Geng et al., 2018, https://doi.org/10.1029/2018JB016169) pressures. We monitored the deformation during stepping creep experiments until sample failure. Our results differ from those of traditional creep experiments and show that the creep failure strength of Tournemire shale samples increased significantly (by ∼64%) at both pressures. Our experiments suggest that at appropriate temperatures, the pressure solution is highly active and is the dominant temporal sealing mechanism in the shale. Using our experimental data and the statistical rock physics method, we modeled the temporal reduction of effective porosity in terms of depth and temperature. Our thermal‐stress coupled modeling results suggest that the pressure solution induced sealing is the most active at middle‐shallow depths (∼3.8 km). We believe that the sealing capacity and creep failure strength of dolomite‐rich shales may change significantly at middle‐shallow depths, indicating an important influence on reservoir fluids transfer and fault gouge strength.