Shihuai Zhang

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Zhang

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Shihuai

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0000-0003-2725-5567

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Publications 1 - 10 of 13

Global Frictional Equilibrium via Stochastic, Local Coulomb Frictional Slips
Zhang, Shihuai; Ma, Xiaodong (2020)
ESS Open Archive
Natural variability of fault friction and slip uncertainty exist in the Earth’s crust. To what extent it influences crustal stress and its evolution is intriguing. We established a quasi-static, 2D model to simulate the stress evolution due to Coulomb frictional slips in the brittle crust. The model simply features randomly-oriented fractures with heterogeneous frictional coefficients. We emphasized the global stress response by summing the contribution of cascades of local frictional slip under specific boundary conditions. We illustrated that the decrease in stress difference manifests as a self-organized process that ultimately leads to frictional equilibrium. The model informs that the frictional equilibrium of a stochastic system can depart substantially from a deterministic estimation. Although the model quantitatively corroborates the notion of frictional equilibrium in places where fracture slip is the dominant mechanism for stress release, it reveals far more profound influence of system heterogeneity on the local and global stress evolution.
Fault slip potential induced by fluid injection in the Matouying enhanced geothermal system (EGS) field, Tangshan seismic region, North China
Feng, Chengjun; Gao, Guangliang; Zhang, Shihuai; et al. (2022)
Natural Hazards and Earth System Sciences
The Tangshan region is one of the most seismically active areas in the North China, and the 1976 M 7.8 earthquake occurred on 28 July near the Tangshan fault zone. The Matouying enhanced geothermal system (EGS) field is located ∼90 km away from the city of Tangshan. Since late 2020, preliminary hydraulic stimulation tests have been conducted at depths of ∼3965-4000 m. Fluid injection into geothermal reservoir facilitates a heat exchanger system. However, fluid injection may also induce earthquakes. In anticipation of the EGS operation at the Matouying uplift, it is essential to assess how the fault slip potential of the nearby active and quiescent faults will change in the presence of fluid injection. In this study, we first characterize the ambient stress field in the Tangshan region by performing stress tensor inversions using 98 focal-mechanism data (ML≥2.5). Then, we estimate the principal stress magnitudes near the Matouying EGS field by analyzing in situ stress measurements at shallow depths (∼600-1000 m). According to these data, we perform a quantitative risk assessment using the Mohr-Coulomb framework in order to evaluate how the main active faults might respond to hypothetical injected-related pore pressure increases due to the upcoming EGS production. Our results mainly show that most earthquakes in the Tangshan seismic region have occurred on the faults that have relatively high fault slip potential in the present ambient stress field. At well distances of less than 15 km, the probabilistic fault slip potential on most of the boundary faults increases with continuing fluid injection over time, especially on the faults with well distances of ∼6-10 km. The probabilistic fault slip potential (fsp) increases linearly with the fluid injection rate. However, the fsp values decrease exponentially with increased unit permeability. The case study of the Matouying EGS field has important implications for deep geothermal exploitation in China, especially for Gonghe EGS (in Qinghai Province) and Xiong'an New Area (in Hebei Province) geothermal reservoirs that are close to the Quaternary active faults. Ongoing injection operations in the regions should be conducted with these understandings in mind.
How does in situ stress rotate within a fault zone? Insights from explicit modeling of the frictional, fractured rock mass
Zhang, Shihuai; Ma, Xiaodong (2021)
We quantitatively investigate the spatial stress variations within fault zones by explicitly incorporating macroscopic fracture networks in a multilayer fault zone model. Based on elastic crack theory, we first derive a unified constitutive relationship for frictional fractures, featuring elastic and plastic shear deformation and shear-induced normal dilatancy. To honor the progressively accumulated damage across a fault zone, we establish a fractured multilayer model including randomly-oriented frictional fractures with varying densities from layer to layer. Under the specific boundary conditions of a fault zone, the global mechanical response of each layer is quantitatively related to the deformation of the interior fractures. Stress variations and effective elastic property changes are systematically studied considering the influences of fracture properties and pore pressure. We show that the major principal stress always rotates toward a limiting angle of 45 with respect to the fault slip direction and that differential stress invariantly decreases with the fracture density. However, mean stress increases for an unfavorably-oriented fault and decreases when the regional major principal stress trends at a small angle (< 45°) to the fault slip direction. Accumulated damage also results in a decrease and increase in the effective Young’s modulus and Poisson’s ratio, respectively. The influences of fracture properties, pore pressure and fracture network can be attributed to their control on the fracture deformation components and relative proportion. Our model can predict continuous variations of stresses and effective elastic properties from intact country rock, through fractured damage zone, to the plastic fault core of a mature fault.
Acoustic Emissions and Seismic Tomography of Sandstone Under Uniaxial Compression: Implications for the Progressive Failure in Pillars
Zhang, Guang; Zhang, Shihuai; Guo, Pei; et al. (2023)
Rock Mechanics and Rock Engineering
This work presents the spatio-temporal variations of microcracking and damage accumulation during the progressive failure in a low-porosity sandstone under uniaxial compression, using active ultrasonic surveys and passive acoustic emission monitoring. The combination of both techniques allows us to perform a joint inversion of source locations and P-wave velocity tomography of the specimen during the failure process. Based on the P-wave first arrivals from both the active surveys and numerous passive AE events, three-dimensional (3D) P-wave velocity structure and its temporal variations can be obtained in high resolution in spite of the limited active ray coverage. The reconstructed P-wave velocity structure indicates that the specimen becomes more heterogeneous due to damage accumulation. Within such a highly anisotropic medium, the ray paths are found to be curved, rather than straight as assumed in a homogeneous material. The curved ray paths are used to calculate the travel times of each AE event so as to obtain the accurate source location. We calculate the energy of the detected AE event and estimate the energy distribution. It shows that about 50% of the events account only for about 2.5% of the total energy while 1% of the events release 47.3% of the total energy. The events with large energy, albeit few, allow to trace the fractures. We also investigate the variations of the P-wave velocities along different characteristic ray paths during the failure process. Based on the joint inversion, the variations of the 3D P-wave velocity structure with deformation have been further investigated. It is found that the emergence of the low-velocity zone (LVZ) does not indicate the onset of fractures and that the LVZ is highly unpredictable in a spatio-temporal sense. The spatial correlation between the LVZ and AE events suggests that AE events usually appear on the edge of the LVZs, where there are high-velocity contrasts. This work suggests that the quantification of P-wave velocity changes has the potential for extracting timely precursory information of pillar bursts.
Time-dependent deformation of Wufeng-Longmaxi shale and its implications on the in situ state of stress
Cheng, Wei; Zhang, Xiaowen; Jin, Juan; et al. (2023)
Frontiers in Earth Science
The stress-strain relationship in shales is generally time-dependent. This concerns their long-term deformation in unconventional reservoirs, and its influence on the in situ stress state therein. This paper presents an experimental investigation on the time-dependent deformation of the Longmaxi shale gas shale. A series of creep experiments subject the shale samples to long-term, multi-step triaxial compression. It is found that the shale samples exhibit varying degrees of time-dependent deformation, which can be adequately described by a power-law function of time. The experimental results establish the relationship between the elastic Young’s modulus and viscoplastic constitutive parameters, which are different from previous those derived from North American shales. Based on this viscoplastic constitutive model, the stress relaxation and the differential stress accumulation over geologic time scales can be estimated. It is found that linear elasticity substantially overestimates the differential stress accumulation predicted in the context of viscoplastic relaxation. The characterized viscoplasticity and stress relaxation are of vital importance for various geomechanical problems in shale reservoirs.
Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes
Zhang, Shihuai; Ma, Xiaodong; Bröker, Kai Erich Norbert; et al. (2023)
Journal of Geophysical Research: Solid Earth
The in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory in the Swiss Alps, the breakouts observed in an array of eight inclined boreholes penetrating a fault zone offer a unique opportunity to characterize the fault-associated spatial stress variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these boreholes are identified, revealing a hierarchy of a major fault zone along with secondary structures. Within the boreholes, breakout rotations occur over multiple scales, spanning individual fractures and the entire major fault zone. We first estimate and rule out the effect of the fracture-induced anisotropy on the breakout rotations, which are attributed mainly to the stress variations. Based on the stress field around a circular borehole and Mohr-Coulomb failure criterion, the observed breakout azimuths are used to invert the stress information. Results show that the stress field outside the fault zone features a stress ratio (quantifying the relative stress magnitude) of about 0.9, an inclined overburden stress (inclination: 12°∼18°), and a maximum horizontal principal stress (SHmax) oriented N100∼120°E. Within the fault zone, a substantial reduction of the stress ratio and complicated stress rotations are constrained, likely induced by the stress drop on local fractures. As a result, less critical stress state inside the major fault zone is expected. Our work provides a semi-quantitative estimation of the in-situ stress variations around fault zones in the absence of direct stress measurements, which is beneficial to a number of scientific and engineering applications
Constraining the stress field and its variability at the BedrettoLab: Elaborated hydraulic fracture trace analysis
Bröker, Kai Erich Norbert; Ma, Xiaodong; Zhang, Shihuai; et al. (2024)
International Journal of Rock Mechanics and Mining Sciences
The measurement of the stress field and its natural heterogeneity is a common challenge in the field of geophysics and underground engineering. This study presents findings from an extensive mini-frac campaign conducted at the Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Six dedicated stress measurement boreholes, spanning 500 m along the Bedretto Tunnel, were utilized to prepare for meso-scale hydraulic stimulation experiments in a granitic rock volume. A detailed analysis of the induced hydraulic fracture trace morphology was carried out using acoustic televiewer logging data, with an interpretation based on the local geological context. The results show that the far field orientation of the maximum horizontal stress (S_Hmax) is on average N112°E and the overburden stress can be assumed to be a principal stress direction. However, the en-echelon shape of the observed hydraulic fracture traces suggests that there is a slight inclination of the greatest principal stress away from the vertical stress direction. The mini-frac test intervals along boreholes inside the enlarged niche of the BedrettoLab indicate a counterclockwise rotated horizontal stress by up to 37° and potentially stronger inclination of the greatest principal stress from the vertical stress direction. The most likely causes of this stress field rotation, along with a considerably larger scatter in stress magnitudes, appear to be the larger laboratory niche dimensions compared to the tunnel and the presence of natural fractures that are preferentially oriented for slip in the far field stress state. Although the uncertainties and variability of the stress field around the BedrettoLab are not yet fully understood, our results are useful for strategic planning and design of meso-scale stimulation experiments in the rock volume. Furthermore, our detailed analysis procedure contributes to advancing the understanding of mini-frac tests in similar geological settings.
Strength and deformability of a low-porosity sandstone under true triaxial compression conditions
Zhang, Shihuai; Wu, Shunchuan; Zhang, Guang (2020)
International Journal of Rock Mechanics and Mining Sciences
A case study of estimating in situ stress at the Bedretto Underground Laboratory, Switzerland
Bröker, Kai Erich Norbert; Ma, Xiaodong; Zhang, Shihuai; et al. (2021)
P-wave velocity tomography and acoustic emission characteristics of sandstone under uniaxial compression
Zhang, Guang; Wu, Shun-chuan; Zhang, Shihuai; et al. (2023)
Rock and Soil Mechanics = 岩土力学 = Yan tu li xue
Pillar burst is one of the most frequent dynamic disasters in deep mining, which poses a serious threat to safe and efficient mining. In this study, the failure mechanisms and precursors of pillar burst are investigated by active ultrasonic survey and passive acoustic emission (AE) monitoring in uniaxial compression tests on Zigong red sandstone. Combining active and passive AE monitoring data, a P-wave velocity tomography inversion is performed to analyse the temporal and spatial variations of P-wave velocity structure during the sample failure. Results show that the velocity structure of the sandstone sample is highly heterogeneous during loading, and a low-velocity zone emerges, within which most of the acoustic emission events are present. The dispersion of P-wave velocity reflects the global variations of P-wave velocity. It changes drastically during the peak stage, and increases with the ongoing loading. The AE events differ significantly between the pre-peak and post-peak stages. In the pre-peak stage, AE events are randomly distributed in the sample, while in the post-peak stage, clustered AE events are identified. In addition, it is found that using the homogeneous velocity structure for AE events location results in a higher positioning error. The decreasing b value before the eventual failure of the sample indicates that large-scale crack activities are intensified, leading to the increase of sample heterogeneity, which also proves the necessity of applying the heterogeneous velocity structure for AE events location. The research results can be further used for on-site pillar stability monitoring, and the periodic P-wave velocity tomography provides precursors for pillar bursts.

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