| 1. |
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| 2. |
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| 3. |
- Bai, Jiahe, et al.
(författare)
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Investigating slurry infiltration in a coral reef limestone based on X-ray computed tomography and CFD-DEM numerical simulation
- 2024
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Ingår i: Powder Technology. - : Elsevier. - 0032-5910 .- 1873-328X. ; 443
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Tidskriftsartikel (refereegranskat)abstract
- Coral reef limestone is a highly fossiliferous and porous sedimentary rock, ubiquitous in continental shelves and deep oceanic zones. To understand the slurry infiltration process from a microscopic perspective, we use X-ray computed tomography (CT) to reconstruct the complex three-dimensional pore structures of a coral reef limestone sample and further perform CFD-DEM simulations to capture the transport behavior of slurry particles passing through the reef limestone skeleton. The simulation results show that the kinetic energy of slurry particles in general follows a linear relationship with the applied pressure differential, which is influenced by the physical characteristics of pore channels. The particle infiltration time is inversely correlated to the pressure differential obeying a power-law function. In addition, statistical analysis of the length and spatial distribution of particle trajectories indicates the dominant role of high-coordination-number pore spaces in slurry suspension infiltration through the reef limestone. Our results have important implications for slurry shield tunneling through coral reef limestone strata.
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| 4. |
- Brixel, B., et al.
(författare)
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Tracking Fluid Flow in Shallow Crustal Fault Zones : 1. Department of Earth Sciences, ETH Zürich, Zürich, Switzerland
- 2020
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Ingår i: Journal of Geophysical Research - Solid Earth. - : American Geophysical Union (AGU). - 2169-9313 .- 2169-9356. ; 125:4
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Tidskriftsartikel (refereegranskat)abstract
- New in situ measurements to constrain the range, distribution, and spatial (meter-scale) variations of permeability in shallow crustal fault zones are reported based on systematic downhole tests at 0.5-km depth in crystalline rock. Single and cross-hole hydraulic packer tests were performed at a new dedicated test facility hosted in the Grimsel Test Site, in the Swiss Alps, following the technical instrumentation and isolation of discrete fault zones accessed by an array of boreholes. Single-hole test results are presented in this paper, while cross-hole experiments are reported in the companion paper. Our results reveal a sharp spatial falloff in permeability, from 10(-13) to 10(-21) m(2), with off-fault distances of 1-5 m and characterized by a power-law relation with fracture density. Fractures linking subparallel faults were detected as high-permeability discrete spots several meters away from off-fault damage. Due to the narrow (centimeter-wide) thickness of fault cores, the hydraulic tests presented in this study do not characterize the permeability of fault core materials. The transmissivity of single fractures spans six orders of magnitude (10(-12) to 10(-6) m(2)/s) and is systematically higher in damage zones. In situ stresses appear to have a minor effect on natural, present-day fracture transmissivity at the borehole scale. We suggest that the geometrical and topological properties of fracture systems instead tend to control the permeability of the shallow crustal faults studied.
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| 5. |
- Cao, Wenzhuo, et al.
(författare)
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Influence of Landscape Coverage on Measuring Spatial and Length Properties of Rock Fracture Networks : Insights from Numerical Simulation
- 2018
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Ingår i: Pure and Applied Geophysics. - : Springer. - 0033-4553 .- 1420-9136. ; 175:6, s. 2167-2179
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Tidskriftsartikel (refereegranskat)abstract
- Natural fractures are ubiquitous in the Earth’s crust and often deeply buried in the subsurface. Due to the difficulty in accessing to their three-dimensional structures, the study of fracture network geometry is usually achieved by sampling two-dimensional (2D) exposures at the Earth’s surface through outcrop mapping or aerial photograph techniques. However, the measurement results can be considerably affected by the coverage of forests and other plant species over the exposed fracture patterns. We quantitatively study such effects using numerical simulation. We consider the scenario of nominally isotropic natural fracture systems and represent them using 2D discrete fracture network models governed by fractal and length scaling parameters. The groundcover is modelled as random patches superimposing onto the 2D fracture patterns. The effects of localisation and total coverage of landscape patches are further investigated. The fractal dimension and length exponent of the covered fracture networks are measured and compared with those of the original non-covered patterns. The results show that the measured length exponent increases with the reduced localisation and increased coverage of landscape patches, which is more evident for networks dominated by very large fractures (i.e. small underlying length exponent). However, the landscape coverage seems to have a minor impact on the fractal dimension measurement. The research findings of this paper have important implications for field survey and statistical analysis of geological systems.
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| 6. |
- Cao, Wenzhuo, et al.
(författare)
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Stress-Dependent Deformation and Permeability of a Fractured Coal Subject to Excavation-Related Loading Paths
- 2021
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Ingår i: Rock Mechanics and Rock Engineering. - : Springer. - 0723-2632 .- 1434-453X. ; 54:8, s. 4299-4320
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Tidskriftsartikel (refereegranskat)abstract
- The deformation and permeability of coal are largely affected by the presence and distribution of natural fractures such as cleats and bedding planes with orthogonal and abutting characteristics, resulting in distinct hydromechanical responses to stress loading during coal mining processes. In this research, a two-dimensional (2D) fracture network is constructed based on a real coal cleat trace data collected from the Fukang mine area, China. Realistic multi-stage stress loading is designed to sequentially mimic an initial equilibrium phase and a mining-induced perturbation phase involving an increase of axial stress and a decrease of confining stress. The geomechanical and hydrological behaviour of the fractured coal under various stress loading conditions is modelled using a finite element model, which can simulate the deformation of coal matrix, the shearing and dilatancy of coal cleats, the variation of cleat aperture induced by combined effects of closure/opening, and shear and tensile-induced damage. The influence of different excavation stress paths and directions of mining is further investigated. The simulation results illustrate correlated variations among the shear-induced cleat dilation, damage in coal matrix, and equivalent permeability of the fractured coal. Model results are compared with results of previous work based on conventional approaches in which natural fracture networks are not explicitly represented. In particular, the numerical model reproduces the evolution of equivalent permeability under the competing influence of the effective stress perpendicular to cleats and shear-induced cleat dilation and associated damage. Model results also indicate that coal mining at low stress rates is conducive to the stability of surrounding coal seams, and that coal mining in parallel to cleat directions is desirable. The research findings of this paper have important implications for efficient and safe exploitation of coal and coalbed methane resources.
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| 7. |
- Gan, Quan, et al.
(författare)
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Induced fault reactivation by thermal perturbation in enhanced geothermal systems
- 2020
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Ingår i: Geothermics. - : Elsevier BV. - 0375-6505 .- 1879-3576. ; 86, s. 101814-101814
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Tidskriftsartikel (refereegranskat)abstract
- Heat extraction by circulating cold water through a geothermal reservoir could potentially induce earthquakes of large magnitudes. In this work, we explore the role of water injection inside a normal fault in triggering seismic slip, under different temperature and rate scenarios of fluid injection into fault damage zones. The resulted non-uniformity of thermal drawdown along the fault damage zones significantly affects the magnitude and timing of induced earthquakes. A non-dimensional expression integrating fault configuration (e.g. length and thickness) and injection condition (e.g. rate and temperature) is used to describe the relationship between the injection schedule and the resulting fault seismic slip event. As the dimensionless parameter increases, suggesting a transition from heat conduction to convection, the dimensionless event timing also grows nonlinearly. The perturbation of fault stress field induced from localized thermal cooling process is pronounced, compared to decoupled hydro-mechanical scenarios. The stress field perturbation in the system due to thermal cooling is characterized through a Coulomb friction ratio analysis for evaluating the stress changes along the fault plane and a tensor-based stress perturbation analysis for quantifying the stress changes in the damage zones and host rocks. The thermal influence acting on local patches along the fault strike not only advances the timing of seismic slip, but also increases the magnitude of induced seismic events, by unloading the fault to prompt seismic rupture. The injection temperature has a significant impact on facilitating the onset of seismic slip, i.e. attempting to accelerate the timing and increase the magnitude of fault reactivation. The injection rate variation will affect the timing by changing the pore pressure field and heat transfer manner. Prior to the onset of fault reactivation, the thermal unloading response increases fault permeability by decreasing normal stress, such that more permeable channels in the fault allow fluid to diffuse. Produced plastic shear strain due to fault slip provide extra positive contributions to increased normal aperture through shear dilation, thereby the fault permeability increases significantly by around two orders of magnitude.
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| 8. |
- Gao, Ke, et al.
(författare)
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Can we estimate far-field stress using the mean of local stresses? : An examination based on numerical simulations
- 2019
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Ingår i: Computers and geotechnics. - : Elsevier. - 0266-352X .- 1873-7633. ; 116
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Tidskriftsartikel (refereegranskat)abstract
- In situ stress is an important parameter in rock mechanics, and reliable estimates of far-field stresses are indispensable for robust rock engineering analysis. Here, by using the combined finite-discrete element method, we simulate a series of stress fields of both synthetic and natural fracture networks to examine whether the Euclidean mean of local stresses can be used to estimate the far-field stress. The calculations show that given a large number of local stress measurements, their Euclidean mean gives a close approximation of the far-field stress state. Whereas when only a limited number of local stresses are available, the probability of obtaining a practically acceptable estimate of far-field stress increases when more stress measurements are involved. The required number of stress measurements for deriving an acceptable estimate varies with the geomechanical condition; in general, the larger the overall stress variability is, the more local stress measurements are needed. Our research findings suggest that given a limited number of stress measurements, which is often encountered in rock engineering projects, attention is needed when deriving the far-field stress state based on them, and simply using their mean as far-field stress for further rock structure design and numerical analysis may yield erroneous results.
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| 9. |
- Gao, Ke, et al.
(författare)
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Influence of boundary constraints on stress heterogeneity modelling
- 2018
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Ingår i: Computers and geotechnics. - : Elsevier. - 0266-352X .- 1873-7633. ; 99, s. 130-136
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Tidskriftsartikel (refereegranskat)abstract
- By employing the effective variance of stress tensors as a scalar-valued measure of stress heterogeneity, we quantitatively analyse the influence of boundary constraint stiffness on numerically derived stress distribution in a fractured rock mass. The results reveal a decreasing trend in the effective variance of stress field with an increasing boundary constraint stiffness. This work demonstrates the efficacy of effective variance for stress heterogeneity quantification, and also indicates that the boundary constraint stiffness can affect stress modelling results. We suggest that quantitative evaluation of the effects of boundary constraints may be needed in geomechanical modelling of fractured rock masses.
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| 10. |
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| 11. |
- Gu, Guansi, et al.
(författare)
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A finite element-based dynamic simulation method for modeling shield-ground interactions : 3D numerical simulations with comparison to physical experiments
- 2024
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Ingår i: Computers and geotechnics. - : Elsevier. - 0266-352X .- 1873-7633. ; 169
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a novel numerical simulation approach based on the finite element method for dynamically modeling the excavation process of shield tunneling is proposed, with the shield-ground interactions well captured. This method is capable of mimicking the alternating modes of advancing and stopping of a shield boring machine during underground construction, with the important effects of the cutterhead rotation and slurry support pressure considered. Under the cutting action, the soil at the excavation face would experience irreversible deformation and damage, such that additional support needs to be provided by the cutterhead blades and slurry to maintain stability. The impacts of key construction parameters are examined, including cutterhead rotary speed, advance rate, and slurry support pressure, on shield tunneling operations and ground responses. The numerical model is rigorously validated against physical model experiments. This work provides useful insights into the mechanistic processes in the stratum during shield tunneling, including the spatiotemporal evolution of ground deformation patterns and stress redistributions. The results offer valuable guidance for optimizing shield tunneling operations and enhancing tunneling safety and efficiency.
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| 12. |
- Huang, Xin, et al.
(författare)
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A tensor-based analysis of stress variability in granular media subjected to various loading conditions
- 2019
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Ingår i: Powder Technology. - : Elsevier. - 0032-5910 .- 1873-328X. ; 356, s. 581-593
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Tidskriftsartikel (refereegranskat)abstract
- This paper uses a novel tensor-based approach to characterize the spatial variability and localization of stress fields in granular media derived from three-dimensional discrete element simulations. We use the Euclidean distance of the stress tensor of each particle to the bulk mean stress tensor to quantify local stress fluctuations and use the effective variance of the stress field to quantify the overall dispersion. We observe that the evolutions of both the local stress fluctuation and the bulk stress dispersion strongly depend on the stress state, packing density and shearing process. However, they become constant, and are uniquely related to the deviatoric stress and the void ratio when the sample is at critical state, under which the stress variability reaches an ultimate condition. Multifractal analysis further shows that the local stress field is highly heterogeneous and exhibits self-organized patterns.
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| 13. |
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| 14. |
- Huang, Xin, et al.
(författare)
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Characterizing stress variability within granular samples upon liquefaction
- 2020
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Ingår i: Computers and geotechnics. - : Elsevier BV. - 0266-352X .- 1873-7633. ; 127, s. 103771-103771
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Tidskriftsartikel (refereegranskat)abstract
- A tensor-based approach is adopted to investigate the evolution of stress variability within granular media based on DEM simulations of both cyclic and static liquefactions. The bulk stress dispersion is evaluated using the effective variance of the entire stress tensor field and the local stress fluctuation is quantified by the Euclidean distance between the stress tensor of individual particles and the global stress tensor. The orientation discrepancies between the local stress tensor and global fabric tensor, and between the local and global stress tensors are examined by their joint invariants. We observe that the bulk stress dispersion decreases but the discrepancy of local stress fluctuation increases upon liquefaction. The initially deviatoric stress-dominated stress variability gradually becomes hydrostatic stress-dominated, and returns to deviatoric stress-dominated again after liquefaction. The liquefaction process is characterized by the degradation of coaxiality between the local stress tensor and the global fabric tensor and that between the local and global stress tensors. There exists a range of threshold values for such coaxiality, below which liquefaction occurs. Particles with coaxiality higher than the threshold value and particles with coaxality lower than the threshold value exhibit different roles in sustaining external loads. However, the discrepancy becomes neutral upon liquefaction.
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| 15. |
- Jiang, Chuanyin, et al.
(författare)
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Anomalous transport and upscaling in critically-connected fracture networks under stress conditions
- 2024
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Ingår i: Journal of Hydrology. - : Elsevier. - 0022-1694 .- 1879-2707. ; 630
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Tidskriftsartikel (refereegranskat)abstract
- Anomalous transport in fractured rocks is of high importance for numerous research fields and applications in hydrogeology and has been widely studied in the last decades. This phenomenon is due to the structural heterogeneities of fractured rocks and the high contrast between the fractures and matrix properties. At the same time, the fracture properties in terms of fracture density and geometrical characteristics are related to in-situ stress conditions that impact the connectivity of the system and the distributions of fracture aperture and length, including the creation of new cracks when the differential stress conditions are strong enough. In order to understand how all these features impact the observed anomalous transport, we study the role of in-situ stress in mass transport through a two-dimensional fractured rock. The fracture network is based on a real outcrop with a connectivity state around the percolation threshold, over which we simulate stress-dependent fracture deformation and propagation, and perform hydrodynamic transport through the deformed rock mass. The impact of the changes in aperture and the creation of new cracks on transport behavior is evaluated for various stress scenarios and reproduced with upscaled representations of transport processes. We also consider matrix diffusion as a source of anomalous transport and demonstrate that this process can be incorporated into the proposed upscaled models. Results showed that traditional upscaling methods with space-Lagrangian velocities description capture well the Gaussian-like breakthrough curves (BTCs) under low-stress ratio conditions, but fail under high-stress ratio conditions with multiple-peak early-times BTCs. To characterize the emergence of strong anomalous transport, we extend the Random Walk model Directed by a Markov Process with space-Lagrangian velocity sampling by incorporating multiple transition matrices conditioned by different initial velocity states. This new transport upscaling model shows high accuracy in predicting complex transport behaviors in critically connected fracture networks.
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| 16. |
- Jiang, Chuanyin, et al.
(författare)
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Fracture Activation and Induced Seismicity During Long-Term Heat Production in Fractured Geothermal Reservoirs
- 2022
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Ingår i: Rock Mechanics and Rock Engineering. - : Springer Nature. - 0723-2632 .- 1434-453X. ; 55:8, s. 5235-5258
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Tidskriftsartikel (refereegranskat)abstract
- We study fracture activation and induced seismicity during the long-term heat production of fractured geothermal reservoirs. The fracture system is modeled by the discrete fracture network approach that realistically represents the distribution and behavior of natural fractures in the system. Using a novel fully coupled thermal-hydro-mechanical model, we systematically investigate the interplay among in situ stresses, injection pressure and temperature on the spatio-temporal evolution of heat production-induced seismicity. Our results show that in situ stress state exerts a fundamental background control on the seismicity occurrence, while injection-induced fluid pressurization and thermal perturbation act as two competing triggering factors. Induced seismic events occur mainly in the near field of the injection well and along the main streamlines of fluid flow through the reservoir. Under a low differential stress condition, fracture activation is dominated by late-stage thermal drawdown, such that the injection temperature controls the timing, magnitude and number of seismic events. However, under a high differential stress condition, the effect of early-stage pressurization dominates with critically stressed fractures activated once or twice. The fracture activation is sensitive to the variation of pressure gradient, whilst the injection temperature attempts to influence the magnitude but in general not the timing and number of thermal drawdown-induced secondary seismic events. Furthermore, these anthropogenic parameters (i.e. injection pressure and temperature) strongly affect the long-term thermal energy output of the reservoir. Our research findings have important implications for the mitigation of seismic hazards by optimizing production strategy design during heat extraction from deep geothermal reservoirs.
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| 17. |
- Jiang, Chuanyin, et al.
(författare)
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The Role of In Situ Stress in Organizing Flow Pathways in Natural Fracture Networks at the Percolation Threshold
- 2019
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Ingår i: Geofluids. - : Hindawi Publishing Corporation. - 1468-8115 .- 1468-8123. ; 2019
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the effect of in situ stresses on fluid flow in a natural fracture network. The fracture network model is based on an actual critically connected (i.e., close to the percolation threshold) fracture pattern mapped from a field outcrop. We derive stress-dependent fracture aperture fields using a hybrid finite-discrete element method. We analyze the changes of aperture distribution and fluid flow field with variations of in situ stress orientation and magnitude. Our simulations show that an isotropic stress loading tends to reduce fracture apertures and suppress fluid flow, resulting in a decrease of equivalent permeability of the fractured rock. Anisotropic stresses may cause a significant amount of sliding of fracture walls accompanied with shear-induced dilation along some preferentially oriented fractures, resulting in enhanced flow heterogeneity and channelization. When the differential stress is further elevated, fracture propagation becomes prevailing and creates some new flow paths via linking preexisting natural fractures, which attempts to increase the bulk permeability but attenuates the flow channelization. Comparing to the shear-induced dilation effect, it appears that the propagation of new cracks leads to a more prominent permeability enhancement for the natural fracture system. The results have particularly important implications for predicting the hydraulic responses of fractured rocks to in situ stress fields and may provide useful guidance for the strategy design of geofluid production from naturally fractured reservoirs.
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| 18. |
- Kang, Peter K., et al.
(författare)
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Stress‐Induced Anomalous Transport in Natural Fracture Networks
- 2019
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 55:5, s. 4163-4185
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the effects of geological stress on fluid flow and tracer transport in natural fracture networks. We show the emergence of non-Fickian (anomalous) transport from the interplay among fracture network geometry, aperture heterogeneity, and geological stress. In this study, we extract the fracture network geometry from the geological map of an actual rock outcrop, and we simulate the geomechanical behavior of fractured rock using a hybrid finite-discrete element method. We analyze the impact of stress on the aperture distribution, fluid flow field, and tracer transport properties. Both stress magnitude and orientation have strong effects on the fracture aperture field, which in turn affects fluid flow and tracer transport through the system. We observe that stress anisotropy may cause significant shear dilation along long, curved fractures that are preferentially oriented to the stress loading. This, in turn, induces preferential flow paths and anomalous early arrival of tracers. An increase in stress magnitude enhances aperture heterogeneity by introducing more small apertures, which exacerbates late-time tailing. This effect is stronger when there is higher heterogeneity in the initial aperture field. To honor the flow field with strong preferential flow paths, we extend the Bernoulli Continuous Time Random Walk model to incorporate dual velocity correlation length scales. The proposed upscaled transport model captures anomalous transport through stressed fracture networks and agrees quantitatively with the high-fidelity numerical simulations.
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| 19. |
- Lei, Qinghua, et al.
(författare)
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A new approach to upscaling fracture network models while preserving geostatistical and geomechanical characteristics
- 2015
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Ingår i: Journal of Geophysical Research - Solid Earth. - : American Geophysical Union (AGU). - 2169-9313 .- 2169-9356. ; 120:7, s. 4784-4807
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Tidskriftsartikel (refereegranskat)abstract
- A new approach to upscaling two-dimensional fracture network models is proposed for preserving geostatistical and geomechanical characteristics of a smaller-scale “source” fracture pattern. First, the scaling properties of an outcrop system are examined in terms of spatial organization, lengths, connectivity, and normal/shear displacements using fractal geometry and power law relations. The fracture pattern is observed to be nonfractal with the fractal dimension D≈ 2, while its length distribution tends to follow a power law with the exponent 2
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| 20. |
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| 21. |
- Lei, Qinghua, et al.
(författare)
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A numerical study of stress variability in heterogeneous fractured rocks
- 2019
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier. - 1365-1609 .- 1873-4545. ; 113, s. 121-133
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Tidskriftsartikel (refereegranskat)abstract
- We conduct numerical simulations to investigate the variability of local stresses in heterogeneous fractured rocks subjected to different far-field stress conditions. A realistic fracture network is constructed based on a real outcrop mapped at the Hornelen Basin in Norway. The heterogeneity of the rock material is modelled using a Weibull distribution of Young's modulus characterised by a homogeneity index m. As m decreases, the rock material becomes less homogeneous. The distribution of local stresses in the fractured rock under far-field stress loading is derived from a hybrid finite-discrete element model, and the stress variability is further analysed using a tensor-based formalism that faithfully honours the tensorial nature of stress data. The local stress perturbation is quantified using the Euclidean distance of each local stress tensor to the mean stress tensor, and the overall stress dispersion is measured using the effective variance of the entire stress tensor field. We show that the local stress field is significantly perturbed when the far-field stresses are associated with a high stress ratio and imposed at a critical direction in favour of intense sliding along preferentially-oriented fractures. The strong correlation between fracture sliding and local stress variability are further revealed from a scanline sampling analysis through the domain. Furthermore, larger perturbation of local stresses can be induced as the inhomogeneity of the rock materials increases (i.e. m decreases). Whether the stress field is dominated by fractures or matrix depends on the far-field stress state, material inhomogeneity, and fracture properties. If the rock material is highly heterogeneous, stress variability is controlled by the matrix when the far-field stress ratio is low; however, the stress distribution becomes more affected by fractures as the stress ratio increases. If the rock material is more homogeneous, the system tends to be more dominated by fractures even under a relatively low stress ratio.
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| 22. |
- Lei, Qinghua, et al.
(författare)
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A Stochastic Dynamical Model of Slope Creep and Failure
- 2023
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 50:11
-
Tidskriftsartikel (refereegranskat)abstract
- We propose a stochastic dynamical model to simulate slope secondary and tertiary creep phenomena. The slope secondary creep is represented by the Kesten process defined as a stochastic affine auto-regressive process involving both multiplicative and additive random variables. The Kesten process can realistically capture the co-existence of a background deformation and intermittent displacement bursts, which are together well characterized by an inverse gamma velocity distribution. The slope tertiary creep is modeled by a nonlinear stochastic dynamical equation embodying a nonlinear feedback mechanism and a nonlinear random effect, which can mimic the development of slow or catastrophic landslides. For catastrophic landslides, the probability density function of slope velocities tends to deviate from the inverse gamma distribution by populating the "dragon-king" regime, although sometimes they may grow undetectably in the "black-swan" regime. Our model provides a quantitative framework to understand, simulate, and interpret complex landslide displacement time series.
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| 23. |
- Lei, Qinghua, et al.
(författare)
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Anderson localisation and reentrant delocalisation of tensorial elastic waves in two-dimensional fractured media
- 2021
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Ingår i: Europhysics letters. - : Institute of Physics Publishing (IOPP). - 0295-5075 .- 1286-4854. ; 136:3
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Tidskriftsartikel (refereegranskat)abstract
- We study tensorial elastic wave transport in two-dimensional densely fractured media using numerical simulations and document transitions from propagation to diffusion and to localisation/delocalisation. For large fracture stiffness, waves are propagative at the scale of the system. For small stiffness, multiple scattering prevails, such that waves are diffusive in disconnected fracture networks, and localised in connected ones with a strong multifractality of the intensity field. A re-entrant delocalisation is found in well-connected fracture networks due to energy leakage via evanescent waves and cascades of mode conversion.
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| 24. |
- Lei, Qinghua, et al.
(författare)
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Correlation Between Fracture Network Properties and Stress Variability in Geological Media
- 2018
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 45:9, s. 3994-4006
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Tidskriftsartikel (refereegranskat)abstract
- We quantitatively investigate the stress variability in fractured geological media under tectonic stresses. The fracture systems studied include synthetic fracture networks following power law length scaling and natural fracture patterns based on outcrop mapping. The stress field is derived from a finite-discrete element model, and its variability is analyzed using a set of mathematical formulations that honor the tensorial nature of stress data. We show that local stress perturbation, quantified by the Euclidean distance of a local stress tensor to the mean stress tensor, has a positive, linear correlation with local fracture intensity, defined as the total fracture length per unit area within a local sampling window. We also evaluate the stress dispersion of the entire stress field using the effective variance, that is, a scalar-valued measure of the overall stress variability. The results show that a well-connected fracture system under a critically stressed state exhibits strong local and global stress variabilities.
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| 25. |
- Lei, Qinghua, et al.
(författare)
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Effects of geomechanical changes on the validity of a discrete fracture network representation of a realistic two-dimensional fractured rock
- 2014
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier BV. - 1365-1609 .- 1873-4545. ; 70, s. 507-523
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Tidskriftsartikel (refereegranskat)abstract
- This paper aims to examine the validity of the discrete fracture network (DFN) method in representing a realistic two-dimensional fractured rock in terms of their geomechanical response to in situ stresses and hydraulic behaviour in a steady state fluid held. First, a real fracture network is extracted from the geological map of an actual rock outcrop, which is termed the analogue fracture network (AFN). Multiple DFN realisations are created using the statistics oldie analogue pattern. A conductivity parameter that was found to have a linear relationship with the conductivity of 2D fracture networks is included to further enhance network similarity. A series of numerical experiments are designed with far-field stresses applied at a range of angles to the rock domains and their geomechanical response is modelled using the combined finite discrete element method (FEMDEM). A geomechanical comparison between the AFN and as DFN equivalents is made based on phenomena such as heterogeneity of fracture-dependent stress contours, sliding between pre-existing fracture walls, coalescence of propagating fractures and variability of aperture distribution. Furthermore, an indirect hydro-mechanical (HM) coupling is applied and the hydraulic behaviour of the porous rock models is investigated using the hybrid finite element-finite volume method (FEFVM). A further comparison is conducted focusing on the hydraulic behaviour of the AFN and DFNs under the effects of geomechanical changes. The results show that although DFNs may represent an AFN quite well for fixed mechanical conditions, such a representation may not be dependable if mechanical changes occur.
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| 26. |
- Lei, Qinghua
(författare)
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Fluid flow and coupled hydromechanical processes in fractured rocks with application to geoenergy and geoengineering
- 2023
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Ingår i: Proceedings of the 20th International Conference on Flow Dynamics. - : Institute of Fluid Science, Tohoku University.
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Konferensbidrag (refereegranskat)abstract
- This paper presents an overview of our recent studies on modelling fluid flow and coupled hydromechanical processes in fractured geological media. The mathematical formulation of a fully-coupled hydromechanical model is given with the model capability further demonstrated through a series of 2D/3D simulation examples of water injection/drainage in fractured rocks. The research findings and insights obtained have important implications for understanding and predicting the behaviour of fractured rocks masses during geoenergy and geoengineering activities.
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| 27. |
- Lei, Qinghua
(författare)
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Impact of fracture normal and shear stiffnesses on the scattering attenuation of P and S waves in a naturally fractured rock
- 2022
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Ingår i: Journal of Applied Geophysics. - : Elsevier. - 0926-9851 .- 1879-1859. ; 206
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, numerical simulations are conducted to study elastic wave transport, scattering, and attenuation in a naturally fractured rock associated with length-correlated fracture normal and shear stiffnesses. The model represents the pattern of a real fracture outcrop in an explicit fashion based on the discrete fracture network approach and computes the dynamical interaction between waves and fractures based on the displacement discontinuity method. A broad spectrum of geologically relevant fracture stiffness values are explored to analyse the impact of fracture normal and shear stiffness components on the wavefield evolution. It is observed that when the fracture normal and shear stiffnesses are both high, the wavefield is a propagative mode dominated by a forward ballistic transport. With the reduction of fracture normal and/or shear stiffnesses, the wavefield becomes diffusive characterised by the emergence and dominance of coda waves. If the fracture stiffnesses are very low, waves become trapped entering the so-called localisation regime associated with an absence of effective transport as well as a profound attenuation. Our results show that the scattering attenuation of S waves tends to be greater than that of P waves in the propagation and diffusion regimes, but becomes similar in the localisation regime. The research findings of this paper have important implications for understanding and predicting the seismic wave attenuation behaviour in naturally fractured rocks for various geophysical applications.
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| 28. |
- Lei, Qinghua, et al.
(författare)
-
Implementation of an Empirical Joint Constitutive Model into Finite-Discrete Element Analysis of the Geomechanical Behaviour of Fractured Rocks
- 2016
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Ingår i: Rock Mechanics and Rock Engineering. - : Springer. - 0723-2632 .- 1434-453X. ; 49:12, s. 4799-4816
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Tidskriftsartikel (refereegranskat)abstract
- An empirical joint constitutive model (JCM) that captures the rough wall interaction behaviour of individual fractures associated with roughness characteristics observed in laboratory experiments is combined with the solid mechanical model of the finite-discrete element method (FEMDEM). The combined JCM-FEMDEM formulation gives realistic fracture behaviour with respect to shear strength, normal closure, and shear dilatancy and includes the recognition of fracture length influence as seen in experiments. The validity of the numerical model is demonstrated by a comparison with the experimentally established empirical solutions. A 2D plane strain geomechanical simulation is conducted using an outcrop-based naturally fractured rock model with far-field stresses loaded in two consecutive phases, i.e. take-up of isotropic stresses and imposition of two deviatoric stress conditions. The modelled behaviour of natural fractures in response to various stress conditions illustrates a range of realistic behaviour including closure, opening, shearing, dilatancy, and new crack propagation. With the increase in stress ratio, significant deformation enhancement occurs in the vicinity of fracture tips, intersections, and bends, where large apertures can be generated. The JCM-FEMDEM model is also compared with conventional approaches that neglect the scale dependency of joint properties or the roughness-induced additional frictional resistance. The results of this paper have important implications for understanding the geomechanical behaviour of fractured rocks in various engineering activities.
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| 29. |
- Lei, Qinghua, et al.
(författare)
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Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media
- 2020
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Ingår i: Journal of Rock Mechanics and Geotechnical Engineering. - : Elsevier. - 1674-7755. ; 12:4, s. 780-792
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically. We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling. The geomechanical response of the fractured rock is simulated using a hybrid finite-discrete element model, which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of discrete fractures and the propagation of new cracks. Under far-field stress loading, the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures. The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment. We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks. A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening, leading to greater rock mass permeability. However, a disconnected fracture network accommodates much less fracture shearing and opening, and has much lower bulk permeability. We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric (i.e. percolation parameter) of fracture networks, which yields an excellent match to the numerical results. We infer that fluid flow through a well-connected system is governed by traversing channels (forming an “in parallel” architecture) and thus equivalent permeability is sensitive to stress loading (due to stress-dependent fracture permeability), whilst fluid flow through a disconnected system is more ruled by matrix (linking isolated clusters “in series”) and has much less stress dependency.
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| 30. |
- Lei, Qinghua, et al.
(författare)
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Modelling coseismic displacements of fracture systems in crystalline rock during large earthquakes: Implications for the safety of nuclear waste repositories
- 2021
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier. - 1365-1609 .- 1873-4545. ; 138
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Tidskriftsartikel (refereegranskat)abstract
- Crystalline rock has been tested/selected by many countries for developing underground nuclear waste repositories at ~500 m depth to achieve geological isolation of high-level, long-lived radioactive waste. During the assessment period of up to one million years, large earthquakes may occur around the repository and trigger coseismic displacements along secondary fractures, jeopardising the integrity of the buffer-waste canister system. It is, therefore, of great importance to understand the coseismic behaviour of the repository site during large earthquakes. In this paper, we develop a finite element method-based seismo-mechanical model to simulate the response of fractured rocks subject to both in-situ stresses and seismic activities. The model can well capture the fracture displacement behaviour under dynamic loadings involving alternating regimes of contact loss, partial slip and total sliding. We model the earthquake-induced displacement field according to the seismic source theory in combination with a generic source time function. We apply the model to the nuclear waste repository site at Forsmark, Sweden, and analyse the coseismic responses of both single fractures and fracture networks during a potential post-glacial earthquake with a moment magnitude of Mw = 5.6. The shear dislocation of a single fracture is strongly dependent on the fracture length and dip angle, while the displacement pattern of a fracture network is dominated by its “backbone” structures. We observe that significant coseismic shear displacement occurs if the fractured rock is close to the hypocentre and located in the dilational quadrant of the primary fault due to reduced shear resistance. However, the earthquake-induced shear displacement decreases drastically with the increased distance to the hypocentre and a distance of ~700 m may be needed for a fracture up to 100 m long to not displace beyond 5 cm. If two repeated earthquakes would occur, a distance of ~1200 m may be necessary.
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| 31. |
- Lei, Qinghua, et al.
(författare)
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Modelling fluid injection-induced fracture activation, damage growth, seismicity occurrence and connectivity change in naturally fractured rocks
- 2021
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier. - 1365-1609 .- 1873-4545. ; 138
-
Tidskriftsartikel (refereegranskat)abstract
- We develop a fully-coupled hydro-mechanical model to simulate fluid injection-induced activation of preexisting fractures, propagation of new damages, development of seismic activities, and alteration of network connectivity in naturally fractured rocks. The natural fracture system is represented by a discrete fracture network. The stress and strain fields of the fractured porous media are solved in the framework of a finite element model, which mimics the damage evolution in rock matrix based on an elasto-brittle failure criterion and simulates the normal/shear displacement of natural discontinuities based on a non-linear constitutive law. The coupled geomechanics and fluid flow processes in the fractured rock are computed honouring essential coupling mechanisms such as pore pressure-induced shear slip of pre-existing fractures, poro-elastic response of rock matrix, and stress-dependent permeability/storativity of both fractures and rocks. We use the numerical model developed to investigate the hydro-mechanical behaviour of two cases of deeply buried fractured rock in response to high-pressure fluid injection, one case with fracture density just below the percolation threshold and the other above the threshold. We observe a strong control of natural fracture network connectivity on the damage emergence, seismicity occurrence and connectivity change in the rock mass subject to hydraulic stimulation. We also highlight the strong poro-elastic effect that tends to drive heterogeneous connectivity evolution of fracture systems during fluid injection. The results of our research and insights obtained have important implications for injection-related geoengineering activities such as the development of enhanced geothermal systems and extraction of hydrocarbon resources.
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| 32. |
- Lei, Qinghua, et al.
(författare)
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Modelling the reservoir-to-tubing pressure drop imposed by multiple autonomous inflow control devices installed in a single completion joint in a horizontal well
- 2020
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Ingår i: Journal of Petroleum Science and Engineering. - : Elsevier. - 0920-4105 .- 1873-4715. ; 189
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Tidskriftsartikel (refereegranskat)abstract
- Autonomous inflow control devices (AICDs) are used to introduce an additional pressure drop between the reservoir and the tubing of a production well that depends on the fluid phase flowing into the device: a larger pressure drop is introduced when unwanted phases such as water or gas enter the AICD. The additional pressure drop is typically represented in reservoir simulation models using empirical relationships fitted to experimental data for a single AICD. This approach may not be correct if each completion joint is equipped with multiple AICDs as the flow at different AICDs may be different. We use high-resolution numerical modelling to determine the total additional pressure drop introduced by two AICDs installed in a single completion joint in a horizontal well. The model captures the multiphase flow of oil and water through the inner annulus into each AICD. We explore a number of relevant oil-water inflow scenarios with different flow rates and water cuts. Our results show that if only one AICD is installed, the additional pressure drop is consistent with the experimentalzly-derived empirical formulation. However, if two AICDs are present, there is a significant discrepancy between the additional pressure drop predicted by the simulator and the empirical relationship. This discrepancy occurs because each AICD has a different total and individual phase flow rate, and the final steady-state flow results from a self-organising mechanism emerging from the system. We report the discrepancy as a water cut-dependent correction to the empirical equation, which can be used in reservoir simulation models to better capture the pressure drop across a single completion containing two AICDs. Our findings highlight the importance of understanding how AICDs modify flow into production wells, and have important consequences for improving the representation of advanced wells in reservoir simulation models.
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| 33. |
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| 34. |
- Lei, Qinghua, et al.
(författare)
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Numerical study of fluid injection-induced deformation and seismicity in a mature fault zone with a low-permeability fault core bounded by a densely fractured damage zone
- 2022
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Ingår i: Geomechanics for Energy and the Environment. - : Elsevier. - 2352-3808. ; 31
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Tidskriftsartikel (refereegranskat)abstract
- We apply a fully-coupled hydro-mechanical simulation tool to study fluid injection-induced pressure diffusion, poroelastic response, fracture slip, damage growth, and microseismicity occurrence in a complex fault zone. The fault zone has a low-permeability fault core bounded by a high-permeability damage zone, which is characterised by a fracture network with the fracture density exponentially decaying away from the fault core. We employ the discrete fracture network approach to explicitly represent the distribution and behaviour of these fractures in the fault zone. We use the finite element method to solve the coupled governing equations of fluid flow, solid deformation, and damage evolution in the fractured porous rock. We explore a number of scenarios with different orientations for the in-situ stress field and different locations for a constant rate fluid injection. We report an interactive control of fracture density, fracture orientations, in-situ stress state, and injection point position on the seismo-hydro-mechanical behaviour of the fault zone subject to fluid injection. Insight from our research findings may have important implications for injection-related geoengineering activities such as the development of enhanced geothermal systems and unconventional hydrocarbon reservoirs.
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| 35. |
- Lei, Qinghua, et al.
(författare)
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On the selection of joint constitutive models for geomechanics simulation of fractured rocks
- 2022
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Ingår i: Computers and geotechnics. - : Elsevier. - 0266-352X .- 1873-7633. ; 145
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Tidskriftsartikel (refereegranskat)abstract
- Fractures such as faults and joints often dominate the mechanical strength and deformation of rock masses. It is thus of central importance to adopt an appropriate joint constitutive model in geomechanics simulations so that the behaviour of fractures can be realistically represented. Over the past decades, various joint constitutive models have been proposed from theoretical/experimental perspectives and implemented into different geomechanics solvers. However, numerical modelling researchers are often confronted and even confused with the question about which joint model to use in their simulations, especially when a compromise needs to be reached between the realism (or complexity) of the selected constitutive model and the difficulty in the numerical implementation. In this Short Communication, we review some of the popular joint constitutive laws that have been used for geomechanics simulations and present a discussion on their suitability and limitations, aiming to provide a guidance for the joint constitutive model selection for computer simulations. We also point out a few unrealistic features of some widely used joint constitutive models with corresponding corrections recommended.
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| 36. |
- Lei, Qinghua, et al.
(författare)
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Polyaxial stress-dependent permeability of a three-dimensional fractured rock layer
- 2017
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Ingår i: Hydrogeology Journal. - : Springer. - 1431-2174 .- 1435-0157. ; 25:8, s. 2251-2262
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Tidskriftsartikel (refereegranskat)abstract
- A study about the influence of polyaxial (true-triaxial) stresses on the permeability of a three-dimensional (3D) fractured rock layer is presented. The 3D fracture system is constructed by extruding a two-dimensional (2D) outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a “through-going” joint set abutted by later-stage short fractures. Geomechanical behaviour of the 3D fractured rock in response to in-situ stresses is modelled by the finite-discrete element method, which can capture the deformation of matrix blocks, variation of stress fields, reactivation of pre-existing rough fractures and propagation of new cracks. A series of numerical simulations is designed to load the fractured rock using various polyaxial in-situ stresses and the stress-dependent flow properties are further calculated. The fractured layer tends to exhibit stronger flow localisation and higher equivalent permeability as the far-field stress ratio is increased and the stress field is rotated such that fractures are preferentially oriented for shearing. The shear dilation of pre-existing fractures has dominant effects on flow localisation in the system, while the propagation of new fractures has minor impacts. The role of the overburden stress suggests that the conventional 2D analysis that neglects the effect of the out-of-plane stress (perpendicular to the bedding interface) may provide indicative approximations but not fully capture the polyaxial stress-dependent fracture network behaviour. The results of this study have important implications for understanding the heterogeneous flow of geological fluids (e.g. groundwater, petroleum) in subsurface and upscaling permeability for large-scale assessments.
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| 37. |
- Lei, Qinghua, et al.
(författare)
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Real-Time Forecast of Catastrophic Landslides via Dragon-King Detection
- 2023
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 50:6
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Tidskriftsartikel (refereegranskat)abstract
- Catastrophic landslides characterized by runaway slope failures remain difficult to predict. Here, we develop a physics-based framework to prospectively assess slope failure potential. Our method builds upon the physics of extreme events in natural systems: the extremes so-called "dragon-kings" (e.g., slope tertiary creeps prior to failure) exhibit statistically different properties than other smaller-sized events (e.g., slope secondary creeps). We develop statistical tools to detect the emergence of dragon-kings during landslide evolution, with the secondary-to-tertiary creep transition quantitatively captured. We construct a phase diagram characterizing the detectability of dragon-kings against "black-swans" and informing on whether the slope evolves toward a catastrophic or slow landslide. We test our method on synthetic and real data sets, demonstrating how it might have been used to forecast three representative historical landslides. Our method can in principle considerably reduce the number of false alarms and identify with high confidence the presence of true hazards of catastrophic landslides.
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| 38. |
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| 39. |
- Lei, Qinghua, et al.
(författare)
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Role of natural fractures in damage evolution around tunnel excavation in fractured rocks
- 2017
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Ingår i: Engineering Geology. - : Elsevier. - 0013-7952 .- 1872-6917. ; 231, s. 100-113
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Tidskriftsartikel (refereegranskat)abstract
- This paper studies the role of pre-existing fractures in the damage evolution around tunnel excavation in fractured rocks. The length distribution of natural fractures can be described by a power law model, whose exponent a defines the relative proportion of large and small fractures in the system. The larger a is, the higher proportion of small fractures is. A series of two-dimensional discrete fracture networks (DENs) associated with different length exponent a and fracture intensity P-21 is generated to represent various scenarios of distributed preexisting fractures in the rock. The geomechanical behaviour of the fractured rock embedded with DFN geometry in response to isotropic/anisotropic in-situ stress conditions and excavation-induced perturbations is simulated using the hybrid finite-discrete element method (FEMDEM), which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of natural fractures, and the propagation of new cracks. An excavation damaged zone (EDZ) develops around the man-made opening as a result of reactivation of preexisting fractures and propagation of wing cracks. The simulation results show that when a is small, the system which is dominated by large fractures can remain stable after excavation given that P-21 is not very high; however, intensive structurally-governed kinematic instability can occur if P-21 is sufficiently high and the fracture spacing is much smaller than the tunnel size. With the increase of a, the system becomes more dominated by small fractures, and the EDZ is mainly created by the coalescence of small fractures near the tunnel boundary. The results of this study have important implications for designing stable underground openings for radioactive waste repositories as well as other engineering facilities that are intended to generate minimal damage in the host rock mass.
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| 40. |
- Lei, Qinghua, et al.
(författare)
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Tectonic interpretation of the connectivity of a multiscale fracture system in limestone
- 2016
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 43:4, s. 1551-1558
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Tidskriftsartikel (refereegranskat)abstract
- This paper studies the statistics and tectonism of a multiscale natural fracture system in limestone. The fracture network exhibits a self-similar characteristic with a correlation between its power law length exponent a and fractal dimension D, i.e., a approximate to D + 1. Contradicting the scale-invariant connectivity of idealized self-similar systems, the percolation state of trace patterns mapped at different scales and localities of the study area varies significantly, from well to poorly connected. A tectonic interpretation based on a polyphase fracture network evolution history is proposed to explain this discrepancy. We present data to suggest that the driving force for fracture formation may be dissipated at the end of a tectonic event when the system becomes connected. However, the "effective" connectivity can successively be reduced by cementation of early fractures and reestablished by subsequent cracking, rendering a variable "apparent" connectivity that can be significantly above the percolation threshold.
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| 41. |
- Lei, Qinghua, et al.
(författare)
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The shape and motion of gas bubbles in a liquid flowing through a thin annulus
- 2018
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 855, s. 1017-1039
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Tidskriftsartikel (refereegranskat)abstract
- We study the shape and motion of gas bubbles in a liquid flowing through a horizontal or slightly inclined thin annulus. Experimental data show that in the horizontal annulus, bubbles develop a unique ‘tadpole-like’ shape with a semi-circular cap and a highly stretched tail. As the annulus is inclined, the bubble tail tends to vanish, resulting in a significant decrease of bubble length. To model the bubble evolution, the thin annulus is conceptualised as a ‘Hele-Shaw’ cell in a curvilinear space. The three-dimensional flow within the cell is represented by a gap-averaged, two-dimensional model, which achieved a close match to the experimental data. The numerical model is further used to investigate the effects of gap thickness and pipe diameter on the bubble behaviour. The mechanism for the semi-circular cap formation is interpreted based on an analogous irrotational flow field around a circular cylinder, based on which a theoretical solution to the bubble velocity is derived. The bubble motion and cap geometry is mainly controlled by the gravitational component perpendicular to the flow direction. The bubble elongation in the horizontal annulus is caused by the buoyancy that moves the bubble to the top of the annulus. However, as the annulus is inclined, the gravitational component parallel to the flow direction becomes important, causing bubble separation at the tail and reduction in bubble length.
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| 42. |
- Lei, Qinghua, et al.
(författare)
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The use of discrete fracture networks for modelling coupled geomechanical and hydrological behaviour of fractured rocks
- 2017
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Ingår i: Computers and geotechnics. - : ELSEVIER SCI LTD. - 0266-352X .- 1873-7633. ; 85, s. 151-176
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Forskningsöversikt (refereegranskat)abstract
- We present a discussion of the state-of-the-art on the use of discrete fracture networks (DFNs) for modelling geometrical characteristics, geomechanical evolution and hydromechanical (HM) behaviour of natural fracture networks in rock. The DFN models considered include those based on geological mapping, stochastic generation and geomechanical simulation. Different types of continuum, discontinuum and hybrid geomechanical models that integrate DFN information are summarised. Numerical studies aiming at investigating geomechanical effects on fluid flow in DFNs are reviewed. The paper finally provides recommendations for advancing the modelling of coupled HM processes in fractured rocks through more physically-based DFN generation and geomechanical simulation.
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| 43. |
- Lei, Qinghua, et al.
(författare)
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Transport and Localization of Elastic Waves in Two‐Dimensional Fractured Media : Consequences on Scattering Attenuation
- 2021
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Ingår i: Journal of Geophysical Research - Solid Earth. - : American Geophysical Union (AGU). - 2169-9313 .- 2169-9356. ; 126:6
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Tidskriftsartikel (refereegranskat)abstract
- We present numerical simulations of elastic wave transport in two-dimensional fractured media. Natural fracture systems, following a power-law length scaling, are modeled by the discrete fracture network approach for geometrically representing fracture distributions and the displacement discontinuity method for mechanically computing fracture-wave interactions. The model is validated against analytical solutions for wave reflection, transmission, and scattering by single fractures, and then applied to solve the wavefield evolution in synthetic fracture networks. We find that the dimensionless angular frequency (omega) over tilde = omega Z/kappa plays a crucial role in governing wave transport, where., Z, and. are the angular frequency, seismic impedance, and fracture stiffness, respectively. When. is smaller than the critical frequency (omega) over tilde (c) (approximate to 5), waves are in the extended mode, either propagating (for small.) or diffusing by multiple scattering (for intermediate.); as. exceeds.c, waves become trapped, entering either the Anderson localization regime (kl* approximate to 1) in well- connected fracture systems or the weak localization regime (kl* > 1) in poorly-connected fracture systems, where k is the incident wavenumber and l* is the mean free path length. Consequently, the inverse quality factor Q(-1) scales with. obeying a two-branch power-law dependence, showing significant frequency dependence when (omega) over tilde < <(omega)over tilde>(c) and almost frequency independence when (omega) over tilde > (omega) over tilde (c). Furthermore, when (omega) over tilde < <(omega)over tilde>(c), the wavefield exhibits a weak dependence on fracture network geometry, whereas when (omega) over tilde >(omega) over tilde (c), the fracture network connectivity has an important impact on the wavefield such that strong attenuation occurs in well-connected fracture systems.
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| 44. |
- Li, Yingming, et al.
(författare)
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Atmospheric distribution of polychlorinated dibenzo-p-dioxins, dibenzofurans and dioxin-like polychlorinated biphenyls around a steel plant area, northeast China
- 2010
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Ingår i: Chemosphere. - : Elsevier BV. - 0045-6535 .- 1879-1298. ; 79:3, s. 253-258
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Tidskriftsartikel (refereegranskat)abstract
- Air monitoring of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and dioxin-like polychlorinated biphenyls (PCBs) was carried out in June 2008 and January 2009 to investigate the concentrations, profiles and estimating potential inhalation risks to the local residents around a steel plant area in northeast China. The air concentrations and WHO-TEQs of PCDD/Fs ranged 94-4944fgm(-3) (average 1352fgm(-3)) and 3-247fgm(-3) (average 81fgm(-3)), respectively. The WHO-TEQ concentrations of dioxin-like PCBs ranged 1-18fgm(-3) (average 5fgm(-3)), contributing to 3.6-26% of the total TEQ. Higher PCDD/F concentrations were observed in the winter, whereas higher dioxin-like PCB concentrations were found in the summer. The seasonal trend can be related to the significant correlation between the concentrations of dioxins and the reciprocal of temperature (positive for PCDD/Fs, P<0.01; negative for dioxin-like PCBs, P=0.05). A significant positive correlation (P<0.0001) was found between the concentration of total suspended particulate (TSP) and PCDD/F concentrations, but not for PCB congeners. Although the steel plant sites showed higher dioxin levels than the residential and background areas, the PCDD/F levels in the atmosphere of the steel plant area was at a relatively low level. The results from this study provides further aid in evaluating the impact of steel plants as PCDD/Fs emission sources to the ambient air in China.
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| 45. |
- Li, Yingming, et al.
(författare)
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Levels and vertical distributions of PCBs, PBDEs, and OCPs in the atmospheric boundary layer : observation from the Beijing 325-m meteorological tower
- 2009
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Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 43:4, s. 1030-1035
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Tidskriftsartikel (refereegranskat)abstract
- Polyurethane foam disk passive air sampling was carried out to investigate the levels, vertical distributions, and potential sources of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and organochlorine pesticides (OCPs) in the atmospheric boundary layer of an urban site in Asia. Sampling was performed at nine heights (15, 47, 80, 120, 160, 200, 240, 280, 320 m) of the 325-m meteorological tower in Beijing, China over three 2-month periods between December 2006 and August 2007. This is the first study to report vertical variations of PBDEs in the ABL and one of only a few studies to investigate vertical distributions of persistent organic pollutants. The levels of sigma19PCBs and sigma8PBDEs were relatively low, ranging from 22 to 65 and from 2.3 to 18 pg m-3, respectively. Air concentrations of gamma-HCH were high, with values in the range of 39-103 pg m-3 in winter, 100-180 pg m-3 in spring, and 115-242 pg m-3 in summer, respectively. alpha-HCH concentrations ranged from 20 to 86 pg m-3, p,p'-DDT between 7.3 and 78 pg m-3, and HCB between 15 and 160 pg m-3. The seasonal variations of PCBs, PBDEs, and OCPs may reflect different sources for these chemicals, such as those related with regional use (gamma-HCH), volatilization/re-emission (PBDEs, PCBs, alpha-HCH), and pesticide impurities (HCB). Although the performance reference compounds (PRCs) were spiked before deployment, the sampling rates showed strong dependency on wind speeds, resulting in large variations in uptake rates in the ABL, ranging from approximately 7.0 m3 day-1 at ground level to 11 m3 day-1 at 320 m. Levels of PCBs, PBDEs, and OCPs decreased with increasing ABL height indicating the potential of Beijing as the local sources.
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| 46. |
- Li, Yingming, et al.
(författare)
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Reduction of atmospheric polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during the 2008 Beijing Olympic games
- 2011
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Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 45:8, s. 3304-3309
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Tidskriftsartikel (refereegranskat)abstract
- A total of 120 air samples were collected at three urban and one rural location in Beijing, China in the summers of 2007-2010, and before, during, and after the Beijing 2008 Olympic Games (BOG), in order to assess the effectiveness of long-term and short-term emission-control measures in reducing polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the atmosphere. During the BOG (August, 2008), the PCDD/Fs concentrations decreased to an average value of 1150 fg m−3 (63 fg I-TEQ m−3), which was reduced by approximately 70% from the average in 2007 and by 29% from that in July 2008, before the Olympic event began. Although 2009-2010 levels of PCDD/Fs were significantly higher than 2008, the overall temporal trend was decreasing for summer months during the sampling campaign period. The apparent half-lives of atmospheric PCDD/Fs were estimated to be 3.2-5.8 years by statistically regressing the logarithm PCDD/Fs concentrations versus the number of years passed since 2006. The air concentrations of total suspended particulates (TSP) during the BOG ranged between 135 and 183 μg m−3, showing a 52% reduction from 2007 and 26% decrease from those prior to the Olympic event. No significant relationships were found between meteorological parameters (temperature, humidity, and wind speed) and PCDD/Fs or TSP during the BOG, whereas the PCDD/Fs concentrations were significantly dependent on the air quality (p < 0.05, positive against TSP and negative against visibility). This work is one of few temporal trend studies of atmospheric PCDD/Fs in mainland China, and provides unique insight into the effects of large-scale control measures in improving air quality and reducing one of the most ubiquitous and toxic organic pollutants in the environment.
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| 47. |
- Liu, Lijun, et al.
(författare)
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Simulating two-phase flow and geomechanical deformation in fractured karst reservoirs based on a coupled hydro-mechanical model
- 2021
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier BV. - 1365-1609 .- 1873-4545. ; 137
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Tidskriftsartikel (refereegranskat)abstract
- Two-phase flow in fractured and karstified porous media subject to coupled hydro-mechanical conditions is an important issue for oil recovery in carbonate reservoirs. However, due to the co-existence of porous media flow, fracture flow and free flow, as well as their couplings with geomechanical deformation, modeling the behavior of fractured karst systems remains challenging. In this work, a novel coupled hydro-mechanical model for simulating the complex behavior of fractured and karstified porous media is developed. Two-phase Darcy's equation is used to describe fluid flow in both matrix and fractures, while the free flow in cavities is considered based on an assumption of phase instantaneous gravity segregation. A modified Barton-Bandis's constitutive model is used to mimic the nonlinear fracture deformation. The cavity deformation is solved based on the fluid pressure on the cavity boundaries. A mixed finite volume-finite element method and a fixed-stress iterative splitting method are adopted to numerically solve the coupled system of equations. The model is then applied to a series of 2D and 3D problems to unravel the impacts of fractures and cavities on two-phase flow and geomechanical deformation in fractured karst systems. The results indicate that cavities hinder water breakthrough due to storage effects, while water may quickly migrate through highly conductive fractures. Cavities tend to dominate the flow and mechanical processes even though fractures are present as well. Significant stress concentration is observed around cavities. Furthermore, the results of 3D cases imply that phase gravity segregation in cavities leads to lower water saturation in the area above cavities and delays water breakthrough.
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| 48. |
- Lyu, Zehao, et al.
(författare)
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A novel approach to optimising well trajectory in heterogeneous reservoirs based on the fast-marching method
- 2021
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Ingår i: Journal of Natural Gas Science and Engineering. - : Elsevier. - 1875-5100 .- 2212-3865. ; 88
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Tidskriftsartikel (refereegranskat)abstract
- To achieve efficient recovery of subsurface energy resources, a suitable trajectory needs to be identified for the production well. In this study, a new approach is presented for automated identification of optimum well trajectories in heterogeneous oil/gas reservoirs. The optimisation procedures are as follows. First, a productivity potential map is generated based on the site characterisation data of a reservoir (when available). Second, based on the fast-marching method, well paths are generated from a number of entrance positions to a number of exit points at opposite sides of the reservoir. The well trajectory is also locally constrained by a prescribed maximum curvature to ensure that the well trajectory is drillable. Finally, the optimum well trajectory is selected from all the candidate paths based on the calculation of a benefit-to-cost ratio. If required, a straight directional well path, may also be derived through a linear approximation to the optimised non-linear trajectory by least squares analysis. Model performance has been demonstrated in both 2D and 3D. In the 2D example, the benefit-to-cost ratio of the optimised well is much higher than that of a straight well; in the 3D example, laterals of various curvatures are generated. The applicability of the method is tested by exploring different reservoir heterogeneities and curvature constraints. This approach can be applied to determine the entrance/exit positions and the well path for subsurface energy system development, which is useful for field applications.
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| 49. |
- Ma, Yueqiang, et al.
(författare)
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Roles of heat and stress transfer in triggering fault instability in conjugate faulted reservoirs
- 2024
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier. - 1365-1609 .- 1873-4545. ; 180
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Tidskriftsartikel (refereegranskat)abstract
- The presence of multiple conjugate but non-intersecting faults in geothermal reservoirs presents issues related to fault interaction in the presence of complex coupled thermo-hydro-mechanical (THM) processes in influencing the triggering of seismicity. We examine alternate strategies in stimulating such a conjugate-faulted geothermal reservoir analogous to that hosting the Mw 5.5 Pohang earthquake (2017). We evaluate the response of the reservoir to both short-term stimulation (1y) and long-term production (10y), both with and without thermal effects - for a large fault (F1) adjacent to a non-intersecting smaller fault (F2) and in a reverse faulting stress regime. Results suggest that the slip on either fault impacts the stress state on the other fault through stress transfer. Reactivation of the minor fault (F2) transfers stress towards to the upper part of primary fault (F1), inducing instability. The slip of the major fault is delayed by positioning the location of injection away from the junction between the two faults - decreasing the injection depth from 4087.5 m to 3712.5 m delays the time to slip by 2.61 y. Furthermore, thermal stress plays a decisive role in prompting late-stage fault reactivation for long-term fluid circulation where pore pressures have already reached steady state. The pattern of thermal unloading follows the path of fluid transport and heat transfer along the faults. Overall, this study not only advances our understanding of mechanisms of injection-induced fault instability in EGS reservoirs with multiple and closely-interacting faults, but also provides insights into how different injection strategies can delay or mitigate induced seismicity.
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| 50. |
- Obeysekara, A., et al.
(författare)
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Modelling stress-dependent single and multi-phase flows in fractured porous media based on an immersed-body method with mesh adaptivity
- 2018
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Ingår i: Computers and geotechnics. - : Elsevier. - 0266-352X .- 1873-7633. ; 103, s. 229-241
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a novel approach for hydromechanical modelling of fractured rocks by linking a finite-discrete element solid model with a control volume-finite element fluid model based on an immersed-body approach. The adaptive meshing capability permits flow within/near fractures to be accurately captured by locally-refined mesh. The model is validated against analytical solutions for single-phase flow through a smooth/rough fracture and reported numerical solutions for multi-phase flow through intersecting fractures. Examples of modelling single- and multi-phase flows through fracture networks under in situ stresses are further presented, illustrating the important geomechanical effects on the hydrological behaviour of fractured porous media.
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