查看更多>>摘要：A comprehensive analysis of crack initiation and propagation in coal is critical for understanding its mechanical behavior and its impact on permeability. The finite element-based digital volume correlation (DVC) of time-resolved X-ray tomography images under cleat-parallel uniaxial compression provides 3D incremental strain fields to understand the evolution and propagation of cracks. In this study, DVC coupled with quantitative image analysis was used to understand the role of cleats in guiding and initiating cracks in coal. Simulation of fluid flow through coal at different loading segments provides dynamic quantification of evolution in permeability. After each loading stage, quantification of strain eigen vectors and image analysis indicates appearance, coexistence, and coalescence of tensile and shear cracks guided by the organic matter in coal. We established that constructive interference between tensile and shear crack systems enhances permeability and decreases tortuosity by almost 3x and 7x respectively in coal by widening crack aperture, however, destructive interference is likely to lead to matrix pulverization and clogging of larger crack apertures. During compression, while one part of the coal matrix shows extension perpendicular to the loading direction, another part shows slippage parallel to the loading direction. Based on the variation in crack propagation behavior, four distinct crack morphology zones were identified and discussed. We found that the tortuosity of the cracks decreases exponentially, while permeability and crack volume fraction increase exponentially up to 2.8 mu m(2) and 28.6% respectively with increasing load. The box-counting-based fractal dimension for the cracks shows a steady increase from 1.58 to 1.78 with loading due to the evolution of existing cracks, and the formation of new cracks.
Li, Charlie C.Zhao, TongbinZhang, YubaoWan, Wenkai...
8页
查看更多>>摘要：Strain burst occurs within highly stressed hard rocks in underground excavations. A prerequisite for strain bursting is that excess energy exists to eject material after rock failure. Both the bursting rock and the surrounding rock mass will contribute to the excess energy. However, the proportion of the energy contributed from each is currently not well understood. This issue will be quantitatively investigated in this study. In this paper, the energy sources for strain bursting and the energy conversion during bursting are first illustrated in two conceptual models. The portions of the energy dissipated and released from the bursting rock and the surrounding rock mass are differentiated by means of numerical modeling. The numerical modeling shows that the proportion of energy released from the surrounding rock mass increases with increasing burst depth. Beyond a critical burst depth, the energy released from the surrounding rock mass becomes higher than the energy stored in the bursting rock. This concept was further verified by an analytical solution to the behavior of a circular opening when its radius is enlarged. Based on the study, it can be concluded that the magnitude of the burst is mainly determined by the strain energy released from the bursting rock itself in a shallow burst event, but it is more associated with the energy released from surrounding rock mass in a large scale burst event.
查看更多>>摘要：In this work we use the peridynamics theory of solid mechanics to simulate fracture in an annular rock domain subject to an in-situ stress and create surrogate models that predict the area of the resulting cracks. Peridynamics is a non-local formulation of continuum mechanics that naturally accommodates material discontinuities. Furthermore, unlike other fracture modeling techniques there is no need to provide information about the crack path. We utilize the peridynamics code Peridigm and take a two-stage approach to fracture modeling. First an implicit solve is performed to compute the in-situ stress state. We then execute an explicit solve where a pressure loading designed to emulate fluid-driven hydraulic fracture is applied at the borehole and transmitted to the pre -stressed rock. We present results from polynomial and single and multi-level Gaussian process surrogate models constructed from a sampling study of the peridynamics model. The surrogates predict crack area given a measure of the in-situ stress anisotropy and rise time and amplitude of the pressure loading. These surrogates take a minuscule fraction of peridynamics model's running time to evaluate and are a step towards enabling advanced optimization and uncertainty quantification workflows that require many model evaluations.
查看更多>>摘要：This paper investigates the effects of the gradient distribution of microdefects on wave propagation through a rock mass. A piecewise three-characteristic lines method is proposed that can consider the gradient distribution of microdefects. A rock mass with a microdefect gradient distribution was explored and compared to wave propagation through a rock mass with a uniform distribution of microdefects. Subsequently, the effects of gradient and frequency on the particle stress transmission coefficient, particle velocity transmission coefficient, and energy transmission coefficient were discussed. The results indicate that a rock mass with a microdefect gradient distribution significantly affects wave propagation. The traditional study of wave propagation in a rock mass with a uniform distribution of microdefects can be considered a special example of the present study. In addition, the particle velocity transmission coefficient and energy transmission coefficient are always less than 1.0. However, when the gradient is large and the frequency is small, the particle stress transmission coefficient is larger than 1.0, which shows that the particle stress amplitude in a rock mass with a microdefect gradient distribution will be amplified.
查看更多>>摘要：A three-dimensional CFD-DEM simulation method was developed to investigate the influence of different reservoir fluids on sand production. The fluid properties and reservoirs were prescribed and modelled after the conditions in the Kenkiyak and Uzen oil fields in Kazakhstan. A cylindrical sandstone sample was modelled using the Discrete Element Method in three stages of the pluviation, diagenesis, and perforation processes as in the oil field. A coupled simulation with the Computational Fluid Dynamics was used to simulate the sand production from the perforated sandstone under different fluid flow conditions with light and heavy oils. A continuous sand production was found to be more severe with the heavy oil flow due to its higher transport capability and a microstructural sanding mechanism that mobilized a higher proportion of the sample's particles with the fluid outflow. Sand production for light oil occurred in two stages of a first transient sand production that was followed by a second continuous sand production. The transient sand production was associated with more significant bond breakage, particle movement and porosity change within the damage zone that was created by the central perforation inside the sandstone sample.
查看更多>>摘要：In this study, we investigated the failure process and evolution mechanisms in circular tunnels under a complex stress path of deep in-situ stress + excavation unloading + stress adjustment using a simulation test based on loading first and then drilling (LFTD) mode. This test was performed on cubic sandstone specimens using a true-triaxial testing system combined with a self-developed drilling unloading test device. For comparison, another set of sandstone specimens, similar to the original specimens but with phi 25 mm circular through-holes, were pro-cessed to perform another set of simulation tests based on drilling first and then loading (DFTL) mode. Under constant lateral stress, compared with the DFTL test, the initial failure vertical stress was lower, and the sur-rounding rock was more prone to failure in the LFTD test, indicating that drilling unloading causes damage to the surrounding rock, thereby inducing a strength-weakening effect. Under constant lateral and vertical stresses, sidewall failure in the LFTD test was more severe. The sidewall's initial failure vertical stress difference between the LFTD and DFTL tests increases with increasing lateral stress or initial hydrostatic pressure. The effect of drilling unloading becomes more apparent under a higher initial stress state. Comparing the same types of simulation tests under different stress states, the initial failure vertical stress increases with the lateral stress or initial hydrostatic pressure, inducing a strength-strengthening effect in the surrounding rock. Comparing the strength-strengthening effect caused by increasing the lateral pressure or initial hydrostatic pressure and the strength-weakening effect caused by drilling unloading shows that the strength-strengthening effect is more evident.
查看更多>>摘要：The DECOVALEX Project is an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. DECOVALEX stands for "DEvelopment of COupled Models and VALidation against EXperiments ". The creation of this here referred to as DECOVALEX-2019, started in 2016 and ended in 2019. Modeling teams from 13 international partner organizations participated in the in-depth overview of these collaborative research efforts and how these have advanced the state-of-the-art of understanding and modeling coupled THMC processes.
查看更多>>摘要：The problems of cost, environmental protection and efficiency of hard rock fragmentation in petroleum exploration, drilling and other projects need to be solved urgently. Electro-pulse-boring (EPB) technology has attracted attention due to its advantages of low rock breaking energy consumption, low wear and low time cost. However, there is still no more accurate description model for EPB and rock fragmentation, and the existing models have limited guidance on the parameter optimization of EPB tools. This paper introduces an electric breakdown model, which realizes the whole electric breakdown process under a single high-voltage pulse by coupling the five fields: circuit field, current field, breakdown field, heat transfer field and solid mechanical field. The multi physics model comprehensively considers the circuit structure parameters of the electric impulse generator (EIG), the initiation and development of electric breakdown, the dependence of electric breakdown strength and time, and the heterogeneity of rock. The model results found that the breakdown channel originates from the weak electric strength in the rock, and the development of the channel is highly dependent on the heterogeneity of the rock medium. The rock fragmentation process with a single pulse is accompanied by local electric fragmentation and hydroelectric fragmentation, When the pulse voltage rise time (PVRT) is greater than 500 ns, the hydroelectric fragmentation mode is dominant. The temperature and thermal stress values of the discharge channel are consistent with the previous studies results, and the temperature distribution at the electrode tip is the highest, which shows that the ablation problem of the electrode in EPB process can not be ignored. This model has the potential to establish the relationship between rock-breakingparameters and rock-breaking efficiency of EPB, and can provide a practical method for the development and parameter optimization of EPB and rock breaking tools.
查看更多>>摘要：High-porosity granular rocks are moisture-sensitive solids, in that their strength and deformability are modulated by the relative humidity of their environment. Here, a novel continuum breakage-damage framework is developed to characterize and simulate the inelasticity of cemented granular materials subjected to changes of relative humidity. A microstructural model is proposed to describe the evolution of the solid-fluid interfaces emerging from grain breakage and cement disintegration. The proposed thermodynamic framework links the microstructural model to the energy dissipation and macroscopic rate-dependence of sandstones. The performance of the model is assessed against experimental data for sandstones subjected to loading under both dry and wet conditions. It is shown that the proposed model can accurately predict the yielding and stress-strain response of sandstones by capturing the moisture-weakening effects. The simulations of the model indicate that the increase of moisture lowers the yield stress and reduces the brittleness of the post-yielding response of variably saturated cemented granular materials. It is shown that the rate of damage and breakage growth control the distortion of the yield surface. When inelasticity is dominated by damage, cement bonds are disintegrated and the yield surface shrinks, thus resulting into augmented brittleness. By contrast, and in agreement with experimental evidence, the response of lightly cemented granular solids is found to be dominated by the breakage of the skeletal grains. As a result, changes in relative humidity are predicted to be accompanied by hardening behavior.
查看更多>>摘要：In cold mountainous regions, the freeze-thaw cycling often leads to rock weathering, which might trigger spalling, significant landslides and rockfalls. In this work, we used combined experimental, theoretical and numerical approaches to investigate the mechanisms of frost cracking as a result of coupled effects of freeze-thaw cycling, confining stress and the interaction of multiple cracks. The experimental facility with a temperature cycling chamber and a high-pressure cell has been developed. We used rock-like materials (rock analogue samples made from cured cement and quartz-sand mixtures) to conduct the experiments. The rock-like samples contain pre-existing single or double initial cracks, which are then water filled and exposed to freeze-thaw cycling. The coupled thermal-hydro-mechanical modeling for analyzing fractures induced by freeze-thaw cycling was achieved by using a code that was previously developed named TOUGH-AiFrac. Good agreements between the TOUGH-AiFrac modeling and the laboratory experiments have been achieved including the exact paths of crack propagation. The experimental and numerical results show that the frost cracks tend to propagate in the direction of maximum principal stress. The results further show that the interaction effect between two frost cracks is significantly influenced by the position, orientation and offset of initial cracks, as well as the orientation of intact rock bridges between the cracks. At last, the stress shadow that was calculated by the TOUGH-AiFrac model between two cracks was discussed.
NSTL
Elsevier