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During underground mining excavation, dynamic disturbances (e.g., blasting) may accelerate rock creeping and induce unstable failure. It is crucial to properly quantify the impact of dynamic disturbances on creeping behavior to understand the long-term effects of underground excavation on rock media. A damage-based creep model was established in this study and implemented in Fast Lagrangian Analysis of Continua in 3D (FLAC3D) to quantify the damage and unstable failure of creeping rock triggered by dynamic disturbance. This model reveals the mechanical behavior of rock under creep-impact loading. It was verified by calculating the viscoelastic deformation around circular excavation, by reproducing the damage process under static loading, and by simulating the rock damage and failure under creep-impact loading. The model was then used to examine the mining-induced creep deformation around an operational shaft in Xincheng Gold Mine, China, where both the mining-induced creep of the country rock mass and dynamic disturbance of blast-induced vibration were considered. Numerical simulation indicated that the dynamic disturbance could accelerate the damage of rock under creep stress, which may be an important factor to trigger the accelerating creep of rock, even though the unstable failure of rock lagged behind the dynamic disturbance. The case study on the shaft stability analysis indicated that the dynamic disturbance could accelerate the time-dependent deformation of the shaft, the first dynamic disturbance induced much more shaft deformation, while in long-term creep (e.g., 5 years), the effect of blasting vibration was not significant. The trend and magnitude of shaft deformation agreed with the in situ measurements, which indicates that the numerical simulation could be used to predict the long-term shaft deformation in future. [ABSTRACT FROM AUTHOR]

Fractures in coal seams are channels for gas migration, and their geometric characteristics can affect permeability, thereby affecting the effectiveness of coalbed methane (CBM) extraction and carbon dioxide sequestration. The surface of natural fractures is rough and composed of minerals with complex shapes. At present, research on rough fracture is mainly focused on surface characteristics, with less attention given to the dynamic effects of rough structure in fractures on coal permeability considering effective stress and matrix adsorption expansion during different stages of methane extraction. In this work, we construct the complex geometric structure in the fracture surface into simplified rough elements. First, a physical model of fracture rough elements was constructed by using the analytical geometry method; meanwhile, the roughness applicable to this model was proposed, and the characterization of roughness degree of different-shaped rough elements was unified. Subsequently, a definition and an approach for selecting and computing effective fracture aperture were proposed. By taking into account the competition between effective stress and matrix adsorption expansion, the change of effective fracture aperture related to pore pressure was determined. Additionally, a novel permeability evolution model was formulated according to the cubic law and compared with the numerical simulation of other researchers for verification. The variation patterns of permeability, gas pressure and fracture aperture under diverse characteristic fracture roughness elements during CBM extraction were obtained by utilizing the proposed model. The findings indicate that the fractures with smaller roughness, higher stagger degree and gentler inclination angle of rough elements are more favorable for CBM extraction. This has inspired a new understanding of the mechanism of CO 2 -enhanced coal bed methane recovery (CO 2 -ECBM) technology in deep coal seams with regard to fracture rough element characteristics, which offers valuable theoretical support for the practical implementation of this technology. • A new physical model of rough-walled fractures is proposed that simplifies each bump on the surface into a regular rough element. • The effective fracture aperture that varies nonlinearly with the extraction process is defined. • Dynamic effect of roughness, stagger degree and shape of rough elements on methane migration is analyzed. • The mechanism of injecting CO 2 into deep coal seams to promote methane production is discussed. [ABSTRACT FROM AUTHOR]

The stress state in backfilled mine stopes is an important issue to assess the behavior of the interaction between the backfill and the surroundings or barricades. Most previous arching analyses have considered only the vertical backfilled stopes in both 3D and 2D conditions, and the 3D stress distribution that results from the arching effect in inclined mine stopes remains unclear. In this paper, based on the limit equilibrium theory, a 3D stress solution that is applicable to vertical and inclined backfilled stopes is proposed to further examine the arching effect. The solution is validated against an available centrifuge model by changing the inclination of the model. The proposed analytical solution is consistent with the numerical simulations, and it is suggested that neglecting the wall inclination causes one to underestimate the arching phenomenon. In other words, the vertical stresses at the bottom of the stope can decrease when the wall inclination is considered. Hence, when the stope is assumed in plain strain conditions, both the vertical and horizontal stresses exerted on the barricades are overestimated. [ABSTRACT FROM AUTHOR]

Relative to water-based fluids, non-aqueous fracturing fluids have the potential to increase production, reduce water requirements, and to minimize environmental impacts. Since the viscosity of supercritical CO 2 is one-tenth that of water, its density is close to that of water, and is capable of promoting sorptive rejection of methane, it is an attractive substitute for water in the extraction of shale gas and coalbed methane. The following defines a geomechanical model accommodating the interaction of fluid flow, adsorption-induced swelling stress, solid deformation and damage to quantify rock-gas interactions during supercritical CO 2 fracturing for shale gas production. The architecture of the shale is accommodated that includes both pore- and micro-crack-based porosity. According to the microcrack model representing shales with low porosity, both analytical and numerical results show that the effective stress coefficient is much smaller than unity. We analyze the potential advantages of fracturing using supercritical CO 2 including enhanced fracturing and fracture propagation, increased desorption of methane adsorbed in organic-rich portions of the shale and the potential for partial carbon sequestration. Rock-gas interactions include both the linear poroelastic response and the chemo-mechanical interaction due to sorption. Simulation results demonstrate that supercritical CO 2 fracturing indeed has a lower fracture initiation pressure and a significantly lower breakdown pressure, as observed in experiments, and that fractures with greater complexity than those developed with liquid CO 2 and water fracturing result. With increasing dynamic viscosity of the fracturing fluids, the predicted breakdown pressure also increases, consistent with experimental observations. [ABSTRACT FROM AUTHOR]

During the injection of CO 2 into coalbed for geologic sequestration, CO 2 is likely both adsorbed and dissolved in coals, and swells them slightly. When coal is swollen due to the adsorption and dissolution, the induced strain is relieved as the coal structure rearranges to a lower free energy state, causing coal damage and resulting in permeability change. This adsorption- and dissolution- induced coal damage is usually ignored in the permeability model, even though this kind of damage has been observed and detected in the past CO 2 injection experiments. In a previous study we generated a sequence of coal permeability profiles from the “V” shape as observed in experiments through a newly dual-permeability model. Based on this work, in this paper, the dual-permeability model is extended to take the coal damage induced by gas adsorption and dissolution into account according to the principle of damage mechanics, and this model is used to examine the permeability evolution during the injections of different gases, including CO 2 , CH 4 and N 2 , into coal specimen. Numerical simulation indicates that the model predicts well the permeability increase during the high injected gas pressure. In addition, since the highest gas adsorption into coal occurs for CO 2 , the coal permeability during CO 2 injection decreases most pronounced, in contrast, the permeability reduction during N 2 injection is the smallest. Also, it is confirmed that the adsorption-induced coal damage is dominated by tensile damage of coal during the injection of CO 2 . [ABSTRACT FROM AUTHOR]

As a stress wave propagates in rock, the amplitude of the stress wave will attenuate, and the waveform will dissipate, which can be equivalently characterized by the stress wave propagation in a viscoelastic material. The stress wave propagation may induce rock damage when the stress is greater than the rock strength. In this study, viscoelastic wave propagation in rock and the damage and failure process of rock induced by viscoelastic wave propagation, which are usually examined separately in existing theoretical and numerical models, were investigated under a unified framework. Based on viscoelastic damage theory and the theory of stress wave propagation in a one-dimensional viscoelastic bar, theoretical equations for spalling fracture of a viscoelastic bar were obtained. Then, the numerical model implemented by the one-dimensional wave propagation program was validated by reproducing the dynamic spalling of green sandstone bar; after that step, the program was employed to study the behavior of viscoelastic bar spalling. Based on the numerical simulation, the condition, position and time of the dynamic spalling induced by quarter sine wave propagation in the viscoelastic bar were examined, and the influence of the amplitude and frequency of the quarter sine wave and the viscosity coefficient and length of the viscoelastic bar on the spalling were quantified. [ABSTRACT FROM AUTHOR]