To study the seepage and deformation characteristics of coal at high temperatures, coal samples from six different regions were selected and subjected to computed tomography (CT) scanning studies. In conjunction with ANSYS software, 3D reconstruction of CT images was used for the establishment of fluid-solid conjugate heat transfer model and coal thermal deformation model based on the microstructures of coal. In addition, the structure of coal was studied in 2D and 3D perspectives, followed by the analysis of seepage and deformation characteristics of coal at high temperatures. The results of this study indicated that porosity positively correlated with the fractal dimension, and the connectivity and seepage performances were roughly identical from 2D and 3D perspectives. As the porosity increased, the fractal dimension of coal samples became larger and the pore-fracture structures became more complex. As a result, the permeability of coal samples decreased. In the meantime, fluid was fully heated, generating high-temperature water at outlet. However, when the porosity was low, the outlet temperature was very high. The average deformation of coal skeleton with different pore-fracture structures at high temperatures showed a trend of initial increase and subsequent decrease with the increase of porosity and fractal dimension. The maximum deformation of coal skeleton positively correlated with connectivity but negatively correlated with the fractal dimension.

Study on seepage and deformation characteristics of coal microstructure by 3D reconstruction of CT images at high temperatures

Rock burst is one of the main coal and rock dynamic disasters in steeply inclined and extremely thick coal seams (SIETCS). Timely identification of potential precursor information enables effective and specifically targeted measures to mitigate hazards. To provide a reference for the determination of precursor information in SIETCS with similar conditions, an in situ investigation that lasted for more than 1 year was conducted on the + 450 horizontal no. B3 + 6 fully mechanized top-coal caving face of the Wudong coal mine. This mine has an average dip angle of 87° and experienced a total of three rock bursts. The investigation focused on the consequences of rock burst (including the source location, energy, damage range, and modes of ensuing damage), the temporal and spatial evolution laws of the monitoring parameters of microseismic (MS) and acoustic emissions (AE), and the relationship between precursory information of both the AE and MS monitoring systems. Furthermore, a comparison of the evolution laws of the precursory characteristics between steeply and gently inclined coal seams was conducted. The obtained results indicate that rock burst failure mainly shows a directionality from south to north of the roadway, and the main factor causing the rock burst was the suspended rock pillar. Prior to the rock burst, sources of MS events gradually clustered and congregated around the rock pillar or coalface, and high-energy events (energy > 103 J) noticeably increased. Similar variations of sharp rise–sharp drop were found for both daily total energy and event counts. Prior to rock burst or large energy mine tremors, the AE energy deviation continued to rise, a peak was observed in the total number of high-value daily AE deviations, and the precursory information of AE was ahead of the precursory information of the MS monitoring system. The evolution laws of MS precursor information prior to rock burst differed between SIETCS and gently inclined coal seams. These evolution laws can be used as precursory warning for rock burst in SIETCS. Integrated monitoring of the AE and MS monitoring systems realizes the real-time monitoring and early warning from the germination stage to the occurrence of rock burst, and can determine the danger area in space. This can improve the early warning efficiency and provides a solid foundation for safe mining in SIETCS.

Precursor of Spatio-temporal Evolution Law of MS and AE Activities for Rock Burst Warning in Steeply Inclined and Extremely Thick Coal Seams Under Caving Mining Conditions

Before a coal mining operation, it is necessary to carry out coal-bed methane (CBM) pre-extraction to prevent coal and gas explosions, causing accidents. However, gas extraction and coal mining will lead to coal damage, which results in a change in the gas migration law. Especially, under deep mining conditions, the gas migration mechanism is more complicated owing to a high ground temperature. Coal permeability is the most important constituent that determines gas flow properties. Therefore, the coal permeability evolutionary law related to damage-induced conditions under different temperatures should be further researched. In this paper, a series of triaxial seepage experiments during the whole stress-induced process, and in the changing of the effective stress process were carried out. The results have shown that during the whole stress–strain process, with an increase in axial strain, coal permeability gradually decreases to a minimum value at first, then increases sharply, and finally keeps nearly constant. A higher temperature resulted in a lower elastic modulus, peak strain, and peak strength, but it caused higher thermal damage. When the coal fractures, coal permeability increases with the increase in temperature. During a change in the effective stress process, higher temperatures resulted in higher permeability. Under higher effective stress, the impact of temperature on permeability was not significant. Based on the above results, a novel damage-based permeability model was developed to describe the permeability evolutionary law caused by damage-induced conditions under compaction, and in a fracturing situation. In the proposed model, an exponential function has been used for the combination between permeability and damage variable. The damage variable is composed of thermal damage and mechanical damage. In addition, the damage variable has been modified by introducing a modified function of the initial damage. Finally, the proposed model has been applied to fit two sets of experimental data available. The fitting results showed that the proposed permeability model could well reflect the permeability behaviors of damage-induced coal at different temperatures.

A Novel Damage-Based Permeability Model for Coal in the Compaction and Fracturing Process Under Different Temperature Conditions

To investigate the acoustic emission (AE) precursors of coarse-grained hard rock instability, an experimental study on the rockburst and slabbing process of granite was carried out using a true triaxial test system. The evolution of the AE signals was monitored and analyzed in terms of the AE hit rate, fractal dimension of the AE hit number, AE count rate, b-value, dominant frequency and microcrack type. The test results show that after rock slabbing occurs, the AE precursors that can be used to predict the final dynamic instability (rockburst) are as follows: indicators such as the AE hit rate and AE count rate suddenly increase and then suddenly decrease; the AE hit rate exhibits a “quiet period”; during the “quiet period”, a small number of high-amplitude and low-frequency hits occur, and the signals corresponding to shear fracture continue to increase. The AE precursors for the final static instability (spalling) are as follows: both the AE hit rate and the b-value continuously decrease, and intermittent sudden increases appear in the high-frequency hits or the AE count rate.

Acoustic emission precursors of static and dynamic instability for coarse-grained hard rock

A rockburst prediction model of the entropy weight grey relational backpropagation (BP) neural network is developed. The model needs to select the evaluation factors according to the engineering practice and establish the sample library. The entropy weight method is used to calculate the objective weight of the characteristic factors, and the similarity between the samples is calculated by the combination of grey relational theory and the entropy method. The training sample of the BP neural network is selected by threshold determination. Finally, we use the trained neural network to estimate the rockburst intensity grade of samples to be tested. This model is applied to the rockburst prediction of Qamchiq tunnel project, and the prediction results are in good agreement with the actual conditions of the subsequent construction, thus verifying the feasibility and effectiveness of the model in the rockburst prediction.

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Rockburst Prediction Model Based on Entropy Weight Integrated with Grey Relational BP Neural Network

A three-parameter permeability model for the cracking process of fractured rocks under temperature change and external loading

Rock burst is a severe disaster in mining and underground engineering, and it is important to predict the rock burst risk for minimizing the loss during the constructing process. The rock burst proneness was connected with the acoustic emission (AE) parameter in this work, which contributes to predicting the rock burst risk using AE technique. Primarily, a rock burst proneness index is proposed, and it just depends on the heterogeneous degree of rock material. Then, the quantificational formula between the value of rock burst proneness index and the accumulative AE counts in rock sample under uniaxial compression with axial strain increases is developed. Finally, three kinds of rock samples, i.e., granite, limestone and sandstone are tested about variation of the accumulative AE counts under uniaxial compression, and the test data are fitted well with the theoretic formula.

Rock burst proneness prediction by acoustic emission test during rock deformation

Heat treatment on shale reservoirs can promote the development of secondary fractures in a matrix on the basis of hydraulic fracturing, forming multi-scale gas–water seepage channels and strengthening the gas desorption. Experimental evidence shows that heat treatment can enhance gas recovery in the same mining life. Heat treatment on a shale gas reservoir is a multi-physical and multi-phase coupling process. However, how the thermal stimulation interacts with nonlinear two-phase flow in heterogeneous shale volume fracturing has not been clear. In this paper, a fully coupled THGM model for heating-enhanced shale-gas recovery in heterogeneous shale reservoirs is proposed. First, the governing equations are formulated for the shale-reservoir deformation involving both gas adsorption and thermal expansion, the permeability evolution model for the cracking process of fractured shale, the gas–water two-phase continuity equation considering the effects of gas solubility and the heat transfer equation for heat conduction and convection. The interactions among stress, temperature and seepage in a heterogeneous shale reservoir were studied. Secondly, a test on shale permeability after 50 °C temperature treatment was conducted. The evolution of temperature, capillary pressure, water and gas saturation and the permeability of shale during the heat treatment of the reservoir were numerically analyzed. Finally, the gas production from a shale gas reservoir was numerically simulated with this THGM model. The numerical results indicated that the thermal-induced fracturing, gas desorption and separation from water make predominant contributions to the evolution of permeability. The heat treatment can enhance cumulative gas production by 58.7% after 27.4 years of heat injection through promoting gas desorption and matrix diffusion.

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A Thermal-Hydraulic-Gas-Mechanical Coupling Model on Permeability Enhancement in Heterogeneous Shale Volume Fracturing

The in-situ combustion–explosion fracturing technology in shale reservoirs can promote continuous fracture expansion with a radial detonation wave first converging into a shock wave and then decaying into an elastic wave. The transformation scale of the shale reservoir is determined by the range of wave propagation during combustion–explosion. As wave propagation paths are usually tortuous and fractal, the previous integer wave models are not competent to describe the wave propagation and estimate the impact range of the combustion–explosion fracturing process. This study develops two fractional wave propagation models and seeks analytical solutions. Firstly, a novel fractional wave model of rotation angle is proposed to describe the process of detonation waves converting into shock waves in a bifurcated structure. The radial displacement gradient of the detonation wave is represented by the internal expansion and rotation deformation of the shale. Secondly, another fractional wave propagation model of radial displacement is proposed to show the process of a shock wave decaying into an elastic wave. Thirdly, the proposed models are analytically solved through the fractional variable separation method and variational iteration method, respectively. Analytical solutions for rotation angle and radial displacement with fractal time and space are obtained. Finally, the impacts of the branching parameter of the detonation wave converge bifurcation system, aggregation order of detonation compression wave, and different types of explosives on the rotation angle of the shock wave are investigated. The propagation mechanism of the primary wave (P-wave) with time and space is analyzed. The analytical solutions can well describe the wave propagation process in fractured shales. The proposed fractional wave propagation models can promote the research of wave propagation in the combustion–explosion fracturing process of shale reservoirs.

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Fractal Analysis for Wave Propagation in Combustion–Explosion Fracturing Shale Reservoir

The paths of a two-phase flow are usually non-linear and discontinuous in the production of shale gas development. To research the influence mechanism between shale gas and water, several integer two-phase flow models have been studied but few analytical solutions have been obtained on shale gas and water pressure. This study first developed a local fractional mathematical model for gas and water two-phase flow in shale gas production. The model thus created considers the effects of capillary pressure, the fractal dimension of the flow pipe, and the discontinuity of the flow path. Second, the local fractional traveling wave method and variational iteration method were applied to this model for the development of iterative analytical solutions. Both shale gas and water pressure were analytically derived. Third, the depressurization process of the shale gas and water was analyzed, and a parametric study was conducted to explore the impacts of fractional dimension, entry capillary pressure, and travel wave velocity on shale gas pressure. Finally, our conclusions are drawn, based on the results of these studies.

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Xiao-ji Shang

Papers

A three-parameter permeability model for the cracking process of fractured rocks under temperature change and external loading

Rock burst proneness prediction by acoustic emission test during rock deformation

A Thermal-Hydraulic-Gas-Mechanical Coupling Model on Permeability Enhancement in Heterogeneous Shale Volume Fracturing

Fractal Analysis for Wave Propagation in Combustion–Explosion Fracturing Shale Reservoir

Fractal Analytical Solutions for Nonlinear Two-Phase Flow in Discontinuous Shale Gas Reservoir