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Pore characteristics are crucial in the occurrence, aggregation, migration, and potential for CO2sequestration in shale gas reservoirs. We employed N2adsorption/desorption, CO2adsorption, mercury intrusion porosimetry (MIP), focused ion beam-scanning electron microscopy (FIB-SEM), and three-dimensional reconstruction of FIB-SEM images to characterize the pore characteristics in the Longmaxi Formation of the Sichuan Basin, southwestern China. These methods allowed for the detailed study of microstructure, porosity, and permeability down to the micropore scale (<2 nm). Meanwhile, N2and CO2isotherm analyses revealed a range of pore sizes, including micropores, mesopores (2–50 nm), and macropores (>50 nm). Micropores significantly contribute to the specific surface area, while mesopores and macropores predominantly contribute to pore volume. MIP results indicated extremely high pore tortuosity and connected porosity less than 1.43%. FIB-SEM and its three-dimensional reconstructions showed significant pore distribution within organic matter. At the FIB-SEM resolution (5 and 10 nm), pore connectivity is notably poor, with many large pores several hundred nanometers in diameter, undetected by the N2isotherm method. Permeabilities estimated by FIB-SEM are 1–2 orders of magnitude lower than those measured by MIP, exhibiting anisotropy. Assessments of gas in place and CO2storage capacity suggest that porosity evaluations via MIP may underestimate the quantifiable gas content in shale formations. The combined use of N2adsorption/desorption, CO2adsorption, MIP, and FIB-SEM techniques for integrating pore size characteristics offers a holistic perspective of the pore size spectrum in shale gas reservoirs, effectively addressing the limitations inherent in each individual method.

In numerical simulation of the mechanical responses and acoustic emission (AE) characteristics of rocks under cyclic loading, the impacts of compositional heterogeneities of mineral grains have barely been considered. This will lead to a poor reproduction of rock's behaviors in terms of stress-strain relationship and micro-seismic characteristics in numerical simulation. This work aims to analyze and reveal the impact of parameter heterogeneity on the rock's fatigue and micro-seismic properties based on PFC3D. Two distribution patterns (uniform and Weibull distributions), are implemented to assign four critical parameters (i.e. tensile strength, cohesion, parallel bond stiffness and linear stiffness) for 32 sets of numerical schemes. The results show that the models with high heterogeneity of tensile strength and cohesion can better reproduce the stress-strain relationship as well as the patterns of cumulative AE counts and energy magnitude. The evolution of the proportion of three-level AE events in the laboratory test is consistent with the numerical results when the highly heterogeneous tensile strength and cohesion are distributed. The numerical results can provide practical guidance to the PFC-based modeling of rock heterogeneity when exposed to multi-level cyclic loading and AE monitoring.

Based on the loose medium flow field theory, the loose top-coal drawing law of longwall top-coal caving (LTCC) mining technology is studied by using self-developed three-dimensional (3D) test device. The loose top-coal drawing test with shields and the controlled test without shields are performed in the condition without any boundary effect. Test results show that shields will cause reduction in drawing volume of coal in the LTCC mining. The deflection phenomenon of drawing body is also observed in the controlled test, which is verified that the deflection of drawing body is caused by shield. It is found that the deflection angle decreases with increasing caving height, with the maximum value of αtailand the minimum value of 0. In addition, the formula to calculate the drawing volume is proposed subsequently. The deflection of drawing body is numerically simulated using particle flow code PFC3Dand the proposed formula to calculate drawing volume in LTCC is also verified.

This review details the state of the art in research on top coal drawing mechanisms in Longwall top coal caving (LTCC) by examining the relevant literature over the last two decades. It starts with an introduction of the brief history and basic procedures of LTCC. The framework of research on the drawing mechanism, basic concepts, and some theoretical models of LTCC are detailed in sect. research framework of top coal drawing mechanism. The authors note that the Top coal drawbody (TCD), Top coal boundary (TCB) and Top coal recovery ratio (TCRR) are key factors in the drawing mechanism. The Body–boundary–ratio (BBR) research system has been the classic framework for research over the last 20 years. The modified Bergmark–Roos model, which considers the effects of the supporting rear canopy, flowing velocity of top coal, and its shape factor, is optimal for characterizing the TCD. A 3D model to describe the TCB that considers the thicknesses of the coal seam and roof strata is reviewed. In sect. physical testing and numerical simulation, the physical tests and numerical simulations in the literature are classified for ease of bibliographical review, and classic conclusions regarding the drawing mechanism of top coal are presented and discussed with elaborate illustrations and descriptions. The deflection of the TCD is noted, and is caused by the shape of the rear canopy. The inclined coal seam always induces a larger TCD, and a deflection in the TCD has also been observed in it. The effects of the drawing sequence and drawing interval on the TCRR are reviewed, where a long drawing interval is found to lead to significant loss of top coal. Its flowing behavior and velocity distribution are also presented. Sect. practical applications of drawing mechanisms for LTCC mines 4 summarizes over 10 cases where the TCRR of LTCC mines improved due to the guidance of the drawing mechanism. The final section provides a summary of the work here and some open questions. Prospective investigations are highlighted to give researchers guidance on promising issues in future research on LTCC.

The stability of coal walls (pillars) can be seriously undermined by diverse in-situ dynamic disturbances. Based on a 3D particle model, this work strives to numerically replicate the major mechanical responses and acoustic emission (AE) behaviors of coal samples under multi-stage compressive cyclic loading with different loading and unloading rates, which is termed differential cyclic loading (DCL). A Weibull-distribution-based model with heterogeneous bond strengths is constructed by both considering the stress–strain relations and AE parameters. Six previously loaded samples were respectively grouped to indicate two DCL regimes, the damage mechanisms for the two groups are explicitly characterized via the time-stress-dependent variation of bond size multiplier, and it is found the two regimes correlate with distinct damage patterns, which involves the competition between stiffness hardening and softening. The numerical b-value is calculated based on the magnitudes of AE energy, the results show that both stress level and bond radius multiplier can impact the numerical b-value. The proposed numerical model succeeds in replicating the stress–strain relations of lab data as well as the elastic-after effect in DCL tests. The effect of damping on energy dissipation and phase shift in numerical model is summarized.

The theoretical equation of initial top-coal boundary (ITCB) is one of the fundamental problems of top-coal drawing law in longwall top-coal caving (LTCC) mining. The granular mechanical model of ITCB is established based on loose medium flow field theory. By analysing the stressed top-coal on ITCB along the section of coal seam thickness and face advancement directions, the theoretical equation of ITCB is deduced respectively in the two conditions of first gangue drawing out and excessive gangue drawing out. The theoretical equation is further modified by a correction coefficient k, which is given by a large number of experimental data. Meanwhile, it is found that the ITCB at excessive gangue drawing out condition is in the special shape that "the top opening is elliptical and the bottom opening is rectangle". The theoretical equation of ITCB can provide meaningful guidance to the improvement of top-coal recovery ratio in LTCC.

Ground support is widely implemented to mitigate dynamic rock failures in underground mines. This paper investigated the ground support requirements in burst-prone mines to mitigate the catastrophic dynamic rock failures of rock and/or coal bursts. First, the ground support principles and considerations in burst-prone conditions are identified. The objective of a ground support system is to increase the capacity to accommodate rock fracturing in a rockburst and, in turn, to minimize the kinetic energy of the ejected material. The support capacities of various yielding rockbolts and integrated support systems are then investigated using the test results in the laboratory. Apart from the energy absorption and yielding deformation capacity, the initial stiffness and energy absorption rate are also critical factors when applying yielding rockbolts in practice. Adding rope lacing and mesh strap to surface support elements can substantially enhance the support performance of the system. In practice, semi-analytical and empirical approaches are often used to determine the ground support elements in burst-prone areas. Semi-analytical methods first evaluate the support demand in burst risk zones and then select support elements according to their laboratory test results. Alternatively, empirical methods determine the ground support elements according to the locally established empirical rating scheme, which usually ranks the support capacities of various support systems based on ground support conditions and damage conditions. The outcomes of this study can provide insights into ground support strategies and assist the mining industry to develop effective coal burst control technologies.

Discrete element calculations of the top-coal drawing process for different gangue-coal density ratios were conducted to investigate the effect of the gangue-coal density ratio on the drawing mechanism in longwall top-coal caving. The effects were analyzed for the drawing body, the top-coal boundary, and the recovery of top coal. The results show that for increasing density ratio, the initial drawing body on the goaf side is farther away from the drawing support and its width and volume gradually increase. The upper part of the sickle-shaped drawing body extends near the initial drawing body with increasing density ratio in the normal cycling stage, and the distance from the drawing body to the initial drawing body is its maximum width. The larger the density ratio, the smaller the height of the top coal above the goaf at the end of the initial drawing process. The height of the top-coal boundary decreases with increasing density ratio, until it reaches a limit. In a normal cycle, due to hysteretic development, the top-coal boundary moves toward the goaf until the density ratio is approximately 2.0, which is consistent with the physical experiment results. Finally, increasing the advance length of the working face is beneficial for increasing the overall recovery of top coal.

This work strives to experimentally unveil the combining effects of freeze-thaw (FT) action and differential cyclic loading (DCL) on the sandstone masonry. The cylindrical sandstones after different FT actions (0, 20, 40 FT cycles) were cyclically compressed by utilizing two loading modes having distinct loading and unloading rates. The testing results are presented from the perspectives of mass loss, P-wave velocity attenuation, rock deformation, energy dissipation, and stress-strain hysteresis. The results indicate that the stress pattern with a higher loading rate and a lower unloading rate corresponds to a lower fatigue strength as well as a shorter fatigue life. In addition, the stress pattern with rapid loading and slow unloading is more favorable yielding a larger amount of dissipated energy within a cycle. More applied FT cycles impose a more damaging effect on sandstone which is characterized by a lower fatigue life and a higher strain rate. Both FT actions and the stress pattern with rapid loading and slow unloading can lead to an evident elastic aftereffect, a phase lag is clearly visible for axial strain at the peak stress. The phase lag is the fundamental to the more dissipated energy under the stress pattern with rapid loading and slow unloading.

Abstract: Stability of coal pillar (walls) is of great importance to security of underground mining. Periodic excavation and water table variation subject coal pillars (walls) to cyclic load with varying stressing rates and wetting–drying (WD) circumstances. Therefore, investigation on mechanical responses of coal exposed to effects of WD and compressive differential cyclic loading (DCL) was conducted to experimentally reveal coal’s mechanical behaviors. 22 cylindrical coal samples, 7 WD schemes (natural, soaking, 0, 10, 20, 30, 40 WD cycles) and 2 DCL loading modes (loading rate is 4 times of unloading rate and vice versa) were applied. The measurements covered mass loss, P-wave velocity, energy dissipation, crack characterization and hysteretic phase shift. P-wave velocity shows an exponential attenuation versus WD cycle. Mode 1 possessing loading rate 4 times of unloading rate corresponds to a higher rate of premature failure, incurring 4 out of 7 samples failed in cyclic stage, whereas all 7 samples survived in cyclic stage in mode 2 with opposite stressing rates. Stressing rate induced impact on deformation and energy dissipation is qualitatively revealed, with 6 out of 7 groups exhibiting more dissipated energy (approx. 10–50% larger than mode 2) in mode 1. More dissipated energy is attributed to a pronounced phase lag incurred by a higher loading rate and elastic-after effect. WD action induced meso-cracks were characterized and the width of crack is proportional to applied WD cycles. The testing results can provide practical guidance for in-situ application, such as evaluating the stability of coal mine based ground water reservoirs. Highlights:

he DCL test is first time applied on coal.

WD action has a pronounced effect on P-wave attenuation for coal.

Dissipated energy is influenced by loading pattern in DCL.

A modulus related parameter is proposed to predict coal failure.

This work aims to investigate the mechanical behaviour of medium-grained sandstones under cyclic loading with different loading/unloading rates. This type of cyclic loading is called differential cyclic loading (DCL) and is considered for testing rock behaviour. In the experiments, constant amplitude and multi-level cyclic loadings were performed. Three loading modes were designed to consider different relationships between loading and unloading rates. Axial strain evolution and energy dissipation were analysed for different loading/unloading rates and maximum cyclic load level. The correlations between P-wave velocities and strengths of rocks deduced from this research are compared with existing published data. The relationships between final strength and axial strain at failure under different loading patterns were also discussed and a rough assessment of the remaining fatigue life is introduced using the predicted value by fitting the axial peak strain.