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• Coal-breaking using water jets at different reservoir temperatures was studied. • When temperatures exceed 75.0 °C, the depth of pit increases by 81.0 %, the specific energy consumption decreases by 52.4 %. • The threshold fracturing pressure presenting an exponential trend as coal temperature increases. • Coal-breaking shifts from a "water wedge" dominated mode to a "water wedge-thermal fracture" as temperature increases. The development of abundant deep coalbed methane (CBM) resources has attracted significant attention due to their commercial development potential. However, developing such resources presents many challenges due to their generation in complex geological environments governed by high temperatures and stresses. Thus, this work investigated coal fragmentation using water jets at different reservoir temperatures. The results show that compared with room temperature conditions, under water jet impact, the threshold fracturing pressure of high-temperature coal is lower, the rock-breaking volume is larger, and thermal stress leads to intensified dynamic damage. Indeed, when the temperature exceeds 75.0 °C, the coal undergoes volume fragmentation, and the depth of water jet-breaking coal increases by 81.0 %, while the specific energy consumption decreases by 52.4 %. The threshold fracturing pressure rapidly decreases as temperature increases and stabilizes, presenting an exponential trend. When the temperature exceeds 75.0 °C, the threshold fracturing pressure is maintained at around 10.0 MPa and no longer decreases with the temperature increase. In addition, three-dimensional (3D) damage field analysis highlights that the coal breaking mode shifts from a "water wedge" dominated mode to a "water wedge-thermal fracture" mode as temperature increases. Finally, the dynamic fracturing mechanism of coal under high-temperature conditions of water jet impact has been revealed. The research results of this paper provide a theoretical basis for the hydraulic permeability enhancement design of deep coal seams. [ABSTRACT FROM AUTHOR]

Deep mineral resources are abundant, and water jet technology has been proven to be an efficient means of developing this resource. In-situ stress is one of the prominent features in deep mines. However, the dynamic damage process of rock under the in-situ stress environment subject to water jets has not been revealed. Therefore, this study aims to fill this gap. For this study, coal breaking tests were performed to reveal the macro fracture patterns of coal under different true triaxial stress conditions. The results show that the bedding angle can significantly affect the coal-breaking characteristics, and when the bedding angle is 45° combines the advantages of length and aperture for jet drilling. When there is no geostress, the width of the cracks on the wall of the fracture pit is 3 μm, with a crack width of 7.5 μm at the bottom of the hole, and no through cracks generated under triaxial stress. Subsequently, further theoretical analysis and numerical simulation showed that the in-situ stress has a hysteresis and inhibitory effect on the dynamic damage evolution of jet coal breaking, hereby affecting the final fracture mode during the period. The results of this study will provide basic theoretical support for the jet drilling of rock in deep layered rock. [Display omitted] • Water jet rock-breaking test was performed under high true triaxial stress level. • The effect of in-situ stress on the dynamic damage mechanism of rock-breaking was revealed. • The macroscopic and mesoscopic damage mechanisms of coal were elucidated. • The influence of bedding on the dynamic damage mechanism of coal has been determined using a new device. [ABSTRACT FROM AUTHOR]

Featured Application: This study can provide guidance for coal and coalbed methane mining processes in multi-layered coal seams. Coalbed methane is not only a clean energy source, but also a major problem affecting the efficient production of coal mines. Hydraulic fracturing is an effective technology for enhancing the coal seam permeability to achieve the efficient extraction of methane. This study investigated the effect of a coal seam reservoir's geological factors on the initiation pressure and fracture propagation. Through theoretical analysis, a multi-layered coal seam initiation pressure calculation model was established based on the broken failure criterion of maximum tensile stress theory. Laboratory experiments were carried out to investigate the effects of the coal seam stress and coal seam dip angle on the crack initiation pressure and fracture propagation. The results reveal that the multi-layered coal seam hydraulic fracturing initiation pressure did not change with the coal seam inclination when the burial depth was the same. When the dip angle was the same, the initiation pressure linearly increased with the reservoir depth. A three-dimensional model was established to simulate the actual hydraulic fracturing crack propagation in multi-layered coal seams. The results reveal that the hydraulic crack propagated along the direction of the maximum principal stress and opened in the direction of the minimum principal stress. As the burial depth of the reservoir increased, the width of the hydraulic crack also increased. This study can provide the theoretical foundation for the effective implementation of hydraulic fracturing in multi-layered coal seams. [ABSTRACT FROM AUTHOR]

Water jet drilling is considered a possible method used during the development of underground unconventional natural gas. Meanwhile, the fracture mechanisms of rock subjected to various in-situ stresses may differ significantly. Thus, laboratory experiments are conducted in this paper using the new jet testing system for the deep reservoir to simulate the drilling process of a water jet in the deep reservoir. The breakage mechanisms of rock are examined using the three-dimensional (3D) reconstruction visualization models of damage distributions, surface morphology, and micro-structural analysis on the influenced zone. The results revealed the dependence of two typical breakage characteristics on the stress state of the tested rock. When the samples were subjected to stress in the triaxial cell, the range of the hole was considerably reduced. Besides, as the horizontal stress difference was increased by 6 MPa, the diameter and depth of the erosion holes increase by 60% and 38.6% respectively. Moreover, the results of the scanning electron microscopy (SEM) showed that the fracturing of the hole wall was mostly caused by tensile failure generated by a water wedge that was easily controlled by high in-situ stress. The in-situ stress state of the reservoir significantly affected the pore size distribution and porosity. The increase of stress inhibited the increase of porosity around the hole. The porosity reaches 6.1% when the stress difference is 6 MPa, which increases by 135%. These results were anticipated to be used as a guide for the utilization of unconventional natural gas in deep reservoirs with complex stress. • The rock-breaking experiments under different true triaxial confining stresses were conducted using sandstone specimens. • As the stress difference was increased by 6 MPa, the diameter and depth of the hole were increased by 60% and 38.6%. • The in-situ stress states of the reservoir subject to water jets affect the pore size distribution and porosity. • The porosity reaches 6.1% when the stress difference is 6 MPa, which increases by 135%. [ABSTRACT FROM AUTHOR]

Water jet technology has been broadly applied to extract coalbed methane (CBM) from low permeability reservoirs. However, the existing studies on the failure and damage characteristics of coal subjected to water jets have neglected the influence of gangue. To study the propagation of stress waves and the fragmentation characteristics of gangue-containing coal impacted by a water jet, a numerical model was developed for gangue- and defect-containing coal based on the smooth particle hydrodynamics (SPH) method, and was verified by using laboratory experiments. The research results indicated that gangue had a significant influence on the propagation of stress waves and the fragmentation characteristics of coal. When the stress wave propagated from coal to gangue, the total reflective and transmissive regions of the stress wave existed at the interface. The interaction process of stress waves reflected and transmitted at the interface led to the formation of shear cracks near the liquid–solid contact region and the formation of tensile cracks in the other regions, which eventually formed the obvious local fragmentation characteristics. Finally, the blocking coefficient M was defined to quantify the resistance of the gangue against the stress-wave propagation. The resistance of the gangue against the stress-wave propagation was uneven, and the maximum and minimum blocking coefficients were 0.48 and 0.15, respectively. This study could lay a foundation for understanding the breaking mechanism of coal by water jets. • There are total reflective and transmissive regions of the stress wave. • A numerical model was developed for gangue- and defect-containing coal subjected to water jets. • There are the obvious local fragmentation characteristics of gangue-containing coal by water jets. • The barrier of gangue to stress-wave propagation was uneven. [ABSTRACT FROM AUTHOR]