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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]

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]

Ultra-high pressure water jets (UHP-WJ) rock breaking is a promising technology for developing unconventional natural gas reserves. In this study, rock-breaking experiments were conducted using typical reservoir rock samples such as sandstone, shale, and coal. The failure patterns in rocks after impact by UHP-WJ were analyzed. Moreover, the 3D damage field model of the rock was reconstructed based on CT scanning, and the internal crack distribution was visualized and examined. Furthermore, the fracture mechanisms of three types of reservoir rocks under the UHP-WJ impingement were studied. The results indicate that intergranular fractures produced due to shear stress are the dominant type of fracture in sandstones, tensile failure due to stress wave was the main type of fracture observed in shale, and coal exhibited the double breaking characteristics of stress wave effect and water wedge effect. The fragments size distribution of samples shows the largest fragments in shale, followed by coal, and the smallest fragments are observed in sandstone. Finally, the evolution of rock pore structure before and after the impact of UHP-WJ was monitored by nuclear magnetic resonance (NMR) methods. The results reveal that the micropores transform into mesopores and macropores in the sandstone, where the total porosity increased by 14.3%–24.8%. As the jet pressure exceeds 100 MPa, the porosity of shale increases significantly, such that the total porosity of the shale sample increased by 7.6%–37.1%. The mesopores transform into macropores and fractures with the influence of the water wedge effect in coal samples, resulting in the reduction of mesopores and the increase of macropores, where the total porosity of the coal sample increased by 14.6%–40%. The complexity and heterogeneity of pores were reduced under UHP-WJ with the increasing jet pressure. The findings provide a theoretical reference for optimizing the recovery from unconventional natural gas reservoirs. [ABSTRACT FROM AUTHOR]

Coalbed methane (CBM) recovery using water jet drilling is regarded as a promising technology for high-efficiency, heat-free, and environmentally-friendly development of deep resources. An investigation of the fracture mechanism of coal subjected to a water jet under triaxial stress is essential for the future application of this technology in a deep environment. In this study, the fracture mechanism was discussed, and laboratory experiments were conducted on coal breaking with a water jet under triaxial stress conditions for the first time. The results indicate that there are distinct fracture patterns with/without triaxial stress. With the increase of triaxial stress, the main component of coal failure transitions from tensile failure due to the water wedge to shear failure due to water hammer pressure. The rock-breaking ability of the water jet was impaired significantly under triaxial stress conditions. The tensile fracture zone induced by the water jet was concentrated in the range of 60° of the maximum principal stress direction. Computed tomography (CT) scanning tests were used to document the fracture morphology inside the coal specimens. CT scanning showed that the cracks around the hole were deflected into the direction of maximum principal stress, and the area of the damage caused by the jet decreased with increasing triaxial stress. Three-dimensional (3D) reconstructions of the impacted coal were established based on slice images for visualization. The results revealed that the weak planes in the coal and the triaxial stress were the dominant factors affecting the damage characteristics. When the average stress increased 5 MPa, the radial damage range decreases approximately 58%, and the axial damage range decreases approximately 57%. The damage is concentrated in the jet impact area. These findings provide a reference for the application of water jet technology in deep CBM recovery. • The rock-breaking experiment under true triaxial stress was performed using raw coal specimens. • The fracture pattern of coal was analyzed due to coupling of the jet and triaxial stress. • Three-dimensional (3D) reconstructions of the damage field in coal were created based on slice images for visualization. • Fracture mechanism and damage characteristics of coal under triaxial stress on coal breaking by jet were discussed. [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]

Rock breaking using a high-pressure water jet is an emerging drilling technique for high-temperature deep-ground resources. The damage characteristics associated with using a water jet in high-temperature hard rocks were significantly different from those at room temperature. In order to explain the fracturing mechanisms associated with this robust drilling technology, the author conducted jet impact tests on granite and shale at varying temperatures to determine the crushing characteristics of these rock samples. The 3D reconstruction technology was utilized to characterize the internal 3D damage field and to analyze the fragmentation mechanisms of two high-temperature hard rock samples. According to this study, the degree of rock fragmentation at high-temperature condition was significantly greater than that associated with the heating-cooling and room temperature conditions. Thermal shock stress promoted large volume lamellar fractures of granite and shale, but there are maximum damage temperature conditions in shale. The stress wave and thermal stress caused by dynamic load act synergistically to cause rock mass peeling. Microscopically, the fracture characteristics of the rock samples were also examined by SEM, and quartz grains of high-temperature shale were observed to be exfoliated from their heterogeneous surroundings. • Lots of experiments on water jets impacting hard rocks with different heat treatment conditions. • The failure mechanisms, erosion depth and area, and cracks distribution of different structural hard rocks have been obtained. • Thermal stress greatly increases the efficiency of hard rock fragmentation. • Synergistic effect of stress wave and thermal stress is the key to high temperature hard rock bulk rupture. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lots of experiments on water jets impinging rocks with different structures were conducted. • Breakage patterns, failure mechanisms, erosion depth and area, and cracks distribution of the eroded rocks with different structures were obtained. • Damage field distribution in eroded rocks were characterized quantitatively. This paper aims to investigate the damage and fracture characteristics of rocks with different structures under water jet impact. Three types rocks including heterogeneous coal, homogeneous sandstone, and transversely isotropic shale, were selected as the impact target rock. Water-jet impingement experiments with jet velocities varying from 447 m/s to 774 m/s were conducted. The rock macro-breakage characteristics were obtained by describing the breakage patterns, measuring the erosion depth and area, and counting the cracks number. A 3D reconstruction method based on the computed tomography and digital image processing technology was proposed to visualize the internal breakage characteristics and quantitatively analyze the damage field distribution. Combined with fracture morphology from scanning electron microscope, the failure mechanisms of rocks with three structures under water jet impact were revealed. The results indicate that coal, sandstone and shale will experience different breakage patterns with the increasing jet velocity: layered transverse cracks → "十"-type crack network → splitting crack, only one spindle-shape erosion pit, surface debris spalling → layered transverse cracks → "T"-type crack network. The effects of jet velocity and rock structure on the evaluation indexes (such as erosion depth and damage area, the number and angle of transverse cracks) were investigated thoroughly. In addition, the relationships between total damage degree based on breakage volume and jet velocity, and between damage variable based on breakage area and erosion depth, were compared and discussed. The failure mechanisms of three rocks impacted by water jets are as follows: (i) the reflection and interference of stress wave combined with the pressured water wedge effect cause the layered breakage and longitudinal splitting breakage of heterogeneous coal; (ii) the grinding effect of back flow mainly accounts for the formation of spindle-shape erosion pit in homogeneous sandstone; (iii) the shock stress wave effect and bedding structure lead to the spalling of large shale block and layered breakage, and the reflected wave tensile accompanied with pressured water wedge effect cause the tensile splitting breakage. [ABSTRACT FROM AUTHOR]