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Use the Previous and Next buttons to navigate three slides at a time, or the slide dot buttons at the end to jump three slides at a time.Lei Qin, Cheng Zhai, … Jizhao XuHewan Li, Siyang Sun, … Ziheng ZhangBo Li, Laisheng Huang, … Yongjie RenYutao Li, Yaodong Jiang, … Pengpeng WangZhen Wei, Ke Yang, … Ji-qiang ZhangLongxiao Chen, Kesheng Li, … Chuanxiao LiuYanbing Wang, Yang Yang, … Jianguo WangXin Ding, Jing Hou & Xiaochun XiaoDongliang Ji, Hongbao Zhao, … Lina GeScientific Reports volume 12, Article number: 18543 (2022 ) Cite this articleIn this study, the surface crack-propagation law and pore damage characteristics of coal samples of different water contents after they undergo leaching in liquid nitrogen are investigated using a 4 K scientific-research camera, HC-U7 non-metal ultrasonic detector, nuclear magnetic resonance testing technology, and self-made multi-functional three axial fluid–solid coupling test system. Experimental investigations are conducted on coal samples of different water contents before and after they undergo liquid-nitrogen freezing and thawing in order to determine the propagation law of surface fissures, the development law of internal micro-fissures, the development process of internal pores, the change law of the pore-size distribution, and the law of coal-sample deformation and gas seepage during the stress process. The test results show that , with the increase in water content in the liquid-nitrogen leaching process, the frost heave force on the coal surface increases, and the greater the increase ratio of the coal porosity, the faster is the development of micro-cracks and pores. Under the action of liquid nitrogen, the number of micro-pores, meso-pores, and macro-pores in the coal sample increased, and with the formation of new cracks and the connection of the original cracks, liquid-nitrogen freezing and thawing can promote the development of the pore structure in the coal body. The permeability changes of coal samples of different water contents during unloading failure exhibit obvious stage characteristics. The above results demonstrate that the moisture content of coal has a significant effect on the development of surface cracks and pore-damage characteristics of coal after liquid-nitrogen freezing and thawing, and there is a positive correlation between the surface crack expansion and internal damage of the coal samples of different moisture contents leached in liquid nitrogen.Coal-bed methane is an associated energy of coal as well as a clean fuel and chemical raw material. Coal mass is usually considered to comprise a coal matrix and natural fracture network. The poor permeability of a coal seam directly restricts the efficient extraction of coal -bed methane. Therefore, it is necessary to use auxiliary means to improve the permeability of the coal seam. In particular, coal-bed methane reservoirs in china have the "three lows" characteristics of low pressure, low permeability, and low saturation, and the permeability of more than 70% of the coal seams is less than 1 × 10−15 m21. Liquid-nitrogen fracturing technology is a green and efficient water-free fracturing technology2. The conventional method involves injecting a large amount of low-temperature liquid nitrogen into the coal seam through surface drilling or down-pore drilling3. As a fracturing fluid, liquid nitrogen has the advantages of no pollution, low cost, and easy preparation. It can be used effectively to address the problems of water wastage, water lock, and water sensitivity. And the cracking effect is more significant. When water-bearing coal rock undergoes liquid-nitrogen freezing and thawing, the water–ice phase transition and frost heave force comprise the main mechanism. Therefore, it is necessary to study the effect of the freezing and thawing of liquid nitrogen on coal and rock of various water contents. In recent years, scholars have conducted a significant amount of research on the effect of liquid nitrogen on the fracturing of coal.The critical vaporization temperature of liquid nitrogen is − 196 °C. If liquid nitrogen is injected into the coal body, the thermal stress generated by the sudden temperature drop of the coal body may change its pore and micro-crack structure, resulting in the change of the micro-defect structure inside the coal body. Qin et al.4,5 realized the fine and quantitative characterization of coal pore distribution in the process of liquid nitrogen cracking based on relaxation spectrum analysis technology and scanning electron microscope technology; Cai et al .6 used the low-temperature nitrogen adsorption method to find that the pore-enlarging, pore-increasing effects and the changing trends of the pore structure at all levels caused by liquid nitrogen-induced cracking of coal are proportional to the water content of the coal sample; Yan et al.7 conducted an experimental study on the propagation law of surface cracks and the change law of internal pore size distribution before and after liquid nitrogen immersion of coal at different prefabricated temperatures by using microscope observation, ultrasonic wave velocity test and nuclear magnetic resonance test technology; Wan et al.8 conducted a post-freeze–thaw NMR test on the rock samples, and obtained the development and expansion of pores of various sizes in the sandstone during the freeze–thaw cycle, and the development of the pores in the sandstone after the freeze–thaw cycle was the highest.Xu et al.9 used nuclear magnetic resonance technology to detect the internal damage changes of water-containing freeze–thaw damaged coal and found that the number of pores increases with the increase of water content; He et al.10 studied that the fractal dimension shows an increasing relationship with the increase of water content; Xu et al.11 analyzed the pore evolution law of freeze-thawed coal samples with different water contents by using sonic velocimeter and nuclear magnetic resonance equipment. The results show that the higher the water content of the coal sample, the better the permeability enhancement effect of liquid nitrogen in the coal seam. Liu et al.12 studied the strength characteristics of frozen sandstone with different initial water content; Zhang et al.13 used a gas injection displacement gas test system for loaded coal to conduct nitrogen seepage tests, and used the permeability growth rate and the average permeability growth rate to characterize the coal permeability growth level; Chen et al.14 conducted NMR detection and shear creep tests on sandstones with different water contents after freezing and thawing, revealing the influence mechanism of freeze–thaw cycles and water content changes on sandstone mesostructure and creep characteristics; Wen et al .15 studied the mechanical properties and mesostructure of rock under the action of water–rock coupling. The results show that with the increase of water content, when the sandstone specimen is tensile failure, the number and area of ​​micro-cracks increase with the increase of the strain rate, increasing trend; Song et al.16 analyzed the deterioration mechanism of rocks under the influence of water content under freezing and thawing under load through scanning electron microscope (SEM) and uniaxial compression test.Zhao et al.17 tested the permeability change of coal before and after liquid nitrogen treatment under the condition of air pressure 0.25 MPa and confining pressure 2.25 MPa. In addition, a series of liquid nitrogen anti-reflection processes are also proposed for the effect of water content. Mcdaniel et al.18 proposed to spray water in liquid nitrogen fracturing coal seams to generate ice crystals in the form of water mist to act as proppant and diverting agent; Zhang et al.19, Zheng et al.20 proposed the method of low-temperature gas-assisted coalbed gas fracturing technology, and expounded that frost heave force is the main mechanism of liquid nitrogen fracturing water-bearing coal rock. Taking advantage of this water–ice phase transition, Feng et al.21 proposed a staged fracturing method for temporary plugging of liquid nitrogen ice crystals in horizontal wells of coal-bed methane based on a single-channel packer. Coal occurs underground and is often in a saturated state due to the action of groundwater. Under the action of low-temperature fluid, the water phase in the coal seam turns into ice, and its frost heave force causes coal cracks to expand. Therefore, it is very important to study the effect of liquid nitrogen freezing and thawing on the damage law of coal samples with different water contents.The above studies all show that water can enhance the effect of liquid nitrogen on coal fracturing. Scholars mainly focus on revealing the changes of pore and fracture characteristics and mechanical properties of coal and rock mass before and after liquid nitrogen. When analyzing the seepage characteristics of coal and rock mass, scholars test the seepage characteristics of coal samples with different moisture content after freezing and thawing by setting fixed confining pressure and axial pressure. Few people can simulate the seepage law of coal samples with different moisture content after freezing and thawing under the influence of mining through experiments. Therefore, on the basis of previous studies, the author carried out liquid nitrogen leaching tests on coal samples under different water content, and emphatically analyzed the changes of coal seepage characteristics and stress sensitivity before and after leaching, in order to provide a reference for the study of liquid nitrogenn fracturing technology and coalbed methane anti-reflection theory.In addition, the application effect of liquid nitrogen fracturing is determined by a variety of influencing factors, and the ground stress of coal seam also changes in real time. In this study, 4 K scientific research camera observation, ultrasonic wave velocity test, nuclear magnetic resonance technology and other comprehensive methods were used to study the fracture development law and pore characteristics of coal with different moisture content under the condition of liquid nitrogen complete immersion, an attempt to solve the microstructure and detailed deformation and failure law of coal with different moisture content under the action of liquid nitrogen leaching. The relationship between surface crack development and internal micro-failure of coal with different water content after liquid nitrogen leaching was discussed. In order to study the stress–strain characteristics and permeability change of coal with different moisture content under the action of liquid nitrogen freeze–thaw and ground stress.In this study, a multifunctional triaxial fluid–structure interaction test system is designed based on the digital image measurement method of subpixel Angle deformation monitoring. This system provides a new method for triaxial measurement of coal samples. This method measures the deformation of coal samples by capturing and tracking the displacement of coal corner points. The measurement results show a high sub-pixel accuracy (0.02 pixels), and the accuracy analysis can reach 10–4 orders of magnitude. This method can obtain more accurate local deformation measurement of coal samples under the action of force, and further study the stress–strain and seepage characteristics of the above-mentioned coal samples under triaxial stress. Based on the stress–strain characteristics of coal and the change of coal permeability, the freeze–thaw fracturing effect of coal with different moisture content in liquid nitrogen is proved.The test coal samples were collected from the coal seam number 3 of Wangzhuang Coal Mine of Shanxi Lu'an Group. In order to reduce the influence of the anisotropy of coal, all the coal samples were obtained from a single large coal block, and the The same bedding direction was maintained during sampling. The coal samples were first wrapped in plastic wrap and shipped back to the laboratory, and their surfaces were then peeled off. The large coal block was processed into a 50 mm × 100 mm cylindrical specimen using a core drilling machine and a core cutting and grinding machine; some of the specimens are presented in Fig. 1. The specimens were then wrapped using plastic wrap and stored in a vacuum oven for later use. The coal samples were industrially analysed according to “Methods for Industrial Analysis of Coal” (GB/T212-2008). The industrial analysis of the experimental coal samples is shown in Table 1. A non-metallic ultrasonic detector was used to ultrasonically test the specimens. According to the test results, the specimens with a longitudinal wave velocity ranging from 1.85 to 1.92 km/s were selected (the specimens in this wave-velocity range have good integrity and a large number), and the coal samples were dried at a constant temperature.As shown in Table 2, the main instruments used in the test include an electric heating incubator, HC-U7 non-metallic ultrasonic detector, MacroMR12-150H-I low-temperature NMR analyser, 4 K scientific-research-grade special camera, and self-developed flow–solid coupling triaxial servo seepage test device containing gas coal.The control range of the electric heating incubator was 10–300 °C at room temperature, the degree of fluctuation was ± 1.0 °C, and the temperature resolution was 0.1 °C.For the ultrasonic detection, an HC-U7 non-metallic ultrasonic detector was used, the sampling period was 0.025 μs, the receiving sensitivity was less than 10 μV, the acoustic time measurement accuracy was 0.025 μs, the amplitude measurement range was 0–170 dB, and the emission pulse width was 0.1–100 μs.For the optical measurement and imaging, a 4 K scientific-research-grade camera was used to measure the mesoscopic damage to the coal samples. The device had a pixel size of 1.85 μm × 1.85 μm and a 200× magnification. It supported the measurement of point spacing, line spacing, and the automatic adsorption of graphic elements to improve measurement accuracy.The NMR test device used was Rec Core2500, which had a magnetic field strength of 1200Gauss, resonance frequency of 2.38 MHz, and maximum echo number of 8000. The liquid-nitrogen freezing and thawing of the coal samples of various water contents would make the transformation of the coal-pore-structure characteristics apparent. The internal pore structure of the coal samples of various water contents before and after the freezing and thawing was studied using NMR technology.The self-developed gas-containing coal fluid–solid coupling triaxial servo seepage test device is presented in Fig. 2. The test device is a multi-functional triaxial fluid–structure coupling test system. It is mainly composed of six parts: an automatic operation platform, a servo loading system, triaxial pressure chamber, gas–liquid seepage control system, data-measurement system, and auxiliary system. There are two methods of controlling the axial load component force and displacement. The parameters such as stress, deformation, gas pressure, and flow rate are collected automatically. The maximum axial pressure was 300 MPa, and the maximum confining pressure is 10 MPa. The force value and deformation test accuracy were ± 1% of the indicated value, and the force value control accuracy was ± 0.5% of the indicated value. The data recording frequency of the stress, strain, and flow rate was 2 times/s. The device could satisfy the research on the mechanical properties of confining pressureunloading and seepage law of the gas-bearing coal and rock. The three-axis coal-sample full-surface-deformation digital-image measurement system consists of a pressure chamber, base, digital image sensor (complementary metal-oxide semiconductor (CMOS) sensor) and lens, camera bracket and sealing cover, lighting device, measurement and control software, reflector, and some other parts, as shown in Fig. 3.With a flat mirror placed in the pressure chamber, the deformation and strain (field) distribution of the entire surface (360°) of the entire sample were measured using a CMOS camera, as shown in Fig. 3. This method makes the local axial The deformation and radial deformation of the coal sample. The measurement information is more abundant and the precision is relatively high. The equipment can measure the deformation of the entire coal sample under the action of force.Composition of the digital image measurement system for full-surface coal sample deformation.As shown in Fig. 3, a computer connected to the pressure gauge and flow meter monitors the upstream pressure, downstream pressure, and gas flow in real time. The axial permeability of the coal body was calculated as shown in Eq. (1):where \(k\) is the penetration (10−15 m2); \(Q\) is the gas flow (cm3/s); \(p_{0}\) is the atmospheric pressure (0.101325 MPa); \( \mu\) is the gas viscosity coefficient (Pa s), the seepage gas used in this test is high-purity nitrogen (\(\mu\) = 0.017805 Pa s); \(L\) is the height of the cylindrical coal sample (cm); \(A\) is the cross-sectional area of ​​the cylindrical coal sample (cm2); \(P_{1}\) and \(P_{2}\) are the upstream and downstream gas pressure values ​​of the system (MPa), respectively; and the downstream port is directly emptied, ie, \(P_{2}\) = \(P_{0}\) ).Preparation process of coal samples with different water saturation: The coal sample was placed in a vacuum drying oven (constant temperature of 100 °C), the weight of the coal sample was measured every 1 h until the last two coal-sample quality errors were less than 0.1%, and the dry-coal sample quality was recorded. The coal sample was placed in a vacuum saturated device with a vacuum pressure of − 0.1 MPa to saturate it with water. The coal samples were removed from the device and weighed every 6 h, until the coal sample quality no longer improved. The coal sample was then considered to be saturated with water. The mass of the saturated water-coal sample \(m_{s}\) was then recorded. The water-saturated coal The samples were again placed in a vacuum drying oven (constant temperature of 100 °C) for drying and were removed and weighed periodically during that period. The weighing time is adjusted according to actual need until the coal sample is taken out after the target dry quality is reached and immediately placed it into a sealed bag and cooled to room temperature for later use. The pre-set coal-sample moisture contents were 0%, 30%, 50%, 70%, and 100%. The calculation formula was \( m = S_{0} (m_{s} - m_{d} ) + m_{d}\) , where \(m\) is the target drying quality of the coal sample, \(S_{0}\) is the pre-set coal sample moisture content, and \(m_{s}\) is the mass of the saturated water-coal sample. The above steps were repeated to prepare the coal samples of various moisture contents.Under the same sound-wave emission frequency, the HC-U7 non-metallic ultrasonic detector was used to test the wave speed of a sound wave propagating in the coal sample.The coal samples of moisture contents of 0%, 30%, 50%, 70%, and 100% were subjected to liquid-nitrogen freeze–thaw treatment (freezing for 1 h). After the coal samples returned to room temperature, the microscopic damage on the surface of the coal samples was optically measured and imaged using a 4 K special camera for scientific research. Through image stitching and binarisation, the surface-damage fissure maps of the coal samples of various moisture contents in freezing and thawing were obtained. The box dimension was introduced to quantitatively describe the complex fracture network.The coal sample was saturated with water in a vacuum water-saturated device with a vacuum pressure of − 0.1 MPa for 12 h, such that the coal sample was fully saturated with water.A low-temperature NMR analyser was used to conduct the NMR test on the water-saturated coal sample, and the T2 distribution curve and porosity of the coal sample were obtained under various water-saturated states.In order to study the permeability of the coal samples of various moisture contents after the freezing and thawing, the experiment was performed using the experimental device presented in Fig. 3. After the coal sample was installed and sealed, the measurement point of the latex map corresponding to the coal sample was selected. After completion, the coal sample was simulated in the in-situ stress state through parameter setting, that is, the confining pressure (9 MPa) was loaded, and the axial pressure was applied as the hydrostatic pressure (9 MPa). The time consolidation method was used in the experiment to load the coal sample. The confining-pressure loading rate was 0.02 MPa/s, a total of 450 s, and a time period of 7.5 min was required for loading to 9 MPa. The hydrostatic pressure of the coal sample was set as 9 MPa.As shown in Fig. 4, at the first stage, the confining pressure and axial pressure were loaded simultaneously. In the second stage, the above pressure environment was maintained, nitrogen adsorption was performed for 24 h, and the gas pressure was 1 MPa; it was also ensured that the buffer tank was sealed and the electronic flow meter value was stable. In the third stage, the axial pressure was increased until the coal sample was close to the strength, stop adding the axial pressure, and the pressure was unloaded at a rate of 0.02 MPa/s according to the confining pressure until it was broken (with the advancement of the working face, the stress around the coal and rock was released, and the pressure was slightly reduced).Figure 5 presents the microscopic freezing swelling damage images of the coal samples having various saturation levels after intrusion and melting, which were obtained using a 4 K scientific-research-grade camera. The developing size was 8.3 mm × 4.7 mm. The pixel size of the 4 K scientific research-grade measurement camera is 1.85 μm × 1.85 μm, and the resolution is 4096 × 2160. The Scale bar of Fig. 6 is 1:500(1 μm:500 μm).Meso-photograph of coal and crack extraction after thermal shock.Extraction of crack network after thermal shock.In order to accurately describe the complexity of the fractures of different scales in the fractal space, the pixel point coverage method can be used to calculate the fractal description factor, and the end-face images of coal samples with different degrees of liquid-nitrogen freeze –thaw saturation can be binarised, as shown in Fig. 5.When calculating the fractal dimension of the fracture, the fracture sketch was place on the evenly divided grid, and the minimum number of grids required to cover the fracture grid fractal was calculated. The mesh was refined in a step-by-step manner based on changes in the required coverage in order to calculate the box dimensions. When the side length of the lattice is \({\text{r}}\) , the space is divided into \({\text{N}}\) lattices in total, then the dimension of the box is as follows, as shown in Eq. (2):In the above formula, \(D_{\text{B}}\) is the box dimension, and \({\text{r}}\) is the division scale. \(D_{\text{B}}\ ) reflects the efficiency with which the entire area is covered with small boxes of the same shape.Through graphic splicing, the evolution map of the entire end-face fracture network was obtained, as shown in Fig. 6. It can be observed from Fig. 6 that there were three types of crack evolutions owing to the thermal cracking of the end face of the coal and rock mass. These evolutions were the growth of primary cracks, development of new single macroscopic cracks, and connection of crack networks. Under the action of the cold shock, frost heave fracture occurs inside the coal rock mass. First, the primary crack expands, and new micro-cracks are created. With the intensification of frost heave rupture, small cracks gradually extend and penetrate, and macroscopic cracks are gradually formed. A large number of cracks are propagated and connected to form a complex crack network. The development process of these three types of fissure structures reflects the basic form of coal-rock frost-heave fracture evolution22.As coal is a heterogeneous material, in addition to the existence of a large number of micro-cracks, micro-pores, and other microscopic features in its interior, temperature changes change the mechanical properties of the basic components that comprise the coal sample material. Furthermore, owing to the inconsistency of its thermodynamic effects, changes are caused in the internal stress distribution of the coal body and also in the pore structure of the coal body, ie, in the generation of cracks and changes in the structural properties.It can be observed from Fig. 7 that, with the difference in the prefabricated water saturation of the coal body, the corresponding characteristic crack width and density of the coal body also changed accordingly. After freezing and thawing with liquid nitrogen, the increase ratio of characteristic fissures of the coal body with a water content of 0% was not apparent, ie, it was only 13.08%. With the increase in the prefabricated water content of the coal body, the characteristic fissure width and density of the coal sample after freezing and thawing in liquid nitrogen exhibited greater changes. When the prefabricated water content of the coal body reached 100%, the characteristic fissure width and density change of the coal body before and after the liquid-nitrogen leaching were greater than those when the water content was 0%. In the process of freezing and thawing, the coal body exhibited a crackling sound, and the cracking of the coal body was apparent.Quantitative depiction factor for cold shock damage.The overall degree of damage of the coal body is indirectly reflected in the measured acoustic-wave velocity of the coal body before and after the leaching. Moreover, the ultrasonic-wave velocity reflects the change in the density of the propagating medium and macroscopically reflects the degree of development of the millimetre-scale gas-seepage crack channel in the coal body. According to the test plan, ultrasonic-wave velocity tests were conducted on coal samples of various saturation levels before and after leaching. The test results are presented in Fig . 8.Relationship between coal samples with different water saturation rates and wave speed before and after leaching.The degree of fissure development of each raw coal sample before and after leaching23. The rate of change of the wave velocity is given in Eq. (3):In the formula, \(\varepsilon\) is the wave velocity change rate; \(v\) is the wave velocity after immersion (km/s); and \(v_{0}\) is the wave velocity before immersion ( km/s) 24.For damaged materials, when damage occurs, the following relational Eq. (4) is satisfied:where \(V_{\text{p}}\) is the longitudinal wave velocity in the coal rock after the damage has occurred (m/s), and \(V_{f}\) is the longitudinal wave velocity in the coal rock before the damage has occurred (m/s).As shown in Fig. 8, the average wave speed of the coal samples of different water contents after leaching was smaller than that before leaching, and the wave speed increments of the coal samples with the water content of 0%, 30%, 50% , 70%, and 100% were − 0.044, − 0.323, − 0.573, − 1.000, and − 1.049, and the wave velocity change rates were 2.16%, 15.80%, 24.93%, 35.32%, and 53.58%, respectively. According. to the wave-velocity increment and wave-velocity change rate of the coal samples of different water contents, with the increase in the water content, the damage in the coal samples after the freezing and thawing becomes increasingly severe.The propagation speed of the ultrasonic wave is mainly dependent on the density and elastic modulus of the isotropic, completely elastic medium25. When there are fractures in the coal body, it is no longer homogeneous and isotropic nor completely elastic. In such a case, the various elastic moduli of the coal change to a certain extent, resulting in a significant change in the propagation speed of the sound wave. The relationship between the crack spacing in the coal body and the acoustic-wave conduction velocity of the coal body can be expressed as shown in Eq. (5):In the formula, \({\text{S}}_{{\text{i}}}\) is the crack spacing (m); \(k_{s}\) is the tangential stiffness of the crack (N /m2); \(v_{s}\) is the shear wave velocity (m/s); and \(G\) is the shear modulus (N/m2).After the leaching and thawing, the frost heave stress in the coal body causes the development and generation of internal cracks, and the coal-body cracks increase or widen on the original basis, and the crack spacing thus decreases (\({\text{ S}}_{{\text{i}}}\) decreases). During leaching, the changes in the coal mass and volume are negligible, and the shear modulus remains unchanged; thus, \(v_{s}\) decreases As shown in Fig. 8, with the increase of the moisture content of the coal after the liquid-nitrogen freezing and thawing, the ultrasonic-wave velocity decreases. The ultrasonic velocity of the coal samples decreased by 30%, 50%, accordingly. 70%, and 100%, respectively, as compared with the coal samples having a moisture content of 0%. It is reflected from the side that the coal body with high moisture content will cause more damage after freezing and thawing. The internal and external primary fissures expand gradually and connect with each other to form secondary fissures. TheseIt hinders the penetration of the ultrasonic waves into the coal, thus resulting in a decrease in the longitudinal-wave velocity of the coal sample, and the relative reduction rate also decreases significantly.It can be observed from Figs. 8 and 9 that, with the increase in the water saturation, both \({\text{S}}\) and the end-face fracture network \(D_{\text{B}}\ ) exhibit an increasing trend. According to the change in \({\text{S}}\) , when the coal undergoes cold shock, the ultrasonic-wave velocity decreases with the increase in the water saturation. The higher the moisture content of the coal, the greater the decrease in the ultrasonic velocity and the greater the \({\text{S}}\) value.Correlation between cold damage quantitative description factor and intensity.