This experimental study probes into the evolution patterns of fine damage and mechanical and seepage characteristics of coal rocks using the coal rock triaxial servo experiment system by employing three types of pretreatment methods, namely heating, freeze-thawing with liquid nitrogen and freeze-thawing with water, with an aim to investigate the effect of temperature on freeze-thaw damage and seepage characteristics of coal rocks.The study results indicate that with an increase in preheating temperature, the degree of damage to the coal sample's end face escalates while the wave speed progressively decreases.As the temperature gradually rises, the structural stability of the coal sample diminishes, leading to increased body strain, shear strain, fragmentation, crack formation, and improved cracking effects.Freeze-thawing with liquid nitrogen causes damage to the coal sample and induces an increase in internal flow. Among the different freeze-thawing methods, the most severe damage and the highest amount of gas seepage are observed when using water-filled liquid nitrogen. This process leads to a gradual increase in gas seepage due to a decrease in perimeter pressure until a breakthrough surge occurs.Hence, engineering practitioners can explore the application of preheating-water injection-liquid nitrogen injection freeze-thaw technology to achieve efficient coal seam penetration and enhance gas extraction efficiency.
Article
Study on deformation and seepage characteristics of preheated freeze-thaw coal rocks
https://doi.org/10.21203/rs.3.rs-3222438/v1
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liquid nitrogen freezing and thawing
triaxial loading
a closer look damage
seepage
translucency
China's coal-bed methane resources are abundant and widely distributed, but there are characteristics such as complex geological conditions of coal fields, the presence of coal seams with high gas content and low permeability [1]. How to safely and efficiently extract coal-bed methane, optimize the energy structure and reduce gas disasters has become the focus of discussion among scholars from all walks of life.
With the research and development of liquid nitrogen fracturing technology, the method of using liquid nitrogen ultra-low temperature fracturing coal seam to extract coal bed methane has received attention from scholars at home and abroad, and scholars have carried out a series of researches around the theory of liquid nitrogen fracturing coal body, fracturing effect and damage law of low temperature freezing and thawing coal body.Coetzee Sansh et al [2] chose liquid nitrogen as the fracturing medium and found that liquid nitrogen was very effective in fracture development and extension, and freeze-thawing with liquid nitrogen under dry saturated conditions revealed a reduction in the number and volume of pores, an enlargement of microfractures and an increase in pore size.Winkler [3] concluded that the freezing and swelling forces generated by the volume expansion of water freezing at low temperatures are a major factor in local damage to coal rocks.Wang Qiao et al [4] used NMR and CT techniques for the freeze-thawing process of liquid nitrogen to obtain the fracture development damage. The results showed that the mechanical strength decreased and the gas permeability increased after freeze-thawing of liquid nitrogen, forming a good fracture development network, and the degree of damage increased with the increase of water content.Wei et al [5] discussed the rupture mechanism, thermal stress distribution and pore water migration in liquid nitrogen freeze-thaw, and the heterogeneity of coal rock damage.Li et al [6] revealed the mechanism of fracture development in coal samples under different freeze-thaw cycles in terms of the variation of mechanical property parameters and pore structure development of coal.Yang Zhaozhong and others [7] studied the effect of low temperature conditions on the permeability of coal rocks, showing that the increase in permeability of coal samples increases exponentially with increasing temperature difference.Wei et al [8] demonstrated that low temperature promoted the development of pore fractures in coal samples, and the pore volume, porosity and permeability of coal samples increased after low temperature treatment.
In contrast to low temperature freezing and thawing, high temperatures can also cause varying degrees of damage and changes to the mechanical properties of the coal rock.Xu Jiang et al [9] warmed up the gas-bearing coal body, and within a certain temperature range, the internal pore fissures increased densely after the coal sample was damaged, the channels for gas flow were widened, the permeability of the coal seam became larger, and the gas could be better extracted.Richter [10] found that the high temperature thermal expansion behaviour of rocks is influenced by multiple factors such as temperature, heating power, porosity and density.Qi Xiaohan et al [11] investigated the effect of thermal shock on the fine-scale damage and mechanical properties of coal rocks.Xu Jiang et al [12] conducted a study of temperature and raw coal permeability under triaxial stress conditions and found that the effect of temperature decreases as the permeability increases, and the effect of temperature diminishes under the effective stress and gas pressure.Li Zhiwei et al [13] studied seepage characteristics based on damage analysis of coal bodies at elevated temperatures and obtained that the magnitude of seepage increases with increasing temperature.Li et al [14] introduced methane and helium for different temperature percolation tests and when the thermal stress was high, the permeability became larger with increasing temperature and decreased vice versa.Wei et al [15] measured the factors influencing water content on percolation at different temperatures and obtained that permeability increases with temperature, and within a certain temperature, enhanced pressure leads to a decrease in permeability.
The existing literature for liquid nitrogen freeze-thaw and different initial temperature coal rocks is more detailed and comprehensive in its studies of the effects of macroscopic damage to coal rocks, but there are fewer studies of combined high and low temperature damage to coal bodies.As both high and low temperatures can damage the structure of the coal rock, in order to further fracture the coal body to extract gas, this paper carries out experiments on the basis of preheated drying of coal rock with liquid nitrogen freeze-thaw treatment, fusing preheated drying and low temperature liquid nitrogen.The enhanced permeability of fractured coal bodies is accompanied by changes to the original structure and properties of the coal rock, which can create safety issues.Therefore, the damage damage characteristics and changes in mechanical properties of coal rocks were investigated by conducting triaxial circumferential pressure unloading experiments before and after freezing and thawing of preheated coal rocks with liquid nitrogen at different circumferential pressures.This paper provides a deeper understanding of the deformation rules and characteristics of liquid nitrogen freeze-thaw coal rocks, simulates the deformation characteristics of coal rocks under the influence of mining stress before extracting gas from pre-mining boreholes after liquid nitrogen freeze-thaw, and provides reference for further improving the theory and technology of liquid nitrogen fracturing coal bodies.
2.1Experimental Equipment.
The specimen preparation and pre-processing equipment include: HZ-15 Electric Core Drilling Machine, SHM-200 Double-Sided Grinding Machine, Electronic Balance, Vernier Caliper, Liquid Nitrogen Freezing and Thawing Tank, Vacuum Saturation Chamber, and Electric Blast Drying Oven.
The parameter measurement devices include: HC-U7 Non-metal Ultrasonic Detector with a sampling period of 0.025 µs, receiving sensitivity less than 10 µV, sound velocity measurement accuracy of 0.025 µs, amplitude measurement range of 0 to 170 dB, and pulse width of 0.1 to 100 µs as shown in Fig. 1 (a). CS200 Gas Mass Controller, which, when connected to the experimental setup, allows real-time monitoring of gas flow rate by the gas from the coal sample outlet, and synchronously acquires data at a rate of one sample per second with the triaxial experimental equipment, as shown in Fig. 1 (b).
The mechanical loading and unloading device is the HC-SPT-100 High-Pressure Triaxial Testing System, as shown in Fig. 2. This system consists of a rock material testing machine and a visual measurement system, capable of simulating the ground stress conditions of deep underground for coal and conducting analyses of the mechanical properties of coal under high confining pressure. It can monitor the real-time changes in coal deformation evolution through the equipment graph measurement function, accurately measure parameters such as stress, strain, volumetric strain, and shear strain under high-pressure conditions. Furthermore, it facilitates research on coal and rock fracture and energy change mechanisms during high-pressure processes.
2.2 Coal Sample Selection and Preprocessing
The raw coal used in this study was taken from the 9106 production face of Wangzhuang Coal Mine in Changzhi City, Shanxi Province. The extracted large pieces of low-grade coal were processed into standard cylindrical specimens with a diameter of 50 mm and a height of 100 mm.
The prepared standard coal samples were uniformly placed in an electric blast drying oven for constant-temperature drying. During this process, a digital electronic balance was used to measure the mass. The coal samples were considered fully dried when their mass remained unchanged after three consecutive measurements. To reduce the influence of original cracks and cleavage variations on the experimental results, a non-metal ultrasonic detector was used to measure the velocity of ultrasonic waves in the coal samples. The samples with consistent wave velocities were selected for the experiments. A total of 12 coal samples were chosen for the study, divided into 3 groups, with 4 specimens in each group. The samples in each group were subjected to different temperature treatments (heated to 50°C, 75°C, 100°C, and 200°C, and held at each temperature for 2 hours). The first group underwent only the heating treatment, the second group was subjected to liquid nitrogen freezing after heating (immersed in liquid nitrogen for 1 hour), and the third group was treated with saturated water and then liquid nitrogen freezing after heating. The ultrasonic characteristics of the coal samples under different preprocessing conditions are presented in Table 1.
Table.1 Ultrasonic characteristic parameters of coal samples with different pretreatment conditions
| Treatment Conditions | Processing Temperature / ℃ | Wave Velocity v / (km·s-1) | Time T / µs | Amplitude A / dB |
|---|---|---|---|---|
| Heating | 50 | 1.81 | 55.32 | 110.04 |
| 75 | 1.69 | 55.40 | 108.63 | |
| 100 | 1.41 | 61.22 | 101.34 | |
| 200 | 1.23 | 70.57 | 96.38 | |
| Liquid Nitrogen Freezing and Thawing | 50 | 1.54 | 62.36 | 101.12 |
| 75 | 1.51 | 63.44 | 93.65 | |
| 100 | 1.35 | 77.38 | 81.26 | |
| 200 | 1.21 | 78.63 | 97.64 | |
| Saturated Water and Liquid Nitrogen Freezing and Thawing | 50 | 1.40 | 80.21 | 85.34 |
| 75 | 1.23 | 81.48 | 83.12 | |
| 100 | 1.02 | 82.43 | 80.87 | |
| 200 | 0.83 | 85.19 | 84.36 |
2.3 Experimental Method
The Wangzhuang Coal Mine has a mining depth of approximately 450 meters. Therefore, the static water pressure is set at 9 MPa, and the gas pressure is set at 1 MPa. The loading path involves confining pressure loading until failure. The experimental procedure is as follows:
The pressure chamber is pressurized at a constant rate of 0.02 MPa/s to raise the static water pressure and axial pressure to 9 MPa.High-purity nitrogen gas at 1 MPa is introduced into the pressure chamber. The gas flow rate is allowed to stabilize.After achieving a stable gas flow, the confining pressure is unloaded at a constant rate of 0.02 MPa/s until failure occurs.The experiment is then terminated, and throughout the process, the parameters of stress, strain, and gas flow rate are automatically recorded and saved by the computer.
During the experiment, the coal samples are subjected to the specified confining pressure and gas pressure conditions to simulate the stress environment at the depth of the coal seam. The loading and unloading process is closely monitored and recorded to study the mechanical behavior and energy change mechanisms of coal under high-pressure conditions.
3.1 Effect of temperature on fracture quality of coal samples
When the coal sample is at room temperature, there are tiny cracks, the length of the cracks is short and the width of the cracks is very small, after the heating treatment, the original micro cracks of the coal sample develop into obvious cracks, and with the further increase of temperature, the crack opening of the coal sample further increases, other parts without cracks also appear small cracks one after another, coal heating moisture loss, organic matter decomposition and volatilisation will make the coal lose weight, and the quality decreases, analysis of the quality decrease after the heating treatment can get the effect of temperature on quality, the formula is as follows.
Where:\(Δ \varvec{m}\)- Percentage of mass loss, %.\({\varvec{m}}_{1}\)-initial mass of the coal sample, g.\({\varvec{m}}_{0}\)-Mass of coal sample after treatment, g.
A coal sample specimen was selected and subjected to a gradual heating process, with the mass loss of the sample set at 0g initially and the degree of mass loss increasing with increasing temperature, as shown in Fig. 3.
As shown in Fig. 3, the quality of the coal sample decreases with the gradual increase in temperature, and the mass loss at 75°C is twice that of the coal sample at 50°C. The same is true for 100°C compared to 75°C. The degree of inclination increases steeply when the temperature of the coal sample reaches 100°C, indicating that the treatment at 100°C leads to serious damage to the coal sample and substantial loss of internal mineral composition, with a high mass loss ratio of 8.53% compared to the initial mass loss ratio of 200°C, and a mass loss ratio of 9.86%, indicating that 100°C has brought absolute damage to the coal sample and the effect of continuing to increase the temperature to impact the coal sample is reduced.
High temperatures have a very significant thermal damage effect on coal rocks, and the high temperature conditions produce a degree of expansion of the crystal particles within the coal rock. Due to the different degrees of response of different mineral particles to temperature, uneven expansion is produced, and the minerals squeeze each other, resulting in interaction forces, which in turn leads to thermal cracking, and the fracture opening of the coal sample increases as the temperature rises, with an increasing number of crack bars.The end faces of the coal samples at different temperature conditions were photographed and analysed at the same scale, and it was found that the degree of pore fracture variation increased with increasing temperature, with the end face fractures gradually appearing meshed, as shown in Fig. 4.
Figure 4 shows the end face of the same coal sample under different temperature conditions. As the temperature of the coal sample gradually increased, numerous tiny fissures sprouted on the end face of the coal sample on the basis of the full development and expansion of the original fissures, and the denseness and width of the fissures increased substantially with the increase in temperature, while the pore fissures on the end face of the coal sample below 100°C developed slowly, the expansion of the fissures was not obvious, and the width of the fissures did not become significantly larger, indicating that the structure The degree of damage is low and there is room for further development.
The fractures on the end faces of the coal samples after heating at 100°C were significantly more dense, forming a dense connected network of fractures, indicating that the internal crystalline particles of the coal samples started to expand unevenly at 100°C, and that the metal and minerals within the structure volatilised and reacted, squeezing and deforming each other under tension, weakening the mechanical properties and severely damaging the structure.
2.2 Analysis of coal sample wave velocity variation
The non-metallic ultrasonic detector was used to measure the change in wave velocity of the coal samples before and after heating, freezing and thawing with liquid nitrogen after heating and freezing and thawing with water-filled liquid nitrogen after heating. The wave velocities of the heated coal samples, the heated liquid nitrogen freeze-thawed coal samples and the water-filled liquid nitrogen freeze-thawed coal samples are shown in Fig. 5.
As can be seen from Fig. 5, with the gradual increase of the temperature to which the coal samples were subjected, the wave velocity curves under all three pre-treatment methods showed a linear decrease, indicating that the temperature affects the internal structure and mineral composition of the coal rock, and the higher the pre-treatment temperature of the coal sample, the more fractures in the coal rock, and the more obvious its fracture-causing effect.
At the same temperature, the wave velocity of the water-saturated liquid nitrogen freeze-thawed coal sample was less than that of the liquid nitrogen freeze-thawed and heated coal samples, indicating that the damage to the coal body was exacerbated by the water-saturated liquid nitrogen freeze-thawing and increased porosity, resulting in a slower ultrasonic transmission rate.
3.1 Surrounding pressure unloading stress variation study
During the reciprocating cutting motion of the coal shearer, the cutting blade experiences stress unloading as it moves in and out. Therefore, the unloading confining pressure experiment is conducted under the condition of stable axial pressure. The study aims to investigate the mechanical characteristics and seepage behavior of coal samples under the influence of mining. To achieve this, coal samples are subjected to various treatments, including heating, liquid nitrogen freezing and thawing after heating, and saturated water and liquid nitrogen freezing and thawing after heating, similar to the conditions of axial pressure loading. Subsequently, the coal samples undergo confining pressure unloading experiments with a consistent unloading rate of 0.02 MPa/s, the same rate used during the confining pressure loading. The stress-strain data of the coal samples under different experimental conditions during confining pressure unloading are presented in Table 2. The confining pressure unloading failure curves for the coal samples are depicted in Fig. 6.
| Experimental conditions | Temperature/°C | Surrounding pressure strength/MPa | Axial strain/% | Radial strain/% | Volumetric strain/% |
|---|---|---|---|---|---|
| Heating | 50℃ | 4.30 | 2.54 | 0.12 | 0.89 |
| 75℃ | 4.09 | 2.75 | 0.26 | 1.55 | |
| 100℃ | 4.05 | 2.60 | 0.45 | 2.42 | |
| 200℃ | 3.85 | 2.17 | 0.56 | 3.22 | |
| Liquid nitrogen freeze-thaw | 50℃ | 4.01 | 2.35 | 0.16 | 1.04 |
| 75℃ | 2.80 | 2.36 | 0.34 | 2.37 | |
| 100℃ | 2.25 | 1.54 | 0.53 | 2.76 | |
| 200℃ | 2.05 | 2.13 | 0.85 | 3.21 | |
| Water-filled liquid nitrogen freeze-thaw | 50℃ | 2.98 | 1.54 | 0.39 | 1.50 |
| 75℃ | 1.62 | 2.44 | 0.55 | 2.02 | |
| 100℃ | 1.25 | 2.89 | 0.89 | 2.18 | |
| 200℃ | 0.97 | 4.58 | 0.90 | 4.34 |
As shown in Fig. 6, the crushing pressure points of heated coal samples, heated liquid nitrogen freeze-thawed coal samples and water-saturated liquid nitrogen freeze-thawed coal samples at the unloading of the peritectic pressure, the size of the peritectic pressure varies for different coal samples unloaded at the same temperature, and the crushing pressure decreases with the increase of the processing temperature of the coal samples. The change pattern of the crushing pressure point of the liquid nitrogen freeze-thaw coal samples and the water-filled liquid nitrogen freeze-thaw coal samples was similar, showing a large decreasing trend.
The perimeter pressure data of the unloading crushing of the 50°C coal sample is the largest, 4.30 MPa, and the rest of the specimens are under the perimeter pressure at the crushing point. The perimeter pressure starts to unload and drop from 9 MPa, and its circumferential constraint force decreases, and the stress of the coal sample specimen originally wrapped and squeezed by the pressure is released, and the stress energy given by the axial pressure starts to expand outward as the constraint effect decreases, and crushes at the corresponding perimeter pressure unloading point of the coal sample. The poorer the structural stability of the coal sample itself, the wider the range of accumulated energy, the less likely it is to break when the circumferential pressure is unloaded, i.e. the lower the circumferential pressure at the breaking point.
3.2 Radial and volumetric strain studies
In the perimeter pressure unloading crushing experiments, the curve changes before the perimeter pressure drops are basically the same as the strain curve during axial pressure loading. The axial strain, radial strain and volume strain of the specimen in the triaxial perimeter pressure unloading experiments reflect the general law of strain of the perimeter pressure unloading specimen, and the axial strain and radial strain of the perimeter pressure unloading at different temperatures are plotted as 7.
Figure 7 shows the axial and radial strains at different temperatures. It can be seen that in the heated coal samples and the liquid nitrogen freeze-thaw samples after heating, the axial strain does not increase with the temperature, as the compressive strength of each coal sample is different, and the axial pressure is only loaded to 40MPa at this time and maintained stable, so there is no obvious pattern in the axial strain in the heated and liquid nitrogen freeze-thaw samples;
In the water-filled liquid nitrogen freeze-thawed coal samples, the strain curves in both the axial and radial directions increased with increasing temperature, especially for the axial strain, indicating that the pore fractures in the vacuum-filled liquid nitrogen freeze-thawed coal samples were fully developed and the fracturing effect of the samples was much better than that of the liquid nitrogen freeze-thawed and heated coal samples.
For the radial strain of the unloaded coal samples, the radial strain of the 50°C coal sample undergoing peritectic unloading was the smallest at 0.12%, due to the relatively intact structure of the coal sample, low brittleness and high stability. The radial strain at 50°C was used as the reference for the radial strain of the peritectic unloading, and the radial strain increases at 75°C, 100°C and 200°C were 116.67%, 275.00% and 366.67%; the radial strain at 50°C of the liquid nitrogen freeze-thawed coal sample was also used as the reference for the radial strain of 0.16, and the radial strain increases with the increase of temperature were 112.50%, 231.25% and 431.25%; the radial strain of the water-filled The increase in radial strain with increasing temperature was 41.03%, 128.21% and 130.77% for the liquid nitrogen freeze-thawed coal sample at 50°C. The increase in radial strain was significant due to the small value of the radial strain itself, and the specific magnitude of the value was much lower than the percentage.The axial and radial strains for the same temperature perimeter pressure unloading are shown in Fig. 8.
As shown in Fig. 8 for comparison of different treatment conditions at the same temperature, i.e. comparison of radial strains between freeze-thawing with liquid nitrogen and freeze-thawing with saturated liquid nitrogen after heating, it can be seen that the radial strains of the saturated liquid nitrogen freeze-thawing coal samples are the largest, with the radial strains at different temperatures, the radial strains of the coal samples at 50°C, 75°C, 100°C and 200°C are 0.12%, 0.26%, 0.45%, and The increase in radial strain under freeze-thawing of liquid nitrogen was 33.33%, 30.77%, 17.78% and 51.79%; the increase in radial strain under freeze-thawing of water-saturated liquid nitrogen was 225.00%, 111.54%, 97.78% and 60.71%. This indicates that under the same temperature conditions, the freezing and thawing of water-filled liquid nitrogen will increase the volume of water freezing to expand the fissures and cause serious structural damage, and the coal samples have the best fracturing and penetration effect.The volumetric strains during unloading at different temperature perimeter pressures are shown in Fig. 9.
As shown in Fig. 9, the volumetric strain curves of the coal samples under different temperature conditions of peritectic unloading, it is obvious that the volumetric strain of the coal samples at 200℃ is the largest and the structural deformation is the largest, and the volumetric strain of the coal samples at 100℃, 75℃ and 50℃ decreases in order, before the coal samples start peritectic unloading, the maximum volumetric strains of the heated, heated liquid nitrogen freeze-thaw and heated water-filled liquid nitrogen freeze-thaw coal samples at 50℃ are The increase in volumetric strain of the heated coal samples was 74.16%, 171.91% and 261.80% with increasing temperature; the increase in volumetric strain of the liquid nitrogen freeze-thawed coal samples was 127.88%, 165.38% and 208.65%; the increase in volumetric strain of the water-saturated liquid nitrogen freeze-thawed coal samples was 34.67%, 44.00% and 189.33%, This indicates that the temperature causes the internal structure of the coal samples to break down, and the coal loses weight due to the loss of moisture and volatilisation of organic matter, which in turn leads to the development and expansion of fissures, and the effect of fracture and permeability increases with increasing temperature.
The volume strains at the initial and crushing moments of the 100°C heated coal sample, 100°C heated liquid nitrogen freeze-thaw and 100°C heated post-saturated liquid nitrogen freeze-thaw coal samples were selected for crushing at peritectic pressure unloading to produce contours as shown in Fig. 10.
The left panel in Fig. 10 shows the volumetric strain at each corner point of the axial pressure loaded coal sample at the initial moment, and the right panel shows the volumetric strain at the moment of crushing. The initial moment has structural integrity and a small range of volume variation, and at the moment of crushing, the volume strain increases. From the distribution of strain contours, it can be seen that the initial moment contour arrangement is relatively sparse and the strain is small; at the moment of breakage, the distribution of strain contours is messy, the pores inside the coal rock are densely distributed or even penetrate to the whole specimen, and the coal body is severely damaged. The high degree of body strain indicates that all parts of the coal rock are deformed, with more penetrating fissures and the coal rock splitting into smaller, more finely divided coal bodies.
3.3 Shear strain studies
The definition of the shear strain classification and the proportion of the integrated degree of deformation during the unloading of the enclosure is the same as during axial loading, and the shear strain data at the final moment of the enclosure is shown in Table 2.The integrated deformation of the shear strain is shown in Fig. 11.
| temperature | experiment condition | 5 ≥ X>0 | 10 ≥ X>5 | X>10 | Comprehensive deformation degree A |
|---|---|---|---|---|---|
| Calef | 50℃ | 129 | 13 | .2 | 19.1 |
| Add temperature liquid nitrogen freeze melt | 127 | 13 | 4 | 19.9 | |
| Add subsistence water and nitrogen freeze melt | 123 | 16 | 5 | 21.2 | |
| Calef | 75℃ | 119 | 19 | 6 | 22.5 |
| Add temperature liquid nitrogen freeze melt | 118 | 21 | 5 | 22.7 | |
| Add subsistence water and nitrogen freeze melt | 111 | 30 | 3 | 24.6 | |
| Calef | 100℃ | 117 | 21 | 6 | 23.1 |
| Add temperature liquid nitrogen freeze melt | 116 | 19 | 9 | 23.7 | |
| Add subsistence water and nitrogen freeze melt | 107 | 27 | 10 | 26.5 | |
| Calef | 200℃ | 113 | 24 | 7 | 24.4 |
| Add temperature liquid nitrogen freeze melt | 110 | 27 | 7 | 25.3 | |
| Add subsistence water and nitrogen freeze melt | 106 | 21 | 17 | 27.5 |
As shown in Fig. 11, the integrated shear strain of the coal sample after crushing by perimeter pressure unloading is defined as the integrated deformation of the coal sample to reflect the situation after crushing, and the curve shows an increasing trend with increasing temperature.Using the comprehensive deformation degree of coal samples at 50℃ as the benchmark, the increase of comprehensive deformation degree of heated coal samples with increasing temperature was 17.80%, 20.94% and 27.75% respectively; the increase of comprehensive deformation degree of liquid nitrogen freeze-thaw coal samples at 75℃, 100℃ and 200℃ was 14.07%, 19.10% and 27.69%; the increase of water-filled liquid nitrogen freeze-thaw coal samples was 16.04%, 25.00% and 29.72%, 25.00%, 29.72%. The size of the integrated deformation indicates the degree of fragmentation of the coal sample at the point of fragmentation, and also reflects the stability of the original experimental coal sample itself, with a large integrated deformation of the coal sample having fully developed pores, good fracturing and permeability, and fractures forming a network with each other, and vice versa.The shear strain distribution is shown in Fig. 12.
Figure 12 shows the distribution of shear strain during unloading crushing, it can be seen that the range of shear strain variation during unloading crushing is mainly distributed in the range of 0%-5%, the number of shear strain exceeding 10% is much smaller than that of axially loaded crushing, indicating that the degree of shear deformation in unloading crushing is smaller than that in axially loaded crushing.However, it can still be found that as the temperature gradually increases, the amount of moderate shear deformation and severe shear deformation data increases, and the amount of shear deformation data increases, indicating that the degree of fragmentation increases, due to the poor structural stability of the coal sample itself, with more cracks and a good cracking effect.
The change of gas flow inside the coal sample, before the perimeter pressure started to unload, was consistent with the axial pressure loading process, i.e. the initial gas flow size depended on the degree of internal structural pore fracture of the coal sample itself, with the increase of the axial pressure, the gas flow decreased rapidly, the perimeter pressure unloading experiment was to load the axial pressure to 40MPa and maintain it stable, the perimeter pressure was unloaded at a uniform speed, the circumferential stress constraint of the coal sample was reduced, the coal sample was subjected to The circumferential stress constraint of the coal sample is reduced and the compression of the coal sample is gradually reduced, therefore the gas flow efficiency is increased, the whole process of gas flow in the perimeter pressure unloading experiment is shown in Fig. 13.
Figure 13, the magnitude of gas flow rate of the heated, post-heated liquid nitrogen freeze-thaw and water-saturated liquid nitrogen freeze-thaw coal samples after treatment at 50°C and 75°C was much smaller than that of the 100°C and 200°C treated coal samples, and the gas flow rate changed to a small extent, i.e. only a slight decrease, during the increase in axial pressure to 40 MPa, indicating that the compressible space of the coal samples under such conditions was small and the gas flow channels were relatively stable, while the 100°C and 200°C coal samples showed a significant drop in gas flow rate during the rise in axial pressure, with the pressure causing the pore fissures to become smaller or even closed.The flow of gas circulating within all coal samples is at its lowest value at the start of the unloading drop in circumferential pressure and rises gradually as the binding force decreases until the sample is broken and the gas flow spikes.
Surrounding pressure unloading coal samples at hydrostatic pressure 9MPa state, gas seepage law and axial pressure loading coal samples gas flow change law consistent, that is, the higher the temperature of the coal samples, the more serious structural damage, the greater the initial gas flow, after heating water-filled liquid nitrogen freeze-thaw coal samples of damage is much greater than the damage of liquid nitrogen freeze-thaw coal samples.
The gas flow rates of the heated, heated liquid nitrogen freeze-thaw and water-filled liquid nitrogen freeze-thaw coal samples at 50°C were 0.39 L/min, 0.51 L/min and 1.05 L/min, as a benchmark, the gas flow rate increase at 75°C was 69.23%, 150.98% and 182.86%, and when the temperature was increased to 100°C and 200°C, the gas flow rate increase of the coal samples were 584.62%, 570.59%, 1031.43% and 3161.54%, 7621.57%, 8942.86% respectively.The seepage volumes during unloading at different temperature perimeter pressures are shown in Fig. 14.
Figure 14 shows a graph of the gas flow rate from the start of the unloading of the perimeter pressure to the crushing of the coal sample at a stress state of 40 MPa axial pressure and 9 MPa circumferential pressure. It can be seen from the graph that the gas flow rate gradually increases as the constraint force decreases, and the gas flow rate grows rapidly for the high temperature coal sample, which also has the largest drop in gas flow rate at the axial pressure constraint due to its own loose structure. The horizontal coordinate in the graph is the circumferential pressure, it can be seen that in the unloading crushing of heated coal samples, the circumferential pressure at the crushing point is similar, the longer the curve indicates that the circumferential pressure is smaller when the circumferential pressure unloading crushing, i.e. the better the coal samples own fracture development, the longer the unloading energy release, reflecting the best effect of fracturing and increasing the permeability of the heated water-filled liquid nitrogen freeze-thawed coal samples, stronger than the heated liquid nitrogen freeze-thawed coal samples, stronger than the heated coal samples, on the other hand, it indicates that as the coal samples are treated On the other hand, it indicates that as the temperature of the coal samples increases, the structure is severely damaged and the cracking and penetration effect is enhanced.
(1)The quality of the coal rock decreases as the temperature increases. The preheat treatment temperature is positively proportional to the degree of end-face damage and inversely proportional to the wave velocity. Under the same temperature conditions, the degree of structural damage is greatest in water-filled liquid nitrogen freeze-thawed coal samples, followed by liquid nitrogen freeze-thawed coal samples and finally heated coal samples.
(2)The higher the preheating temperature, the more serious the structural damage, and the increase in body strain, radial strain and shear strain of the coal sample.
(3) In the process of unloading the perimeter pressure, the perimeter pressure at the breaking point of the coal sample is negatively correlated with the temperature, the freezing and thawing of liquid nitrogen leads to structural damage of the coal sample, and the damage of liquid nitrogen freezing and thawing is the most serious after full water, so the gas seepage is the largest, and the decrease of the perimeter pressure leads to a slow increase of the gas seepage until the breaking surge.Therefore, the use of preheating - water injection - liquid nitrogen freeze-thaw technology can be explored in engineering practice to achieve efficient coal seam penetration, providing a reference for the application of liquid nitrogen freeze-thaw fracturing technology for coal bed methane extraction.
Author Contributions: P.W. and T.Z. was responsible for the conception of experimental conditions and data analysis to write the manuscript; X.Q. was responsible for providing experimental funding, experimental guidance, and revision of the manuscript; Y.L., S.H., X.W., and X.Y. were responsible for the experiments, data collection, and collation. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Key Laboratory Open Fund Project of the Ministry of Education (Project No. JSK202010) and the National Natural Science Foundation of China (Project No. 52274205).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The experimental data used to support the results of this study are available from the corresponding authors upon request.
Conflicts of Interest: The author declares no conflict of interest in this article.
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No competing interests reported.