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The geothermal resource originating from the hot dry rock is a form of clean and renewable energy, which has the advantages of stable, efficient, and permanent. The foremost obstacle hindering its development resides in the construction of fracture channels for seepage and heat transfer within deep reservoirs characterized by low porosity and permeability. The conventional hydraulic fracturing method has high initiation pressure, single fracture shape, and difficult expansion under the constraint of deep high stress. Therefore, a liquid nitrogen cyclic cold shock method was proposed in this study. Liquid nitrogen is injected into the high-temperature rock periodically, inducing robust thermal stress through the temperature shock effect, thereby forming a complex fracture network. To study the damage mechanism of hot dry rock influenced by the temperature shock effect, the hot dry rock was subjected to a cyclic liquid nitrogen cooling treatment. The changes in temperature distribution, pore structure, and mechanical characteristics were tested and the cascade failure mechanism of “temperature shock-pore development-mechanical damage” was analyzed. The main conclusions are as follows: liquid nitrogen cold shock causes a temporal-spatial drastic change in the temperature field, marked by a high cooling rate and a large temperature gradient, and the induced thermal stress reaches up to 6.75 MPa. The number of pores increases, the size expands, and the increase in micro and mesopores is most significant, with a maximum porosity of 10.45%. Under the influence of high-temperature differences and multiple cold shocks, the number of macropores and fractures increases, giving rise to the formation of an interconnected network of fractures. This further increases the plasticity of the hot dry rock, reducing the tensile strength to only 1.70 MPa with a smaller damage threshold. Temperature shock induces mineral grain shrinkage, generating tensile stress at the boundaries between grains. When its value exceeds the tensile strength, tensile cracks will be generated. Pores and cracks mainly occur at the boundaries and within the interior of quartz grains. Temperature is the main controlling factor of hot dry rock damage, and the cyclic cold shocks serve to ensure sustained damage. The differences in the thermal expansion coefficients of mineral grains, the rapid cooling of liquid nitrogen, and the fatigue damage of cyclic shock are the main reasons for the damage of hot dry rock.

Currently the law of heat and mass transfer and the analysis of coal damage in the process of injecting low-temperature liquid nitrogen (LN2) into the borehole of coal seam are the key problems to be solved urgently for the application of LN2 in the field of coal seam permeability enhancement. By using flowmeter, endoscope, thermocouple, mass balance and ultrasonic detector, the initial accumulation process and liquid level changes of LN2 during its injection into a coal borehole were visualized and analyzed. On this basis, the variation laws of the actual liquid level, net liquid level, and injection efficiency of LN2 in the borehole with time at different injection rates were investigated, and the coal body was ultrasonically detected for damage at three different times. It was found that the initial accumulation of LN2 will form an obvious Leidenfrost effect at the interface of the coal at the bottom of the borehole. With the continuous reduction of the coal temperature at the bottom of the borehole, the radiation heat transfer coefficient decreases, and then the thickness of the vapor film decreases. At different injection rates, the maximum increment of the net level in a 1 m borehole is about 35 cm, which means that when LN2 is injected into a long borehole, it should be injected segmentally, and the increase of injection time of LN2 will lead to the waste of LN2. The ratio of actual liquid level to net liquid level decreases first and then tends to be stable. The ratio of two liquid levels is similar in stable state. The change rule of LN2 injection efficiency with time at different rates is reduction-increase-stability-reduction. The injection efficiency of LN2 is higher at low injection rate. When the injection rate is increased, the nitrogen gas with phase change per unit volume of LN2 will have shorter heat exchange time in the borehole and lower discharge temperature outside the borehole, thus increasing the loss of LN2 and lowering the injection efficiency. Ultrasonic detection results show that as LN2 penetrates the fractures, the cold shock effect can freeze and shrink the fractures in both sides. As a result, the original fractures grow wider and their tips are stretched and deformed by the freezing and shrinking forces on both sides. Consequently, new fractures are formed.

The range of the cold shock of low-temperature liquid nitrogen on coal, the variation law of thermal stress in coal caused by temperature difference and the mechanical action mechanism of coal damage are the key issues to be studied urgently in the application of liquid nitrogen in the field of coal and rock permeability enhancement. Using the non-contact full-field strain measuring device, infrared temperature distribution measuring device and the thermocouple temperature measuring device, the variation laws of the coal temperature field and strain field around the borehole with time are measured after the liquid nitrogen cold shock on the borehole of coal samples in the open environment. By establishing a thermal-stress model including elastic modulus, Poisson’s ratio, thermal expansion coefficient, temperature difference of measuring points and radius parameters of measuring points, the experiments are designed to measure the parameters of models and the parameters are input into the models for solution. The thermal stress at different positions in coal and rock and its variation law with cold shock distance and time are calculated, and the above laws of three types of coals with different metamorphic degrees are compared. The results show that in the process of liquid nitrogen cold shock, the temperature at each measuring point of three types of coal samples all shows a variation law of rapid decrease-slow decrease-stability, and the thermal stress at each measuring point has experienced three stages: rapid growth-slow growth-zero growth. The closer to the cold shock borehole, the greater the temperature difference between adjacent measuring points, and the greater the thermal stress. After 1 800 s of liquid nitrogen cold shock, the thermal stress at each measuring point of three kinds of coal samples basically reached a stable stage, and the thermal stress at 1 cm from the cold shock drilling hole was ≥ 1.84 MPa. The cold shock of liquid nitrogen can cause four kinds of deformation in the coal, and then damage the coal. Among them, the type II, III, IV deformation caused by “pressure-induced tension” and the compound deformation of three are the fundamental reasons for the formation of complex cracks in the coal. However, although the range of cooling and deformation of different coals is different, the overall range of action is limited, and it can only produce damage around the borehole. Therefore, the liquid nitrogen cold shock technology is suggested to be applied in combination with fracturing technologies, and the damage around the borehole after cold shock can effectively reduce the initiation pressure and make the fracturing fracture more complicated.

Liquid Nitrogen (LN2) cold shock on coal reservoir is a promising technology, and its complex fracture networks are directly related to the temperature effect of each component in coal caused by temperature change. Among them, the typical temperature effects include the expansion difference of different mineral particles, the evaporation of pore water and the volume expansion of gas heated, and the phase transition of water into ice. The above-mentioned effects will all occur in the process of cold shock and temperature returning of coal samples, and further influence the generation of fractures. In this paper, the relationship between minerals, moisture and porosity of six kinds of coals in the range of −30 °C–40 °C and the overall thermal expansion coefficient of coal samples is explored by using low-temperature thermal expansion coefficient tester, low-field nuclear magnetic resonance tester, XRD diffraction analyzer, CT scanner and muffle furnace industrial analyzer. It is found that the main minerals in coal are generally lower than the thermal expansion coefficient of the whole coal. The moisture and porosity of coals are proportional to the overall thermal expansion coefficient of coal samples. Among them, the porosity of small holes accounts for the highest proportion, and its internal moisture and gas have the greatest influence on the overall thermal expansion of coal, which indicates that the temperature effect of pore water evaporation and gas volume expansion in pores plays an important role in the process of coal temperature change.

The hard roof difficult to collapse easily causes gas accumulation, which threatens the production safety of coal mine. Therefore, roof pre-cracking is required. Although blasting and hydraulic fracturing can also crack the roof, blasting can easily induce rock bursts, whereas hydraulic fracturing needs complex equipment. In contrast, soundless cracking demolition agents (SCDAs) with noise-free, dust-free, and safe characteristics have obvious advantages. The main component of SCDA is calcium oxide, which reacts with water to produce higher expansion pressure. In this paper, focused on the angles of the borehole, the effect of SCDA is analyzed by numerical simulation based on Pingdingshan coal mine. The research results showed that the azimuthal angle α (between borehole projection and the roadway direction) does not significantly affect the efficacy of SCDAs, whereas the influence of borehole elevation angle β is far more significant than that of the azimuthal angle. Therefore, the angle β is a dominant factor influencing the effect of SCDAs. Based on different effects of SCDAs at different angle of boreholes, the weakening unit was established, so the SCDAs could give full play to roof fracturing. Moreover, field tests validated the importance of borehole angle on weakening the hard roofs.

Liquid nitrogen (LN2) cracking is a new waterless cracking technology, and the efficiency of cracking coal from different angles is obviously different. Therefore, the boiling heat transfer of LN2 in contact with media with different stratification angles is studied in this paper. It is found that the longitudinal and transverse wave velocities of ultrasonic waves are positively correlated with the bedding angle of coal samples. With the increase of bedding angle of coal samples, the rate of temperature reduction of coal samples at the initial stage of LN2 quenching first drops and then rises, and the coal temperature decreases fastest in parallel bedding direction. The central temperature of coal samples from different bedding angles has experienced four stages: rapid decline-slow decline-rapid linear decline-slow decline, among which stage 3 lasts the longest and the temperature reduction rate exceeds 50%. The quenching effect parameters are defined, and the quenching effect of four stages is stage 3 > stage 4 > stage 2 > stage 1. Prolonging the time of stage 3 can improve the quenching efficiency of LN2. According to the RPI model, the change of the side wall temperature of coal sample is rapidly decreasing-slowly decreasing, and a low temperature layer with high thermal resistance is formed after rapid decrease, which can inhibit the heat transfer from coal to LN2.

With the rapid development of the economy, underground utility tunnel has become an important infrastructure and “lifeline” for major cities. Through the investigation of the fire form of the pipe gallery, it was found that the most dangerous fire in the underground utility tunnel is the integrated pipe gallery power cabin, which is mainly represented by cable fires. This paper mainly focuses on the special T-shaped structure of the underground pipe gallery. The FDS software is used to simulate the occurrence of cable fires, analyzing the temperature field change of the corridor and the temperature of the ceiling of the T-shaped corridor with the changing of the fire source power. The cable fire is how to spread at the T-junction. The situation results provide a certain reference for the actual fire extinguishing of underground T-shaped cable duct corridors. Keywords: T-shaped utility tunnel, Cable fire spread, FDS numerical simulation, Temperature field