# Study of Rock Crack Extension under Liquid Nitrogen Low-Temperature Fracturing
###### Abstract
There are also some fracturing studies that use liquid nitrogen to produce a low-temperature effect, e.g., [PERSON] et al. [9] pumped liquid nitrogen into Devonian shale wells at conventional flow rates and pressures. The results showed that the initial production of the well increased by 8%, and thermal cracks orthogonal to the main crack surface were expected to occur. [PERSON] et al. [10; 11; 12; 13; 14; 15] evaluated the rock damage and crack effect caused by the low-temperature cooling of liquid nitrogen via scanning electron microscopy, nuclear magnetic resonance, the acoustic emission (AE) test, and uniaxial compression. The damage degree was affected by lithography, porosity, and water saturation. [PERSON] et al. [16; 17] used an experimental device that they designed to study the permeability changes in rock treated with liquid nitrogen at a low temperature under true triaxial confining stress and the characteristics of microcrack propagation that it displayed. The above-mentioned experiments show that the injection of liquid nitrogen will generate new cracks and promote the propagation of existing cracks, increasing the fracturing effect inside the rock. Therefore, understanding the mechanism of this phenomenon and the law of crack propagation in low-temperature fracturing is of great significance for the field application of low-temperature fracturing. However, crack formation and propagation under liquid nitrogen cold impact on shale are regarded as fast and highly complex processes, and the real-time acquisition of data related to the liquid nitrogen low-temperature fracturing process is difficult to control. [PERSON] et al. [18] conducted a numerical simulation of this phenomenon to obtain real-time data of the temperature field and stress field in the process of rock fracturing at a low temperature with liquid nitrogen. [PERSON] et al. [19] simulated and predicted the productivity of gas wells using this technology and analyzed the economic benefits of 10 years of production. Additionally, the subsurface conditions in which shale gas is located are very complex, and there will be discrepancies between realistic and experimental models [20]. Both [PERSON] [21] and [PERSON] [22] have proposed integrated strategy approaches which use well data, 3D seismic data, and regional geological information to perform numerical simulations. This notion indicates that numerical simulation can overcome adverse factors in laboratory conditions, and it is an effective method to understand the mechanism. Therefore, with the shale gas reservoir as the research object, our study examines low-temperature fracturing technology combined with visualization, laboratory tests, and numerical simulation methods, uses the means of RFPA software and a high-speed camera, and analyzes the different homogeneous degrees and heat transfer coefficients of shale conditions, such as the internal stress field, temperature field, and the change rule of the law of crack extension. During the simulation of the low-temperature fracturing of shale with liquid nitrogen under cold impact, the law of crack initiation and propagation is highlighted, the influence of sensitive factors such as the heat transfer coefficient and in situ stress on crack propagation are studied, the relationship between each factor and crack development mode is obtained, and how to improve fracturing effect and increase energy production is discussed. The paper contains the following four sections: a visual physical experimental analysis of the liquid nitrogen effect; a feasibility study on low-temperature frozen shale using a numerical simulation method; the numerical simulation of liquid nitrogen low-temperature fracturing; and the effect of in situ stress on crack propagation.