液态CO₂压裂是一项具有广阔应用前景的无水压裂技术。针对液态CO₂压裂诱导产生复杂裂缝和降低起裂扩展压力的机制，以往研究通常归结于液态CO₂的低粘、低表面张力和强扩散性等性质，鲜有考虑到液态CO₂注入煤层时的低温冲击效应，其形成的温度应力场效应改变了煤体的力学特性，造成室内实验和现场应用中选取的关键参数可靠性低。因此，为揭示液态CO₂低温冲击对煤体裂缝起裂扩展的强化作用机制，本文依据拉应力准则和有效应力原理，构建考虑液态CO₂低温冲击的煤体裂缝起裂和扩展准则；基于前人的研究基础，以煤体相似材料模拟煤层为研究手段，围绕液态CO₂压裂过程中温度应力对煤层起裂压力的影响因素和对裂缝扩展的增强作用开展实验。研究成果如下：

（1）基于岩石力学、弹性力学和断裂力学的基本原理，系统分析了液态CO₂注入过程中低温冲击作用下钻孔周围应力的分布规律，明晰了液态CO₂与煤体间温差形成的温度应力可有效降低煤岩起裂扩展压力；根据应力叠加原理，以修正的拉应力破坏准则为煤体起裂扩展的判别条件，确定了裂缝起裂扩展表征关系式，建立了考虑温度应力的裂缝起裂扩展数学模型。

（2）基于相似定律，对标原煤的力学参数，筛选了模拟煤层的相似材料，通过分析不同配比的相似材料的力学性质和破裂形态，得出最接近于煤体力学性能的相似配比，即水泥：石膏：沙子：煤粉=4:1:3:2；基于De pater相似原理，计算模拟实验系统的相似因子，结合相似因子得出了液态CO₂压裂的实验方案。

（3）开展了液态CO₂低温冲击强化煤层起裂实验，探究了不同温度作用下，温度应力对煤体试件起裂压力和表面裂缝特征的影响规律。研究发现液态CO₂与煤体的温差越大，对煤体起裂压力的影响越显著，有效降低了起裂压力，压注温度从0 ℃下降至-20 ℃，起裂压力降幅高达9.9 %，其中温度应力所占比例从10 %增长至18 %。单位温差条件下，起裂压力降幅和温度应力增幅均为0.017 MPa，低温冲击破裂产生的裂缝数量随温差增大而增加，裂缝形态更复杂。

（4）开展了液态CO₂低温冲击强化诱导裂缝扩展相似模拟实验，探究低温液态CO₂对煤层诱导裂缝的增强作用。研究发现注液温度降低，其形成的温度拉应力越大，诱导裂缝的扩展强化作用越显著，有效降低了诱导裂缝扩展的峰值压力，液态CO₂诱导裂缝的扩展峰值压力10 ℃比-10 ℃低10.5 %，温度应力占比为10 % ~ 23 %，诱导裂缝压力峰值和温度应力随温差增大的变化率均为0.054 MPa；

（5）在诱导裂缝扩展过程中，高能量声发射事件与宏观裂缝具有较高的一致性，裂缝产生动态扩展。液态CO₂注入温度越低，产生偏转和分支多裂缝，裂缝形态相对复杂且延伸较长，裂缝数量更多，声发射瞬间释放能量的最大值和累计能量越高，CO₂注入温度为-10 ℃的累计能量比注入温度为10 ℃的高1个数量级，更容易形成裂缝网络。

Liquid CO₂ fracturing is an anhydrous fracturing technology with broad application prospects. Regarding the mechanism of liquid CO₂ fracturing that induces complex fractures and reduces the fracture extension pressure, previous studies usually attribute it to the properties of liquid CO₂, such as low viscosity, low surface tension, and strong diffusivity, but seldom take into account the low-temperature impact effect when liquid CO₂ is injected into coal seams, and the effect of the temperature stress field formed by liquid CO₂ alters the mechanical properties of the coal, which results in the low reliability of the key parameters selected in indoor experiments and on-site applications. . Therefore, in order to reveal the enhancement mechanism of the low-temperature impact of liquid CO₂ on the crack initiation and expansion of coal seams, this paper constructs the crack initiation and expansion criterion of coal seams considering the low-temperature impact of liquid CO₂ based on the tensile stress criterion and the effective stress principle; based on the research basis of the previous research, the coal seam simulated by the similar material of coal seams is used as a research tool, and the influence factors of the temperature stress on the fracture initiation pressure and the enhancement effect on crack extension of coal seams in the process of fracturing by liquid CO₂ are investigated. and the enhancement effect on crack extension during the liquid CO₂ fracturing process. The research results are as follows:

(1) Based on the basic principles of rock mechanics, elasticity mechanics and fracture mechanics, the distribution law of stress around the borehole under the low-temperature impact in the process of liquid CO₂ injection was systematically analyzed, and it was clarified that the temperature stress formed by the temperature difference between the liquid CO₂ and the coal body could effectively reduce the cracking and expansion pressure of the coal rock; according to the principle of superposition of stresses, and with the modified tensile stress damage criterion as a discriminant condition for the cracking and expansion of the coal body, a fracture expansion characterization relation was established to take account of temperature stress. Based on the principle of stress superposition and the modified tensile stress damage criterion as the discriminant condition of coal body crack initiation and expansion, the characterization relation equation of crack initiation and expansion was determined, and the mathematical model of crack initiation and expansion considering temperature stress was established.

(2) Based on the law of similitude and benchmarking the mechanical parameters of virgin coal, similar materials for simulating coal seams were selected. By analyzing the mechanical properties and fracture patterns of similar materials with different proportions, the most coal-like mechanical performance ratio was derived, which is cement: gypsum: sand: coal powder at a ratio of 4:1:3:2. Utilizing the De Pater similarity principle, the similarity factors for the simulation experimental system were calculated, and, combining these factors, an experimental plan for liquid CO₂ fracturing was developed.

(3) Experiments on enhancing coal seam cracking through the impact of liquid CO₂ at low temperatures were carried out to explore the influence of temperature stress on the fracture pressure and surface crack characteristics of coal specimens under different temperature conditions. The study found that the greater the temperature difference between the liquid CO₂ and the coal body, the more significant the effect on the fracture pressure of the coal body, effectively reducing the fracture pressure. With the injection temperature dropping from 0 ℃ to -20 ℃, the fracture pressure decreased by as much as 9.9 %, where the proportion of temperature stress increased from 10 % to 18 %. For each unit of temperature difference, the reduction in fracture pressure and the increase in temperature stress were both 0.017 MPa. The quantity of fractures generated by the low-temperature shock increased with the temperature difference, and the fracture morphology became more complex.

(4) Experiments on the simulation of similar crack propagation induced by the impact of liquid CO₂ at low temperatures were conducted to investigate the enhancement effect of low-temperature liquid CO₂ on induced cracks in coal seams. The study found that a decrease in the injection temperature increases the tensile stress caused by temperature, thus significantly enhancing the crack propagation. This effectively reduces the peak pressure required for expanding the induced cracks. The expansion peak pressure of the cracks induced by liquid CO₂ at 10 ℃ is 10.5 % lower than at -10 ℃, with temperature stress accounting for 10 % to 23 %. Both the peak pressure of induced fractures and the temperature stress increase at a rate of 0.054 MPa as the temperature difference increases.

(5) During the process of induced fracture expansion, high-energy acoustic emission events show high consistency with macroscopic fractures, leading to dynamic expansion of the fractures. The lower the injection temperature of liquid CO₂, the more complex, extended, and numerous the fractures become due to increased deflection and branching. The maximum and cumulative energy released by acoustic emissions are higher. The cumulative energy at an injection temperature of -10 ℃ is an order of magnitude higher than at 10 ℃, facilitating easier formation of fracture networks.

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