我国煤层渗透率普遍较低，硬厚煤层资源储量丰富，开采过程中存在回采效率低、动力灾害频发以及煤尘浓度大等问题，针对低渗硬厚煤层开采过程中煤体软化破碎、卸压与增透防灾难题，本文以煤层水压致裂软化技术原理为指导，采用理论分析、物理实验、数值模拟与现场试验等方法，对CO2水耦合作用性质、CO2耦合致裂煤体力学特性与孔隙结构的演化特征、CO2耦合致裂煤层起裂与扩展规律进行系统地研究，对低渗硬厚煤层安全高效开采具有重要的理论与应用价值。研究主要结论如下： （1）CO2耦合溶液特性。CO2耦合溶液密度、粘度与CO2质量分数正相关，与温度负相关，其密度与压力正线性相关；表面张力、水煤接触角与CO2质量分数均负相关，CO2耦合溶液对煤体的毛细管力是水的2.61~2.78倍，对煤体渗吸作用显著增强。 （2）CO2耦合致裂煤体的力学特性。CO2耦合致裂煤体的抗压强度与弹性模量均减小，随着CO2质量分数、孔隙压力与作用时间增加，煤体的抗压强度与弹性模量降幅增大，软化系数减小；而温度作用相反，煤体的软化系数随着温度增加而增大；建立了CO2耦合致裂煤体弹性模量的损伤函数。CO2耦合致裂煤样的单轴抗压强度减小，压密过程增长，塑性应变增大，峰值轴向应变增大，且峰值后有多个峰值点，破坏后产生更多裂隙，发生张拉-剪切破坏；CO2耦合致裂煤样劈裂破坏主裂隙呈锯齿状，并有次生裂隙，峰值载荷、抗拉强度、峰值应变与能率均减小，呈现出塑性变形的破坏特征。 （3）CO2耦合致裂煤体孔隙结构的演化特征。CO2耦合溶液与煤体中的矿物组分发生溶解、溶蚀作用，煤体的孔隙形态发生变化。CO2耦合致裂煤体过程中，存在微孔、介孔向大孔转化，平均孔径变大，且产生微孔，比表面积与孔容均减小。基于分形理论对CO2耦合致裂前后煤体孔隙结构进行定量表征，CO2耦合致裂煤体的孔隙体积分形维数减小，而孔隙表面积分形维数增大；孔隙表面分形维数降低，孔隙表面变的更为光滑，煤体对瓦斯的吸附性能减弱1.4%~4.3%，煤体的渗透率提高77%~122%。 （4）CO2耦合致裂煤体机制。在注入水压与毛细管力作用下，CO2耦合溶液与煤体发生物理化学作用，煤体中的孔隙形态、结构发生变化，煤体强度降低，在孔隙压力作用下煤体孔隙间基质的压应力减小，劈裂作用促使孔隙自身扩展，与附近大孔隙连通，形成微裂隙（或原裂隙增长）；在水压力、孔隙压力与地应力作用下裂隙起裂、扩展连通天然裂隙，形成裂隙网络。 （5）CO2耦合致裂起裂扩展规律。基于自研多相耦合压裂试验系统，开展CO2耦合致裂试验，将煤体CO2耦合致裂过程分为孔腔填充、CO2-水-煤相互作用与压裂破坏三个阶段，验证了CO2耦合致裂起裂压力预测模型的可靠性；模拟分析了不同压裂液致裂，CO2耦合致裂过程中物性参数、地应力差对起裂压力、裂隙形态与孔隙压力的影响规律，结果表明CO2耦合致裂起裂压力更低，裂缝更大，且微裂隙发育，渗透范围可达20m，有效渗透范围增加了3倍。 （6）现场试验。以新疆金川煤矿和陕北常家梁煤矿为试验对象，提出低渗硬厚煤层CO2耦合致裂设计原则与系统工艺，验证了CO2耦合致裂煤层技术工艺在综放开采与大采高综采的技术效果。

The permeability of coal seam in China is generally low, the reserves of hard and thick coal seam resources are abundant, and there are some problems in the mining process, such as low mining efficiency, frequent occurrence of dynamic disasters and high concentration of coal dust. In order to solve the problems of coal body softened and broken, pressure relief and permeability enhancement and disaster prevention in the process of low permeability hard and thick coal seam mining, this paper adopts the methods of theoretical analysis, physical experiment, numerical simulation and field test under the guidance of the principle of hydraulic fracturing and softening technology in coal seam. The properties of CO2 water coupling, the mechanical properties and pore structure evolution characteristics of CO2 coupling cracked coal seam, and the initiation and expansion law of CO2 coupled cracked coal seam are systematically studied. It has important theoretical and application value for safe and efficient mining of low permeability hard and thick coal seam. The main conclusions of the study are as follows: (1)Characteristics of CO2 coupled solution. The density and viscosity of CO2 coupling solution are positively correlated with the mass fraction of CO2, negatively correlated with temperature, and positively linearly correlated with pressure. The surface tension and the contact angle of water coal were negatively correlated with the mass fraction of CO2. Compared with clean water, the capillary force of CO2 coupled fracturing fluid on coal was increased by 1.61~1.78 times, and the imbibition of coal was significantly enhanced. (2) Mechanical properties of CO2 coupled cracked coal. The compressive strength and elastic modulus of cracked coal body caused by CO2 coupling decrease. With the increase of CO2 mass fraction, pore pressure and action time, the decrease of compressive strength and elastic modulus of coal body increases, and the soften coefficient decreases. On the contrary, the soften coefficient of coal increases with the increase of temperature, and the damage function of elastic modulus of cracked coal caused by CO2 coupling is established. The uniaxial compressive strength of CO2 coupled cracked coal sample decreases, the compaction process increases, the plastic strain increases, the peak axial strain increases, and there are many peak points after the peak value, more cracks occur after failure, and tension-shear failure occurs.The main fracture of CO2 coupled fractured coal sample is serrated, and there are secondary fractures, peak load, tensile strength, decrease of peak strain and energy rate, and present the characteristics of plastic deformation and failure. (3) The evolution characteristics of micro-pore structure of CO2 coupled cracked coal. The CO2 coupling solution dissolves with the mineral components in the coal body, and the pore morphology of the coal body changes. In the process of CO2 coupling cracking coal body, there are micropores, mesoporous to macroporous, the average pore size becomes larger, and micropores are produced, and the specific surface area and pore volume are reduced. Based on fractal theory, the pore structure of coal before and after CO2 coupling cracking is quantitatively characterized. The fractal dimension of pore volume of CO2 coupled cracked coal decreases, while the fractal dimension of pore surface area increases. The fractal dimension of pore surface decreases, the pore surface becomes smoother, the adsorption performance of coal for gas decreases by 1.4%~4.3%, and the permeability of coal increases by 77%~122%. (4) The mechanism of CO2 coupling cracking coal body. Under the action of injecting water pressure and capillary force, the physicochemical interaction between CO2 coupling solution and coal body takes place, the pore morphology and structure of coal body change, the strength of coal body decreases, and the compressive stress of inter-pore matrix of coal body decreases under the action of pore pressure. The fracturing process causes the pores to expand themselves, connect with the nearby macropores, and form microfissures (or the growth of the original fissures). Under the action of water pressure, pore pressure and ground stress, the fracture begins to crack, the natural fracture is extended and connected, and the fracture network is formed. (5) The Law of crack initiation and propagation caused by CO2 Coupling. Based on the self-developed multiphase coupling fracturing test system, the CO2 coupling fracturing test is carried out. The CO2 coupling fracturing process of coal body is divided into three stages: cavity filling, CO2-water-coal interaction and fracturing failure. The reliability of CO2 coupling crack initiation pressure prediction model is verified. The effects of physical parameters and ground stress difference on the initiation pressure, fracture morphology and pore pressure in the process of fracturing with different fracturing fluids and CO2 coupling are simulated and analyzed. The results show that the fracture initiation pressure of CO2 coupling is lower and the fracture is larger. With the development of microfissures, the permeability range can reach 20 m, and the effective permeability range is increased by 3 times. (6) Field test. Taking Jinchuan Coal Mine in Xinjiang and Changjialiang Coal Mine in Northern Shaanxi as experimental objects, the design principle and system technology of CO2 coupling cracking in low permeability hard and thick coal seam are put forward, and the effect of CO2 coupling fracturing coal seam technology method in fully mechanized caving mining and large mining height fully mechanized mining is verified.