我国西南地区煤层赋存条件复杂且煤层厚度薄，在实施水力压裂过程中，水力裂缝更容易受到顶板和底板岩石以及压裂参数的影响，出现扩展不稳定的情况，而水力压裂效果与水力裂缝的形态密切相关。因此，本文针对具有开采价值的低透气性薄煤层，通过理论分析、数值模拟和工程试验相结合的方式开展薄煤层水力压裂过程中裂缝演化规律研究。全面深入的分析了薄煤层水力压裂过程中裂缝的起裂和扩展规律，以及在顶、底板影响下水力裂缝的演化特征，并设计了薄煤层水力压裂试验方案进行工程试验，为后续薄煤层水力压裂设计和评估水力压裂效果提供依据。主要研究内容和结论如下：

        建立了水力裂缝扩展的力学模型，推导了裂缝在煤层中扩展的临界水压P₁、进入顶板和底板岩石的临界水压P₂和水力裂缝在岩石中扩展的临界水压P₃ ，明确了影响裂缝演化规律的因素。以水力裂缝的临界水压作为判定条件，水力裂缝扩展至煤层顶板和底板岩石内后有4中扩展形式分别为：P＞P₁＞P₃ 时水力裂缝在煤层和岩石中同时扩展；P₁＞P＞P₃ 时水力裂缝在岩石中扩展，在煤层中不能扩展；P₃＞P＞P₁时水力裂缝在煤层中扩展，在岩石中不能扩展；P₁＞P₃＞P时水力裂缝在煤层和岩层中均不能扩展。

        根据现场施工要求，基于Cohesive单元建立了含钻孔的水力压裂二维数值模型，分析了水力压裂过程中水力裂缝的起裂和扩展过程，探讨了水平最大地应力、弹性模量、渗透系数以及泊松比对水力裂缝扩展规律的影响。整个水力压裂过程可以分为三个阶段，分别为：水力裂缝起裂阶段、主裂缝形成阶段和主裂缝稳定扩展阶段；水平最大地应力与水平最小地应力差值大于2MPa时，水力裂缝的形态以压裂钻孔为原点在水平最大地应力的两个方向上近似呈对称分布；低弹性模量、低渗透系数和高泊松比的煤层在裂缝扩展过程中容易产生剪切破坏，裂缝扩展路径会发生改变，但整体扩展方向沿水平最大地应力方向。

        为进一步探究顶、底板影响下水力裂缝的演化特征，建立了水力压裂三维单裂缝扩展模型，分析了顶、底板影响下水力裂缝的演化特征，探讨了煤层与顶、底板岩石的水平应力差和顶、底板岩石的抗拉强度以及注液参数对水力裂缝演化特征的影响。注液压力稳定期间出现压力下降的现象，说明水力裂缝已向煤层顶、底板延伸，压力下降越大水力裂缝在高度上向煤层顶、底板延伸越大；煤层与顶、底板的水平应力差，以及顶、底板的抗拉强度是影响薄煤层水力压裂效率的主要因素；压裂液参数选用大流量、低粘度的注入方式可提高薄煤层的压裂效率和压裂效果。

        根据理论分析和数值模拟的研究结果结合K1煤层的地质情况，设计了水力压裂试验方案：压裂液选择清水，流量设计为10m³/h~30m³/h，保证压裂过程中注液压力不低于水力裂缝在煤层中扩展的临界水压11.38MPa。现场注液压力曲线特征与数值模拟结果一致，说明数值模拟结果对现场有良好的指导作用。通过瞬变电磁法测试了水力压裂后的影响范围，水力压裂后在压裂观察孔和压裂孔之间的距离均为低阻异常区，水力压裂在预定扩展路径上取得了理想的效果，表明了试验方案的合理性。

        The occurrence conditions of coal seams in Southwest China are complex and the thickness of coal seams is thin. In the process of hydraulic fracturing, hydraulic fractures are more susceptible to the influence of roof and floor rocks and fracturing parameters, and the expansion is unstable. The effect of hydraulic fracturing is closely related to the shape of hydraulic fractures. Therefore, aiming at the thin coal seam with low permeability and mining value, this paper studies the fracture evolution law in the process of hydraulic fracturing of thin coal seam by combining theoretical analysis, numerical simulation and engineering test. The law of crack initiation and propagation in the process of hydraulic fracturing in thin coal seam and the evolution characteristics of hydraulic fractures under the influence of roof and floor are analyzed comprehensively and deeply. The hydraulic fracturing test scheme of thin coal seam is designed for engineering test, which provides a basis for the subsequent hydraulic fracturing design and evaluation of hydraulic fracturing effect in thin coal seam. The main research contents and conclusions are as follows:

        The mechanical model of hydraulic fracture propagation was established. The critical water pressure P₁ of fracture propagation in coal seam, the critical water pressure P₂ entering roof and floor rock and the critical water pressure P₃ of hydraulic fracture propagation in rock were deduced. The factors affecting the evolution law of fracture were clarified. Taking the critical water pressure of hydraulic fracture as the judgment condition, there are four expansion forms of hydraulic fracture after extending to the roof and floor rocks of coal seam: when P＞P₁＞P₃, hydraulic fracture expands in coal seam and rock at the same time; when P₁＞P＞P₃, the hydraulic fracture expands in the rock, it stops expanding in the coal seam; when P₃＞P＞P₁, the hydraulic fracture will expand in the coal seam and stop expanding in the rock; when P₁＞P₃＞P, the hydraulic fracture will stop expanding in the coal seam and rock strata.

        According to the requirements of field construction, a two-dimensional numerical model of hydraulic fracturing with boreholes was established based on Cohesive elements. The initiation and propagation of hydraulic fractures during hydraulic fracturing were analyzed. The effects of horizontal maximum ground stress, elastic modulus, permeability coefficient and Poisson 's ratio on the propagation of hydraulic fractures were discussed. The whole hydraulic fracturing process can be divided into three stages: hydraulic fracture initiation stage, main fracture formation stage and main fracture stable expansion stage. When the difference between the horizontal maximum in-situ stress and the horizontal minimum in-situ stress is greater than 2MPa, the shape of the hydraulic fracture is approximately symmetrically distributed in the two directions of the horizontal maximum in-situ stress with the fracturing borehole as the origin. Coal seams with low elastic modulus, low permeability coefficient and high Poisson 's ratio are prone to shear failure during crack propagation, and the crack propagation path will change, but the overall propagation direction is along the direction of horizontal maximum ground stress.

        In order to further explore the evolution characteristics of hydraulic fractures under the influence of roof and floor, a three-dimensional single fracture propagation model of hydraulic fracturing was established. The evolution characteristics of hydraulic fractures under the influence of roof and floor were analyzed. The horizontal stress difference between coal seam and roof and floor rock, the tensile strength of roof and floor rock and the influence of liquid injection parameters on the evolution characteristics of hydraulic fractures were discussed. The phenomenon of pressure drop occurs during the stable period of liquid injection pressure, indicating that the hydraulic fracture has extended to the roof and floor of the coal seam. The greater the pressure drop, the greater the hydraulic fracture extends to the roof and floor of the coal seam in height. The horizontal stress difference between coal seam and roof and floor, and the tensile strength of roof and floor are the main factors affecting the hydraulic fracturing efficiency of thin coal seam. The injection method of large flow and low viscosity can improve the fracturing efficiency and fracturing effect of thin coal seam.

According to the results of theoretical analysis and numerical simulation, combined with the geological conditions of K1 coal seam, the test scheme of hydraulic fracturing was designed: the fracturing fluid was clear water, the flow rate was designed to be 10m³/h~30m³/h, and the injection pressure during the fracturing process was not lower than the critical water pressure of 11.38MPa for the expansion of hydraulic fractures in the coal seam. The characteristics of the injection pressure curve in the field are consistent with the numerical simulation results, which shows that the numerical simulation results have a good guiding effect on the field. The influence range after hydraulic fracturing is tested by transient electromagnetic method. After hydraulic fracturing, the distance between the fracturing observation hole and the fracturing hole is a low-resistance anomaly area. Hydraulic fracturing has achieved ideal results on the predetermined expansion path, indicating the rationality of the experimental scheme.