水力压裂是提高低渗透高瓦斯煤层抽采效率和瓦斯产量的有效方法之一。由于煤层中存在丰富的天然裂缝，天然裂缝与水力裂缝相互作用会使水力裂缝的起裂和扩展形式呈现出复杂多样性，从而影响煤层的增透效果。为研究水力压裂煤在天然裂缝作用下的起裂和扩展规律，利用自主研发的多功能真三轴流固耦合试验系统开展了煤的水力压裂试验。基于“孔壁应力集中诱发拉伸破裂”理论研究了水力压裂煤的裂缝起裂规律，并结合断裂力学从细观角度揭示了水力压裂煤的裂缝扩展机制。研究结果表明：天然裂缝的存在会诱导水力裂缝沿着天然裂缝方向扩展，极大降低了煤岩的起裂压力。不含明显天然裂缝煤岩压裂所需的起裂压力与根据拉伸破坏起裂准则计算的理论起裂压力结果相近，符合“孔壁应力集中诱发拉伸破裂”准则；含明显天然裂缝煤岩压裂所需的起裂压力均小于根据拉伸破坏起裂准则计算的理论起裂压力，且当天然裂缝方向垂直于最小主应力方向时，所需的起裂压力较小，为3.355 MPa；当天然裂缝平行于最小主应力时，所需的起裂压力仅大于最小主应力，为7.902 MPa。水力裂缝类型为I型时，不含明显天然裂缝煤岩的实测最小和最大扩展压力值均大于理论计算最小和最大扩展压力值，差值范围分别为2.043～6.845 MPa和3.951～8.576 MPa；当煤岩含明显天然裂缝且天然裂缝方向平行于X方向时，水平应力差的存在将引起煤岩的实测扩展压力值小于理论计算的扩展压力值。水力裂缝类型为Ⅱ型或I-Ⅱ型时，随着水力裂缝长度的增加，裂缝扩展所需压力逐渐减小。

Hydraulic fracturing is one of the effective methods to improve the extraction efficiency and gas production of low-permeability coal seams with high gas content. Due to the existence of abundant natural fractures in the coal seam, the interaction between natural fractures and hydraulic fractures will make the initiation and propagation of hydraulic fractures show complexity and diversity, thereby affecting the effect of permeability enhancement of coal seam. In order to study the initiation and propagation of hydraulic fractured coal under the action of natural fractures, hydraulic fracturing tests on coal were carried out by using the self-developed multifunctional true triaxial fluid-solid coupling test system. Based on the theory of “pore wall stress concentration induced tensile fracture”, the fractures initiation law of hydraulic fractured coal was studied, and combined with fracture mechanics, the fracture propagation mechanism of hydraulic fractured coal was revealed from a mesoscopic perspective. The results shown that, the presence of natural fractures can induce hydraulic fractures to propagate along the direction of natural fractures, which greatly reduced the initiation pressure of coal. The required fracture initiation pressure for fracturing coal without obvious natural fractures was similar to the theoretical initiation pressure calculated according to the initiation criterion of tensile failure, which met the criterion of “tensile fracture induced by stress concentration in hole wall”. The required fracture initiation pressure for fracturing coal with obvious natural fractures was all less than the theoretical fracture initiation pressures calculated according to the initiation criterion of tensile failure, and the required fracture initiation pressure was smaller when the natural fracture was perpendicular to the direction of the minimum principal stress, which was 3.355 MPa. When the natural fracture was parallel to the minimum principal stress, the required initiation pressure was only greater than the minimum principal stress, which was 7.902 MPa. When the hydraulic fracture type was type I, the measured minimum and maximum propagation pressure values of coal without obvious natural fractures were greater than the theoretically calculated minimum and maximum propagation pressure values, and the difference ranges were 2.043～6.845 MPa and 3.951～8.576 MPa, respectively. When the coal contained obvious natural fractures and the direction of the natural fractures was parallel to the X direction, the existence of horizontal stress difference will cause the measured propagation pressure value of coal to be less than the theoretically calculated propagation pressure value. When the hydraulic fracture was type II. or type I-II., the pressure required for fracture propagation gradually decreased with the increase of the hydraulic fracture length.