间接压裂是提高碎软低渗煤层地面井煤层气产量的新技术之一，其成功与否的关键在于顶板水力裂缝能否有效穿透煤岩界面并进入煤层，核心要素是碎软低渗煤层的韧性破坏与渗流耦合响应特征。将二维问题的Park-Paulino-Roesler势能函数扩展到三维应力空间，结合立方定律，得到煤结构面的韧性断裂-裂缝切向渗流关系;构建煤基质塑性变形与拉、压损伤的Helmholtz 自由能表达式，结合达西定律，导出煤基质在塑性变形、张拉损伤变量影响下的渗透系数(或滤失系数)表达式。在数值计算中，将煤基质与结构面的力学属性分别赋予实体单元和零厚度的黏聚力单元，通过共享节点的方式连接并由此实现2者间的应力-渗流耦合。采用断裂力学实验识别材料参数，结合水力压裂实验验证碎软煤层的韧性破坏－渗流(DF-S)耦合本构方程组的合理性。在此基础上，模拟研究了多因素影响下水力裂缝从顶板向煤层延伸的过程，包括水平井和界面的间距D，竖向与最小水平地应力之差Δσ，煤岩界面摩擦因数fc,r。结果表明:① DF-S本构方程组可以较好地反映间接压裂过程中碎软煤层的韧性破坏－渗流耦合响应特征;② 煤岩界面会在很大程度上阻碍水力裂缝扩展，其机理在于水力压裂引起煤岩界面处煤结构面的韧性断裂、煤基质的塑性损伤、压裂液滤失引发的水力能量耗散，导致实际用于水力裂缝扩展的弹性能(或裂缝表面能)最低只占到水力能量的2%;③ 数值模拟条件下，水力裂缝穿透煤岩界面的临界fc,r与D呈正相关，与Δσ呈负相关关系。D对临界fc,r的影响更大，将D控制在1 m之内有望使18%~30%的水力能量用于水力裂缝扩展，提高间接压裂煤层技术的成功率。水力裂缝穿透碎软煤层与顶板界面的临界条件成果在山西晋城矿区赵庄矿得到应用，实现碎软煤层地面井煤层气增产，为煤层间接压裂方程设计提供了理论支撑。

Indirect fracturing is one of the new technologies to improve the output of coal bed methane (CBM) in surface wells in broken soft and low-permeability coal bed.The key to its success lies in whether hydraulic fractures (HFs) in roof can effectively penetrate the coal rock interface and enter the coal seam.And the core factor is the coupled response characteristics of ductile failure and seepage for the coal.The Park Paulino Roesler potential energy function of the twodimensional problem is extended to the three dimensional stress space,combining with the cubic law,the ductile fracture tangential seepage relationship of coal discontinuities is obtained.The Helmholtz free energy expression considering coal plastic deformation,tensile and compressive damage is constructed,combining with Darcy’s law,the permeability coefficient (or filtration coefficient) expression of the coal matrix under the influence of plastic deformation and tensile damage variables is derived.In the numerical calculation,the mechanical properties of coal matrix and discontinuities are assigned to the solid elements and the zero thickness cohesive elements,respectively,and they are connected by sharing nodes to realize the stress seepage coupling.The fracture mechanics experiment is used to identify the material parameters,and the rationality of the ductile failure seepage (DF-S) coupled constitutive equations of the coal is verified by experiments of fracture mechanics and hydraulic fracturing.On this basis,the process of HFs extending from the roof to the coal bed under the influence of multiple factors are simulated,including the distance between the horizontal well and the interface D,the difference between vertical and minimum horizontal in situ stress Δσ,and the coal rock interface friction coefficient fc,r.The results show that:① DF-S constitutive equations can well reflect the ductile failure seepage coupling response characteristics of the coal during indirect fracturing;② The coal rock interface will hinder the HFs expansion,and the mechanism of this phenomenon lies in the hydraulic energy dissipation caused by the ductile fracture of the coal discontinuities,plastic damage of the coal matrix,and the leakage of fracturing fluid,resulting in a minimum of 2% of hydraulic energy used for HFs propagation;③ Under numerical simulation conditions,the critical fc,r for HFs to penetrate the coal rock interface is positively correlated with D,and negatively correlated with Δσ.D has a greater impact on the critical fc,r,and 18%-30% of hydraulic energy is used for HFs propagation if  D is controlled within 1 m,thus increasing the success rate of indirect fracturing technology.The critical D,Δσ,fc,r condition of HFs penetrating the coal rock interface has been applied in Zhaozhuang mine of Jincheng,so as to improve the CBM production of surface wells in broken soft and low-permeability coal seam,which provides theoretical support for indirect fracturing design.