大尺寸煤岩组合体水力裂缝越界形成缝网机理及试验研究

为了实现煤层气资源高效开采,针对我国煤层气储层"高储、低渗、成缝困难"的赋存特征,通过对裂纹尖端能量释放率、裂纹尖端应力场屈服区域数值分析以及煤岩组合体越界压裂试验,对煤岩组合体水压致裂过程中缝网形成机理及裂纹扩展特征进行研究。结果表明:同等应力条件下,煤体裂纹尖端塑性屈服区域明显大于砂质泥岩;裂纹在砂质泥岩中起裂瞬间释放的应变能可在煤体中产生约10.69~25.53倍当量裂纹面积或长度,有利于裂缝的延伸及多裂缝结构形成;通过数值分析可知,在达到近似临界煤岩拉破坏值时,砂质泥岩裂缝尖端在XX方位的最大集中应力值(7.05×10~5)约为煤体(1.98×10~5)的3.56倍;裂纹从坚硬砂质泥岩到软煤扩展试验过程中,形成了明显的复杂缝网结构;水力裂缝从覆岩到煤扩展过程中,在煤岩界面影响下出现贯穿、转向或偏转等现象,裂纹的转向或偏转导致裂纹形态复杂,有利于缝网结构的形成。

Mechanism and experimental investigation of the formation of hydro-fracture system by fracturing through the interface of large-size coal-rock

Taking the efficient exploitation of coalbed methane resources as the object,the mechanism and experiment on the formation of hydro-fracture system were investigated by the analysis of energy release rate of the fracture tip,the numerical analysis of the fracture tip yield fields and the experiments of fracturing through the interface of large-size coal-rock combination specimens,according to the characteristics of high storage,low permeability and difficult to form fracture in the coal seam in China.The results indicate that the plastic yield area of the fracture tip of the coal is larger than that of sandy mudstone under the same stress condition.The released strain energy of the fracture tip per unit length in the sandy mudstone could make 10.69 to 25.53 times area or length of fracture in the coal mass under the ideal conditions.It is convenient for the fracture propagation and the formation of the fracture system.In the numerical analysis,the maximum concentrated stress (7.05×105) of the crack tip of the sandy mudstone in the XX orientation is about 3.56 times of that of the coal (1.98×105) when the critical pressure is reached.In the process of hydraulic fracturing from the hard sandy mudstone into the coal mass,the complex fracture system was formed on the coal-rock combination.The penetration,diversion or offset occur at the interface between the coal and the rock under the influence of the interfacial cohesive force.The diversion or offset leading to the fracture patterns complex,will improve the fracture system.