基于真三轴物理模拟实验的裂缝扩展规律研究

为了研究不同起裂方式下水力裂缝三维扩展特征,采用真三轴水力压裂物理模拟试验机,开展定向压裂与钻孔压裂试验,结果表明：定向压裂裂纹从预制裂纹尖端起裂,向中间主应力方向转向,裂纹呈双翼弯曲形态,其整体仍平行于最大主应力方向。钻孔压裂裂纹在钻孔轴向对称位置起裂,呈椭圆形自相似扩展特征,裂纹倾向垂直于最小主应力。裂纹最终扩展方位不受起裂方式主导,而是由地应力场决定。水力压裂过程呈现憋压起裂和稳压扩展两个典型阶段。憋压起裂阶段,泵压急剧上升达到破裂压力后又迅速跌落,声发射能量骤增且波动剧烈。稳压扩展阶段,泵压曲线呈锯齿状波动发展并趋于平稳, 声发射能量水平相对较低。研究结论可为煤矿坚硬顶板压裂施工提供参考。

Study on propagation of hydraulic fracture based on true triaxial physical simulation experiment

To study the three-dimensional propagation of fractures under different initiation modes, true triaxial experimental system was used to perform directional fracturing and borehole fracturing tests. The results show that the crack starts from the tip of the prefabricated crack and turns to the direction of the intermediate principal stress. The crack presents a double-wing bending shape and is still parallel to the direction of the maximum principal stress as a whole. In the borehole fracturing test, the fracture initiation is at the axial symmetric position of the borehole, and the fracture propagation tends to perpendicular to the minimum principal stress, and the crack is elliptical and self-similar. There are two typical stages of fracture propagation: pressure-cumulative and pressure stabilization stage. During pressure-cumulative stage, the pump pressure rises sharply before reaching the initiation pressure. Acoustic emission energy surges and fluctuates sharply. During pressure stabilization stage, the pump pressure curve develops in a zigzag pattern and tends to be stable. Acoustic emission energy levels are relatively low in this stage. The research can provide reference for fracturing of hard roof in coal mine.