Fracture Mechanics Study of Shear Type Fracture Process Zone in Microcracking Materials

这项研究的目的是发展一种断裂力学方法，为下一代地热开采提供一种预测深部岩体中水力驱动裂隙扩展的方法。每项研究任务(I-IV)的研究结果如下。任务(I)：岩石的断裂准则在围压为150 Mpa和孔压为50 Mpa的情况下，进行了艾迪尔花岗岩的拉伸和压缩试验。根据试验结果，得出了拉伸软化和剪切弱化曲线。结果表明，所推导的实验曲线与围压和孔压无关，可以用来分析复合型裂纹的扩展。任务(II)：模拟水力压裂试验在不同围压下对厚壁圆筒试件进行了微型水力压裂试验。试验表明，当差应力小于100 Mpa时，产生张开型裂纹；当差应力大于100 Mpa时，产生剪切型裂纹。这一观察结果表明，深层岩体中的剪切破裂可能是由水力注入引起的。任务(III)：分析水力驱动裂纹扩展的有限元程序采用嵌入裂纹单元的有限元程序。任务(I)中确定的裂缝判据被纳入裂缝扩展条件的程序中。任务(IV)：水力压裂和人工油藏形成过程的数值分析。利用有限元程序对典型构造应力条件下的深部水力裂纹扩展进行了分析。结果表明，裂纹扩展在较浅的区域是张开型的，在较深的区域预计会发生剪切断裂，然后过渡到张开断裂。

The objective of this research is to develop a fracture mechanics method for predicting the growth of hydraulically driven cracks in deep-seated rock masses for next generation geothermal energy extraction. The research results are described below for the each research task (I-IV).Task (I) : Fracture criterion of rocksTension and compression tests of Iidate granite were conducted at confining pressures up to 150 MPa and pore pressure up to 50 MPa. Tension softening and shear weakening curves were deduced from the test results. It was shown that the deduced experimental curves were independent of the confining and pore pressures used in this study, and that the relationships can be employed for analyzing the mixed mode crack growth.Task (II) : Simulated hydraulic fracturing experimentMini-hydraulic fracturing tests were performed on thick-walled cylindrical specimens under a variety of confining pressure. The tests indicated that an opening type crack was created when the differential stress was less than 100 MPa and a shear type crack was initiated if the differential stress exceeded 100 MPa. This observation suggests that shear fracture may be induced by hydraulic injection in deep-seated rock masses.Task (III) : FEM code for analyzing the growth of hydraulically driven cracks A FEM code was newly developed by employing an embedded crack element. The fracture criterion determined in Task (I) was incorporated into the code the crack growth condition.Task (IV) : Numerical analysis of hydraulic fracturing and formation process of artificial reservoirs. The hydraulic crack growth at depth was examined using the FEM code under a typical tectonic stress condition. It was shown that the crack growth was opening type at shallower regime and at greater depth shear fracture was predicted to occur followed by the transition to open fracture.