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

点击翻译按钮获取中文摘要

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.