The Research On The Characteristics Of Evolution Of Borehole Strain Behavior Induced By Hydraulic Fracturing

The proposed three-axis coal and rock hydraulic fracturing simulation system was developed and launched hydraulic fracturing tests with reshaped coal and raw coal, while the borehole initiation behavior is analyzed from the view of microscopic.The borehle strain-pressure curve was analyzed during the hydraulic fracturing process;based on the development of the law of the borehole strain, analyzing the "three states "initiation characteristics.The borehole strain-pressure curve of reshaped coal under diferent water flow was found, while the relationship among fracturing process initiation pressure&destruction of the test block and water flow was analyzed.The results show that the strain of borehole is obvious during the increasing process of water stress, and the strain cannot recover while the water stress is unloaded. The borehole shows two types of strain, the tensile strain and the compression strain. Two type of strain region appeared during the hydraulic fracturing process, and the compression strain occurred in compress region could recover much of strain when the water stress is unloaded. According to strain curve, three stages occurred in the hydraulic fracturing process, including the micro-damage appears stage, damage region appear stage and borehole deform stage. At micro-damage appears stage, the water with high pressure compressed the gas into rock body, thus the micro-damage occurred. At the nest stage, the tensile region and compress region appeared in the surface of borehole, and the tensile strain could continued until the borehole is deformed while the compression strain could recovered much. Therefore, the residual tensile strain is obvious when the borehole is broken but the residual compression strain is not obvious.With the water flow increases, the initiation pressure and borehole strain rate also increased and the destruction of the test block tends to be more complicated and diverse. The pumped flow pressure is the essence of the energy was inputted and the hydraulic fracturing process is the essence ofenergy’s inputting, storage, transferring, dissipation and release process,surrounding rockelastic’s strain energy accumulation and release causes borehole strain’s change.The strain’s rapid growth of the test block before the initial stage indicated the energy’s accumulation, after the initial rupture occurs, each fluctuations of borehole’s strain curve reflects the crack extension. The paper is of great importance in study on fracture mechanism of borehole and the evolution of the energy during the process.