13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Table 1 Experimental stress data The first fracturing The second fracturing Sample number xσ yσ zσ xσ yσ zσ 2# 3 3 3 3 4 5 4# 4 4 4 1 2 3 6# 3 4 5 For example, when the confining pressure was 3MPa for 2# sample, a horizontal main fracture formed after the first fracturing. The highest pump pressure was 6.5MPa. The second fracturing was carried out after changing the size of stress. A vertical main crack was created along the maximum horizontal principal stress direction. The highest pump pressure was 2.4MPa affected by the defects. The strength of the rock is dispersed for defects of various scales in the rock, and the ability of resisting external force is weakened [13]. Under external loads, the defects of the sample will be activated. The activation defects extend under the load, and finally lead to mutual merger and samples are destructed. Therefore, when the loads are up to a certain threshold, random defects are activated in the defect reservoirs during the experiments. And more complex fractures may produce. The first fracturing z x The second fracturing Defect z y x Figure 3 Experimental image of 2# sample after hydraulic fracturing For 4# sample, under the higher uniform confining pressure, the artificial fracture closed. And a new fracture produced along the direction perpendicular to the preset fracture which affected by the defects. The highest pump pressure was 6.2MPa. When the size and orientation of the stresses were changed, the direction of the new fracture would not change, which still followed the direction of the maximum horizontal principal stress.
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