Pressurization Rate Effect on Breakdown Pressure of Hydraulic Fracturing

摘要

A test site with rare frequency of joints was selected in a granitic area in China. Total number of discontinuities were 37 from the surface down to the bottom of a 300 m long borehole. Brazilian strength of the core sample was almost constant along the entire borehole. Pressure pulse tests revealed that there are three regions with low, medium and high permeability along the borehole.In each of the three permeability regions, hydrofracturings at slow and fast pressurization rates were performed. Most fractures induced were vertical. The direction of them were consistently N6E +-15deg. The breakdown pressure Pb was about 14 MPa in the shallower part, and 20 MPa in the deeper region.In the high permeability region, Pb at slow pressurization rate was clearly lower than that at fast rate. On the other hand, Pb at both fast and slow rates were almost the same in the low permeability region. This result suggests that the increase of pore water pressure around the borehole decreases Pb. Based on this point, normalized pressurization rate has been introduced. At normalized pressurization rate larger than 0.1, the change of pore water pressure is negligible, and does not affect the result of Pb. While Pb decreases considerably at a lower normalized pressurization rate such as 0.001.