Abstract: The breakdown pressure of the pressure-time curve obtained from the in-situ test is an important parameter for calculating the tectonic stress. In order to investigate the influence of pumping rate on breakdown pressure and pressurization rate, four large-scale hydraulic fracturing experiments on low-permeability hard and brittle limestone with varying constant pumping rate were conducted. Acoustic emission monitoring technique was used to investigate how varying pumping rate affected breakdown pressure, failure mode and complexity of fracture network as well as internal relationship between pumping rate and pressurization rate. The experimental results show that the pumping rate has a significant effect on the breakdown pressure. A larger pumping rate results in a higher breakdown pressure and a lower complexity of fracture network. The typical pressure-time curve consists of a slow boosting stage, a rapid boosting stage, a stable boosting stage and a sudden drop stage. Pressure increases linearly with time during the stable boosting stage, and there is a clear linear relationship between the pressurization rate and the pumping rate. Using the linear relationship between the pumping rate and the steady-state pressurization rate, the Ito theory can quantitatively explain the dependence of the breakdown pressure on the pumping rate. The theoretical predictions are in good agreement with the experimental results.

Key words: hydraulic fracturing, pumping rate, pressurization rate, breakdown pressure, low permeability hard and brittle rock