The effect of pulse frequency on the acoustic emission characteristics in coal bed hydraulic fracturing

In order to fully understand the deformation and damage characteristics and to further reveal the evolutionary process of coal. In this article, experiments on acoustic emission characteristics of the Pulse Hydraulic Fracturing (PHF) process under different pulse frequencies, are carried out by the large-size true tri-axial hydraulic fracturing test system and the PAC AE instrument. The acoustic emission behavior evolution characteristics, the damage evolutionary rule, the AE location distribution and the dynamic feature of b-value are analyzed. The results show that the evolution of acoustic emission behavior stimulated by PHF in coal bed can be divided into four phases with time: gentle period, acceleration period, boosting period and steady period. The damage evolution process can be divided into three phases: initial damage period, damage development period and damage accelerating development period. Our results show that AE information can reasonably reflect the damage evolution character of coal. Besides, a lower frequency corresponded to a stronger AE energy release and a higher intensity of fracture extension inside the samples during the PHF process. Also in this case, a number of AE events were produced at early stage, where AE location distribution was more homogenous. For the b-value, during the whole PHF process, its variation was similar under different frequencies.