Acoustic Emission Response Mechanism of Hydraulic Fracturing in Different Coal and Rock: A Laboratory Study

To examine the influence of lithology on the microseismic response mechanism during the process of coal and rock hydraulic fracturing (HF), we selected four lithological samples of coal, sandstone, shale, and mudstone, which are commonly found in coal-measure strata. The acoustic emission (AE) cumulative counts, AE energy, crack classification, AE peak frequency, and AE position of the four samples under natural conditions were studied, and the laws of crack propagation and evolution in the process of coal and rock HF were explained. We discovered the phenomenon of “bimodal frequency bands” during the HF of coal and rock samples. The results showed that the pressure curves and AE cumulative counts of the coal samples exhibited the largest fluctuations. The power-law distribution of AE energy has a discrete effect in the high-energy region and a plateau effect in the low-energy region. The AE energy power-law indexes of the mudstone samples are the largest, with the values of 0.64 and 0.61, whereas those of the shale samples are the smallest, with the values of 0.44 and 0.45. The crack classification of the samples of coal is mainly shear cracks, and the crack classification of the samples of sandstone, shale and mudstone is mainly tensile cracks. It is proposed to regard frequency band II as the dominant frequency band for HF in the coal, sandstone, and mudstone samples and frequency band I as the dominant frequency band for HF in shale samples.