Acoustic Emission Source Location Monitoring of Laboratory-Scale Hydraulic Fracturing of Coal Under True Triaxial Stress

Abstract

Crack monitoring is an important issue in coal and rock hydraulic fracturing (HF). In this study, we conducted experimental coal HF acoustic emission (AE) monitoring tests under true triaxial stress by using a self-developed experiment system. The results showed that the AE signal statistical parameters had a good positive correlation with the water pressure curve. Based on the STA/LTA, improved AIC and simplex algorithm, we developed a new method for AE event automatic identification and source location for coal HF. By using this new method, we were able to detect and locate more accurately the AE events induced by HF. Hydraulic crack propagation and temporal-spatial AE event distribution were located with high accuracy. According to the AE event temporal-spatial distribution of AE events, coal hydraulic crack propagation was not uniform. In the middle and late HF stages, especially when hydraulic cracks approach the coal sample boundary, the hydraulic crack propagation speed was accelerated significantly. The important factors that affected hydraulic crack initiation, propagation, steering, and acceleration included the triaxial stress state, original joints and cracks, and local stress disturbance caused by hydraulic cracks. These results offer a reference for designing optimized, safe and effective HF programs, which are important in preventing coal/rock dynamic disasters and ensuring safe and efficient mining.