Rupture properties and safety assessment of raw coal specimen rupture process under true triaxial hydraulic fracturing based on the source parameters and magnitude

Abstract

To realize the safety assessment and risk management of the raw coal specimen rupture process under true triaxial hydraulic fracturing, the AE parameters and source parameters were obtained based on the AE monitoring of raw coal specimen rupture under true triaxial hydraulic fracturing. Throughout the process of hydraulic fracturing, the variation rule of AE parameters and the spatio-temporal distribution of the AE events induced by hydraulic fracturing of raw coal under true triaxial stress were studied. Then, based on the Brune model and grid search method, the source parameters and moment magnitude of raw coal specimens rupture under true triaxial hydraulic fracturing were obtained, and the relationship between the seismic moment and the other source parameters was analyzed. Finally, the source parameters relating AE events in raw coal specimens rupture under true triaxial hydraulic fracturing and micro-earthquakes induced by hydraulic fracturing were compared and investigated; the relationship between Mw (moment magnitude) and ML (local magnitude) of AE events of raw coal specimens rupture under true triaxial hydraulic fracturing was revealed. The results show that: the AE locating results are generally consistent with the actual rupturing of raw coal specimens. The source displacement spectrum of raw coal specimens rupture under true triaxial hydraulic fracturing is in good accordance with predictions made using the Brune model. By calculating the source parameters and moment magnitude, the source scale of hydraulic fracturing of raw coal specimens under true triaxial stress can be evaluated accurately. Source parameters such as corner frequency and apparent stress of raw coal specimen decrease linearly with the increase of seismic moment, and the source radius and source energy increase linearly with the increase of seismic moment; the stress drop tends to fluctuate around a certain value. The source properties of hydraulic fracturing of raw coal specimens under true triaxial stress can be evaluated. The seismic source properties relating AE events in raw coal specimens rupture under true triaxial hydraulic fracturing and micro-earthquakes induced by hydraulic fracturing are different. Considering Mw, ML1 can be better used to calculate the magnitude of AE events of raw coal specimens rupture during true triaxial hydraulic fracturing. These results can reveal the source properties throughout the rupturing of raw coal specimens during true triaxial hydraulic fracturing, and evaluate the safety of rupture process.