Fracture evolution of deep coals in true tri-axial hydraulic fracturing experiment

Deeper exploitation of coal resources faces higher possibility of rockburst and mining earthquake. Hydraulic fracturing (HF) proved to be an effective solution in coal mines but its monitoring and evaluation remains unexplored. This study presents a true triaxial hydraulic fracturing experiment with acoustic emission (AE) recording, conducted on a 150 mm cubic coal sample from a deep underground mine. Fracture evolution was analysed based on 539 localized events using a modified simplex method. According to the injection history, AE counts and energy evolution, the coal undergoes initiation, intersection, and breakdown stages. The AE energy peak lags behind the water pressure peak due to the dilatancy effect. Rapid fluctuations in water pressure trigger peak AE event rates, often followed by silent periods with minimal AE activity, which can serve as precursors for crack prediction. The distribution of AE events indicates that fractures originate from the naked borehole and propagate upward, predominantly accumulating in the middle and upper regions of the coal sample. At low water pressure, fractures extend primarily along the maximum principal stress direction, while at high water pressure, they diffuse spherically. The uneven transition and storage of crack energy during injection lead to alternating shrinkage and expansion of AE event distribution as water pressure increases. In addition, coal heterogeneity plays a significant role in fracture formation, resulting in tortuous hydraulic fracture planes that deviate from alignment with the maximum principal stress. • Coal fracturing exhibits a silent period and dilatancy effect before rupture. • Discontinuous fluid supply induces alternating shrinkage and expansion of cracks. • Stress state and coal heterogeneity control fracture initiation and propagation.