Fracture behaviour and seismic response of naturally fractured coal subjected to true triaxial stresses and hydraulic fracturing

Hydraulic fracturing of coalbed methane wells has been widely practised as an effective method to increase drainage efficiency in low-permeability, low-pressure and low-saturated coal seams. To investigate hydraulic fracture performance and associated seismic response in coal, hydraulic fracturing experiments were carried out on two cubic coal blocks containing a host of natural fractures using a true triaxial rock testing machine equipped with loading, injection and acoustic systems. The acoustic system uses transducers with active sources to repetitively generate and receive ultrasonic P/S wave pulses for characterising mechanical properties of the coal blocks and revealing fracture growth. Silicon oil was injected into the middle of coal blocks to create hydraulic fractures under deviatoric stress conditions, and the stress and displacement, borehole pressure and volume, and seismic response were recorded over the injection process. X-ray computed tomography (CT) was conducted before and after the experiments to identify the location and geometry of hydraulic and natural fractures. Results have shown that the fracturing behaviour, the drawdown period of borehole pressure and the intrusion of fracturing fluid are dominated by the complexity and insulation offered by internal natural fracture networks of coal blocks. In addition, seismic spectrograms captured both fracture initiation and its subsequent interaction with natural fractures, which indicates that the induced fracture and fracturing fluid interfere with the propagation of seismic waves and influence ultrasonic seismic characteristics. Seismic velocity tomography of ultrasonic acoustic signals recorded also provided the spatial information of fractures, such as approximate locations of pre-existing fractures and injection-disturbed regions.