This study proposes a new fractal permeability model for fractured rocks that comprehensively accounts for the geometric fracture characteristics and the fluid transport mechanism. Then, the permeability changes of fractured rocks are analyzed using discrete fracture networks (DFNs) with or without roughness and different geometry parameters in the DFN modeling and finite element simulation. The results show that the proposed permeability model well agrees with the experimental data, and the established DFN numerical model more realistically reflects the fracture network in fractured rocks. Fluctuation of tortuous fracture lines (rough fractures) increases the fracture intersection probability, consequently increasing the fracture intersection area or connecting adjacent fractures. Moreover, permeability increases with the fractal dimension D_f, porosity ϕ, maximum fracture length l_max, and proportionality coefficient β, and it decreases with increasing fractal dimension D_Tf of fracture tortuosity. When the fracture proportionality coefficient is 0.001 ≤ β ≤ 0.01, different DFNs yield similar simulation results for permeability. However, with increasing fracture network complexity, the predictive model created using conventional DFN (C-DFN) increasingly overestimates the fractured rock permeability. Thus, building a permeability model for a fractured rock using rough DFN (R-DFN) is more effective than that using C-DFN. Our findings are helpful for real permeability predictions via DFN and analytical modeling.

To explore the occurrence mechanism of compound dynamic disasters in coal rocks, this study conducted a true triaxial test simulating gas extraction and stress loading and unloading conditions. To differentiate behaviors among disaster types, the effects of acoustic emission energy, temperature and impact force were analyzed during disaster incubation. The results revealed that different simulation depths lead to varying types of compound dynamic disasters. Compared to rockburst-outburst compound dynamic disasters, outburst-rockburst compound dynamic disasters exhibited higher relative outburst intensity and critical gas pressure. Deep coal rock disasters were characterized by long incubation and short excitation. As a threshold for disaster type transformation, a critical gas pressure range of 2.2-2.8 MPa was identified. During incubation, the temperature generally increased, with greater variation in the coal seam than at the coal-rock interface. During excitation, the temperature dropped sharply, with smaller variation in the coal seam. Outburst-rockburst disasters consistently showed higher temperature variation than rockburst-outburst disasters. Impact force evolution in roadways followed a similar pattern across disaster types: initial impact → intensification → peak → attenuation, with a peak effect. The peak impact force increased linearly with critical gas pressure, with outburst-rockburst peak growth rates being 47.76 times higher than rockburst-outburst peak growth rates. This study provides important insights into the multi-parameter evolution characteristics of deep coal rock compound dynamic disasters, offering a scientific basis for disaster prediction and control. Document Type: Original article Cited as: Zhang, X., Tang, J., Pan, Y., Ren, L., Huang, L., Zhang, Z. Experimental study on compound dynamic disaster in deep coal rock under gas and stress loading and unloading. Advances in Geo-Energy Research, 2025, 16(1): 60-76. https://doi.org/10.46690/ager.2025.04.07

Coal and gas outburst is one of the geological disasters that seriously threaten the safety of coal mines production. In recent years, with the increase of mining depth, outbursts become frequent. To further explore the occurrence mechanism of deep coal and gas outburst, a self-developed true triaxial coal and gas outburst simulation device was used to simulate the coal and gas outburst at different depths. The results show that with the increase of simulation depth, the critical gas pressure of outburst gradually decreases, and the unit outburst intensity increases sharply. The gas threshold of deep coal and gas outburst is lower. During the incubation and excitation, gas pressure has three special variation rules, namely self-increasing characteristic, stage and instantaneous. In the early incubation, acoustic emission (AE) energy is at a low level, low energy frequency is dominant; in the later incubation, AE energy increases greatly, high energy frequency is dominant. From the perspective of AE energy, a quantitative index that reflects the danger of coal and gas outburst in the incubation is defined, which provides a scientific reference for the prediction and prevention of disasters of coal and gas outburst in deep mining.