Fracture development and permeability evolution of overburden rocks containing faults

The morphology, spatial distribution, and development of mining-induced overburden fractures within fault-affected areas are extremely complicated. These fractures play a dominant role in governing gas migration between rock strata. Understanding the overburden fractures evolution is essential for improving gas extraction efficiency. As a result, a systematic analysis of the mining activities in East 301 mining face of Dashuitou Coal Mine was performed. Numerical model of mining-induced coal seam containing faults with varying inclination angles was developed by Universal Distinct Element Code. As a result, the mining-induced fractures were statistically analyzed, and the evolution of fracture frequency (f), angle (θ), opening (b), and fractal dimension (D) of overlying rock within the fault-affected areas was obtained. The results show that significant reconstruction and propagation of the fracture network occurred in the fault-influenced region due to its tectonic features and localized stress concentration effects. The mining activity across faults leads to increased fracture frequency (f), angle (θ), and length (L), thereby enhancing the gas migration. A methodology for assessing fracture permeability based on overburden fracture characteristics including f, θ, b, and L is proposed and combined with field observations by in situ peeping technique to determine the optimal fracture network area. The optimal drilling layout is considered to be the fracture area with the opening (b) of >2 mm, penetration degree (T) of 0–0.6, and angles ranging from 35°–60° or 115°–140°. The aim of our research is to realize safe and efficient mining and gas extraction in the fault-affected area.