Study on the characteristics of water-gas two-phase seepage in the overlying rock fracture network of goaf in abandoned mines

In the context of the realization of the “dual carbon” target and the deepening of the supply-side reform of coal, a large number of coal mines with depleted resources and backward production capacity have been gradually eliminated, and the number of abandoned mines in China has continued to increase. The fractures in the overlying rock have a certain control effect on the migration of fluids in the goaf. In order to reveal the two-phase seepage laws of water and gas in the fracture network in the overlying rock in the goaf during the long-term closure of the abandoned mine, this paper utilizes digital image processing technology to identify and extract the characteristics of physical similarity model overlying rock fracture network. The geometric parameter characteristics of longitudinal breakage fractures and interlayer separation fractures are statistically analyzed and identified. A geometric model of the overlying rock fracture network is established, and the two-phase flow theory is used to explore the flow law of the flow pattern of gas floating up and water seeping down from the separation layer in the overlying rock fracture network. The research results show that the overlying rock fractures in the goaf are in the shape of a “trapezoid” as a whole, and the longitudinal breakage fractures and interlayer separation fractures are crisscrossed and connected with each other. The opening, area, and perimeter of the two types of fractures all decrease exponentially with the increase in frequency. The distribution trend of the two types of cracks is logarithmic normal distribution in trace length and normal distribution in angle. Numerical simulation reproduces the process of water inflow, gas displacement and water discharge in overlying rock fracture network in the goaf, and reveals the formation mechanism of the dominant seepage channels in the fractures on both sides of the goaf. Due to the non-equilibrium seepage velocity, there are gas retention areas in high interlayer separation fractures, the two ends of the fractures are blocked, and there are deviations in flow at intersections. The research results have certain guiding significance for the secondary development and utilization of resources left behind in abandoned mines.