Long-term evolution of overlying rock fractures in mined-out areas and its effect on gas flow conductivity

Abstract

Abandoned mines are often closed for long periods, and the overlying rock in mined-out areas is subjected to creep due to constant stress, which affects the overlying rock fracture morphology. The fractures in these rocks have a conductive effect on the gas flow, and previous studies less consider the influence of the long-term evolution process of the fractures on the gas distribution in these areas. Long-term experiments were conducted using a physical similarity experimental model of the mined-out area. To further explain the evolution law of the overlying rock displacement and fracture fields, a method for quantifying the percentage of the fracture field of the overlying rock based on computer image recognition technology is proposed. The results show that the long-term evolution of the overlying rock fractures have the characteristics of transfer and closure, and the change process determines the gas flow trajectory of overlying rock. The long-term evolution of the overlying rock is clarified, which incorporates the three-stage development of the delamination fractures and the stage density increase of the broken fractures. The gas storage capacity of the overlying rock first increases, then decreases, before stabilizing, and the gas diffusion capacity gradually increases. The gas distribution range fluctuates and increases with the evolution of the overlying rock fractures at different times. The time-varying law of the long-term evolution of overlying rock fractures in the mined-out area is explained and theoretical support is provided for clarifying the time-varying law of gas distribution in the overlying rock.