Permeability Evolution and Failure Behavior of Coal Under Coupled Oil–Gas Porous Flow and Mining-Induced Stress

Dynamic instability and disaster of coal mass fundamentally originate from internal damage, failure, and porous flow-field alteration driven by underground engineering disturbances. In coal–oil–gas co-reservoirs, the coupled action of crude oil/reservoir gas porous flow and mining-induced stress can trigger sudden oil/gas inflows or even blowouts, posing a critical threat to the safety of coal mining. Thus, in this study, the porous flow-stress experiment was conducted to investigate the permeability evolution of coal specimens subjected to coupled porous flow pressure and in situ stress conditions with the media of crude oil, reservoir gas, and oil–gas mixture. Based on the in situ stress in the coal–oil–gas co-reservoirs in Ningdong coalfield, porous flow-stress experiments were performed on the coal specimen at an axial pressure of 8 MPa, a confining pressure of 5 MPa, and porous flow pressures of 0 MPa, 2 MPa, 4 MPa, and 6 MPa. The mechanical properties of coal specimen (i.e., triaxial compressive strength, axial, circumferential and volumetric deformation) were systematically measured under different porous flow pressures and various media (i.e., crude oil, reservoir gas, oil–gas mixtures). The results show that under constant triaxial stress and porous flow pressure, the peak strength of the coal specimens is highest in the reservoir gas medium, intermediate in the crude oil medium, and lowest in the oil–gas mixture. Moreover, the trends of axial strain, volumetric strain, and circumferential strain are consistent with the variation in peak strength, revealing that the damage induced by the porous flow of oil–gas mixture is the most severe, followed by crude oil, and least for reservoir gas. Fracture evolutions and the corresponding stress characteristics further are identified, revealing the progressive failure mechanism of coal under different fluid/gas media and the evolution of permeability with deviatoric stress. The permeability at peak strength follows the order: oil–gas mixture > reservoir gas > crude oil, demonstrating that oil–gas coupling disasters pose a greater threat to mine safety than the isolated porous flow of either crude oil or reservoir gas.