Investigation on the coupling response of stress-fracture-seepage field during oil-bearing coal mining

The migration and distribution of oil in oil-bearing coal significantly influence the gas drainage and gas-outburst control during mining activity. To quantitative describe the oil/gas migration and morphology in mining-induced fractured, the triaxial seepage experiment was conducted considering the mining-induced redistributed stress, and the self-developed online low field nuclear magnetic resonance triaxial seepage system (LF-NMR) was employed. The stress-fracture-seepage field evolution was monitored and corresponding coupling mechanism was analyzed based on the T2 spectrum, MRI and stress-strain response. The pore-fracture was composed of macropore (>10 m s), mesopore (1–10 m s) and micropore (0.01–1 m s), and experienced rapid compaction, stable reduction and rebound stage, which was mainly contributed by the macropore. The coal experienced compression, elastic, elastic-plastic, plastic-failure state with the increase of vertical stress, and continuously created tensile stress-dependent, tensile stress and shear stress-dependent, and shear stress-dependent pore-fracture structure under the confined pressure of 18 MPa, 10/14 MPa, and 2/6 MPa. The vertical stress triggered the pore-fracture development, and confined pressure controlled the pore-fracture distribution and morphology. As the unloading of confined pressure, the horizontal deflection of fracture angle was increased, fracture size was decreased, and the conductivity channel for oil/gas migration experienced “compacting channeling " (10–18 MPa), “optimal channeling" (10 MPa) and “gas channeling " (2–10 MPa) with corresponding oil recovery of 35 %, 40 % and 20 %. The gas drainage experienced lower stable state, dynamic peak state, and re-lower state, and the maximum methane concentration of 70 % and discharge of 0.9 m3/min presented ahead of the mining face around 20 m in engineering practice. The findings provide insight into the co-mining of coal, gas and oil.