Evolution of pore-fracture structure and permeability of coal by microwave irradiation under uniaxial compression

In a natural coal reservoir environment, the coal seam is constrained by in-situ stress and gas pressure. Damage on the coal microstructure due to microwave irradiation (MI) differs significantly different from that under no-load conditions. In this study, the effect of MI on the pore-fracture structure and seepage characteristics of load-constrained coal is investigated using a custom-developed microwave fracturing experimental device, nuclear magnetic resonance test device, and permeability test device. Based on the relationship between microwave and pore-fracture structure parameters and the permeability of loaded coal, the pore-fracture structure evolution and permeability growth law of loaded coal under MI are determined. The results show that the number of micropores in coal decreases and the T₂ curve of micropores is shifted to the right under the combined effect of MI and external stress load. The numbers of mesopores, macro-pores, and micro-fractures increase, and the T₂ curve exhibits a broader peak span. The pore-fracture structure evolution effect of loaded coal increases with the microwave power and MI time. Under high-power MI, the pore-fracture structure evolution of the loaded coal shows a “decrease - increase – decrease” trend as the stress load increases, whereas a “decrease – increase” trend is shown under low-power MI. Under the same microwave parameters, the permeability of unloaded and loaded coal increases by a maximum of 15.7 and 364.7 times, respectively. In particular, the permeability increases by 3.1–11.4 and 17.8–49.7 times under external stress loads of 4 and 2 MPa, respectively. The combination of high MI power and short MI duration under the same microwave energy facilitates the development of the pore-fracture structure and increases the permeability of loaded coal. Microwaves have a differential thermal effect on mineral in coal, which reduce the physical properties of the coal. Pore-fracture structure and permeability are further enhanced by the stress load.