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

Abstract

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.