Search

Microwave heating contributes to coal fracturing and gas desorption. However, problems of low penetration depth, local overheating and fracture closure exist. Coal demineralisation by acids has advantages in coal unblocking and permeability improvement, while it is difficult for acid to enter microcracks. Microwave-asisted acidification may offer an alternative. In this work, XRD, 1H-NMR, and SEM were used to evaluate the effect of microwave-assisted acidification on the microstructure of coal. Results show that kaolinite, calcite, and dolomite can be dissolved by acid. After microwave irradiation, the graphitization of microcrystalline structure of carbon improves. Microwave-assisted acidification erodes minerals in coal and enhances the graphitization degree of microcrystalline structure. Compared to individual microwave irradiation or acidification, the pore volume and pore connectivity can be greatly enhanced by microwave-assisted acidification. The NMR permeability of coal increased by 28.05%. This study demonstrates the potential of microwave-assisted acidification for coalbed methane recovery.

Coalbed methane (CBM) reservoirs usually require stimulation to enhance the permeability. Characterized by the transformation of electromagnetic energy into thermal energy, microwave heating may be a promising CBM stimulation approach. In this work, the microstructure evolution of coal affected by microwave heating was investigated by Fourier transform infrared spectroscopy (FTIR), low-temperature nitrogen adsorption (LT-N2GA), and scanning electron microscopy (SEM). In addition, the methane adsorption property of coal was evaluated using a high-pressure adsorption instrument. The results indicate that microwave heating would cause the pyrolysis of ether groups, oxygen-containing groups, and aliphatic hydrocarbon in coal. After microwave treatment, the volume and connectivity between mesopores and micropores increase. The decrease in the methane adsorbability of coal may be attributed to the reduction of the methane adsorption site induced by microwave heating. The outcome of this study implies that microwave heating can be used to promote CBM recovery.