Evaluation of different microwave heating parameters on the pore structure of oil shale samples

Abstract

The pore structure of oil shale during microwave heating significantly determines the flow behavior, the transformation products and the ultimate recovery regardless of ex situ retorting or in situ development methods. In this experimental study, the effects of microwave heating parameters including heating time, reaction temperature and output power on the heating behavior, surface morphology and pore roughness of oil shale were investigated based on the nitrogen adsorption/desorption and scanning electron microscope (SEM) experiments. Fractal dimension D1 and D2 (at relative pressures of 0-0.5 and 0.5-1, respectively) were calculated according to the fractal Frenkel-Halsey-Hill model. The relationships between specific surface area, cumulative pore volume, average pore diameter, and fractal dimension have been studied. The results demonstrated that the pore structure of oil shale was greatly influenced by microwave heating parameters due to the decomposition of kerogen, the internal pressure caused by the steam and volatiles, the complicated reactions and the thermal stress induced by microwave. Moreover, fractal dimension also changed correspondingly with different heating parameters. The variations of fractal dimension D1 was consistent with the evolvement of specific surface area and the changes in fractal dimension D2 showed negative correlation with average pore volume. The relationships between D1 and surface roughness, D2 and pore structure were verified based on the comparison of SEM images. The fractal theory is reliable to describe the surface morphology and pore structure of oil shale and thus supports the application of microwave heating on the oil shale development.