Extracting coalbed methane is challenging due to the strong gas adsorption capacity and low matrix permeability of the coalbed. Recently, thermal recovery methods have been tested to promote methane recovery. In this study, anthracite samples were heated to different temperatures to investigate the internal pressure variation and microstructure changes. It was found that higher temperature resulted in higher internal pressure. At low temperatures, the increase in the internal pressure was mainly due to gas desorption. At 500°C, thermal cracking gases provided the main contribution to the high internal pressures, as more gaseous products were generated at the higher temperature. In addition, the microstructure of coal significantly changed after combustion, including the increased pore volume, the increased specific surface area, and the generation of microfractures. These changes could potentially increase the porosity and permeability of coal. Thus, high‐temperature thermal treatments not only provided energy for gas desorption and organic matter decomposition but also improved conditions for gas transport.
Recently, thermal recovery technologies such as combustion have been studied for shale gas recovery. Thus, understanding of the microstructure of combusted shale is essential for evaluating the effects of thermal treatment on shale gas transport capacity. In this study, the effect of combustion on shale microstructure changes was investigated. Firstly, different-sized shale samples were combusted at 450 °C for 30 min. Afterward, shale microstructure properties including surface topographies, porosity and permeability of the raw and combusted shale samples were measured and compared. It was found that the pore volume and specific surface area increased after combustion, especially for small pulverized samples. According to surface topography obtained from atomic force microscope, more rough surfaces were obtained for the combusted shale due to larger pores and generation of thermal fractures caused by the removal of organic matter. Based on the mercury intrusion porosimetry measurements, the porosity of the shale samples increased from 2.79% to 5.32% after combustion. In addition, the permeability was greatly improved from 0.0019 to 0.6759 mD, with the effective tortuosity decreased from 1075.40 to 49.27. As a result, combustion treatment can significantly improve the gas transport capacity.
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