Influencing Mechanism of Temperature on Coal Pores during the Artificially Thermal Evolution

Abstract

The pores in coal are the primary storage sites of coalbed methane (CBM), and temperature is the critical factor affecting pore evolution during the thermal evolution. This paper investigates the evolution characteristics of different pores and the role of related molecular structures through the artificially thermal evolution of coal. The pore size distribution and morphology of unheated and heated coals were detected by combining the fluid injection technology and image analysis. The molecular structures of each sample are semi-quantitatively analyzed by X-ray diffraction and Fourier transform infrared spectroscopy. The results indicated that 300 °C is a turning point for the evolutions of micropores and macropores. Their respective volume remains steady approximately before 300 °C. Subsequently, the micropore volume increases gradually, while the macropore volume soars suddenly after 300 °C. Meanwhile, the mesopore volume constantly decreases during the artificially thermal evolution. On the other hand, the most mesopore morphology alters from the slit pore and wedge pore to the bottle-neck pore and cylindrical pore, while the macropore morphology alters from the wedge pore into the cylindrical pore and bottle-neck pore. Furthermore, during the artificially thermal evolution, the major factor affecting the evolution of micropore and macropore is the loss of −CH2 side chains from their edges, while the main factor affecting the mesopore evolution is the polycondensation of −CH2 side chains and carbonyl/carboxyl at its edge. These results could help understand the pore variation in CBM reservoirs during coalification.