Gas expansion energy of coals with different metamorphic degrees: Evolutionary characteristics and their implications for the outburst prediction

To achieve an elevated accuracy in the prediction of coal and gas outbursts, this study conducted mercury injection capillary pressure (MICP) tests and isotherm adsorption experiments on selected six coal samples with different metamorphic degrees. Furthermore, it calculated the gas expansion energy of coals and analyzed the evolutionary characteristics of the gas expansion energy of coals with different metamorphic degrees, as well as their relationships with the prediction indices of coal and gas outbursts. The results of this study are as follows: (1) The differences in the pore structures and adsorptivity of coals with different metamorphic degrees led to different gas expansion energy. (2) The total gas expansion energy and the adsorbed-gas expansion energy increased with gas pressure, with the increasing trend gradually slowing down. Under the same gas pressure, a higher metamorphic degree of coals was associated with higher total gas expansion energy and adsorbed-gas expansion energy. (3) The free-gas expansion energy increased exponentially with gas pressure. Under the same gas pressure, the free-gas expansion energy increased with the pore volume per unit mass of coals. (4) When R_max > 1.6%, the free-gas expansion energy corresponding to the gas pressure of 0.74 MPa was roughly equivalent to the critical values (42.98 mJ/g) of initially released expansion energy that induced outbursts. This further verifies that free gas plays a major role in the triggering stage of outbursts and also provides a scientific basis for the rationality of using 0.74 MPa as the critical gas pressure value for outburst prediction. When R_max ranged between 0.6%‒1.6%, the free-gas expansion energy corresponding to the gas pressure of 0.74 MPa was below 42.98 mJ/g, leading to exaggerated outburst potential of coals and increased efforts paid to outburst prevention. When R_max < 0.6%, the free-gas expansion energy corresponding to 0.74 MPa exceeded 42.98 mJ/g, leading to the occurrence of outbursts under a low index. Therefore, it is necessary to fully consider the influence of coals’ metamorphic degrees on coal and gas outbursts. This study will provide a novel scientific basis and a method reference for the prediction of coal and gas outbursts.