Influence of Thermal Cracking Permeability Enhancement in Spontaneously Combustible Coalbed Methane Reservoirs on the Characteristics of Coal Spontaneous Combustion

Abstract

ABSTRACT Thermal cracking permeability enhancement (TCPE) is a method to improve the coalbed methane (CBM) extraction efficiency and yield in low-permeability CBM reservoirs, but this method can change the physical and chemical properties of coal and thus affects the characteristics of coal spontaneous combustion. In order to explore the influence mechanism of TCPE on the characteristics of coal spontaneous combustion, the evolutions of oxygen adsorption capacity and chemical groups of coal under different thermal cracking parameters were measured by experiments. The following research results were obtained: On the whole, the pore and free radical structures of coal are changed after TCPE at low temperature and for a short time. The oxygen adsorption capacity and chemical activity of coal are increased, which leads to the increase of the risk of coal spontaneous combustion. However, when the coal is subjected to long-term high-temperature TCPE, the oxygen absorption capacity, and chemical reactivity are weakened. Ultimately, the risk of coal spontaneous combustion and the intensity of oxygen composite reaction become lower compared with the original state of coal. Based on these findings, it is suggested to consider long-term high-temperature thermal cracking during TCPE in CBM reservoirs.