Effects of physical property dynamic change on heat transfer characteristics of coal seam during in-situ pyrolysis of tar-rich coal

Tar-rich coal has the features of high tar yield and great potential for oil production. The underground in-situ pyrolysis technology is an effective way to accomplish the green and efficient utilization of tar-rich coal resources. The pore structures of coal seam have great change in the actual in-situ pyrolysis process. Hence, the physical properties of coal seam also vary with pyrolysis temperature, which affects the heat transfer performance of coal seam. However, previous studies on heat transfer characteristics of coal seam have been seldom paid attention to the physical property dynamic change during the in-situ pyrolysis, which possibly causes deviation between the simulated heat transfer characteristics and the actual performance. It is still unclear whether the deviation is acceptable in practical engineering practice at present. In this article, a modification scheme for the physical property dynamic change of coal seam was proposed. The effects of physical property dynamic change on heat transfer characteristics of coal seam during in-situ pyrolysis of tar-rich coal were analyzed using numerical simulation approach. In addition to single factor correction, the effects of three-factor coupling modification on heat transfer performance of coal seam were also systematically investigated. The simulation results indicated that both the average temperature and the effective heating area of the coal seam increased after the specific heat capacity was corrected individually. Whereas, the heat transfer of coal seam was weakened after the endothermic effect of pyrolysis and the thermal conductivity coefficient were modified, respectively. When the three factors were coupled for correction in the single well and single fracture mode, the correction factor that played a leading role was the thermal conductivity coefficient, accounting for 90%. However, an increase in the number of cracks enhanced the heat transfer, resulting in the specific heat capacity was dominant among the three correction factors. The present study can offer theoretical guidance and technical support for realizing large-scale in-situ pyrolysis of tar-rich coal.