Influence of magma intrusion on gas outburst in a low rank coal mine

The effect of magma intrusion on gas outburst is illustrated by a case study of the exposed magma intrusion in the 313 mining area, upper coal seam Number 3, in the Qiwu Mine located in Shandong province. Vitrinite reflectance, mercury injection, and maceral statistical analysis are used to characterize the coal. The aspects of coal metamorphism include changes in micro-components as well as in coal structure, the formation of new substances, and changes in gas absorption and storage. The results show that vitrinite reflectance increases within the region influenced by magma intrusion. The metamorphosed region may be divided into a weakly affected belt, a medium affected belt, a strongly affected belt, and a completely affected belt. Compared to the unaffected coal the total pore volume, as well as the amount of big and middle sized holes, increases while the number of transition holes and micro-pores decreases. This diminishes the absorption capacity of the matrix but enlarges the total gas storage space. Vitrinite content initially decreases slightly but then increases rapidly while the inertinite content increases at first but then decreases. Exinite content decreases, then increases, and finally drops to zero. Higher vitrinite, and a lower inertinite, content increase gas absorption ability. This balances reduced adsorption caused by changes to pore structure. Consequently, gas adsorption capacity is not substantially reduced as the coal rank increases. Thermal metamorphism of the coal produces CH4 and other hydrocarbons that increase the total gas content in the coal seam. Asphaltene migrates into the medium and weakly affected regions filling in the pores and fractures there. This plugs the pathway for gas transport. A barrier is formed that hinders gas flow. CO2, H2S, N2, and other gases carried in by the magma react to produce CO2, which increases in relative concentration and enhances the risk of gas outburst. The two barriers, magma intrusion on one side and the medium and weakly affected belts on the other, as well as the unaffected coal seam itself, trap a large amount of gas during the thermal activity. This is the basic reason for gas outburst. These conclusions can enlighten activities related to gas prevention and control in a low rank coal mine affected by magma intrusion.