Influence of thermal metamorphism on CBM reservoir characteristics of low-rank bituminous coal

As mining depths increase, new challenges have emerged, such as abrupt changes in coalbed methane (CBM) reservoir rules and gas outbursts, particularly in coal/sill contact metamorphism zones. To investigate the influence of magmatic-contact metamorphism on CBM reservoirs, fifteen samples were selected from the N1708 coalface in the Daxing Mine at different distances from a sill in the Tiefa Coalfield, China. This sill-form intrusive body had a positive effect on the properties of the CBM reservoir. The thermal effect of the igneous sill remoulds the pore structures of thermally metamorphosed coal (TMC). More generally, the seepage pores (pores >100 nm) and the surface areas of the TMC samples were more developed than those of the unaltered samples, suggesting that contact metamorphism might have significant implications for the excellent gas flow/adsorption capacity of TMC. The Langmuir volume of TMC sample #3 was measured as 36.1 m3/t. TMC samples showed higher initial desorption rates and lower moisture; in sample 2, the proportion of gas desorbed (cumulative volume/total volume) reached almost 0.5 in only 90 s. The diffusion coefficients (D) for four TMC samples were within the range 1.41 × 10−11-2.59 × 10−10 m2/s. These results show that the high initial gas desorption rate and low moisture of the TMC have a positive influence on gas diffusion and CBM drainage, but lead to high gas outburst index. The gas content and gas pressure beneath the sill were obviously greater than those in the unaltered coals. The CH4 levels of TMC were slightly higher than those of the unaltered samples. However, approaching the sill, it is significant that CO2 levels showed an obvious increase. It is postulated that the higher CO2 levels of TMC are likely derived from the breakdown of calcium carbonate under high-temperature conditions. These results indicate that the TMC covers dual gas (CH4 and CO2) outburst potential, thus gas pre-drainage before mining is necessary in contact metamorphism zones.