Effect of coalification and maceration on pore differential development characteristics of high-volatile bituminous coal

Abstract

The coal type of high-volatile bituminous coal (HBC) is an important component of coalbed methane (CBM) reservoirs in China. Moreover, the accurate characterization of pore structures of coal is crucial to the exploration and development of CBM. In this work, the pore structure of vitrain and durain in high-volatile bituminous coals was detected using a scanning electron microscope, mercury intrusion porosimetry, low-temperature N2 adsorption, and low-temperature CO2 adsorption across multiple length scales and the effect of coalification and maceral on pore differential development law of HBC was analyzed. The findings reveal that at different pore size scales and the pore characteristics of different coal lithotypes under the influence of coalification are clearly different. In the same sample, the specific surface area (SSA) and total pore volume (TPV) of supermicropores in vitrain are higher than those of durain but the SSA and TPV of micropores and transition pores in vitrain are lower. The fractal results reveal that with the increase in pore size, the complexity of pore structure of coal increases. The relationship between the vitrain and durain pore fractal dimensions in different scale sections will change. At different pore size scales, the difference in pore structure in the vitrain and durain exhibits a trend of initially increasing and then decreasing with pore size. The inertinite content is positively correlated with the adsorption pore volume and SSA of vitrain and durain, and the vitrinite content is negatively correlated. Physical compaction considerably reduces the pore volume of vitrain during the early evolution stage of HBC but has no effect on durain. The pore parameters of vitrain and durain initially increase and subsequently decrease with the increase in vitrinite reflectances. The maximum value is obtained near the first coalification jump point. The evolution trend of the pore volume of vitrain and durain is consistent at different pore size scales; however, the total variation range of durain is higher than that of vitrain. The adsorption pore structure is mainly affected by asphaltification and polycondensation, whereas physical compaction has a greater impact on the seepage pore structure.