Pore structure and its influence on gas mass transfer of low rank coal in Eastern Junggar Basin, Xinjiang

The complexity of pore structure of low rank coal has a deep impact on methane storage status, gas transfer and estimated coalbed methane (CBM) resources. According to coal petrology research, the result shows that the coal rank is mainly sub-bituminous A (Ro=0.42%-0.59%), the components of maceral and its properties are mainly constituted by vitrinite (51.7%-79.0%), followed by inertinite (15.2%-45.5%), and it has sporadic sapropelinite, generally the volatile matter content accounts for more than 30%. However the inertinite content and the moisture content of lignite are far greater than that of long flame coal. The characteristic and distribution of pore in both lignite and long flame coal were determined by using mercury injection and nitrogen adsorption, and the methods of methane mass transfer in different scales of pore throat were analyzed. The results show that the pore features and distribution of lignite and long flame coal show obvious differences in mercury intrusion and nitrogen adsorption testing, although the lignite coal and long flame coal are classified as the low rank coal. Lignite coal gives priority to macroporous development, micro pore volume is limited. The pore feature of long flame coal is dominated by micro pore, which has less unit pore volume than lignite. The reservoir space of lignite mainly distributes with pore throats ranging from 0.1 μm to 2 μm. The pore volume of long flame coal is mainly made up of micro pore by nitrogen adsorption test. Based this study, combining with the law of gas mass transfer, the study identified four types of mass transfer at multi-scale pore configuration in the coal reservoir, including Darcy flow, slippage effect flow, transition flow and molecular diffusion. The different pore characteristics of long flame coal and lignite make the mass transfer modes be not the same. The limited growth of micro pore of lignite, in which the seepage pores are well connected, gives priority to Darcy flow and slip flow. The micro pore developed well in long flame coal, but the seepage pores are not well connected, and the mass transfer manners dominated by Darcy flow, transition flow and molecular diffusion. The thickness of adsorption methane and slippage effect both have significantly effect on nano pore seepage ability in long flame coal, which could provide a useful support to a stable production at the later period of CBM development. © 2016, China Coal Society. All right reserved.