Changes in the pore structure of lignite after repeated cycles of liquid nitrogen freezing as determined by nitrogen adsorption and mercury intrusion

As a non-aqueous medium for increasing permeability in coal seams, liquid nitrogen fracturing has been widely studied. The changes of the pores in coal fractured by liquid nitrogen have important effects on coalbed methane (CBM) migration. It is difficult to thoroughly characterize the pore structure in coal using a single method. Therefore, this study carried out a detailed study of the pores in coal samples fractured by liquid nitrogen using both nitrogen adsorption and mercury intrusion. The results show that combining these methods can accurately determine pore sizes and specific surface areas in the samples tested. The maximum liquid nitrogen adsorption capacity and the injected mercury volume in the samples were positively correlated with the freezing times and freeze-thaw cycles. This indicated that the number of pores in the coal gradually increased. Cumulative total pore and seepage pore volumes in the samples showed a positive exponential correlation with freezing time. The volume increases also correlated with the number of freeze-thaw cycles and this increase followed a quadratic function. The cumulative specific surface areas also varied with freezing time but the areas first rose and then fell as the freeze-thaw cycles increased. Liquid nitrogen freezing time significantly affects the micropores and specific surface area. However, freezing time has only a minor effect on the larger seepage pores and the total pore volume. Liquid nitrogen freeze-thaw cycles help the formation of connections between micropores and larger pores and thus promote the development of fracture networks. This provides favorable conditions for CBM production.