Liquid nitrogen fracturing efficiency as a function of coal rank: A multi-scale tomographic study

Abstract

This study explored the potential of liquid nitrogen fracturing in three different coal ranks. X-ray computed tomography results in micro- (μ-CT) and macro (medical-CT) scales revealed a poor performance of LN2 fracturing in anthracite. In contrast, porosity evolution for bituminous and sub-bituminous coal in microscale was 14 % and 119 %, respectively. The porosity enhancement in macroscale for these coals was also significant. Interestingly, in both coals, thoroughgoing fracture planes originated from the initial cleat network following freezing. Furthermore, connectivity analysis through skeletonization and Euler number analysis suggested that sub-bituminous connectivity increased by 20-fold for small pores (<3 μm), while it doubled in bituminous. SEM and digital microscopy also revealed the inefficiency of LN2 fracturing in anthracite, but promising efficiency in the other two coals. In macroscale, a large thoroughgoing fracture (140 μm opening) appeared following treatment in bituminous coal. Sub-bituminous coal also illustrated numerous new cracks and interconnections. Another visualisation analyses yielded the appearance of fracture bridges in cleat network. Further, the results suggested that the mechanical properties of the original coal revealed a direct relationship with coal rank, increasing from 2.13 GPa for sub-bituminous to 5.46 GPa for anthracite. The reduction in mechanical properties due to damage was remarkable for lower coal ranks, at 18 % and 31 % in the treated bituminous and sub-bituminous, respectively, but only 4 % in anthracite. Finally, experimental permeability evolution analysis showed a more than double increase in sub-bituminous (0.026–0.064 mD) and bituminous (0.011–0.027 mD) permeability, while anthracite passed no flow. Numerical permeability study, through Lattice Boltzmann simulation, showed a similar trend for the coals. Therefore, this study presented the results of LN2 fracturing technique in a range of coal ranks, which can be useful in terms of long-term enhanced gas recovery plans for coalbed methane extraction in different seams.