Coal cleat network evolution through liquid nitrogen freeze-thaw cycling

•Promising coal cleat network evolution through liquid nitrogen freeze–thaw cycling.•More significant efficiency of latter freezing cycles compared to the first cycle.•Permeability and connectivity enhancement through freeze–thaw cycles.•Considerable coal structure damage by this fracturing technique. This study investigated the potential of liquid nitrogen (LN2) freeze–thaw process in coal fracturing, focusing on the effect of different freezing cycles. μ-Computed Tomography (μ-CT) images revealed a promising efficiency in cleat network evolution after three freezing cycles. The initial coal showed some fractures with the maximum opening of 15 μm, where the treated coal demonstrated several new fractures with the maximum opening of 10 μm, almost all of which were interconnected to the cleat network. The volume and length of the largest fracture more than doubled, 2.6 × 108 to 5.9 × 108 μm3 and 8 × 105 to 1.9 × 106 μm, due to interconnection with new fractures and isolated fractures. Connectivity analysis illustrated that the number of pores increased by 50% (92715 → 142650), where the number of interconnected pores almost doubled (42060 → 78905). The porosity of the coal also doubled from 0.6% to 1.2% based on μ-CT scan results. SEM along with μ-CT images highlighted more encouraging efficiency of second and third freezing cycles, particularly in terms of enhancing fractures interconnection. SEM images revealed the generation of a thoroughgoing fracture, which increased in aperture size through successive freeze–thaw cycles (17 μm to 48 μm) and extended a cleat network in the coal, particularly in latter cycles. Atomic Force Microscopy demonstrated an increase in the area roughness, which was in a direct relationship with freezing cycles. Mechanical properties analysis revealed more significant damage in the coal in the latter freezing cycles, being initially 3.49 GPa and decreasing to 2.81, 2.11 and 1.52 GPa through three freezing cycles. Finally, the permeability of the coal under 1000 kPa confining pressure increased from 0.035 mD to 0.18 mD, with larger increments in later cycles. Numerical permeability study ran in different directions, where resulted in the most promising enhancement in the Z direction (3.7 to 14 md, 277%).