Permeability Evolution of Anthracite Subjected to Liquid Nitrogen Treatment under Repeated Loading-Unloading Conditions

This study conducted a triaxial stress cycle loading-unloading experiment on anthracite coal samples to investigate the effect of different cycle numbers of liquid nitrogen cold dips (LNCDs) on the permeability characteristics of coal bodies under repeated loading-unloading conditions. The permeability of anthracite coal samples treated by LNCD and repeated loading-unloading was enhanced by 2-4 times, and the permeability evolution patterns of LNCD-damaged samples for different numbers of loading-unloading cycles were obtained. To evaluate the permeability enhancement effect of coal samples under single and multiple loading-unloading cycles, the permeability enhancement efficiency index and the LNCD influence coefficient were determined, indicating that the compound effect of LNCD treatment and repetitive stress loading-unloading significantly increased the coal's permeability. The greater the number of liquid nitrogen fracturing is, the higher the permeability of coal samples is under repetitive loading-unloading conditions. LNCD treatment advanced the transition point of coal samples from the compaction state to the enhanced permeability state. The composite repeated loading-unloading-LNCD treatment fracture was subdivided into four stages according to the internal structural changes of the coal body. The model of the composite fracture of the coal seam under the LNCD cycles and loading-unloading was constructed and experimentally validated.