Mesoscopic structural damage and permeability evolution of Shale subjected to freeze-thaw treatment

To study the mesoscopic damage and permeability evolution characteristics of rock under freeze-thaw (F-T) cycles, freeze-thaw cycle experiments were carried out of shale under different F-T temperatures and numbers of cycles, and nuclear magnetic resonance (NMR) and permeability experiments of shale were conducted thereafter. On the basis of these experiments, the pores and permeability of the F-T shale were analyzed, and the existing permeability model is modified and improved; Therefore, the mesoscopic damage evolution characteristics and permeability evolution law of the F-T shale are obtained. It was found that with increasing number of cycles, the pore structure of the rock samples changed as the pore size expanded and the number of pores increased, and the average porosity also increased correspondingly. The influence of the F-T cycle temperature on the shale permeability was not as notable as that of the number of F-T cycles. Based on the SDR-REV permeability model, the spectral area ratio parameters of large pores and fractures in the T₂ spectrum were considered for correction, and a direct relationship between the permeability, F-T temperature and number of cycles was obtained via regression analysis. Compared to the experimental results, it was found that the modified model achieved a good applicability. The damage and permeability characteristics of shale under different F-T conditions were analysed from a microscopic perspective, which could yield an important reference for engineering construction in frozen soil areas.