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.

Mudstone creep experiment and nonlinear damage model study under cyclic disturbance load.

To study the creep characteristics of mudstone under disturbed load, creep rock triaxial compression disturbance tests under different disturbance amplitudes and frequencies are conducted using a self-made triaxial disturbed creep test bench for rock. The influence of different factors on the creep deformation law of each stage is analyzed. The results show that the disturbance effect has a significant impact on the creep properties of mudstone, and various factors have different effects on the creep stages. The instantaneous deformation variable, creep decay time, and steady creep rate change exponentially with the increase in axial pressure, and increase linearly with the increase in disturbance amplitude and disturbance frequency. The disturbance amplitude has a more significant effect on the instantaneous deformation, steady-state creep rate, and accelerated creep. According to the analysis of the test results, a nonlinear disturbance creep damage model based on Burger's model is established. The model is identified and calculated by the improved least squares method based on pattern search. The influence of different disturbance factors on the creep parameters is analyzed. The model fitting results and experimental results are compared to demonstrate that the model is used to simulate different disturbances. It was observed that rock creep under certain conditions exhibits certain adaptability. It is of great significance to carry out rock disturbance creep experiments and study the theory of disturbance creep to ensure the long-term stability of deep rock mass in complex environment.