Abstract: This study selected shale from the Longmaxi Formation in Sichuan as the research object. The shale samples were subjected to microwave irradiation at 2,450 MHz and 800 W for 0, 15, 30, 60, and 90 s. By integrating computed tomography (CT), uniaxial compression tests, and numerical simulation of the temperature field, the microwave-induced evolution of the pore structure and the degradation of macroscopic mechanical properties were systematically analyzed, and their response relationship between them was revealed. The results show that: (1) Microwave irradiation significantly promotes the generation and propagation of pores in shale. After 90 s of microwave irradiation, the number of pores increases by about 88.4%, the porosity rises from 4.1% to 10.7%, both the maximum and average pore radii increase markedly, and the pore size distribution tends to be multi-scale. The thermal effect induced by microwave irradiation is the dominant factor controlling pore development, showing significant regional and heterogeneous characteristics, and is significantly correlated with the degradation of macroscopic mechanical properties. (2) Microwave irradiation significantly enhances pore connectivity. The three-dimensional pore-throat network model shows that after 90 s of microwave irradiation, the number of throats increases by about 109.1%, the average pore-throat radius increases by about 11.7%, and the coordination number increases by 103.3%; the pore connectivity and complexity improve simultaneously, and the pore structure evolves from discrete micro-nano pores to a mesoscopic connected structure. (3) The pore structure evolution of shale under microwave irradiation reflects its structural damage characteristics, and the damage variable constructed based on porosity can effectively reflect the structural deterioration process of shale under thermo-mechanical action. The modified multivariate regression model shows that the pore number, throat number and coordination number can predict the damage evolution process of shale with high accuracy. (4) Microwave irradiation leads to continuous deterioration of shale strength and stiffness, and the failure mode gradually transforms from splitting failure characterized by a single main crack to multi-crack intersecting failure. The results can provide theoretical and methodological support for microwave-assisted unconventional oil and gas extraction and directional weakening of rock mass.

Key words: microwave irradiation, shale, computed tomography scanning, pore structure, damage variable, uniaxial compression