Thermal damage evolution and mechanical deterioration of granite under microwave irradiation-liquid nitrogen jet cycles

Liquid nitrogen and microwave are emerging alternative methods for HDR reservoir fracturing, yet the combined application in rock fracture remains unclear. To investigate the combined effects, granite specimens with boreholes in the centre were cyclical treated by in-hole microwave irradiation (3 kW) and liquid nitrogen jet cooling. Physical and mechanical tests indicated that both P -wave velocity ( V p ) and uniaxial compressive strength (UCS) of the specimens decreased with increasing irradiation time and cycles number. Compared to air-cooled specimens, liquid nitrogen cooling caused a greater reduction in V p and UCS by 100.6% and 79.2%, respectively. Image analysis revealed that crack propagation length, fracture intensity (P 21 ) and fractal dimension (D f ) were directly proportional to both irradiation duration and number of cycles. Nuclear magnetic resonance (NMR) tests confirmed that the porosity of rock specimens increased from 13.8% to 39.8% after liquid nitrogen cooling, mainly due to the formation of micropores and mesopores. Additionally, a composite damage factor ( D C ) was proposed to better characterise thermal damage extent in granite. The present findings can serve as a reference for geothermal energy extraction from granite reservoirs.