Fracturing effect and damage behaviors for microstructure in shale under microwave irradiation

Microwave stimulation technology is regarded as a promising new method for shale gas development due to its heating,high energy conversion efficiency and environmental friendly. Aimed at exploring the feasibility and effec- tiveness of microwave fracturing to shale,using the self-developed microwave irradiation device,we carried out some experiments on microwave fracturing to shale under different microwave parameters. The P-wave velocity of shale be- fore and after microwave radiation was tested with the ultrasonic method,for study the damage behaviors of microstruc- tures and fracturing characteristics of shale. And the microwave fracturing mechanism to shale was initially revealed. The results show that the temperature of shale under microwave irradiation increases rapidly and presents non-uniform distribution,and the higher the power,the stronger the non-uniformity. The damage degree of shale microstructure un- der microwave radiation is related to the microwave power,the irradiation times,the order of energy applying and the water saturation of shale. Under the same microwave energy,the number of microstructures of fully saturated shale in- creases more than that of dry shale. There are microwave energy threshold phenomena in the microstructure evolution and the macroscopic cracks development of shale samples. Under the same microwave energy,with the increase of radi- ation times,the number of shale microstructures shows a trend of significant increase,slight increase and smaller in- crease. Under the same irradiation time condition,the number of shale microstructures shows a trend of slight increase, significant increase and smaller increase,and it increases linearly with the decrease of microwave power. When the to- tal microwave energy is less than 120 kJ,the order of applying microwave energy from small to large is more favorable to the microstructure development of shale. When the total microwave energy reaches 180 kJ,the microstructure devel- opment of shale is not sensitive to the order of microwave energy application. This study indicates that the thermal stress and the vapour pressure in shale induced by microwave irradiation can cause the mineral chip breaking,lead the rigid particles and the particle interface in shale to produce microcracks,showing a significant fracturing effect. This is conducive to improving the permeability of shale gas reservoirs.