The investigation of impact of temperature on mixed-mode fracture toughness of shale by semi-circular bend test

Abstract

Unconventional oil and gas resources are important and have recently attracted attention. High temperature stimulation can effectively release the damage of water phase trapping, which was caused during the drilling and completion of hydraulic fracturing of shale gas reservoirs. The changes of physical and mechanical properties of shale under high temperature is of significance to the safety and efficiency of hydraulic fracturing. This study investigated the effect of temperature on seismic velocity, porosity, permeability, and mixed-mode fracture toughness of shale. Shale samples are separated into two groups, one group of 12 samples to test porosity, permeability and velocity, and the second group of 12 samples to test the fracture toughness of shale. The results showed that as the temperature increased, the color of the shale sample lightened, and seismic velocity decreased while the permeability and porosity increased. The results also revealed that as the temperature was raised to 400 °C–600 °C, seismic velocity, shale permeability, and porosity all increased. Furthermore, the mixed-mode fracture toughness was reduced, indicating that the threshold temperature of the shale was around 400 °C. The threshold value obtained can provide a basis for field evaluation and the potential application of high-temperature treatment. A numerical model was developed to simulate fracture propagation in SCB (semi-circular bend) samples subjected to thermal treatment. The numerical simulation agreed with the experimental results, indicating that the high temperature promoted fracture propagation and had a higher stress concentration at the fracture tip.