Mixed-mode fracture behavior in deep shale reservoirs under different loading rates and temperatures

Abstract

In the last years, shale gas has gradually substituted oil and coal as the main sources of energy in the world. Compared with shallow shale gas reservoirs, deep shale is characterized by low permeability, low porosity, strong heterogeneity, and strong anisotropy. In the process of multi-cluster fracturing of horizontal wells, the whole deformation process and destruction modes are significantly influenced by loading rates. In this investigation, the servo press was used to carry out semi-circular bend (SCB) mixed-mode fracture experiments in deep shales (130, 160, 190 °C) with prefabricated fractures under different loading rates (0.02, 0.05, 0.1, 0.2 mm/min). The fracture propagation process was monitored using acoustic emission. The deformation characteristics, displacement–load curve, and acoustic emission parameters of shale under different loading rates were studied during the mixed-mode fracture propagation. Our results showed that during the deformation and fracture of the specimen, the acoustic emission energy and charge significantly increased near the stress peak, showing at this point the most intense acoustic emission activity. With the increase in loading rate, the fracture peak load of the deep shale specimen also increased. However, the maximum displacement decreased to different extents. With the increase in temperature, the effective fracture toughness of the deep shale gradually decreased. Also, the maximum displacement decreased. Under different loading rates, the deformation of the prefabricated cracks showed a nonlinear slow growth–linear growth trend. The slope of the linear growth stage increased with the increase in loading rate. In addition, as the loading rate increased, an increase in tension failure and a decrease in shear failure were observed. Moreover, the control chart showing the relationship between tension and the shear failure under different temperatures and loading rates was determined.