Effects of Loading Rate on Fracture Modes and Acoustic Emission Characteristics of Interbedded Shale Under Tension

For complex shale-sandstone interbedded reservoirs, a full understanding of the fracture modes and propagation behavior of fractures connecting multi-lithologic production layers plays a crucial role in effective fracturing design. In this regard, by means of acoustic emission (AE) monitoring technology, the Brazilian splitting tests were conducted to study the tensile failure characteristics of the specimens of shale-sandstone combination under different loading rates. The effects of loading rate on the fracture mode and fracture morphology at the shale-sandstone interface were investigated, and the variation of AE characteristic parameters during the loading process was systematically analyzed. The experimental results show that the increase in loading rate leads to an increase in the tensile strength, and the fractures become straighter and closer to the loading direction. The fractures at the shale-sandstone interface in this study can be divided into three typical morphology patterns, namely activation, deflection, and crossing. The dynamic changes of AE counts, AE amplitude, and b value provide a reference for analyzing the influence mechanism of loading rate on fracture behavior in interbedded shale specimens. At low loading rates, the AE amplitude range is wide and the distribution is relatively uniform, and the b value shows a large variation.