Temperature effect on mode Ⅰ fracture behavior of tight sandstone

The fracture complexity of tight sandstone could work to improve the efficiency of tight gas reservoir fracturing.In this paper, three-point bending loading tests was used to characterize the fracture behavior, mechanical properties, fracture path and fracture energy characteristics of tight sandstone at room temperature(RT)and low temperature(LT)of liquid nitrogen(LN₂), and to interpret the pre-fracture mechanism of tight sandstone at LN₂.The results show that: 1)The fracture load and fracture toughness of the tight sandstone treated with LN₂ are significantly lower than those at RT, while the parameters of the samples under parallel bedding slightly increased, which may be related to the enhancement of the bedding plane by LN₂.2)After low temperature treatment with LN₂, the complexity of fracture propagation paths increases, the number of fracture increases, and the fracture bifurcation and convergence occurs.The fracture complexity of sandstone is low at RT, and the effect of LN₂ precracking on the fracture complexity of tight sandstone is significant.3)After LN₂ pre-splitting, the fracture energy of tight sandstone decreases significantly, with a maximum decrease of 46.98 %.4)It is found that the fracture paths of tight sandstone mainly develops along the mineral grain boundary and forms intergranular fracture at RT, and the rock transformation degree is low.However, the complex fracture forms are formed by intergranular fracture, transgranular fracture and intergranular fracture, and the formation of transgranular fracture and complex fracture network are promoted under low temperature.The results of this study suggest that low-temperature during fracturing of tight sandstone reservoir can effectively reduce the fracturing pressure, realize the toughening-brittle transformation of rock and improve fracture complexity.