Influence of Various Control Factors on Fracture Toughness and Fracture Energy of Sandstone Subjected to Liquid Nitrogen Cooling

Abstract

Liquid nitrogen (LN2) cryogenic fracturing can significantly enhance the permeability of tight-sandstone gas reservoirs. In order to evaluate the control factors influencing the changes in the crack toughness and fracture energy of the sandstone in LN2 cryogenic fracturing, the influences of the LN2 cooling state, freeze–thaw cycle (FTC) times, moisture content of the sandstone, and notch inclination angle were studied using combined three-point bending experiments. At the same time, the internal structural damage and induced fracture morphology are employed to analyze the failure mechanism. The results show that the LN2 freeze–thaw treatment can decrease the crack toughness and fracture energy of the sandstone, while these values increase for the LN2 freezing treatment. The water saturation condition can enhance the LN2 cooling effect, but it has a larger influence on LN2 freezing. With the increase of the notch angle, both the crack toughness and the energy release rate (ERR) of the saturated sandstone significantly reduce, but the applied energy only has a slight increase. The crack toughness, ERR, and applied energy of the saturated sandstone gradually decrease with the increase in the LN2 FTC, and their values after 30 LN2 FTC times decrease by 52.67, 76.04, and 41.15%, respectively. The strength feature and fracture energy of the sandstone heavily depend on the internal microstructure and crack propagation mechanism of the sandstone.

Effect of Clay Minerals on Tensile Failure Characteristics of Shale

“… 3 Understanding the failure mechanism and failure process from the micro-level to the macro-level is a benefit to the unconventional gas/oil exploration and exploitation. 4 Therefore, the effects of shale’s inherent heterogeneity on its mechanical properties and fracturing behaviors become hot issues in the field of hydraulic fracturing, 5 , 6 especially in recent years.…”