An experimental investigation into the effect of liquid nitrogen cooling on fracture behaviors of bedding shale

Abstract

Liquid nitrogen (LN₂) fracturing has been proposed to be used in unconventional gas engineering, including shale reservoirs. However, the effect of LN₂ cooling on the fracture behaviors of bedding shale has not been well studied. In this study, three-point bending tests were conducted on shale samples at room temperature and under LN₂ cooling condition, respectively. Different bedding angles were considered in these tests. The phenomenon of acoustic emission was monitored during the fracture initiation and propagation of bedding shale. The results showed that the tensile strength, fracture toughness, bending elastic modulus and fracture energy decreased with the increase of bedding angle under the same temperature condition, but the bedding angle has no significant effect on energy storage ratio and speed. There was a linear relationship between the tensile strength and fracture toughness of bedding shale. The crack initiation was inconsistent with the loading reference line except in 90° bedding shale, and then turned to the direction parallel to the loading reference line, resulting in various fracture morphology of bedding shale. The acoustic emission signals were mainly concentrated in the failure stage because of the severe brittleness of bedding shale. Under LN₂ cooling condition, the tensile strength, fracture toughness, bending elastic modulus, energy storage ratio, fracture energy and AE cumulative ringing counts of bedding shale with the same bedding angle significantly increased, but the energy storage speed decreased. The crack length along bedding direction was longer and the crack inflection point was clearer than those of bedding shale at room temperature.