Extension mechanism of fracturing in liquid nitrogen fractured coal rock dominated by phase change expansion pressure

Abstract

In this paper, the liquid nitrogen (LN2) fracturing test research using LN2 phase expansion instead of N2 pressurization was carried out in response to the neglected high phase expansion rate characteristic of LN2, an anhydrous fracturing medium. Expansion pressure measurements, infrared temperature measurements, DIC full-field strain measurements and acoustic emission (AE) signal measurements were used as measurement means. The phase change gas expansion pressure, specimen end-face field, internal fracture signal and end-face temperature changes during the fracturing process were explored. The experimental results show that there was a significant difference in the performance of LN2 cold shock damaged specimens and undamaged specimens during fracturing. The change rule of LN2 phase change expansion pressure under no peripheral pressure went through linearly increasing first and then stabilizing, the slope of the linear phase was 0.029 MPa/s, the expansion pressure in the stabilizing phase was 1.2 MPa and lasted for 12s, and then the specimen ruptured. The initiation pressure of the gas fractured specimen was 2.2 MPa. The surface strain of the specimen was significantly increased during the stabilization stage of expansion pressure, and the maximum values of transverse strain and longitudinal strain were 0.059% and 0.071%, respectively, which were increased by 43.9% and 73.2% within 12 s. At the same time, the number of AE localization points increased from 12 to 78, and the location of the localization points gradually expanded from the two sides of the horizontal slot to the top of the horizontal slot and the outer wall of the specimen. There was a low-temperature zone with obvious temperature gradient in the extended area and no such change existed in the gas fractured specimens. Therefore, the fracture initiation pressure of the damaged specimen is lower than that of the undamaged specimen, and the cracks are more likely to expand and become continuous during the fracturing process, thus producing complex cracks. The use of LN2 with high phase change expansion rate can replace the high-pressure pumping of N2, which is of great significance for the reduction of the implementation time and cost of the LN2 fracturing process.