Quantification of liquid nitrogen injection efficiency and phase change expansion boost laws in coal seam borehole

Abstract

The implementation of liquid nitrogen injection into coal seam boreholes is a prerequisite for its application to increase coal seam permeation. Exploring the efficiency of liquid nitrogen injection is of great significance in optimizing the settings of key injection parameters, thereby reducing costs. By using high-pressure liquid nitrogen tank, flowmeter, endoscope, thermocouple and mass balance, experiments of different liquid nitrogen injection rates were carried out in 1 m long coal seam borehole. The rules for changing the liquid nitrogen net level in the borehole at different injection rates were obtained: slow growth-linearly growth-slowly growth to stability, and the maximum net liquid level could reach 60 cm. The effective convection coefficient and radiation coefficient during the transient film boiling at the early stages of liquid nitrogen injection were calculated, and it was found that convective heat transfer dominated this stage. The liquid nitrogen injection efficiency at different injection rates was quantitatively investigated, and the maximum efficiency was 8.75 %, and the injection efficiency increases with the increase of injection rate. After stopping the injection, the net liquid nitrogen level in the borehole was analyzed, and they all decreased exponentially. A liquid nitrogen phase change expansion boost model was established and experimental verification passed. Further, it was calculated that the maximum value of liquid nitrogen phase change expansion pressure in a closed borehole of 1 m long coal seam was 45.8 MPa. The expansion pressure can play a dominant role in fracture propagation during the fracturing process, and combined with the damaging effects of liquid nitrogen cold shock on coal, it has a great development prospect in the field of coal seam permeability enhancement.