Mechanism of Unfrozen Water Content Evolution during Melting of Cryogenic Frozen Coal Body Based on 2D NMR

Abstract

The content of unfrozen water in the freezing process of coal body affects the microscopic pore structure and macroscopic mechanical properties of coal body and determines the permeability-enhancement effect of coal seam and the extraction efficiency of coal mine gas. To investigate the evolution mechanism of unfrozen water content in the melting process of lignite, this paper takes the melting process of lignite liquid nitrogen after freezing for 150 min as the research object and quantifies the spatial change process of unfrozen water distribution based on two-dimensional nuclear magnetic resonance technology. Through the accurate interpretation of the superimposed signals of different fluids, the 2D NMR technique can more easily obtain the spatial distribution of different fluids and even the specific content of fluids in different pores in coals. The results show that at −196 °C, the unfrozen water mainly existed in the small coal pore and the small ice pore in the large pore. As the temperature rose, the pores melted, and free water began to be produced. The mathematical model analysis shows that there was intermolecular potential energy between fluid molecules and the coal pore wall, and the pore wall exerted a part of pressure on its internal fluid, and the pressure affected the melting point of pore ice with pore diameter and melting temperature, resulting in the difference of unfrozen water content.