Mechanism of pore water phase change, freezing expansion, and pore enlargement of coal samples under low-temperature liquid nitrogen action

Abstract

Liquid nitrogen fracturing, a technology for enhancing coal seam permeability without water, garnering considerable attention due to the intricate network of fractures induced by the low-temperature impact of LN2 on coal reservoirs. The phase transition freezing expansion of water-ice plays a pivotal role in determining the efficacy of coal damage. As direct measurement of water signal change during freezing is impractical, studying the variation in unfrozen water content during coal sample re-warming subsequent to cold shock is beneficial. This approach aids in elucidating the phase transition expansion and pore expansion mechanisms at different pore scales. The findings reveal the sequence of pore ice melting in coal samples frozen by LN2: micropores exhibit initial water appearance, followed by medium-sized pores, with micropores reaching peak levels first. Subsequently, water migrates from micropores to medium-sized pores, leading to their peak levels, before macropores show water presence. The temperature gradient from the coal sample center to the coal wall, owing to thermal resistance, results in the part melting first also freezing first. Analysis indicates that the porosity signal of lignite and bituminous coal increased by 203 % and 467 % during medium-sized pore melting, emphasizing the prominence of the medium-sized pore melting stage. It is further inferred that the phase transition freezing expansion and pore expansion of coal sample water induced by LN2 primarily occur in medium-sized pores.