Liquid nitrogen fracturing is a technology that enhances coal permeability and supports unconventional oil and gas exploitation. This review systematically covers the history, research topics, factors influencing efficiency, mechanism, and engineering processes of LN2 fracturing. It outlines how LN2 fracturing alters coal and rock pore structure and mechanical properties, thereby improving extraction efficiency. The review identifies the need for further studies in theoretical aspects of two-phase nitrogen flow, technical equipment development, and efficiency monitoring and evaluation. It concludes with a call for future research to focus on multiphase flow and field equipment and technology.

The article presents a detailed analysis of the bonding characteristics and dipole moments of AeF− anions (where Ae ranges from Be to Ba). It discusses the shape of the highest occupied molecular orbitals (HOMOs) and their implications on the bonding/antibonding character, as well as the polarity of the orbitals. The study uses various methods such as isodensity plots, Mulliken overlap population, and Electron Localization Function (ELF) to examine the charge distribution and bonding nature. It also explores the role of (n-1)d orbitals in bonding, particularly for heavier Ae atoms, and the implications on dipole moments. The findings suggest that common indicators for bonding character may not always be reliable and that the anions exhibit strong polar dative bonds with multiple orbital contributions.

The study investigates the mechanical strength changes in coal bodies under different peripheral pressures after liquid nitrogen (LN2) treatment, which is crucial for enhancing coalbed methane extraction. Experiments using an MTS universal testing machine provided stress-strain curves of coal under varying freezing times and pressures after LN2 action. Results showed a positive correlation between the uniaxial compressive strength and peak strain of coal samples in a frozen state, while the modulus of elasticity decreased before 100 minutes of LN2 action. The frozen-thawed coal samples exhibited decreased uniaxial compressive strength and modulus of elasticity before 100 minutes of LN2 action. The study also observed that the elastic strain energy of frozen coal samples was positively correlated with freezing time before 80 minutes and negatively correlated after. The water-ice phase transition and temperature-thermal stresses caused by LN2 significantly damaged the internal structure of coal, especially under in situ ground pressure.