Failure Mechanism of Coal after Cryogenic Freezing with Cyclic Liquid Nitrogen and Its Influences on Coalbed Methane Exploitation

This study explores how liquid nitrogen (LN2) freezing affects the physical pore and fracture structure of coal. Under lab-controlled conditions, coal specimens were frozen with LN2 under different conditions and thawed, and then the uniaxial compressive strengths, acoustic emissions, and ultrasonic wave velocities of the different specimens were compared. After 60 min of freezing for one set of specimens and 30 freeze thaw cycles for another set, the elastic moduli of the coal specimens decreased by 47.8% for the 60 min freezes and by 76.2% for the 30 cycles. For the tested two sets of the same specimens, the uniaxial compressive strengths and longitudinal wave velocities dropped by 13.4% and 40.2% and by 47.8% and 76.2%, respectively. At the same time, the coal porosities and Poisson's ratios increased by 17.5% and 68.1% and by 7.14% and 28.6%, respectively. Owing to the reduction of the coal's mechanical strength, the elastically straining stage was shortened and the peak yield point and the plastic deformation were accelerated. By establishing a relational model for an elastic modulus based damage variable D and the LN2 freezing conditions, it was found that variable D increased to and stabilized at 0.12 with the single freezing experiments. However, the damage to the coal caused by cyclic freezing and thawing was continuous and damage accelerated after 20 freeze thaw cycles. By modeling the state of stress in fractures of LN2 treated coal, the theoretical governing equations for the tension in a single fracture were derived. In addition, the expression regarding the volumetric strain of ice under the effect of tension for a single fracture was obtained. The results showed that the proposed model and expressions were in good agreement with the experimentally obtained data.