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

Abstract

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.

Experimental study of coal fracture dynamics under the influence of cyclic freezing–thawing using shear elastic waves

“…Passive measurements involve the analysis of acoustic emissions in samples under the influence of mechanical or thermal test impacts. The most common method involves the cyclic F-T of coal samples and their subsequent uniaxial loading up to failure together with the registration of acoustic emission parameters (Qin et al 2016(Qin et al , 2018. Other physical effects are sometimes used instead of uniaxial loading, such as rapid heating of a sample with simultaneous acoustic emission registration (Novikov et al 2018).…”

Microcrack evolution and permeability enhancement due to thermal shocks in coal

“…Nevertheless, CBM is very difficult to be drained without external disturbances because methane molecules can tightly adsorb on the micro-pore surface. Over the past decade, many studies have focused on waterless thermal-shock fracturing techniques [3][4][5], because hydraulic fracturing technology consumes a large volumes of water and may affect public health and the environment [6].…”

“…For example, the coal permeability increased by 48.89%-93.55% after LN 2 cooling treatment, and the compressive strength of cool-treated coal specimens decreased by 16.18%-33.74% compared with intact specimens [4]. Under laboratory conditions, Qin et al [5,10] studied the effect of LN 2 freezing on pores and fracture structures in coal. After LN 2 freezing-thaw cycles, they found that the coal specimens' elastic moduli, uniaxial compressive strengths (UCSs) and longitudinal wave velocities dropped while the porosities and Poisson's ratios increased.…”

“…Scanning electron microscope (SEM) and computer tomography (CT) scanning technologies are effective means to investigate the growth and expansion of cracks in geological materials under thermal treatments [42,43]. Cai et al and Qin et al studied the crack propagation in coal using SEM tests and found that LN 2 freezing stretched and increased the number of cracks, and accordingly improved the initial damage degree of the coal [5,[8][9][10]. The change in coal pore structure under increasing temperatures was investigated using SEM technology [44][45][46][47].…”

Effect of low temperature damage on tensile strength of coal under the liquid nitrogen freezing

“…Guo [15] investigated the laws of the crack growth and permeability evolution of coal under the LN 2 freezing. Qin et al [16] investigated the effects of LN 2 freezing on mechanical and physical properties of coal. The results showed that LN 2 freezing has an obvious weakening effect on the wave velocity and compressive strength of coal.…”