Editorial Department of Coal Science and Technology
Doi
Abstract
The permeability of coal reservoir is generally low in China, how to effectively improve the permeability of coal reservoir is a key and difficult point of coalbed methane exploitation, Liquid nitrogen fracturing technology as a kind waterless fracturing technology has received extensive attention in recent years. In order to reveal the influence of liquid nitrogen freezing and freeze-thaw on the mechanical properties of coal, the temperature distribution of coal samples was monitored by infrared thermal imaging technology, and uniaxial compression and acoustic emission tests were performed on the coal samples after the liquid nitrogen freezing and freezethaw, the P-wave velocity, porosity, acoustic emission and energy evolution characteristics of coal samples before and after the freezing and freeze thaw were analyzed. The research result showed that: ①After 360 min freezing and 12 freeze-thaw cycles, the P-wave velocity of coal samples decreased by 58.2% and 64.7%, respectively. The P-wave velocity does not decrease significantly during the initial freezing and freeze-thaw cycle stages, the velocity gradually decreases with the increase of freezing time and freeze-thaw cycles. ②The temperature of the coal sample gradually decreases with increase of freezing times. The surface temperature of the coal sample drops below -60°C after the liquid nitrogen frozen for 180s, the temperature distribution fluctuations at the center of the coal sample occurs due to the different thermal conduction coefficient of the coal particles. ③After liquid nitrogen freezing and freezethaw, the elastic modulus of coal sample decreases exponentially, while the porosity gradually increases. The increase in porosity of the coal sample after liquid nitrogen freeze-thaw is greater than that after liquid nitrogen freezing. ④The acoustic emission activity of coal samples during uniaxial loading is divided into development phase, active phase and severe phase, the maximum acoustic emission ringing count and cumulative acoustic emission ringing count of coal samples increase with the increase of freezing time and freeze-thaw cycles. ⑤Liquid nitrogen freezing and freeze-thaw will weaken the energy storage limit of coal sample, resulting in the reduction of the total energy, elastic energy and dissipated energy at the peak point during the uniaxial loading process
