Characteristics of Brazilian Splitting Failure and Acoustic Emission Evolution of High-temperature Coal After Liquid Nitrogen Cooling Treatment
The use of fracturing technology to enhance the permeability of coalbed methane reservoirs is an essential approach to strengthen coal-bed methane development.For deep and high-temperature coalbed methane reservoirs,liquid nitrogen fracturing technology injects low-temperat-ure liquid nitrogen into the coal seam,generating significant thermal stress.The"cold impact"effect effectively induces fracturing and increases permeability in coalbed methane reservoirs.This study examines the influence mechanism of liquid nitrogen treatment on the mechanical proper-ties and damage characteristics of high-temperature coal-rock.Ultrasonic testing analyzes the changes in ultrasonic characteristics of high-temper-ature coal samples after liquid nitrogen cooling.Based on the Brazilian split test and acoustic emission monitoring,the influence of liquid nitro-gen cooling on the physical and mechanical properties of high-temperature coal samples and the characteristics of acoustic emission are analyzed.In addition,the fractured characteristics of the damaged coal samples are quantitatively characterized based on the three-dimensional morphology scanning of the fracture surface and the apparent crack distribution to explore the influence mechanism of liquid nitrogen cooling on the failure mode of high-temperature coal samples.The experimental results indicated that the mechanical properties of high-temperature coal samples de-teriorate significantly after liquid nitrogen cooling.The P-wave velocity decreases,and there is severe differentiation in ultrasonic frequency,with the primary frequency decreasing from 51.10 to 40.88 kHz.The coal samples gradually transform from brittle to ductile failure,with the highest decrease in the Brazilian split modulus and brittleness index reaching 67％and 75％,respectively.Under the Brazilian split load,the internal acoustic emission signals of the coal samples exhibit low RA values and high AF values,indicating that the internal failure mode of the coal samples is mainly tensile failure,followed by shear failure.With the increase in the initial temperature of the coal samples,multiple undulating re-gions form on the fracture surface of the coal samples,and the height difference between the maximum peak and the maximum valley in the three-dimensional morphology scanning reaches up to 37.60 mm.The liquid nitrogen cooling induces the cross-extension of microcracks,forming a complex crack network,which further leads to the formation of localized failure zones and the complexity of the fracture surface of the coal samples.The fractal dimension of the apparent crack distribution reaches its maximum value when the initial heating temperature is 100 ℃,in-creasing from 0.50 at the initial temperature of 20 ℃ to 1.83.The research results provide a theoretical reference for the design of liquid nitrogen fracturing in deep and high-temperature coalbeds.
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