| The development of coalbed methane has the three attributes of new energy utilization,coal mine safety and environmental protection.However,the low permeability of coal seams in China has always restricted the efficient extraction of coalbed methane.In order to improve the permeability of coal seams,it is necessary to modify the permeability.In recent years,liquid nitrogen fracturing technology has become a research hotspot due to its green environmental protection and remarkable effects,and has broad application prospects.In this paper,low-rank lignite from the Buzhaoba open-pit mine in Yunnan Province,the middle-rank bituminous coal from the Xutuan mine in Anhui Province and the high-rank anthracite coal from the Yangquan coal mine in Shanxi Province as the research objects.The liquid nitrogen leaching experiments,cycle times and temperature gradients were carried out at different times.Under the liquid nitrogen leaching experiment,a high-definition camera was used to record the changes in macroscopic cracks on the surface of the coal sample before and after the treatment,and 99.999% high-purity nitrogen was used to test the permeability of the coal sample.The effect of liquid nitrogen leaching on the cracking and permeability enhancement of coal samples of different coal ranks was compared and analyzed,and the liquid nitrogen leaching treatment method suitable for different rank coals was explored.And use the finite element simulation software Ansys to establish a numerical model to study the heat transfer process and thermal stress distribution of coal under liquid nitrogen leaching.The main conclusions drawn in this article are as follows:(1)Under the same confining pressure,the permeability of the coal sample shows a " U "-shaped change trend with the increase of gas pressure,and the fitting effect of the quadratic function formula is good.Through the analysis of the trend of the fitted curve,it is found that liquid nitrogen leaching treatment will increase the sensitivity of coal permeability to pore pressure.In addition,combined with the results of liquid nitrogen leaching experiments and numerical simulations at different times,the process of liquid nitrogen leaching of a 50×50 mm cylindrical coal sample is divided into three stages according to the fracturing efficiency: high-efficiency fracturing stage(0-158 s),Normal fracturing stage(158-450 s),inefficient fracturing stage(after 450 s).(2)Observing the macroscopic cracks on the surface of the coal sample under the liquid nitrogen cycle leaching treatment,it is found that with the increase of the number of treatments: 1)New cracks are gradually formed in the coal sample;2)The original cracks and new cracks increase with the number of treatments Continue to expand and extend;3)The fissures on the coal sample are gradually connected and connected to each other to form a fissure through network;4)The overall structure of the coal sample is fractured and destroyed.The seepage tests of coal samples at different treatment stages show that under the same confining pressure and air pressure,the permeability of the coal samples has been significantly improved by each cycle of thermal shock treatment.The experimental results of liquid nitrogen leaching of coal samples with different initial temperatures show that the higher the temperature gradient,the better the effect of liquid nitrogen leaching treatment on the cracking and permeability enhancement of coal samples.(3)The relationship of the cracking effect of liquid nitrogen leaching treatment on coal samples of different ranks is: lignite > bituminous coal > anthracite.For low-rank lignite,a single cold treatment with liquid nitrogen can achieve a significant antireflection effect;for medium-rank bituminous coal,increasing the number of cycles of liquid nitrogen treatment will greatly improve the cracking and anti-reflection effect of the coal sample;for high-rank anthracite,increase liquid nitrogen combining the processing temperature gradient or increasing the temperature gradient and increasing the number of cycle processing can make the coal sample be effectively cracked and enhanced.(4)Summarizing and discussing the relevant field tests and practical applications of liquid nitrogen fracturing technology,it is believed that liquid nitrogen fracturing technology has: 1)good production increase effect;2)high safety performance;3)good fracturing fluid flowback effect;4)It has advantages such as low stratum pollution,and has good development and application prospects.In addition,two new technologies based on liquid nitrogen fracturing have been proposed.This thesis contains 39 figures,18 tables,and 116 references... |
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