| China is rich in gas（coalbed methane）resources,but the low permeability of coal seam seriously restricts the efficiency of gas development.It is of great significance to actively explore new methods of gas production increase,under the new situation that the country needs natural gas to“increase reserves and production”and the whole coalbed methane industry is in the“bottleneck”stage.The research shows that the plasma based on physical discharge is characteristized by high energy density and strong destructiveness,which has significant effect in the field of coal seam fracturing,permeability enhancement and plug removal,and has been highly concerned by the industry.In this dissertation,theoretical analysis,laboratory experiments and numerical simulation methods are used to comprehensively,an integrated experimental system of plasma induced fracturing and permeability enhancement of loaded coal and rock was created,so as to realize the integrated operation of plasma induced fracturing and quantitative characterization of permeability of coal under in-situ conditions.The complex pore fracture structure and permeability evolution law of plasma breakdown coal are studied quantitatively and visually.The electrical response,physical property evolution and gas storage and transportation in porous media involved in the process of plasma enhanced gas drainage are the main research lines.The development model of plasma flow in coal body,fracture initiation and pore fracture structure evolution model,and gas cross-scale migration mechanism are also analyzed.The research results are as follows:The influence of ionic solution and ground stress on the critical breakdown voltage of coal was clarified,a theoretical model of improving coal conductivity by ionic solution was proposed,the quantitative expression between the concentration of ionic solution and the critical breakdown voltage of coal was established.Results show that once the coal is saturated with ionic solution,the original“conduction dead zone”changes to“conduction continuous zone”,and the critical breakdown voltage decreases with the increase of ionic solution concentration.There is a negative exponential function between the breakdown field strength and the concentration.Axial pressure can reduce the difficulty of coal breakdown,while confining pressure can limit the expansion of plasma channel in coal body.Under the action of plasma,the loaded coal is mainly composed of new cracks,supplemented by pore structure changes.The typical voltage and current waveforms in the breakdown process of loaded coal were discussed,the influence of voltage,breakdown times and other key factors on electrical parameters was studied,the electrical response mechanism of plasma breakdown of loaded coal was revealed.The results indicate that increasing the breakdown voltage is helpful to accelerate the coal polarization speed and reduce the pre-breakdown period,the peak current changes linearly with breakdown voltage.The first time breakdown of coal sample is the most difficult,and the pre-breakdown period will drop or even disappear with the increase of breakdown times in a short time.The peak current increases first and then remains stable with the increase of breakdown times.The influence of plasma on the electrical properties of coal is beneficial to the next breakdown.The critical breakdown voltage and times show a trend of linear decrease at first and then tend to be stable.The dynamic process of plasma breakdown of coal was studied,the mechanism of selective fragmentation of coal by plasma was clarified,the streamer development model of plasma in coal was established.The results demonstrate that the plasma channel gradually develops from positive to negative.The higher the voltage,the higher the energy density carried by the plasma channel,and the stronger the impact damage effect on coal.The electrical field intensity decreases in the medium with high permittivity,and increases in the medium with low permittivity,which leads to the selectivity of plasma to crack propagation.The development of plasma in coal is in the form of streamer,and the secondary electron avalanche is the key to the development of streamer.The variations of connectivity pore fracture of breakdown coal body was studied quantitatively and visually,the equivalent topological network model of breakdown coal was constructed,the mechanism of multi-scale pore fracture structure damage and crack formation in coal burst was revealed,the damage evolution model of the pore fracture structure of the coal was found.The results show that the high temperature carried by plasma will reduce the content of oxygen-containing functional groups（hydroxyl and carbonyl）and decompose some aliphatic hydrocarbons.The cumulative porosity and fractal dimension increase with the increase of breakdown voltage.The depth and width of breakdown coal surface crack can reach tens to hundreds of microns.The space fracture network was formed in the coal body,which had a great inducing effect on the fluid migration.There is a negative exponential function between the compressive strength and the breakdown voltage.There are four modes of coal body expansion,hole breaking,perforation and crack expansion by plasma.The mechanism of gas trans scale migration（adsorption,diffusion and seepage）in breakdown coal was revealed,the control mechanism of the breakdown voltage and the number of times on the conductivity of the hole crack was clarified.The results show that the ability of gas adsorption of breakdown coal decreases affected by the high temperature of plasma.The higher the breakdown voltage is,the faster the diffusion rate increases.The original coal permeability is only 10-2 m D,but the breakdown coal permeability can reach several m D levels,which can increase the permeability of coal dozens to hundreds of times.The permeability of coal increases with the increase of breakdown voltage,increasing the number of breakdown can break the coal into smaller pieces,but also cause the gas migration channel to be blocked.The research results obtained in this dissertation have improved the research theory of plasma induced cracking and permeability increasing mechanism of coal,further promotes the application of plasma technology in the field of improving coal seam permeability.Based on the above research results,during the doctoral period,I worked as the person in charge of the relevant content to complete three items,including Fundamental Research Funds for the Central Universities,published 12related academic papers（7 SCI papers in JCR zone 1 and 8 top journals）as the first author and corresponding author,with a cumulative impact factor of 47.272,and authorized 16 invention patents.131 figures,6 tables and 340 references were included of this dissertation. |
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