Study On Macro And Micro Properties Of Displaying Gas And Wetting Coal With CO₂ And Alkaline Solutions

Gas disaster is a very important problem,which affecting the safety of mine.With the increasing of mining depth and in-situ stress of coal seam,gas disaster becomes more urgent.CO₂ displacement and coal seam water injection technology are effective measures to control gas of mine,but there are some obvious problems.For example,CO₂ displacement is easy to cause gas enrichment and outburst risk,water injection of coal seam is difficult to enter the pore of coal,resulting in poor water injection effect.In this thesis,the macro and micro wetting characteristics of CO₂-alkaline water biphase displacement of coal were studied.After CO₂ displacement of coal seam,alkaline water was injected into it,which could not only improve the gas extraction effect,but also effectively reduce the risk of coal and gas outburst.The capillary permeability method and two-phase displacement test were used to study the law of alkaline water wetting coal.The microstructure influence of alkaline water on the surface of coal was analyzed by contact Angle method and theoretical calculation.The migration and adsorption law of alkaline water in coal was studied in combination with the pore structure and the evolution law of functional group.The synergistic mechanism of CO₂-alkaline water biphase displacement of coal was revealed.Firstly,a CO₂-alkaline water biphase flooding test system was established in this thesis,which was used to study the wetting law of coal by raw coal waterflooding and CO₂-alkaline water biphase flooding.It was found that DL,QN and TC coal samples of CO₂-alkaline water biphase flooding showed higher pore pressure,water injection and water injection rate than raw coal.During biphase flooding,the water injection of coal sample increased by 16.6%,17.2%and 38.6%compared with raw coal waterflooding,and the final water content of coal sample reached more than 4%.It indicates that alkaline water can reduce the internal permeability resistance of coal,improve the effect of water injection,and eliminate or reduce the risk of coal and gas outburst.Secondly,the influence of different gas,gas pressure,alkaline water concentration and pore permeability on the wettability of coal was studied by capillary permeability method.The results show that the coal samples of DL,QN and TC have the best effect of absorbing 1%Na OH solution under 1 MPa gas pressure,and the wetting height of coal samples of CO₂ displacement increases by 25%,50%and 100%compared with N₂ displacement.The wetting effect of alkaline water on the closed pores of coal is significantly higher than that of the open pores,which verifies the coal affinity of alkaline water from a macroscopic perspective.The influences of different displacement pressure,overlying pressure and water injection pressure on the internal pore pressure and wettability of coal were studied by CO₂-alkaline water displacement experiment.It is found that with the increase of displacement pressure and overlying pressure,the pore pressure of coal increases,the permeability decreases,and the effect of alkaline water injection becomes worse.With the increase of the injection pressure of alkaline water,the effect of water injection of coal seam becomes better.Then,the hydrophilicity of coal samples after CO₂-alkaline water dual-phase displacement was studied.The results showed that the contact Angle of DL,QN and TC coal samples decreased,the surface free energy increased and the hydrophilicity was greatly improved after the dual-phase displacement.The microstructure of DL,QN and TC coal samples after dual-phase displacement was studied by N₂ adsorption experiment and infrared spectroscopy experiment.The results show that the number of open pores increases after biphasic displacement,the pore connectivity becomes better,the average pore diameter and maximum aperture of coal increase.Biphasic displacement reduces the proportion of hydrophobic groups and increases the proportion of oxygen-containing functional groups,among which the increase of hydroxyl content is the most obvious,indicating that the ability of coal to bind water has increased.Combined with the analysis of the damage mechanism of two-phase displacement on coal,it is concluded that the erosion of alkaline solution of coal makes the pore structure more developed,and the dissolution of coal makes the chemical bond more active.It promotes the adsorption between coal and water molecules,which reveals the microscopic migration and adsorption law of alkaline water in the coal.Finally,based on the above research,the synergistic mechanism of CO₂-alkaline water biphase displacement in coal was analyzed and clarified.During biphase flooding,the synergistic effect of CO₂ and alkaline water greatly increases the moisture content of coal,and the reaction of them turns the original water injection resistance into the driving force of negative pressure.The existence of pressure difference enables alkaline water to smaller micropores,which greatly improves the effect of alkaline water hydrating coal.After biphase displacement,alkaline water enters the micropores in the coal under the effect of capillary force,which causes pore expansion and dissolution in coal.And it increases the content of oxygen-containing functional groups,which greatly improves the hydrophilic ability of coal.The synergistic effect of CO₂ and alkaline water integrates the advantages of gas displacement and water injection in coal seam,which not only improves the efficiency of gas extraction,but also improves the water content of coal.It can achieve the purpose of increasing permeability and eliminating risk of outburst in coal seam.This thesis contains 55 figures,17 tables,and 90 references.