Study Of Displacing Gas And Wetting Coal With CO₂ And Alkaline Solutions

CO₂ displacement and coalbed water injection are efficient engineering technologies for mine gas control to eliminate outburst danger.However,there are some obvious problems at present,such as the strong adsorption of CO₂ cannot fundamentally eliminate the outstanding danger.Water can not enter the micropores and cracks of coal seam,and it can not obtain a good result with water injection.Therefore,the technology of displacing gas and wetting coal with CO₂ and alkaline solutions is proposed.After the gas is replaced by high-pressure CO₂,the injection of alkaline solution will chemically harden CO₂.On the one hand,it will absorb CO₂,and on the other hand,alkaline water will replace the CO₂ adsorption space,thereby entering the microporous fissures to improve the water injection effect and achieve the purpose of increasing permeability and eliminating outburst.Therefore,through experimental testing and theoretical analysis,the pore pressure and gas migration law of CO₂ displacement was studied in this paper,the change characteristics of coal body outburst risk index after CO₂ displacement and water invasion characteristics of raw coal containing CH₄ was analyzed.Thus,the technology of displacing gas and wetting coal with CO₂ and alkaline solutions is proposed in this paper.The variation law of pore pressure and water content in this technology was further studied.The influence law of CO₂-alkaline water dual effect on coal microstructure was analyzed,and the process and mechanism of displacing gas and wetting coal with CO₂ and alkaline solutions were revealed.Self-organized the simulation experiment system of displacing gas and wetting coal with CO₂ and alkaline solutions.Experiments of gas phase displacement,coal seam high-pressure water injection,radial steady flow and simulation experiment of displacing gas and wetting coal with CO₂ and alkaline solutions can be conducted.It can meet different overburden pressures,CO₂ and CH₄ gas switching,different displacement gas pressuresand and penetration test simulation experiments,as well as real-time monitoring of coal pore pressure changes.Carried out the simulation experiment of displacing CH₄ by CO₂ and study on characteristics of gas-bearing coal dynamic water invasion.First,the changes of pore pressure and gas outlet concentration in the process of CO₂ displacing were tested.Then,the changes of pore structure,permeability and gas partial pressure of coal samples before and after the flooding were tested.Finally,the water invasion pressure of gas-bearing coal was tested.And the law of moisture content change.Results show that,the pore pressure of the coal near the gas injection port presents a change characteristic of first decreasing and then increasing.The gas outlet concentration presents a fluctuating and rising trend.After displacement,the mean pore-size of the coal sample decreases,the average pore size increases,and the permeability and the partial pressure of CH₄decreases to different extent.The difference in the amount of adsorption of CO₂and CH₄ by coal is compared,and the ratio is between 1.66:1 and 4.06:1.The dynamic water invasion experiment of gas coal showed that the pore pressure of the coal body increased slowly,the water content increased insignificantly,and the water injection effect was poor.Based on the experimental results,the technology of displacing gas and wetting coal with CO₂ and alkaline solutions is proposed in this paper.The pore pressure,water injection volume and water content of CO₂-alkaline water and CO₂-distilled water were compared under different coal samples,in-situ stresses and water injection radii.Results show that,the pore pressure and water injection rate increase significantly,and the effect of water content distribution was better.Displacement effect and coal water content increase significantly with the decrease of overburden pressure and increase of water injection aperture.The experiment simulates the effect of CO₂-alkaline water on the microstructure of coal with different degrees of metamorphism.The evolution laws of pore structure,minerals and oxygen-containing functional groups in coal were qualitatively and quantitatively analyzed by using low-temperature N₂ adsorption,SEM,XRD and infrared spectrum test experiments.Results show that,the swelling effect of the alkaline solution on the coal and the shedding of minerals in the original pore structure promoted the increase of the pore and fissure structure in the coal,and the original pore structure in the coal was"expanded",so that The pore volume and average pore diameter of coal have increased significantly.Minerals were significantly reduced,especially in clay minerals and calcite with poor water stability.Finally,the change law of water content of XT coal sample was analyzed by the change law of oxygen-containing functional groups in infrared spectrum.The effects of displacing gas and wetting coal with CO₂ and alkaline solutions on the microstructure of coal are revealed.The swelling effect of CO₂-alkaline water on the coal body,the collapse of pore structure clay minerals and the dissolution of minerals promote the increase of pore and fissure structure in the coal body,"expand the pore"effect on the original pore structure in the coal body.Changed the composition and proportion of minerals and oxygen-containing functional groups in coal.It reveals the mechanism of increasing coal water content after CO₂-alkaline water two-phase flooding.On the one hand,the hydrophilic functional groups of coal with low metamorphism have a strong ability to bind water;on the other hand,a large amount of CO₂ is adsorbed in the micropores after gas phase flooding.After the alkaline solution is injected,the CO₂ in the macropores is absorbed through water dissolution and chemical reactions,and the OH^-ions in the weakly alkaline water reacts with the CO₂in the micropores to produce a reaction penetration force,so that the water slowly penetrates into the micropores.This technology combines the advantages of CO2flooding gas and conventional coal seam water injection,promotes coal seam gas drainage,improves coal seam wetting effect,and reduces the risk of coal seam outburst.This paper contains 32 figures,5 tables,and 92 references.