Study On The Formation Kinetics Of Carbon Dioxide Hydrate In Porous Media Above Freezing Point

With the development of industry and the massive consumption of fossil fuels,the greenhouse effect has intensified and the resulting global warming problem has become increasingly serious.As one of the main greenhouse gases,carbon dioxide gas is the main factor caused global warming.The emission reduction of carbon dioxide has become the major issue of global concern and the key issue restricting the sustainable development of economy.The storage and solidification technology based on hydrate method is one of the effective methods to reduce the greenhouse-gas emission.At present,scholars at home and abroad have been conducted a lot of research on the formation and decomposition process of gas hydrate in porous media system,but the existing research mainly focuses on non-liquefaction conditions,the influencing mechanism of liquefaction conditions on the formation of carbon dioxide hydrate in porous media is still unclear,and the kinetic data of carbon dioxide hydrate formation in porous media is still lacking.In this paper,quartz sand is used to simulate the porous media conditions of the formation sediments,the formation process of carbon dioxide hydrate was studied in the porous media system under the condition of liquefaction and non-liquefaction,respectively.And the effect of temperature,pressure and particle size of porous media on the induction time,average formation rate and gas storage capacity of carbon dioxide hydrate formation were analyzed.The main research contents and results are as follows:（1）Under the two conditions of liquefaction and non-liquefaction,four different particle sizes of silica sand（particle sizes of 700μm,500μm,380μm and 250μm）were employed and the influence of particle size on the induction time of carbon dioxide hydrate formation was studied in the porous media system.The results showed that,under non-liquefaction condition,the increase of particle size had no obvious influence on the induction time within a certain particle size range during the hydrate formation process.But when the particle size of quartz sand increased to around 700μm,the induction time would be increased significantly,reaching to 49.6 h.Under the liquefied condition,the induction time of carbon dioxide hydrate formation was gradually shortened with the decrease of the particle size of porous media,and when the particle size was reduced to around 380μm,the induction time of hydrate formation would be decreased sharply,reached to 5.6 h.（2）Under two conditions of liquefaction and non-liquefaction,the effects of particle sizes on the average formation rate of carbon dioxide hydrate and the gas storage capacity were studied in the porous media system respectively.The results found that,under the non-liquefaction condition,the reduction of particle size had little effect on the formation rate of carbon dioxide hydrate and gas storage capacity in porous media within a certain particle size range.But when the particle size reduced to around 250μm,the promoting effect of the porous media on hydrate formation would be enhanced significantly.Under the condition of liquefaction,there was a critical size for the particle size of porous media influencing the formation rate and gas storage capacity of hydrate.When the particle size of quartz sand was around 380μm,the liquefied conditions had the best promoting effect on the formation of carbon dioxide hydrate.The formation process of carbon dioxide hydrate under different phase conditions showed that,the formation process of carbon dioxide hydrate was relatively fast in the porous media system under the liquefying conditions and had better gas storage performance.The maximum formation rate and gas storage capacity reached 12.35×10^-4mol/h and 30.599 L/L,respectively.（3）Under the condition of the same particle size,the effects of different temperatures and pressures on the average formation rate of carbon dioxide hydrate were studied in the quartz sand porous media.The results showed that,under the conditions of non-liquefaction or fresh liquefaction at the same experimental temperature,the average formation rate of carbon dioxide hydrate showed an overall increasing trend.As the amount of carbon dioxide liquefaction continues to increase,the formation rate of CO₂ hydrate in porous media would be increased greatly.However,when the CO₂ was completely liquefied in the reaction system,the formation rate of CO₂ hydrate would be dropt sharply,even lowering than that under non-liquefaction conditions.Under the conditions of non-liquefaction or fresh liquefaction at the same initial pressure,the average formation rate of carbon dioxide hydrate showed a decreasing trend overall with the increase of the experimental temperature.As the amount of carbon dioxide liquefaction continues to increase in the reaction system,the experimental temperature and the average rate of gas hydrate formation no longer had a universal correspondence.And there was an optimal liquefied amount affecting the formation rate of hydrate formation under the experimental conditions.（4）Under the condition of the same particle size,the influence of different factors on the formation kinetics of carbon dioxide hydrate was further studied in the porous media system.The results indicated that the driving force formula derived from the temperature and pressure changed can better characterize and reveal the variation trend of the average formation rate of CO₂ hydrate under the conditions of non-liquefaction and fresh liquefaction.With the further increase of liquefaction amount,the leading role of the driving force on hydration reaction had gradually weakened during the CO₂ hydrate formation process in porous media.Through the experimental research and theoretical analysis,it was found that the average formation rate of CO₂ hydrate is the comprehensive effects of the driving force,armor effect,adsorption,dissolution characteristics,and CO₂ liquefaction in the porous media system.Furthermore,under the condition of different temperature and pressure,the influenced degree of each factor on CO₂ hydrate formation of was also different with the different dominant factors in the porous media system.The innovation of the paper concludes that:（1）The influence of different particle sizes of quartz sand quartz sand on the induction time,average generation rate and gas storage capacity of carbon dioxide hydrate were revealed under liquefied and non-liquefied conditions through experiments.（2）The relationship between the average formation rate of carbon dioxide hydrate and the driving force of the hydration reaction under the influence of temperature and pressure is established in porous media,and the influence mechanism and law of different factors on the formation rate of carbon dioxide hydrate are revealed in the porous media system.