Study On The Formation Characteristics And Controlling Factors Of CO₂ Hydrate In Porous Media Below Freezing Point

With the massive consumption of fossil fuels,the problem of the greenhouse effect is intensifying,and energy and environmental issues have become the focus of attention around the world.CO₂ is the main factor leading to global warming and greenhouse effect,and it is imminent to reduce and control the emission of greenhouse gas CO₂ in the atmosphere.The capture and geological storage of CO ₂ gas is considered to be an effective way to achieve CO ₂ emission reduction,which has attracted extensive attention of scholars at home and abroad.The CO₂ storage technology using natural gas hydrate method can effectively achieve CO ₂ emission reduction,which is one of the research hotspots in the field of natural gas hydrate application technology.The CO₂ gas is stored in the formation sediments in the form of hydrate,which can effectively realize the emission reduction and storage of CO ₂gas.At present,a large number of studies on the formation process of CO ₂ hydrate in porous media systems by domestic and foreign scholars mainly focus on the conditions above the freezing point.However,the porous media conditions below the freezing point have an important impact on the formation characteristics and formation methods of CO₂ hydrates.The formation characteristics and kinetic mechanisms of CO₂ hydrates under these conditions still need further study.In this paper,the mixed system of quartz sand and ice powder is used to simulate the porous medium conditions of the formation sediments in the permafrost strata sediments.And the effects of temperature,initial pressure,quartz sand particle size,ice powder particle size and ice content on the formati on characteristics of CO₂hydrate are studied respectively.The quantitative effects and laws of different factors on the formation rate of CO₂ hydrate,the conversion rate of ice to hydrate,and the gas storage per unit volume of ice are revealed.The main experimental contents and the obtained results are as follows:（1）The process of ice powder and CO₂ gas forming hydrate conforms to the shrinking core model in the porous media system below freezing point,which is divided into two stages:hydrates form rapidly on the surface of the ice powder and gas diffusion through the hydrate shell on the surface of the ice particles continues to react with the unreacted ice in the shell.（2）The effects of three ice powder particle sizes of 250μm,500μm and 700μm on the formation characteristics of CO₂ hydrate in porous media systems were investigated experimentally.The results show that the conversion rate of ice to hydrate in the 700μm ice particle size experimental system and the gas storage per unit volume of ice are the largest,reaching 49.69%and 92.19 L/L,respectively.But in the initial stage of the experiment（0-17 h）,the formation rate of CO₂hydrate and the conversion rate of ice to hydrate in 250μm ice powder are the highest.（3）The study found that the particle size has an important influence on the formation of CO₂ hydrate in porous media below the freezing point.In the experimental system with an ice content of 11.11%,the quartz sand with a particle size of 500μm is the most favorable for the formation of hydrates,and the quartz sand with a particle size of 700μm has the worst formation effect of hydrate.In the experimental system with ice content of 20.00%and 33.33%,hydrate conversion rate and gas storage capacity increase with the increase of quartz sand particle size.（4）In the quartz sand system with different particle sizes,the influence of ice content on hydrate formation characteristics shows a certain regularity.In the experimental system of 250μm and 500μm parti cle size quartz sand,the smaller the ice content（the range of 11.11%-33.33%）,the higher the conversion rate of ice to hydrate and the gas storage capacity per unit volume of ice;at 700μm In the experimental system of particle size quartz sand,the hyd rate formation effect is the worst when the ice content is 33.33%,and the hydrate formation is most favorable when the ice content is 20.00%.In addition,the results show that the porous medium is more favorable for the formation of CO₂ hydrate than the pure ice-powder system under the conditions below the freezing point.（5）At the same temperature,the formation rate and conversion rate of CO ₂hydrate and the gas storage capacity per unit volume of ice increase with the increase of the initial pressure.Under the same initial pressure,when the temperature is in the range of 268.15-272.15 K,the temperature around 270.15 K is most favorable for the formation of hydrates,and the conversion rate of ice to hydrate and the gas storage capacity per unit volume of ice are up to 38.07%and 38.07%,respectively.70.64 L/L.（6）The effects of surfactants on the formation of CO₂ hydrate were investigated experimentally,and it was found that there is an optimum concentration of surfactant to promote the formation of CO₂ hydrate below the freezing point.When the SDS concentration was in the range of 0-1.24 g/L,the highest conversion rate of ice to hydrate and the largest gas storage capacity were obtained when the SDS concentration was 0.24 g/L,reaching 43.05%and 80.06 L/L,respectively.At the same time,the research results also show that under different temperature conditions,suitable concentrations of SDS can increase the final gas storage capacity,and can also shorten the time required to reach 90%of the CO₂ gas absorption(t₉₀).The innovation of the paper concludes that:The formation process of CO₂ hydrate in different systems was studied through experiments,and the influence of different factors on the formation process of CO ₂hydrate in porous media below freezing point was elucidated.The effects of factors such as porous media and ice powder particle size,ice content,temperature and pressure conditions,and additive content on the formation of CO ₂ hydrate were revealed.