Replacement Characteristics And Strengthening Mechanism Of CO₂-CH₄ Hydrate In Porous Media Below Freezing Point

As a new type of clean and non-polluting energy,natural gas hydrate（NGH）has great energy potential and resource prospect,which has attracted wide attention from scholars at home and abroad.According to the existing studies,NGH had been found widely in the deep-sea sediment layer at the edge of the offshore continental shelf and the tundra of the terrestrial plateau,which the reserves are extremely abundan,i.e.it’s more than twice than that of conventional energy sources.The safe extraction of NGH is the prerequisite and basis for its efficient utilization,which is of great significance for effectively solving global problems such as conventional energy depletion and climate warming.The carbon dioxide（CO₂）replacement method can not only achieve methane（CH₄）extraction in gas hydrate reservoir,but also store CO ₂ underground in the form of hydrate,which is a very promising method for gas hydrate extraction.However,the current research on gas hydrate replacement mining is mainly focused on the submarine sedimentary environment above the freezing point,while the research on gas hydrate replacement mining in permafrost region below the freezing point is still in the preliminary stage.Hence,the kinetic data of CO₂-CH₄ hydrate replacement process are still lacking,which is of great significance to reveal the intensification mechanism of the replacement process and its kinetic law.It is a great strategic importance that realize the safe and efficient extraction of gas hydrate in the permafrost region to promote the sustainable development of energy in China.In this paper,the mixture of quartz sand and ice particles is used to simulate the occurrence environment of porous medium of NGH in permafrost area.Below the freezing point,the quantitative effects of quartz sand particle size,ice content and displacement pressure on the displacement characteristics of CO ₂-CH₄ hydrate were studied respectively;the strengthening process of CO₂-CH₄ hydrate replacement caused by the liquid CO₂,small molecule gas of N₂ and intermittent heating were carried out,which further analyzed the influence mechanism and law of different factors on the CH₄ hydrate replacement process.Therefore,the strengthening mechanism of CO₂-CH₄ hydrate replacement process in porous media below freezing point was revealed.The main research contents and results are as follows:（1）The formation process of CH₄ hydrate in sub-freezing porous media was investigated using the mixture of ice particles and quartz sand.During the procedure,no significant induction time was observed and the ice particles acted as a template for CH₄ hydrate generation,as well as the growth reaction of hydrate began immediately when the system reaches the phase equilibrium conditions of formation.（2）By studying the effect of quartz sand grain size on the CO ₂-CH₄ hydrate replacement process,it was found that larger mass transfer channels would exist among the larger quartz sands,which would facilitate the diffusion and migration of CO₂ and CH₄ gas through the pore channels of porous media.When the quartz sand particle size is 830μm,the larger mass transfer channels facilitate the gas exchange,and the methane recovery and CO₂ sequestration rates are at a high level.（3）At the temperature of 271.65 K,as the replacement pressure starts from just exceeding the CO₂ hydrate phase equilibrium curve to the replacement pressure approaching the CO₂ liquefaction pressure,the CH₄ recovery rate shows a pattern of increasing and then decreasing,while the CO ₂ sequestration rate increases continuously.The comparison between liquid CO₂ and gaseous CO₂ reveals that liquid CO₂ has a very weak promotion effect on methane recovery due to the rapid formation of the CO₂ hydrate layer by liquid CO₂ and unreacted ice particles,and the CO₂ sequestration rate under liquid CO₂ condition is only 10%.Therefore,NGH in porous media systems below freezing point is not suitable for replacement extr action by liquid CO₂.（4）Through the replacement process of CO₂/N₂-CH₄ hydrate,it was found that the addition of N₂ during the replacement of hydrate below the freezing point is an effective way to improve the methane recovery rate.With the increase of the N₂ ratio,the methane recovery rate gradually increases while the CO₂ sequestration rate increases and then decreases.Based on ensuring the methane recovery rate,the influence of N₂ on CO₂ sequestration should also be considered.Notably,choosing the appropriate ratio of CO₂ and N₂ for different reservoir conditions is the key to replacement mining.（5）With the combination of intermittent heating with CO ₂ replacement for mining NGH,it is revealed that intermittent heating can improve methane recove ry and CO₂sequestration to a certain extent,and increasing the heating frequency can lead to higher methane recovery,but continuously heating has less improvement on methane recovery compared to twice every 24 h.Hence,intermittent heating is more in line with the need for energy conservation in commercial mining.The closer the replacement temperature is to the freezing point,the thicker the quasi-liquid layer between hydrates and the more favorable the storage of CO ₂ as hydrate.In addition,the combination of CO₂+N₂ and intermittent heating is also found to enhance the replacement process of CO₂-CH₄ hydrate in porous media below freezing point,in which the CH₄ recovery rate and CO₂ sequestration rate are keeping in a high level.