The Stability Of CO₂ Hydrate In The Atmosphere Of Methane And In Unsaturated Water

The exploitation of natural gas hydrate?NGH?by the replacement with CO₂ is definitely a very promising method for NGH production among the methods proposed currently,as it offers a win-win situation by replacing the methane gas with CO₂ in hydrate reservoirs.To improve the efficiency of gas production and stablize the goaf of gas hydrate reservoir,direct depressurization and CO₂ replacement were combined in the preliminary works.During the CO₂ migration to the CH₄ hydrate dissociation zone,the CO₂ hydrate formation frontier will be exposed to an highly concentrated CH₄ atmosphere and unsaturated formation water which will cause the formed CO₂ hydrate dissociation inversely.In order to explore the stability of CO₂ hydrate cap based above ideas,experimental studies about the law of degeneration of the cap under the atmosphere of methane and in unsaturated water,were conducted in a novel one-dimensional visual hydrate simulator designed by our laboratory and Raman spectroscopy,respectively.This work is mainly composed of three aspects:?1?A series of experiments were conducted in the hydrate simulator to study the CO₂ hydrate evolution in an atmosphere with different CH₄ concentrations.The composition changes of gas phase during dissociation process of CO₂ hydrate were investigated by gas chromatograph.Morphological changes of CO₂ hydrate during the dissociation process were observed by visible windows of the apparatus.And the results show that the reformation of CO₂ hydrate occurs in the initial stage of dissociation process,then the hydrate will gradually dissociate until the three-phase equilibrium of gas-liquid-solid is achieved.However,the initial rate of this dissociation process is slower than one in the direct depressurization process under the partial pressure of CO₂ in the atmosphere.?2?The interaction of CO₂ hydrate with CH₄ gas and on line monitoring for the states of cavities occupied by CH₄ were investigated by in situ Raman spectroscopy.Spectrums obtained show that CH₄ molecular can occupy the large cavity while the partial pressure of CH₄ below hydrate equilibrium formation pressure,and the ratio of large cavities and small ones increase with time.While at low CH₄ concentration,CH₄ molecular can occupy the big cavity with shorter time compared with high one.?3?The CO₂ solubility in 3.5% seawater at CO₂ hydrate–seawater–gas equilibrium was investigated by Raman spectroscopy,as well as in pure water.And the relationship between peak area ratio PAR?ACO₂/AH₂O?and composition was presented.