Numerical Simulation Of Methane Recovery From Gas Hydrate By CO₂-N₂ Mixture Replacement

Natural gas hydrate is abundant in China and will become an important target of energy development and utilization in the future.In May 2017,China carried out flammable ice test production in the Shenhu sea area of the northern South China Sea for a week,and became the first country in the world to successfully test gas hydrate production at sea.This marked a breakthrough in the development and research of gas hydrate in the sea area.Because of the special reservoir-forming environment of natural gas hydrate,thermal shock mining,depressurization mining and injection inhibitor methods may cause seafloor landslide or methane leakage,which may cause greenhouse effect.Some scholars have proposed CO₂-CH₄（or CO₂-N₂）replacement method to exploit natural gas hydrate,which can not only produce CH₄ supplementary energy supply,but also store CO₂ to reduce greenhouse effect,thus bringing better economic and social benefits.On the basis of previous studies and based on the principle of hydrate decomposition kinetics,this paper establishes a mathematical model for gas hydrate by CO₂-N₂ mixture replacement,taking into account the pressure,temperature and composition changes in the process of gas hydrate exploitation.A two-dimensional coordinate system is used to discretize the mathematical model based on rectangular mesh system,and a semi-implicit method is used to solve the model.They are obtained for the formation pressure,temperature,phase saturation and gas component mole fraction at different mining time.The production dynamic characteristics of gas hydrate production by CO₂-N₂ mixture replacement were studied.The effects of injection pressure,water saturation and the ratio of CO₂-N₂ mixed gas on production dynamic were analyzed.It is compiled by using MATLAB for the numerical simulation program of gas hydrate recovery by CO₂-N₂ mixture replacement.The simulation results show that:with the increase of injection pressure,the rate of formation pressure decline slows down,and the reaction time of CO₂-N₂ replacing CH₄becomes longer,so the cumulative production of CH₄ is higher.With the increase of water saturation,more and more CO₂ hydrates are formed with water,which consumes the amount of CO₂ gas used to replace CH₄,thus the cumulative production of CH₄is reduced.With the increase of the proportion of N₂ in the CO₂-N₂ mixture,the CO₂-N₂ mixture is more easily to enter the CH₄ hydrate cage to replace CH₄,thus the higher CH₄ production can be obtained,and the simulation results show that the cumulative gas production of CH₄ is the highest when the proportion of CO₂-N₂ mixture is 25%-75%.The research conclusions have important guiding significance for the gas hydrate exploitation by CO₂-N₂ mixture replacement.