Gas Composition Differentiation during Natural Gas Hydrate Formation and Its Geological Significance

The different gas sources have different gas compositions under geological conditions,which have different effects on formation conditions of natural gas hydrate reservoir.In this paper,the simulation experiments of gas hydrate formation were carried on one-dimensional gas hydrate experiment device in China University of Petroleum.The two experiment samples are conventional natural gas.The gas compositions,including original gas samples,free gas samples after gas hydrate formation,and decomposition gas of gas hydrate,were analyzed.The experimental results show that CH4 and N2 volume content reduces in the decomposition gas of hydrates,and C2H6,C3H8,iC4H10,nC4H10,CO2 volume content increases,and it is contrary in free gas.This means that C2H6,C3H8,iC4H10,nC4H10,CO2 more easily form gas hydrate compared with CH4 and N2 under the condition of the same temperature and pressure.Based on relative volume variation of the decomposition gas to the original gas composition,it can be concluded that the ability of forming hydrate from hydrocarbon ordinarily increases from methane,ethane,propane to isobutene in experimental temperature and pressure conditions(temperature range is from 4 to 10 ℃,gas inlet pressure is 5 MPa).The different hydrocarbons have different hydrate formation conditions,which results in gas compositions differentiation during hydrate formation.The content of methane decreases in hydrate,but that of wet gas increases,which is contrary in the free gas.The hydrate zone can be divided into dissolved gas zone,gas hydrate stable zone and free zone(or conventional natural gas zone) from top to bottom.The methane content is middle-low-high,and wet gas and dioxide content is low-high-middle in the three zones.The results show that the same gas source have different gas composition characteristics under different zones.