天然气水合物形成过程中的气体组分分异及地质启示

自然地质条件下不同气源的天然气体由于其组成不同，对天然气水合物的成藏条件产生不同影响。以2个常规天然气样品为例，在中国石油大学自行研制的水合物成藏一维模拟实验装置上进行了水合物成藏模拟实验，并对实验前后的原始气样、水合物形成后的游离气、分解气进行了气体组分分析。实验结果表明：水合物分解气中CH₄、N₂含量降低，而C₂H₆、C₃H₈、iC₄H₁₀、nC₄H₁₀、CO₂含量增大，游离气中各组分的变化趋势刚好相反，这意味着同等的温度压力条件下，C₂H₆、C₃H₈、iC₄H₁₀、nC₄H₁₀、CO₂等与CH₄、N₂相比更易于形成水合物；通过计算分解气体各组分相对于原始气体的相对变化量发现，在实验温度压力条件下（高压釜温度范围为4~10 ℃，气体进口压力为5 MPa），烃类气体与水结合形成水合物的能力由甲烷、乙烷、丙烷、异丁烷依次增加；由于不同烃类气体与水合物结合的条件不同，导致水合物形成过程中气体组分发生分异，水合物中甲烷含量减少、湿气含量增大，而游离气中气体变化相反，在自然地质条件下形成由水合物稳定带上部溶解气带、水合物稳定带及下部游离气带（或常规气藏）甲烷含量呈中-低-高特点，湿气和二氧化碳含量呈低-高-中的三层结构分布模式，因此，同一气源气体在不同带内表现出不同的气体组分特征。

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.