雅克拉凝析油气田油水界面甲烷天然释放及其源示踪

Natural Emissions of Methane and Source Identification from Oil-Water Interface of Yakela Condensed Oil/Gas Field

为评价油气田天然释放CH4对大气CH4源与汇的贡献,采用静态箱法实地监测了新疆塔里木盆地雅克拉凝析油气田油水界面处甲烷的释放通量,并采用在线大气甲烷碳同位素制样系统与稳定同位素质谱仪联机测试了通量箱甲烷碳同位素组成.结果表明,由于油水界面边水活跃程度不同,甲烷通量在空间变化很大,最高的日释放通量达2.28mg/m2.d,最低-1.32mg/m2.d,日平均释放通量0.51mg/m2.d,标准偏差达1.23mg/m2.d.油水界面处甲烷通量日变化规律基本相同,凌晨至清晨时达到相对高点,随后逐渐降低,下午至傍晚时段为释放低值甚至负值,夜晚时分又逐渐增加.通量箱中甲烷δ13C组成白天随甲烷浓度的线性降低而逐渐偏重,夜晚δ13C随着甲烷浓度的线性增加而逐渐偏重.可见,油水界面边水活跃,其上方的土壤形成相对氧化的环境,油气藏甲烷及烃类在向地表运移的过程中不断被土壤吸收氧化,仅有少量运移至地表并逸散到大气中,局部甚至均被吸收氧化,而成为大气甲烷的汇.

In order to assess correctly the flux of CH4 emitted naturally from oil/gas basin, a static flux chamber method was applied to study natural emissions of methane to the atmosphere in oil-water interface of Yakela condensed oil/gas field in Tarim basin, Xinjiang. Using an online method which couples a gas chromatography/high-temperature conversion/isotope ratio mass spectrometry （GC/C/MS） together, ^13 C/^12C ratios of the methane in flux chambers were measured. The results demonstrate that oxidation environments of soil and water are different in the oil-water interface because chambers lie in different places. So, fluxes of methane emission in the chambers are very different. The maximum daily flux of methane emission is 2. 28 mg/m^2· d and the minimum -h 32 mg/m^2· d. The average daily flux of methane emission is 0. 51 mg/m^2· d with the standard deviation of h 23 rng/m^2· d. Although there are large differences in flux of methane emission in the oil-water interface, daily change laws are similar： the methane emission flux reaches the maximum at 5：00-6：00 am, and then it decreases gradually, and reaches the minimum at 17：00-18：00 pm, and then it gradually increases. Measurement of carbon isotope of methane shows that δ^13C of methane becomes heavier with decreasing concentration in the afternoon and with increasing concentration in the morning. Thus it can be seen that the soil above the oil-water interface becomes a more oxidation environment because water contains O2. Methane is absorbed and oxidated constantly during migration to the earth＇s surface from deep oil/gas reservoir through microseepage and pervasion. Only a little methane which is not oxidized and degraded is discharged into the atmosphere through diffusing and convection. Even all methane is absorbed by soil in some areas, which leads to the sink of atmospheric methane.