Natural Emissions of Methane and Source Identification from Oil-Water Interface of Yakela Condensed Oil/Gas Field
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摘要: 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2.28 mg/m2·d, 最低-1.32 mg/m2·d, 日平均释放通量0.51 mg/m2·d, 标准偏差达1.23 mg/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.Abstract: In order to assess correctly the flux of CH 4 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, 13C/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/m2·d and the minimum-1.32 mg/m2·d.The average daily flux of methane emission is 0.51 mg/m2·d with the standard deviation of 1.23 mg/m2·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 δ13C 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.
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Key words:
- oil-water interface /
- methane emission /
- stable carbon isotope /
- flux chamber
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0. 引言
全球气候变暖的研究焦点是识别和正确地估算温室气体的天然及人为源和汇.CH4是仅次于CO2的温室气体和化学活性气体, 以单位分子和单位质量计算, 每分子CH4温室增温潜力是CO2的23倍(EIA, 2003), 在过去300年大气CH4的体积分数已增长了一倍多(Dlugokencky et al., 1994), CH4浓度达到1 751 mg·m-3 (Dlugokencky, 2003).因此, 对大气CH4的源与汇进行正确地估算和预测, 已成为目前全球环境变化及温室效应研究的一个热点.
近二十年来, 地质CH4天然释放源被认为占全球大气CH4源的极小部分, 甲烷水合物是最重要的地质CH4天然源(Michael, 1990; Lelieveld et al., 1998; Wuebbles and Hayhoe, 2002; IPCC, 2003), 其释放量仅为3~10 Tg·a-1 (Lelieveld et al., 1998).然而, 自2001年以来已有研究显示: 地质成因天然释放的CH4更重要来源于地壳内部, 它包括了地下深处油气藏烃类产物通过构造断层、岩石裂隙以及可渗透地层微渗漏扩散释放的CH4、陆地和海底泥火山的CH4释放、海洋CH4的渗漏以及地热/火山岩浆系统释放的CH4 (Etiope and Klusman, 2002, 2009; Etiope, 2004, 2008; Judd, 2004; Etiope et al., 2006, 2007a, 2007b, 2008).这挑战了长期以来一直认为地质成因的CH4天然源仅为甲烷水合物的观点.根据大气甲烷14C含量和新的地质成因CH4源的发现, 一些学者对全球大气CH4源强进行了修订, 并对放射性大气CH4和地质成因CH4重新进行了评估、更新和比较, 提出全球大气CH4源强为582±87 Tg·a-1, 其中地质成因CH4源强占30±5%, 这包括了天然和人为释放的地质CH4, 得出地质成因CH4天然源强约为40~60 Tg·a-1, 占全球CH4源强的8.5% (Etiope, 2004; Etiope and Milkov, 2004).同时认为在全球大气CH4源汇衡算模式中, CH4源与汇的不平衡可能是由于缺失了地质成因的CH4天然源所致, 因此在大气CH4预算中应包括地质CH4天然源(Denman, 2007; Lassey et al., 2007; Etiope et al., 2008).这些研究为IPCC和欧洲温室气体研究会提供了地质成因CH4天然释放的可靠依据.在2007年第四次IPCC全球气候变化温室气体评估报告中, 这个新的地质CH4天然源被确认为仅次于湿地的第二个重要的CH4天然源(Denman, 2007).并且, 在2008~2009年, 该成果作为欧洲环境署和美国联邦环境保护署制定《大气排放数据库指南》的主要参考(Denman, 2007; Etiope et al., 2008), 地质CH4释放的天然源被作为全球CH4释放的一个新的重要来源.
含油气盆地地下深处油气藏的烃类微渗漏是地质CH4的一个重要来源(Klusman and Jakel, 1998; Etiope and Milkov, 2004; Etiope et al., 2008).大量地球化学研究证明, 深部油气藏中甲烷等气体可以通过渗滤和扩散运移至地表, 其中未被氧化和降解的甲烷可以通过扩散、对流的方式排放到大气中(Klusman and Jakel, 1998; Etiope, 2004, 2008; Abrams, 2005).全球已知的含油气盆地面积多达8 000万km2, 约占全球陆地面积的15% (USGS, 1995), 其中地球化探已探明的存在甲烷微渗滤的面积约为43 366 000 km2 (Etiope, 2004).可见, 含油气盆地在大气甲烷源汇过程中起到不可忽视的作用.国际上关于含油气盆地CH4微渗漏对大气影响的研究及其通量的评估刚刚起步, 相关研究仅见于欧洲的意大利、罗马尼亚、希腊和美国(Klusman and Jakel, 1998; Klusman et al., 1998, 2000; Klusman, 2003a, 2003b, 2006; Etiope, 2004, 2008; Etiope and Milkov, 2004; Chilingar and Endres, 2005; Etiope et al., 2006, 2008; Baciu et al., 2008; Etiope and Klusman, 2009).国内相关方面的研究刚刚起步(Tang et al., 2006, 2007, 2008), 正确评估含油气盆地以天然方式向大气排放甲烷的通量, 对于深入了解岩石圈和大气圈的碳交换作用, 完善大气甲烷源汇平衡, 掌握大气甲烷在区域尺度上源汇变化规律, 有着十分重要的理论意义和科学价值.本文就新疆塔里木盆地雅克拉凝析油气田油水界面区微渗漏CH4进行了全面地监测和研究, 为系统地评估油气田CH4的释放通量, 客观地评价油气田区天然释放CH4对大气CH4源与汇的贡献提供了依据.
1. 研究区概况
作为世界上重要的石油生产国之一, 我国至今已发现大中型油气田33个, 其中塔里木、准噶尔、吐-哈等许多大型含油气盆地分布在西北干旱地区, 因此, 该地区已成为我国重要的油气田集聚区(宋岩等, 1998).由于西北地区气候干燥, 土壤蒸发量、渗透率很高, 盐碱性反应较强, 土地生物量生产率较低, 地表生物成因CH4的释放量十分有限, 减少了对油气盆地CH4天然释放的影响.因此, 西北地区不失为研究干旱气候条件下含油气盆地CH4天然释放通量的理想场所.
本研究区雅克拉凝析油气田是近年来国内发现的中大型凝析油气藏之一.大地构造位置隶属新疆塔里木盆地北部沙雅隆起东段的轮台-雅克拉断隆带.该断隆是一个夹持在南侧的轮台断裂与北侧的牙哈断裂之间的断垒块体(图 1).雅克拉构造带上发现了白垩系卡布沙良群披覆背斜凝析气藏、中下侏罗统岩性构造复合型凝析油气藏和下奥陶统丘里塔格上亚群、上寒武统阿尔塔格组及震旦系3个潜山凝析油气藏.区内凝析气均产于碳酸盐岩和碎屑岩两类储层中.盖层岩性主要是在低能环境下形成的泥质岩类(泥岩与泥质粉砂岩等) 及少量的蒸发岩, 在古生界至中新生界的多套盖层中, 以吉迪克组、卡布沙良群及下侏罗统为主(李南豪等, 1991; 钟广法等, 2006).雅克拉凝析气田大量的油气化探研究结果证实, 该区埋深在4 000~6 000 m的油气藏中的烃类组分仍可垂向运移到地表; 在垂向微运移过程中, 轻组分和较重组分优先上移; 深层烃类通过微垂向运移, 可在油气藏上方及其地表形成多种可测量到的物化探异常(钱铭云, 1998; 张百灵等, 1998; 侯卫国和苏江玉, 2001; 刘福江等, 2008).
图 1 塔里木盆地各类天然气藏分布示意图(据黄传波, 1999)Fig. 1. Sketch showing distribution of natural gases in Tarim basin1.1 样品采集
观测点设在雅克拉凝析油气田油水边界区(图 2), 根据油气藏的分布资料可知, 该处位于凝析气藏边部, 宽约300 m, 位置最低, 由于重力分异及成藏作用的影响, 使其气油比最低, 凝析油密度最大(张宁等, 2001).本研究为避免人为因素的干扰, 选择在距S6井向西约300 mW处布阵(图 2), 横穿油水界面将7个通量箱从北向南每间隔10 m布置一个, 以保证对油水界面区天然排放/吸收甲烷的变化情况进行全天候的连续观测.其中第5通量箱的坐标为: 41°34′45″N, 83°32′00″E.同位素样品采自5号通量箱.采样点处地表基本为光裸的第四纪土质、砂土质盐碱地, 盐碱地泥裂发育, 泥裂深度平均约20 cm, 局部零星生长了超旱生植物如红柳灌丛、灌木和多汁木本盐类.根据整个油水界面区地质地貌较均匀的特点, 笔者将采样点选择在无植物生长的大片空地处, 由于泥裂层土壤非常干燥, 基座插入会造成泥裂层劈开, 而使通量箱密闭性差, 因此先将泥裂层剔除, 然后再将通量箱基座插入下面的土壤深处.对油水界面区甲烷通量进行了24 h的日变化观测, 同时对通量箱中气体甲烷的δ13C变化作了监测.
甲烷通量采用圆柱形固定型静态箱式方法测定(Mosier et al., 1991).静态箱为自行设计的8 mm厚的透明有机玻璃制成, 箱体由底座和箱盖两部分组成.底座内径为33 cm, 高10 cm, 底座上部外缘制成槽状, 槽中可充满水密封; 箱盖内径为37 cm, 盖顶中央有一直径约4 mm的采样孔, 孔中嵌上橡胶隔垫, 采样针头可通过弹性隔垫插入箱中取样.气体观测时将底座下部7 cm插入土壤使静态箱固定嵌在土壤中, 并用蒸馏水倒入底座外缘的凹槽中, 盖住箱盖以密封静态箱, 安装过程注意要尽可能地减少对观测表面的扰动.采气时, 将采用器针头插入橡胶垫抽取箱内气体.采样时间为罩箱0、10、30、50和70 min, 每次抽取观测箱内气体样品约100 mL, 分别置于密闭铝箔气样袋中用于甲烷浓度分析.在每次监测完后, 都将箱盖取掉, 使箱内空气与大气充分交换, 以保证下一时段的测试.同时, 还采集了白天和夜晚时间段同位素气体样品各一组, 采样间隔同上, 采样量为500 mL.
1.2 样品分析
甲烷浓度用惠普5890气相色谱(FID) 分析, 测量前后均用2.04×10-6的甲烷国家标准气体进行标定.气相色谱工作条件为: 不锈钢填充柱(2 mm×2 m), 内填60/80目13X分子筛, 载气为N2 (99.999%), 流速30 mL/min, 柱温55 ℃, FID检测器温度200 ℃, 测量误差为0.11%~0.25%.甲烷碳同位素组成用Thermo Finnigan生产的Delta Plus XP稳定同位素质谱计在线分析测试系统(GC/C/MS) 和本项目研制的大气甲烷碳同位素的前处理制样装置联机在线分析(唐俊红等, 2006).质谱仪工作条件: 离子源高压3.0 kV, 离子源发射电流1.5 mA, 离子源加热电流6.0 mA; 色谱: 使用C-2000色谱柱, 柱长为25 m (25 m×0.53 mm×20 μm), 载气为高纯He (99.99%), He流量12.0 mL/min, 色谱仪程序升温:
以分离CH4、CO2和CO等气体, 氧化炉(NiO/CuO/Pt) 温度为940 ℃, 测量误差小于0.4‰.甲烷浓度的测试由中国科学院大气物理所大气边界层物理和大气化学国家重点实验室完成; 甲烷碳同位素组成采用作者研制的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的, 本项工作在中国科学院兰州地质所气体地球化学国家重点实验室完成的.
甲烷释放通量按下列公式(Mosier et al., 1991) 计算:
(1) 式(1) 中, E.通量; ρ.被测痕量气体密度; V.采样箱顶空容积; A.采样箱底面; dc/dt.浓度随时间的变化率.
2. 结果与讨论
2.1 雅克拉凝析油气田油水界面区甲烷释放的日变化特征
雅克拉凝析油气田油水界面区甲烷天然释放通量昼夜监测结果如表 1和图 3所示(8:00~9:10、11:00~12:10、14:00~15:10、17:00~18:10、20:00~21:10、23:00~0:10、2:00~3:10和5:00~6:10).由表 1可知, 在油水界面处, 甲烷的释放通量很低甚至为负值, 平均仅为0.02 mg/m2·h, 换算成日平均释放通量为0.51 mg/m2·d, 显然油水界面为含油沉积盆地低渗滤区(Etiope and Klusman, 2008).并且, 油水界面区甲烷的吸收/排放明显不均匀, 同一时段处于油水界面不同位置的各通量箱甲烷通量差异大, 标准偏差达0.04~0.07 mg/m2·h; 1、2、3和7号通量箱各时段监测结果显示主要向大气释放甲烷, 而4、5和6号则主要表现为吸收大气甲烷; 从各箱的日平均通量来看, 1、2、3和7号通量箱为大气甲烷源, 而4、5和6号则为大气甲烷汇, 其中2号通量箱日平均释放通量最大, 为2.28 mg/m2·d, 6号通量箱最小为-1.32 mg/m2·d, 两者相差达3.60 mg/m2·d, 日标准偏差达1.23 mg/m2·d.可见, 由于各通量箱在油水边界所处位置不同, 甲烷的吸收/排放存在着明显的差异, 这可能是由于在油水边界不同位置边水活跃程度不同, 造成了地表土壤氧化环境存在着明显的差异.因此, 尽管油水界面区土壤特征相同, 均为光裸的第四纪土质、砂土质沉积平地, 但其甲烷的吸收/排放在空间上却存在着较大的差异.总的来看, 在油水界面区, 由于边水活跃, 地表水所携带的氧气进入地下水, 其上方的土壤形成相对氧化的环境, 随着油气藏甲烷向地表的运移, 甲烷不断地被土壤吸收氧化(Barker and Fritz, 1981), 仅有很少部分运移至地表, 局部甚至均被吸收氧化, 而成为大气甲烷的汇, 因此整个油水界面区甲烷的释放通量远远低于油气区和断层区(Tang et al., 2007, 2008).
表 1 油水界面区各通量箱不同时段甲烷排放/吸收通量(mg/m2·h)Table Supplementary Table Flux of methane emission of every flux chamber in different times in interface of oil and water尽管油水界面区各通量箱甲烷的吸收/排放存在着较大的差异, 但它们的日变化规律基本相同(图 3), 即在白天甲烷的释放率低于夜晚, 下午17:00左右出现释放低值甚至负值, 随后各通量箱甲烷的释放速率逐渐增加, 并在凌晨至清晨达到相对高值.从各时段甲烷平均通量来看, 下午17:00~18:10油水界面区表现为大气甲烷的汇, 而其他时段虽然为甲烷源, 但其释放通量很低甚至为零.这说明油水界面区边水活跃造成土壤的氧化环境使深部油气藏甲烷及烃类在向地表运移的过程中不断地被氧化吸收, 仅有少量运移至地表, 在白天由于强日照、大气温度高导致土壤温度增高, 地表甲烷氧化菌的氧化活动增强, 从而使这少量的甲烷也被土壤基本或完全吸收氧化(Nesbit and Breitenbeck, 1992; Wang et al., 2000), 成为大气甲烷较弱的源甚至为汇; 而夜晚大气温度的降低导致土壤温度的降低, 从而抑制了地表甲烷氧化菌的氧化, 使得运移至地表的少量甲烷释放到大气中.
从地理位置上看, 新疆地区与北京地区存在着时差, 研究区在下午14:30前后日照最强, 大气温度最高, 土壤温度较大气温度滞后, 在下午17:00至傍晚达到最高, 此时土壤甲烷氧化菌氧化作用最强, 从而使得油气藏渗漏和扩散到油水界面地表处的少量甲烷基本被吸收氧化.凌晨, 大气温度达到最低值, 土壤温度随后在清晨也降至最低值, 且由于研究区为荒漠地区, 温差很大, 最大日差在25 ℃以上, 土壤甲烷氧化菌氧化作用明显降低, 少量渗漏和扩散到地表的甲烷释放到大气中.
2.2 雅克拉凝析油气田油水界面甲烷的碳同位素组成
近年来, 各国科学家已认识到甲烷同位素示踪技术在揭示甲烷的产生、传输及释放的微环境机理和确定相对贡献率方面具有明显的优越性(Rust, 1981; Tyler et al., 1988).并且随着同位素测试方法的不断完善, 大气甲烷和各甲烷源中甲烷的稳定碳同位素组成(13C/12C)、稳定氢同位素组成(D/H) 和放射性碳(14C) 含量已开始应用于示踪大气甲烷源汇和预算全球及区域性甲烷源强(Stevens and Rust, 1982, Stevens and Engelkemeir 1988; Wahlen et al., 1989; Hilkert et al., 1999; Rice, 2001; Keita et al., 2003).然而, 迄今为止地表各种来源的甲烷碳同位素的加权平均值相对于大气甲烷的碳同位素值却偏轻(Michael, 1990; Etiope and Klusman, 2002).这就迫使国内外学者一方面对甲烷光化学反应的同位素动力学分馏系数进行更精确的测定; 另一方面, 对地表甲烷的碳同位素组成开展更深入研究, 这既包括了解已知源的同位素组成在不同时空条件下的可能变化, 也包括探索未知的重同位素甲烷来源.作为甲烷重碳源的重要代表, 含油气盆地天然释放甲烷对大气源汇的影响已引起了各国学者的关注(Klusman, 2003a, 2003b; Etiope, 2004, 2008; Denman, 2007).
为了进一步证实油水界面天然释放甲烷来源, 在油水界面采用静态箱实地测量CH4通量的同时, 检测了其CH4的δ13C昼夜变化规律, 以进一步证实油水界面天然释放甲烷来源.图 4a-c为昼、夜通量箱在罩箱0、10、30、50和70 min时甲烷浓度和相应的碳同位素组成.可见, 在白天, 随着罩箱时间的增加, 甲烷浓度缓慢下降, 而其碳同位素组成则以对数函数形式从-45.8‰和-45.9‰分别上升到-44.5‰及-44.1‰ (图 4a, b), 即随着甲烷浓度的缓慢下降, 箱中剩余甲烷的碳同位素组成逐渐富积13C, 说明此时土壤为甲烷的汇, 并且在土壤吸收甲烷的过程中, 由于同位素分馏效应, 轻同位素12C优先被土壤吸收, 使得箱中剩余的甲烷相对富积13C.而在夜晚, 随着罩箱时间的增加, 甲烷浓度缓慢的上升, 其碳同位素组成以对数函数形式从-45.7‰上升到-43.4‰ (图 4c), 即随着甲烷浓度的增加, 其碳同位素组成逐渐变重, 显示存在热成因甲烷源; 这与细菌甲烷释放所观察现象恰好相反, 即随着通量箱甲烷浓度的增加, 其δ13C是逐渐偏轻.石油地质研究已证明雅克拉凝析气田深部天然气甲烷的碳同位素组成分布在-42‰~-31‰, 均为热解成因气, 属油田伴生气至凝析气范畴(汤玉平和刘运黎, 2002), 那么通量箱中的甲烷碳同位素组成随浓度的逐渐增加而偏重, 显示其明显受到来自深部凝析油气藏中的重碳甲烷等气体的渗漏和扩散的影响, 即油水界面区甲烷的释放存在来自深部凝析油气藏中的热成因甲烷.
3. 结论
含油气盆地深部地下油气藏中的轻烃类气体, 尤其甲烷能够通过渗滤和扩散运移至地表.雅克拉凝析油气田油水界面处边水活跃, 地表水所携带的氧气进入地下水, 其上方的土壤形成相对氧化的环境, 随着油气藏甲烷及烃类向地表的运移, 甲烷不断地被土壤吸收氧化, 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.因此, 在油水界面处甲烷的释放通量很低, 日平均释放通量为0.51 mg/m2·d.而且, 在油水边界不同位置处由于边水活跃程度不同, 造成地表土壤氧化环境存在着明显的差异, 甲烷的吸收/排放在空间上也存在着较大的差异, 最高日平均释放通量达2.28 mg/m2·d, 最低-1.32 mg/m2·d, 两者相差3.60 mg/m2·d, 整个油水界面日释放通量标准偏差达1.23 mg/m2·d.尽管油水界面区甲烷的吸收/排放在空间上存在着较大的差异, 但其日变化规律基本相同, 凌晨至清晨时达到相对高值, 随后下降, 正午17:00到傍晚20:00时段为释放低值甚至负值, 夜晚时分又逐渐增加.油水界面区甲烷δ13C组成变化在白天随着通量箱甲烷浓度的缓慢下降而逐渐偏重, 显示此时土壤为甲烷的汇; 而在夜晚, δ13C组成变化随着通量箱甲烷浓度的线性增加而逐渐偏重, 证实存在着来自地下油气藏的热成因甲烷.
致谢: 工作中受到中石化新疆石油管理局苏江玉教授级高工的大力支持和协助, 在此敬表谢意. -
图 1 塔里木盆地各类天然气藏分布示意图(据黄传波, 1999)
Fig. 1. Sketch showing distribution of natural gases in Tarim basin
表 1 油水界面区各通量箱不同时段甲烷排放/吸收通量(mg/m2·h)
Table 1. Flux of methane emission of every flux chamber in different times in interface of oil and water
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