油气藏微渗漏甲烷对温室气体的贡献及其排放机理研究

Geologic methane microseepage,an important contributor to atmospheric "greenhouse gases" methane budget, has recently become a research focus of global environmental change. Methane emission fluxes from different types of oil/gas reservoirs were significant difference. The Dawanqi field in Kuche depression (Tarim Basin, Xingjiang Province, China), as a typical shallow oil/gas reservoir, will be selected as the area of study. Firstly, by monitoring temporal and spatial variation of methane emission from the Dawanqi reservoir, emission fluxes of microseepage methane in the shallow oil/gas reservoir will be used for developing prediction and evaluation models.Secondly, based upon our previous methane flux studies of microseepage methane from the deep oil/gas reservoirs in the Tarim Basin, the overall contribution to atmospheric methane budget from the Tarim Basin will be predicted reasonably.Finally,in order to shed some lights on the emission mechanisms of microseepage methane from oil/gas reservoirs with varying geologic features and controlling factors in oil/gas reservoirs,computer simulation method will be adopted to evaluate the processes of microseepage and oxidation of methane in saturated zone and unsaturated zones, by taking into consideration the methane fluxes, environmental soil factors,soil gas composition and isotopic characteris- tics. Successful completion of this study will not only provide an important complement to Chinese and global database of microseepage methane flux, but also lay theoretical foundation for more accurately assessing contribute of natural methane emission flux from oil/ gas basins to atmospheric methane budget.

CH4是地球大气中的主要温室气体之一，其中地质CH4微渗漏对大气CH4源汇的贡献近年来成为全球环境变化研究的热点。不同类型油气藏，其CH4释放通量具有明显差异。在前期评估的深层油气藏CH4通量对大气CH4源汇贡献基础上，拟选择典型的浅层油气藏大宛齐油田为研究区，监测其CH4通量时空变化，建立浅层油气藏CH4释放通量预测方程和评估模型，系统地评估浅层油气藏CH4通量；并结合前期预测的深层油气藏CH4通量，合理地评价塔里木盆地CH4天然释放对大气CH4的贡献。同时，根据油气田CH4通量、土壤环境影响因子变化以及不同深度壤中气组成及同位素特征，针对不同地质构造条件和油气藏分布控制因素，结合计算机模拟方法，研究轻烃在地表的扩散和氧化过程，探索油气藏微渗漏CH4排放机理。研究结果将补充和完善中国及全球微渗漏CH4通量数据库，并为正确系统地评估含油气盆地微渗漏CH4释放对大气CH4的贡献提供科学的依据

甲烷是一种潜在的温室气体，估算和预测全球甲烷的源和汇，对全球环境保护和气候变化具有重要的意义。近年来，地质甲烷微渗漏对大气甲烷源汇的贡献逐渐成为了全球环境变化研究的热点之一。探索油气藏微渗漏甲烷的排放机理，评估不同类型的油气藏，对丰富全球地质甲烷渗漏数据库，完善全球甲烷源与汇的平衡，具有重要的理论意义和科学价值。.本项目选取新疆塔里木盆地北部库车凹陷大宛齐油田为研究对象，通过监测地表甲烷通量的和壤中气的测试分析，研究了该油田包括断层区、油气区、背景区微渗漏甲烷的释放。研究显示，甲烷微渗漏普遍存在，甲烷平均释放通量为17.45 mg m-2 d-1，最高可达329.9 mg m-2 d-1；地下3m壤中气和热释烃分析结果显示，甲烷和各类烷烃浓度远远高于背景地区；通量箱中甲烷的δ13C同位素值为-30.7‰ ~ -46.3‰，明显重于大气背景值；夏季微渗漏明显高于冬季，这可能是由于冬季厚达5米的冻土层阻碍了微渗漏。由上可知，大宛齐油田地下深处有机物裂解的烃类通过微渗漏的方式运移至地表并释放到大气中。大宛齐油田甲烷通量表现出显著的空间差异性，但是各采样点的土壤物性差异很小。在断层区，甲烷释放通量明显高于非断层区。通过整合油气勘探资料和剖面采样结果，对比地表和地下甲烷浓度发现，甲烷异常高值多分布在断层附近，并呈现沿构造带展开的趋势，说明了断层为控制甲烷微渗漏最主要的因素。以渗漏因子的方法，建立浅层油气藏CH4释放通量预测方程和评估模型，估算大宛齐油田甲烷通量为112.2 t/y。.大宛齐油田与雅克拉凝析气田对比发现，浅层油气藏大宛齐油田甲烷微渗漏平均释放通量明显高于雅克拉凝析气田。前者断层更为发育，而且油气层埋藏较浅、封盖条件较差，烃类气体更容易渗漏到地表。该研究结果补充和完善了中国及全球微渗漏甲烷通量数据库，并为正确系统地评估含油气盆地微渗漏甲烷释放对大气甲烷的贡献提供科学的依据的意义。