四川威远页岩气区浅层地下水中溶解甲烷的成因、时空分布及其与页岩气开采活动的关系

The environmental risks of shale gas exploitation have attracted extensive attention worldwide in past few years with methane contamination of shallow groundwater being the most controversial issue. It has been illustrated that shallow groundwater in some shale gas production areas in the U.S. was contaminated by dissolved methane. The cause of methane contamination and its relationship with shale gas exploitation, however, is still in debate due to the lack of baseline data of groundwater before shale gas drilling. We are at the beginning of large-scale shale gas development in China and have the advantage to investigate the relationship between methane contamination of groundwater and shale gas exploitation due to the chance to define baseline quality of groundwater and to do the groundwater quality monitoring during shale gas development. However, the study of methane contamination of shallow groundwater in shale gas production areas has not been conducted in our country, which inhibits our understanding of environmental risks of shale gas development. In this study, we will conduct a research on spatial and temporal distribution and origin of dissolved methane in groundwater in Weiyuan shale gas area in order to investigate the cause of methane contamination of groundwater, the impact of different stages of shale gas exploitation on methane concentration in groundwater and finally the relationship between methane contamination of groundwater and shale gas exploitation. On the other hand, we will build the baseline of groundwater quality in areas where shale gas will be extracted in the future.

近年来，页岩气开发的环境风险在全球范围内备受关注，其中以浅层地下水的甲烷污染最具争议。研究表明：美国部分页岩气开采区内浅层地下水存在甲烷污染现象，但由于缺乏页岩气开发前的地下水水质基线数据，目前对于地下水甲烷污染的来源及其与页岩气开采活动的关系尚存在较大争议。我国正处于页岩气规模化开发初期，具备建立地下水水质基线并进行地下水水质追踪对比的条件，是研究地下水甲烷污染与页岩气开采关系的有利地区。然而，目前我国关于页岩气区浅层地下水甲烷污染的系统研究尚未展开，严重限制了我们对页岩气开发环境效应的认识。本研究拟通过对四川威远页岩气区浅层地下水中溶解甲烷时空分布及成因的研究，明确页岩气开采区浅层地下水甲烷污染的来源，探究不同阶段页岩气开采活动对地下水溶解甲烷特征的影响，判断地下水甲烷污染与页岩气开采活动的关系。同时，建立威远页岩气区拟开采区块浅层地下水水质基线，为后续地下水甲烷污染研究提供对比依据。

近年来，页岩气开采区浅层地下水的甲烷污染现象备受关注，对于甲烷污染的来源及其与页岩气开采活动的关系存在较大争议。本研究揭示了四川威远页岩气区浅层地下水中溶解甲烷成因、时空分布及其主控因素，明确了不同阶段页岩气开采活动对地下水溶解甲烷特征的影响。研究发现：研究区浅层地下水中溶解甲烷浓度介于0.001~10mg/L，其中主体介于0.001~1mg/L，低浓度甲烷普遍具有较轻的碳氢同位素组成，主要为生物成因；断层发育处浅层地下水中溶解甲烷浓度升高，且碳氢同位素变重，表明存在深部热成因甲烷的混入。研究区地下水溶解甲烷含量主要受控于浅层地下水埋深和断层发育情况，其中，深层热成因甲烷沿断层的迁移对地下水溶解甲烷浓度影响最为显著。页岩气开采区地下水溶解甲烷时空分布特征及平台处地下水监测结果显示，页岩气钻探-完井-压裂-生产过程中均未出现热成因天然气的逸散泄露，未对地下水溶解甲烷特征造成显著影响。与此同时，我们建立了川南页岩气区拟开采区块浅层地下水水质基线，为后续地下水甲烷污染研究提供对比依据。.