The relationship between methane migration and shale-gas well operations near Dimock, Pennsylvania, USA
The relationship between methane migration and shale-gas well operations near Dimock, Pennsylvania, USA
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DOI：
10.1007/s10040-015-1332-4
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发表时间：
2016-03-01
影响因子：
2.8
通讯作者：
Hammond, Patrick A.
中科院分区：
文献类型：
--
作者：
Hammond, Patrick A.
宾夕法尼亚州迪莫克镇附近散失的甲烷气体的迁移一直是美国页岩气钻探和水力压裂安全辩论的中心问题。该研究将甲烷和乙烷分子浓度和稳定同位素组成的时间变化与页岩气井活动（即垂直/水平钻井、水力压裂和补救措施）联系起来。这是通过分析州和联邦机构以及气井运营商在2008年至2012年间收集的数据来完成的。在某些情况下，甲烷运移在水力压裂之前就开始了。受污染水井的甲烷浓度由于当地水井的自然变化而比其浓度高出一到几个数量级。同位素分析表明，所有样品都具有不同成熟度的热成因，但都来自水力压裂的马塞勒斯页岩上方的地层。初始水井样品的结果与环空气值相似，但与生产气值不同。这表明，套管水泥密封的泄漏很可能造成了冲击，而不是生产套管壁的破裂。挤压固井的补救措施在一定程度上缓解了天然气运移的影响。在一些补救措施导致甲烷水平大幅下降的情况下，同位素值也发生了重大变化，这提供了两种来源的证据，一种是自然来源，另一种是人为来源。在抽气井的同时对水井取样似乎是确定甲烷是否发生迁移的一种经济有效的方法。
Migration of stray methane gas near the town of Dimock, Pennsylvania, has been at the center of the debate on the safety of shale gas drilling and hydraulic fracturing in the United States. The presented study relates temporal variations in molecular concentrations and stable isotope compositions of methane and ethane to shale-gas well activity (i.e., vertical/horizontal drilling, hydraulic fracturing and remedial actions). This was accomplished by analyzing data collected, between 2008 and 2012, by state and federal agencies and the gas well operator. In some cases, methane migration started prior to hydraulic fracturing. Methane levels of contaminated water wells sampled were one to several orders of magnitude greater than the concentrations due to natural variation in water wells of the local area. Isotope analyses indicate that all samples had a thermogenic origin at varying maturity levels, but from formations above the hydraulically fractured Marcellus Shale. The results from the initial water well samples were similar to annular gas values, but not those of production gases. This indicates that leakage by casing cement seals most likely caused the impacts, not breaks in the production casing walls. Remediation by squeeze cementing was partially effective in mitigating impacts of gas migration. In several cases where remediation caused a substantial reduction in methane levels, there were also substantial changes in the isotope values, providing evidence of two sources, one natural and the other man-induced. Sampling water wells while venting gas wells appears to be a cost-effective method for determining if methane migration has occurred.