Effect of Different Sampling Methodologies on Measured Methane Concentrations in Groundwater Samples
Effect of Different Sampling Methodologies on Measured Methane Concentrations in Groundwater Samples
复制标题
不同采样方法对地下水样品中测得的甲烷浓度的影响
作者：
L. Molofsky;S. Richardson;A. Gorody;F. Baldassare;June A. Black;Thomas E. McHugh;J. Connor
供水威尔斯井中溶解的轻烃气体浓度（主要是甲烷和乙烷）的分析通常用于在页岩气和石油开采区域中建立钻井之前和之后的条件。目前有几种方法用于从供水威尔斯井中采集溶解气体分析样品;然而，这些方法所得结果的可靠性尚未量化。本研究比较了在阿巴拉契亚盆地钻前和钻后取样程序中使用三种取样方法收集的地下水样品中测量的溶解甲烷和乙烷浓度。其中包括开放系统收集方法，即在与大气接触时直接灌装40 mL挥发性有机分析（VOA）小瓶（直接填充VOA）和两种替代方法：（1）半封闭系统方法，其中40 mL VOA小瓶在水头下倒置时灌装（倒置VOA）和（2）相对较新的（2013）封闭系统方法，其中样品在不直接接触吹扫水或大气的情况下收集（IsoFlask®）。这项研究表明，在没有荧光的情况下，三种采样方法之间的甲烷浓度差异相对较小。然而，当甲烷浓度等于或超过20 mg/L（研究区域中发生液化的近似浓度）时，IsoFlask®（封闭系统）样品产生的甲烷浓度显著高于直接填充VOA（开放系统）样品，倒置VOA（半封闭系统）样品产生的浓度较低。这些结果表明，开放和半封闭系统样品采集方法适用于非分离样品。然而，封闭系统收集方法的使用为在所有条件下测量溶解的烃类气体提供了最准确的方法。
Analysis of dissolved light hydrocarbon gas concentrations (primarily methane and ethane) in water supply wells is commonly used to establish conditions before and after drilling in areas of shale gas and oil extraction. Several methods are currently used to collect samples for dissolved gas analysis from water supply wells; however, the reliability of results obtained from these methods has not been quantified. This study compares dissolved methane and ethane concentrations measured in groundwater samples collected using three sampling methods employed in pre‐ and post‐drill sampling programs in the Appalachian Basin. These include an open‐system collection method where 40 mL volatile organic analysis (VOA) vials are filled directly while in contact with the atmosphere (Direct‐Fill VOA) and two alternative methods: (1) a semi‐closed system method whereby 40 mL VOA vials are filled while inverted under a head of water (Inverted VOA) and (2) a relatively new (2013) closed system method in which the sample is collected without direct contact with purge water or the atmosphere (IsoFlask®). This study reveals that, in the absence of effervescence, the difference in methane concentrations between the three sampling methods was relatively small. However, when methane concentrations equaled or exceeded 20 mg/L (the approximate concentration at which effervescence occurs in the study area), IsoFlask® (closed system) samples yielded significantly higher methane concentrations than Direct‐Fill VOA (open system) samples, and Inverted VOA (semi‐closed system) samples yielded lower concentrations. These results suggest that open and semi‐closed system sample collection methods are adequate for non‐effervescing samples. However, the use of a closed system collection method provides the most accurate means for the measurement of dissolved hydrocarbon gases under all conditions.