A Toolbox for the Accurate Detection of the Sources of Fugitive Methane Associated with Development of Unconventional Hydrocarbon Resources

The recent expansion of hydrocarbon exploration from unconventional reservoirs has generated some public concern regarding potential environmental impacts including contamination of groundwater due to migration of fugitive gas. Analyzing the isotopic compositions of methane and higher alkanes (where abundant) provides a promising tool to detect the sources of fugitive gas under the following conditions: 1) reliable baseline assessments of dissolved gases in groundwater have been conducted; 2) produced gases have been characterized chemically and isotopically; 3) gases in the intermediate zone have been isotopically fingerprinted; 4) no methane oxidation has occurred in the critical zone. The primary objective of this study was to chemically and isotopically fingerprint gases from the production, intermediate and shallow groundwater zones in areas of production of unconventional hydrocarbon resources in Western Canada to assess whether they are isotopically sufficiently distinct for tracer purposes. In addition, we investigated to what extent the isotopic fingerprint of the fugitive methane is subsequently modified during partial methane oxidation in shallow aquifers, and which additional parameters should be measured to quantify methane oxidation.

Produced gases from select shale gas plays in Western Canada have quite variable wetness parameters and δ¹³C values of methane typically > -46 ‰. In shallow groundwater at baseline conditions, methane was found to be rather ubiquitous with an average δ¹³C value of -67 ‰. This marked difference in carbon isotope ratios makes the isotopic tracing of fugitive gases from the production zones into shallow aquifers highly feasible, provided that no methane oxidation occurs. The latter process enriches ¹³C in the remaining methane and can lead to false claims of thermogenic methane leakage or erroneous assessments of the depth of gas leakage. Therefore, we have developed a toolbox based on the chemical and isotopic compositions of methane, DIC and sulphate in shallow groundwater that reveals the extent of methane oxidation in aquifers. The proposed approach is suitable for determining the accurate δ¹³C value of the leaking, non-oxidized methane, and hence for accurately determining the sources of the fugitive methane and its most likely leakage depth.