GEOCHEMICAL AND ISOTOPIC APPROACHES TO ASSESS POTENTIAL IMPACTS FROM SHALE GAS DEVELOPMENT ON SHALLOW GROUNDWATER

With the rapid expansion of natural gas exploitation from unconventional reservoirs including shale gas, there is public concern about potential future contamination of shallow groundwater with stray gases, formation waters or chemicals used during hydraulic fracturing. In order to enable a scientifically sound assessment of potential future deterioration of freshwater resources in shallow aquifers, it is essential to first establish a baseline of groundwater quality including its dissolved or free gases. In addition, it is desirable to obtain chemical compositions and carbon isotope ratios of mudgases from the intermediate zone below the shallow aquifers and above the shale gas target formation during drilling of energy wells. Finally, the chemical and isotopic compositions of the produced gases should be known. Since 2006, we have conducted monitoring programs determining the chemical and isotopic compositions of water, its dissolved constituents, and of gases obtained from shallow groundwater and formation fluids collected from coalbed methane and shale gas plays in Western Canada. For groundwater samples, we placed special emphasis on determining the sources of dissolved and free gases using isotope techniques to assess whether gases produced from shale gas plays or potentially leaking from the intermediate zone are isotopically distinct from those in shallow aquifers. Methane and ethane in free gas samples obtained from shallow aquifers (n = 24) were found to have mean δ¹³C values of -72.4 ‰ and -50.2 ‰, respectively. These values are similar to those of mudgas samples in very shallow formations of the Western Canadian Sedimentary Basin (WCSB) at depths of less than 250 meters, but are markedly different from the much higher δ¹³C values of methane and ethane in deeper portions of the WCSB and in shale gas plays. Therefore, it appears highly feasible to identify the formation from which potential gas leakage may occur provided that baseline data for shallow groundwater have been determined, and that chemical and isotopic data for mudgases and production gases are available. Repeat baseline sampling of free gas from selected wells revealed that the variability of δ¹³C values of methane and ethane is typically less than ± 2 ‰ over periods of several years.