Methane Baseline Concentrations and Sources in Shallow Aquifers

Article pubs.acs.org/est

Methane Baseline Concentrations and Sources in Shallow Aquifers from the Shale Gas-Prone Region of the St. Lawrence Lowlands (Quebec, Canada) Anja Moritz,† Jean-Francois Hélie,‡ Daniele L. Pinti,‡ Marie Larocque,‡ Diogo Barnetche,‡ Sophie Retailleau,‡ René Lefebvre,§ and Yves Gélinas*,† †

GEOTOP and Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke West, Montreal, Québec, Canada, H4B 1R6 ‡ GEOTOP and Département des sciences de la Terre et de l’atmosphère, Université du Québec à Montréal, C.P. 8888, succursale Centre-ville, Montréal, Québec, Canada, H3C 3P8 § INRS-ETE, 490 de la Couronne, Québec, Québec, Canada, G1K 9A9 S Supporting Information *

ABSTRACT: Hydraulic fracturing is becoming an important technique worldwide to recover hydrocarbons from unconventional sources such as shale gas. In Quebec (Canada), the Utica Shale has been identified as having unconventional gas production potential. However, there has been a moratorium on shale gas exploration since 2010. The work reported here was aimed at defining baseline concentrations of methane in shallow aquifers of the St. Lawrence Lowlands and its sources using δ13C methane signatures. Since this study was performed prior to large-scale fracturing activities, it provides background data prior to the eventual exploitation of shale gas through hydraulic fracturing. Groundwater was sampled from private (n = 81), municipal (n = 34), and observation (n = 15) wells between August 2012 and May 2013. Methane was detected in 80% of the wells with an average concentration of 3.8 ± 8.8 mg/L, and a range of −50‰, indicating a potential thermogenic source. Localized areas of high methane concentrations from predominantly biogenic sources were found throughout the study area. In several samples, mixing, migration, and oxidation processes likely affected the chemical and isotopic composition of the gases, making it difficult to pinpoint their origin. Energy companies should respect a safe distance from major natural faults in the bedrock when planning the localization of hydraulic fracturation activities to minimize the risk of contaminating the surrounding groundwater since natural faults are likely to be a preferential migration pathway for methane.

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INTRODUCTION The interest in shale gas extraction and exploitation has been increasing worldwide during the past decade.1 In 2010, shale gas accounted for 23% of the total dry natural gas production in the United States, a proportion projected to increase to 49% by 2035.2,3 © 2015 American Chemical Society

Received: Revised: Accepted: Published: 4765

January 25, 2015 February 23, 2015 March 9, 2015 March 9, 2015 DOI: 10.1021/acs.est.5b00443 Environ. Sci. Technol. 2015, 49, 4765−4771

Article

Environmental Science & Technology

Figure 1. Map of sampled area and the St. Lawrence Lowlands geology and tectonic structures. Methane concentrations are represented with the size of the circles and methane δ13C signatures by their color. Note that the “fracked” wells (orange stars) appearing on the map are private exploration wells fracked before the moratorium in 2010. They are sealed and cannot be accessed to collect gas samples.

Shale gas extraction utilizes hydraulic fracturing, or “fracking”, to release the entrapped gas by fracturing the source rock. This is done by pumping large amounts of a mixture of water, sand and additives into the well at high pressure.4,5 Some have suggested that hydraulic fracturing of the rock could induce methane migration toward the surface, potentially contaminating groundwater resources.6−10 Yet, the most probable pathways of groundwater contamination by methane or flow-back waters (i.e., fracking fluids and recovered saline groundwater) are leaks through badly cemented well casings, and flow-back waters spillage on the surface.5−11 Studies carried out in the Marcellus shale have reported high thermogenic methane concentrations in groundwater located within a distance of 1 km from fracked wells, as suggested by carbon (δ13C) and hydrogen (δ2H) stable isotope analyses.8−12 It is unclear however whether the gas migrated through fracking-induced fractures or leaky casings.7−11 Li and Carlson (2014) found no correlation between dissolved methane concentration and distance to oil/gas well or well density in Northeastern Colorado. However, these authors have shown that the number of shallow groundwater wells with methane concentrations >5 mg/L decreased as the distance to an oil/gas well became greater than 700 m.13 Darrah et al. (2014) recently reported that contaminated wells in the Barnett and Marcellus shales were linked to gas leakage from intermediatedepth strata through failures of annulus cement, faulty production casings, or an underground gas well failure, ruling

out upward migration from depth through overlying geological strata.7 Few studies worldwide report methane concentrations and sources before hydraulic fracturing, although local legislations requiring companies to assess local baseline methane concentrations before drilling are progressively being implemented.14 In New York State, Kappell et al. (2012) reported natural methane levels in groundwater but did not assess its source,15 while McPhillips et al. (2014) found that methane concentrations were mostly correlated to groundwater chemistry, with little influence from valleys versus upslope location of the wells, distance from a conventional gas well, or geohydrologic units.16 McIntosh et al. (2014) reported that dissolved methane in groundwater of southwestern Ontario (Canada) was almost exclusively microbial in origin and that its concentration was linked to bedrock geology.17 To our knowledge, the only study reporting the analysis of groundwater quality before and after hydraulic fracturing is that of Boyer et al. (2011).18 These authors reported no statistical difference in groundwater−methane concentrations before and after drilling with or without hydraulic fracturing (approximately 50% of the wells were fracked). The dissolved methane concentration was higher in one well following drilling, but this well had not been hydraulically fractured.18 Longer monitoring periods maybe required to understand potential risks to shallow groundwater owing to the slow migration rate of contaminants (gas and fluid) through the well casing, the 4766

DOI: 10.1021/acs.est.5b00443 Environ. Sci. Technol. 2015, 49, 4765−4771

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Table 1. Average, Median, Range of Methane, Ethane and Propane Concentrations (in mg/L), and Methane δ13C Signatures (in ‰ vs. VPDB)a average Nb median maximum minimum limit of detectionc limit of quantificationc

methane (mg/L)

ethane (mg/L)

propane (mg/L)

methane (‰)

3.8 ± 8.8 117 0.1 45.9 ± 0.8 −50‰) had a low methane concentration (7 mg/L) and less depleted in 13C (δ13C > −50‰) appears to be of thermogenic origin. There were nine cases where the gas wetness ratio suggested a nonthermogenic source whereas the δ13C signature of methane was typical of a thermogenic one. Three processes can alter the gas wetness and/or the δ13C signature of methane: (1) mixing of different sources; (2) oxidation of methane; and (3) migration or diffusion of gas.38−43 Mixing of biogenic and thermogenic gases can be represented with different mixing curves that vary according to the gas wetness ratio and the δ13C signature of the end-members. Two such theoretical mixing curves are represented in Figure 5 with different thermogenic and biogenic end-members.

Oxidation of methane results in an enrichment in 13C of the residual methane. It also results in a decreasing C1/(C2+C3) ratio39−42,44 since the oxidation kinetics of 12C−CH4 are higher than those of 13C−CH4, and the oxidation kinetics of methane are higher than those of ethane and propane.39 On the other hand, the migration of gas mainly results in a higher C1/ (C2+C3) ratio since the diffusion rate through the bedrock is higher for the lighter methane compared to heavier ethane and propane. In most studies published so far, migration was found to only slightly affect the δ13C signature of methane compared to the C1/(C2+C3) ratio.41,42 Samples that plot outside of the biogenic or thermogenic window could have been affected by one of more of these processes (i.e., mixing, oxidation and migration). For example, three wells showed high concentrations of methane with a δ13C > −50‰ (Figure 2) although their C1/(C2+C3) ratio was >100 (Figure 5), suggesting an alteration of the original gas composition. The available data is however insufficient to draw definitive conclusions on the processes taking place for each sample. Methane in samples falling within the thermogenic and biogenic windows could also have been affected by these processes to some extent. The determination of the δ13C signature of ethane and/or the δ2H signature of methane would be necessary to determine the exact processes that affected the gas wetness or δ13C signature of methane in these samples. Despite the fact that most of the gas measured in the samples was biogenic in origin, thermogenic sources also contributed to some extent to the groundwater pool of light hydrocarbons in the area. This thermogenic gas could be mostly associated with the Lorraine silty shale outcrops. Here, biogenic gases formed at shallower depths in anoxic environments, likely in semiconfined or confined fractured aquifers where methanogens can proliferate,45,46 possibly mix with thermogenic gases formed in the deeper horizons of the Lorraine Shale or even the Utica Shale. Although speculative at this point, major faults could be a preferential path for this deeper thermogenic component to migrate upward7 and mix with biogenic shallower methane (Figure 2). 4769


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to minimize the risk of contaminating the surrounding groundwater in case a fracked well becomes faulty.

The hypothesis of an increased methane concentration in confined aquifers is supported by the relation found between the measured concentration of methane and well water chemistry (Figure 6). Freshwater recently recharged in nonconfined

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ASSOCIATED CONTENT

S Supporting Information *

Sampling and analysis of methane, ethane and propane dissolved in groundwater, plus a table listing all the data acquired in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 514-848-2424 ×3337; fax: 514-848-2868; e-mail: yves. [email protected]. Author Contributions

Sampling was carried out by A.M., D.B., and S.R., while the concentration and isotopic analyses were done by AM. Calibration of the reference gases for isotopic analysis was done by A.M. and J.F.H. A.M. and Y.G. wrote the first draft of the manuscript with inputs from D.P., J.F.H., M.L., and R.L.. All authors have given approval to the final version of the manuscript.

Figure 6. Methane concentrations measured in groundwater of different chemical types.

Notes

The authors declare no competing financial interest.

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aquifers having a Ca,Mg(HCO3) and Na-SO4-type chemistry are extremely depleted in methane (CH4 median values of 0.06 and 0.02 mg/L, respectively). Methane concentration increases rapidly in more evolved groundwater affected by ion Ca−Na exchange (Na-HCO3) and exchange with saline waters (Na−Cltype) in confined aquifers (CH4 median values of 0.34 to 7.6 mg/ L, respectively). This net increases of methane concentration is probably related to the change from oxygenated shallow environments to deeper and anoxic ones where methanogens can proliferate and methane oxidation and loss is strongly limited.45−47 Methane is thus a natural component of groundwater in the St. Lawrence Lowlands and can be present at concentrations that exceed solubility under conditions encountered in the wells. Depleted δ13C measurements suggest that methane found in these shallow fractured bedrock aquifers is mostly produced by methanogenic bacteria, although the gas composition may have been altered by processes such as migration and/or mixing with deeper-seated thermogenic sources and bacterial oxidation. Additional analyses such as the δ13C signature of ethane and propane as well as δ2H of methane are required to pinpoint the exact sources of this gas and the processes that may have altered it. The relationship between methane concentrations and groundwater chemistry suggests that methane levels are controlled to a large extent by the composition of the bedrock, local redox conditions, as well as water flow patterns and confinement (residence time). Natural faults in the bedrock are likely to be a preferential migration pathway for methane, especially in the Lorraine formation, as shown by the inverse trend between methane concentrations and distance from the faults. This is an important finding as a faulty fracked well located in the vicinity of natural faults could lead to much greater contamination of the groundwater compared to wells operated in compact intermediate bedrock. Energy companies, which target the area where the bedrock of the Lorraine group is located, should thus respect a safe distance from major natural faults in the intermediate and superficial bedrock when locating fracked wells. In view of the current study, this is a reasonable precaution

ACKNOWLEDGMENTS We thank the Strategic Environmental Assessment Committee on Shale Gas and the Quebec Government for entrusting us this project (Project CÉES no. E3-9 and FQRNT “Initiatives Stratégiques d’Innovation” Project no. 171083). This research was funded by grants from the FRQ-NT, NSERC, and CFI.

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Methane Baseline Concentrations and Sources in Shallow Aquifers

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