Environ. Sci. Technol. 2000, 34, 4727-4732
Bacteriogenic Ethane in Near-Surface Aquifers: Implications for Leaking Hydrocarbon Well Bores S . W . T A Y L O R , †,‡ B . S H E R W O O D L O L L A R , * ,§ A N D L. I. WASSENAAR† National Water Research Institute, Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan, Canada S7N 3H5, and Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1
The δ13C values of methane and ethane from two nearsurface aquifers in western Canada are depleted in 13C as compared to the carbon isotopic compositions of the underlying economic natural gas reservoirs and are indicative of in situ bacteriogenesis. The δ13CC2 values ranged from -45.4 to -73.9‰ (PDB), among the most depleted ever reported. These data can be used to resolve the longstanding uncertainty concerning the isotopic signature of bacteriogenic ethane and to differentiate between bacteriogenic and thermogenic hydrocarbon gas. For well bore leakage cases in the Western Canadian Sedimentary Basin, the characterization of a bacteriogenic end-member permits the quantitative resolution of the contribution of shallow aquifer gas versus deeper commercial reservoir gas to well bore leakage. Bacteriogenic near-surface aquifer ethane can account for 4-28% of the ethane from the production casing and for 29-36% of the ethane in gas bubbles traveling to the surface along the outside of the surface casing. The ability to quantify the contribution to well bore leakage from near-surface aquifer gases versus deeper commercial gas reservoirs using stable carbon isotope signatures is critical to implementing successful remediation strategies for soils and groundwaters contaminated due to aging and abandoned oil and gas wells.
Introduction Traditionally, δ13C signatures of CH4 have been used to distinguish between bacteriogenic hydrocarbon gas and thermogenic gas. Interest in discriminating between these gas types has intensified recently in the context of differentiating between bacteriogenic gas and proposed lowtemperature, thermally generated gas or so-called “incipient” thermogenic gas (1, 2). Similarly, distinguishing between near-surface bacteriogenic gas and deeper thermogenic natural gas reservoirs is a critical issue in hydrocarbon gas production fields throughout North America where well bore leakage is an economic and environmental problem (3, 4). Well bore leakage refers to the uncontrolled leakage of gas from aging and abandoned oil and gas wells, resulting in * Corresponding author e-mail:
[email protected]; tel: (416)978-0770; fax: (416)978-3938. † Environment Canada. ‡ Present address: Isotope Science Laboratory, Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada T2N 1N4. § University of Toronto. 10.1021/es001066x CCC: $19.00 Published on Web 10/06/2000
2000 American Chemical Society
contamination of shallow groundwater and soils. Eliminating this contamination is estimated to cost the oil and gas industry hundreds of thousands of dollars per site (5). A major challenge facing current remediation strategies is determining whether the source of these leaking gases is (i) production gas released through casing ruptures at depth (i.e., the gas originates in natural gas pools either in the production interval or in overlying strata) or (ii) gas from near-surface aquifers entering the surface casing or production casing through near-surface ruptures and migrating upward inside the wells. In addition, near-surface gases may simply use the well bores as pathways for upward migration along the outside of the well casings (2, 6). Bacteriogenic hydrocarbon gases are typically composed of >99 vol % methane (CH4) with small amounts of ethane (C2, typically
