Issues associated with hydraulic fracturing of reservoir rocks have centered on the potential impact of produced waters on local groundwater resources. Hence it is very important that forensic methods be employed that can discrminate between oilfield waters and natural groundwater. Naturally-occurring stable isotopes of strontium (87Sr/86Sr) have been shown to be very effective tracers of natural water for many reasons: (1) isotopic fractionation is negligible; (2) contamination by other anthropogenic sources of Sr is typically not a concern; and (3) the 87Sr/86Sr ratio can be measured very precisely. This presentation will address three case studies, two in southern California and one in the midwest, where discriminating between produced oilfield waters and local groundwater became an issue.The first involved discriminating between locally derived irrigation water and Monterey Formation groundwater in order to identify the potential trigger for slope instability in La Conchita, CA. Based upon the measuredt 87Sr/86Sr ratios, ~ 0.7085, it was concluded that the main source of water was natural rather than anthropogenic, i.e., irrigation water. The second case study focuses on the source of chlorinity in groundwater in Long Beach, CA. Here seawater intrusion and/or historic Monterey Formation oilfield production with associated oilfield brine evaporation pond-derived salts may be the source of the elevated chloride concentrations in groundwater. Lastly, hydrocarbon-contaminated groundwater in the midwest was believed to be the result of leakage from evaporation ponds present at the site of two producing oilfields owned by two oil companies. Samples were collected from each firm's evapoaration ponds. The 87Sr/86Sr ratios of the evaporation pond waters averaged ~ 0.7093 for Operator 1 and ~0.7114 for Operator 2. Ratios measured in contaminated groundwater implicated Operator 2's evaporation ponds as the likely source of groundwater contamination. Given the environmental issues arising with hydraulic fracturing, the use of Sr, as well as other isotopic and geochemical fingerprints of produced oilfield waters, offers a scientifically-defensible way to assess contributions, if any, from oilfield operations to local groundwater systems.

Discrimination of produced formation waters from local groundwater using stable strontium isotopes: Examples from southern California and the Midwestern U.S

251st National Meeting (2016)

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