STRONTIUM ISOTOPES AS TRACERS OF PRODUCTION WATER IN THE ENVIRONMENT

The use of hydraulic fracturing in oil and gas production has raised concerns about groundwater contamination proximal to oilfields. Developing methods whereby we can assess and quantify potential impacts from produced oilfield water is, therefore, extremely important.

Strontium isotopes (⁸⁷Sr/⁸⁶Sr) have been used to trace and discriminate among various sources of water (e.g., seawater, oilfield brines, local groundwater, etc.) in the the environment. In this presentation, three case studies will be discussed that demonstrate the utility of this isotopic system as a means of assessing contributions, if any, of oilfield production water to local ecologic/hydrologic systems.

At a seawater intrusion barrier near Long Beach, CA, historic oil production from the Miocene Monterey Formation and the use of evaporation ponds to dispose of oilfield production water suggested the latter could be contributing to local salinity. If true, proposed extensions of the barrier systems would have to be redesigned; ⁸⁷Sr/⁸⁶Sr results indicated the source of salinity was seawater not brines. The barrier was extended as originally planned.

In Oklahoma, local groundwater was being impacted by saline waters suspected of originating from oilfield water evaporation/storage ponds. Local groundwater ⁸⁷Sr/⁸⁶Sr ratios, ~ 0.7087, were lower than the production water, 0.711. Analyses of 6 impacted groundwater samples were identical to those of the storage pond water; the client initiated remediation procedures on impacted groundwater.

Lastly, during production testing of a lower Miocene play southeast of Hong Kong (Pearl River Mouth Basin), the volume fraction of water produced with the oil increased over time. Prior to abandoning the well, samples of the produced water were analyzed to evaluate the source. Lower Miocene seawater is expected to have a ⁸⁷Sr/⁸⁶Sr ratio of ~ 0.7084; analyses of the produced water, however, were identical to those of modern seawater, 0.70923. As a result the well was not abandoned, the leak in the production piping was discovered and repaired. Subsequently investigations identified a reservoir with more than one billion barrels of oil.

Used in conjunction with other geochemical/isotopic tools, Sr isotopes afford an effective means of identifying and monitoring sources of production water.