CHARACTERIZATION OF WATER WITH STABLE ISOTOPES IN UNCONVENTIONAL FIELDS, ARGENTINA

In the Neuquén Province, Argentina, water associated with hydrocarbons production is injected through "disposal" wells into a receiving geological unit, in this case, the Centenario Formation (Fm.). Near each of the disposal wells, the provincial regulatory authority requires a "monitor" well, whose scope is the formation overlying the Centenario Formation (Rayoso Fm.). The purpose of these wells is to monitor the waters and ensure the protection of the shallow aquifers of hydrological interest located in the Neuquén Group (Gpo.). This study focuses on using stable isotopes of water molecules (H and O) as natural tracers and their potential use as a monitoring tool in the development activities of unconventional fields.

To carry out this work, water samples were taken from different geological units, Neuquén Group, Rayoso Fm., and Centenario Fm. The isotopic values (δ‰) of 18O and δ 2H was used to determine the origen of the different water samples and thus evaluate the possible inflows from the Centenario Fm. to the Rayoso Fm.. In addition, injection water was sampled, and isotopic data from the Neuquén River were used. All results were plotted on δ 18O vs. δ 2H graphs comparing the analyzed samples with each other and, in turn, with the global meteoric line.

The results reflect that all samples fall below the meteoric line, presenting different degrees of evaporative fractionation and generating three well-differentiated groups. On the one hand, the samples from the Neuquén Gpo., Rayoso Fm., and the Neuquén River have a similar isotopic signature with δ 18O values between -7‰ and - 13‰, and δ 2H values between -75‰ and -100‰. On the other hand, the Centenario Fm. and some of the injection water samples exhibit a similar but different signature from the previously mentioned units, with δ 18O values between 0‰ and -8‰ and δ 2H values between -30‰ and -70‰. Finally, with positive δ 18O values and δ 2H values between - 20‰ and -35‰, the remaining injection water samples are isolated.

Therefore, it is concluded that there is no cross-contamination from the Centenario Fm. to the Rayoso Fm., safeguarding the shallow aquifers of the Neuquén Gpo. and, validating the use of stable isotopes of the water molecule as a great natural monitoring tool.