A multidisciplinary evaluation on shale gas exploitation risks, Neuquen Basin, Argentina

DAPEÑA, CRISTINA; ARMELLA, CLAUDIA; FAVETTO, ALICIA; CABALERI, NORA G.; CRAVERO, FERNANDA; OSTERA, HECTOR; CAGNONI, MARIANA; POMPOSIELLO, MARIA CRISTINA

Puerto Vallarta

Simposio; 10th South American Symposium on Isotope Geology; 2016

Universidad Nacional Autonoma de Mexico

Since a few years ago, shale gas rush begun in the Neuquen Basin. Based on public concerd on the coeval risks and lack of information, a multidisciplinary Project is carried on in the core área of shale gas exploitation to determine the environmental baseline in aquifers that could be affected by gas migrations, fracking injection, pipeline and facilities leaks. Multilayer aquifers are located in the Neuquen Group, a sandstone-claystone cristal sequence that overlays the main play, Vaca Muerta Formation. These aquifers are very important due to the aridity of climate and dryness, because they sustain the sparse local population and cattle. A few consuming and industrial wells are drilled, with poor (high salinity) water quality. Isotope analyses reflect that fracking water, which comes from the Neuquen River, has a distinctive and well defined isotope composition. On the other hand, coproduced water shows systematic isotopic trends revealing that the whole basin hydrodynamics is poorly constrained at present. Upper and deeper layers in the Neuquen Group aquifers reflect recharge from the Neuquen River close to it, and to the north limits of the area (near Auca Mahuida) only deeper (confined) levels. Local recharge of surficial aquifers in the same area is recognized through isotopic and geophysical evidences. Gas migration in main structures is evaluated by radon and carbon dioxide surficial flux measurements which reveals that they are natural migration pathways that should be considered. Shale gas isotope composition is markedly different from conventional reservoirs (also determined) thus allowing the identification of possible leaks and their sources through Neuquen Group and associated aquifers. Complementary studies like numerical modeling of phreatic aquifer and contamination hypothesis (including degradation) in the vicinity of populated areas expose the environmental risks associated with facilities. Casing protection of the whole extension of the Neuquen Group is suggested, taking into account the measured frackability in selected profiles, the presence of freshwater and the existence of discontinuities in clay layers. Due to the isotopic and chemical differences between surficial water, injection water and groundwater, it is demonstrated that both measurements can detect any mirror contribution of coproduced waters to the others and will bring the basis for a more complete knowledge of basin hydrodynamics. The isotopic contrasts between conventional or non conventional reservoirs, lead to gas source identification and discrimination in case of gas leaks associated to exploitation and natural migration pathways. In summary, the isotopic and geochemical baseline serves to fix the main issues of concern for exploitation sustainability.