A framework to determine sensitive inorganic monitoring indicators for tracing groundwater contamination by produced formation water from shale gas development in the Fuling Gasfield, SW China

摘要

Potential environmental impacts on shallow groundwater from shale gas development facilitated by horizontal drilling and hydraulic fracturing is a widely debated "hot topic". In terms of potential aqueous phase contamination (including flowback fluids and produced water), there is a large gap in knowledge of the indicators for routine monitoring and contamination tracing, which should be considered critical and should be prioritized for analysis. Since formation water from shale formations is the main source of flowback fluids and produced water, and there are significant differences in some specific inorganic geochemical and isotopic compositions between shallow groundwater and formation water, this study has provided a framework to determine sensitive monitoring and diagnostic indicators for tracing potential groundwater contamination from produced water using end-member analysis. The results from a case study of the Fuling Gasfield, Sichuan Basin, SW China as the first and largest commercial shale gas development site in China, shows that produced authentic formation water with similar Br/Cl and Na/Cl ratio with seawater and low delta H-2 and delta O-18 values compared to the evaporated seawater might originate from evaporated seawater modified by water-rock interactions and be mixed with fresh meteoric water. The inorganic geochemical and isotopic indicators, such as Ba, Li, Na, Cl, Br, Sr-87/Sr-86 (as epsilon(sw)(Sr)) and delta B-11 are sensitive to the detection of contamination of fresh shallow groundwater by produced formation water, even in very small fractions (0.05%). Meanwhile, we present a groundwater contamination case related to shale gas development in the Fuling Gasfield. The robust conservative geochemical (Cl and Br) and isotopic (delta B-11 and epsilon(sw)(Sr)) mass balances show that 0.2-0.9% of formation water entered the shallow groundwater causing contamination. The case has also confirmed the effectiveness of those indicators. Findings from this study may help to improve groundwater monitoring and environmental regulations in countries with shale gas exploration and development.