A FRAMEWORK FOR EVALUATION OF GROUNDWATER ENVIRONMENTAL EFFECTS RELATED TO SHALE GAS DEVELOPMENT: A CASE STUDY IN THE XISHUI SHALLOW SHALE BLOCK IN SW CHINA

Thanks to the advancement of hydraulic fracturing and horizontal drilling technology, the shale gas revolution in the United States has changed the energy structure of the country and affected the global energy landscape. Other countries(such as China, Poland, South Africa, Australia, Canada) have also actively followed suit. In 2023, China's shale gas production reached 25 billion cubic meters. However, large-scale hydraulic fracturing may produce a series of environmental and geological problems, including groundwater pollution, induced earthquakes, and water consumption. Currently, there is no systematic framework for evaluating the environmental effects of shale gas development on groundwater, and the sources of major pollutants in groundwater are not clearly understood. This paper takes the Xiushui shallow shale gas block in Guizhou Province as an example. It establishes the baseline value of groundwater environment before shale gas development, determines the water circulation process and groundwater pollution sources during hydraulic fracturing, establishes the method for determining groundwater sensitivity monitoring and pollution tracer indicators, and reveals the influence of key geochemical processes on isotopic tracers in pollution tracing, and initially forms an evaluation framework for the groundwater environmental effects of shale gas development. The study showed that the TDS of shallow groundwater in the Xiushui shallow shale gas block ranged from 102 to 480 mg·L^-1, and the chemical types of groundwater were mainly HCO₃-Ca and HCO₃-Ca·Mg. The chemical composition of water was mainly controlled by the dissolution of carbonates and silicates. The methane content in groundwater was less than 0.01 mg·L^-1, and the volume ratio of dissolved gas(CH₄)in water ranged from 0.0025% to 0.3754%. The δ¹³C-CH₄ is generally less than-50‰, indicating biogenic methane. The inert gas isotopic ratios of ³He/⁴He and⁴He/²⁰Ne were within atmospheric ranges. Field hydraulic fracturing tests and results from long-term commercial extraction wells indicated that shale formation water exists throughout the shale gas development and production process, serving as the main component of flowback fluid and produced water, which is the main source of shallow groundwater pollution. According to the differences in hydrogeochemical characteristics between shale formation water and shallow groundwater, a method for determining groundwater sensitivity monitoring and pollution tracer indicators was established to identify the groundwater sensitivity indicators(Cl^-, Na⁺, Ba²⁺, Li⁺, B, ⁸⁷ Sr/⁸⁶ Sr, and δ¹¹B)in the study area. For gas pollution, end-members of isotopic ratios of CH₄(δ¹³C-CH₄) and noble gas isotopes(³He/⁴He and⁴He/²⁰Ne) in dissolved gas and shale gas(as groundwater pollutants) were established. At the same time, the impact of sorption-desorption on isotopic tracing was shown when there are clay minerals in a water-rock system. In the study area, the desorption of clay mineral B resulted in an increase in B content and a decrease in δ¹¹B in water, and the isotopic re-equilibrium between dissolved Sr and adsorbed Sr increased the ⁸⁷ Sr/⁸⁶ Sr ratio in water. This study is of great significance for the environmental protection of groundwater in shale gas development and is helpful to improve the assessment of groundwater environmental effects during shale gas development.