页岩气开发的地下水环境效应评估——以贵州习水浅层页岩区块为例

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

大规模水力压裂可能产生一系列环境地质问题,包括了地下水污染、诱发地震以及水资源消耗。目前尚未系统地建立页岩气开发的地下水环境效应评价框架,且对地下水主要污染物来源认识不清。本文以贵州习水浅层页岩气区块为例,通过了解页岩气开发前地下水环境背景状况、确定水力压裂中水循环过程以及地下水污染源、建立地下水敏感性监测及污染示踪指标确定方法、揭示污染溯源中关键地球化学过程对同位素污染示踪的影响,初步形成了页岩气开发的地下水环境效应评估框架。本研究确定了研究区浅层地下水背景值状况,包括水化学、同位素以及溶解性气体指标。现场水力压裂试验和具有长周期商业开采井的结果表明页岩地层水存在于整个页岩气开发和生产过程,并作为返排液和产出水的主要组成部分,是浅层地下水污染的主要来源。根据页岩地层水和浅层地下水水文地球化学特征的差异,建立了确定地下水敏感性监测和污染示踪指标的方法,识别出研究区地下水敏感性指标(Cl^(-)、Na^(+)、Ba^(2+)、Li^(+)、B、87Sr/86Sr和δ11B)。针对气体污染(甲烷作为地下水污染物),建立了浅层地下水溶解气和页岩气甲烷同位素(δ13C-CH_(4))及惰性气体同位素(3He/4He和4He/20Ne)组成的端元。同时揭示了当有黏土矿物存在的情况下,吸附-解吸对同位素示踪的影响。在研究区,黏土矿物B的解吸造成水体B含量的增加及δ11B的减少;溶解态Sr与吸附态Sr的同位素再平衡,增加了水中87Sr/86Sr比值。本项研究对于页岩气开发的地下水环境保护具有重要的意义,有助于完善页岩气开发过程中地下水环境效应的评估。

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 HCO_(3)-Ca and HCO_(3)-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-CH_(4) 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 and 4He/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 inδ11B 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.