Study On Impact Mechanism Of CO₂ Leakage On Groundwater And Monitoring Indexes At Geological Storage Site

Carbon capture,utilization and storage technology,including CO₂ enhanced coalbed methane recovery（CO₂-ECBM）technology,has become one of the important technologies to cope with global climate change and effective energy utilization.However,once CO₂leaks,it may affect the groundwater environment,which is a key issue for environmental impact assessment and monitoring of CO₂ geological storage projects.This paper,on the basis of summarizing the research on the impact mechanism of CO₂ leakage on the groundwater environment at home and abroad,relying on the CO₂-ECBM demonstration project in southern Shizhuang area,Qinshui basin,China united coal bed methane co.LTD.The roof rock sample of no.3 coal seam and the seam fracture water of the Permian Shanxi formation were selected.Two groups of experimental conditions,30℃/10MPa and 30℃/7MPa,were set to simulate water rock reaction of liquid phase CO₂ and gas phase CO₂respectively.The main chemical indexes and chemical component concentrations of water samples were measured at the initial,3,7 and 12 days respectively,and the change rule and reaction mechanism were analyzed.Finally,the monitoring indexes of the impact of CO₂leakage on groundwater environment were put forward.This study concluded that:After the CO₂ leaks and dissolves in water,a large amount of H⁺is generated and separated,resulting in a significant decrease in pH.Later,due to the dissolution of minerals,some H⁺is consumed and the pH value rises.The consumption of H⁺promotes further ionization of carbonic acid,and the HCO₃^-concentration continues to increase.The dissolution and precipitation of Ca²⁺,Ca²⁺and Mg²⁺lead to an increase or decrease in the concentration,and eventually an increase or decrease in the total hardness.The changes of TDS,EC and salinity were not obvious.Due to the dissolution of clay minerals such as illite and kaolin into polysilica muscovite and muscovite,a large amount of K⁺is consumed first,and the K⁺concentration is reduced.Later,due to the dissolution of K-containing minerals such as illite,the K⁺concentration increases.Ca²⁺,Na⁺and Mg²⁺concentrations increased first and then decreased,mainly because sodium feldspar and illite dissolve in the early stage of the reaction,after that,complexation with HCO₃^-or adsorption of clay minerals may occur,and the concentration decreases.Total Fe concentration continued to increase,mainly caused by the dissolution of siderite.The high concentration of CO₂ has an inhibitory effect on excess Cl^-and NO₃^-,so the Cl^-and NO₃^-concentrations decrease rapidly in the early stage of the reaction,and then due to competitive adsorption of HCO₃^-,it increased slightly,and the concentration of SO₄^2-changed in the opposite direction.Based on the data analysis of two groups of CO₂-water-rock reactions,the 11 indexes with the most obvious response to CO₂ leakage are identified as the indicative monitoring indexes for the impact of CO₂ leakage on groundwater environment in this CO₂-ECBM demonstration project.Among them,the main chemical indicators include pH,DO,EH,total hardness,and the concentration of HCO₃^-.The main chemical component indicators include the concentration of K⁺,Na⁺,Ca²⁺,Mg²⁺,TFe and NO₃^-.Finally,the indirect monitoring technology and direct monitoring technology of CO₂ leakage are summarized,and the optimal monitoring scheme is proposed to provide support for the optimization of similar engineering monitoring schemes.