Evolution Of Fracturing Fluid Composition And Its Effect On Groundwater In The Development Of Sichuan Shale Gas

With increasing demand in natural gas,shale gas,as a new type of unconventional natural gas resource,is gaining more and more attention.Shale gas stands for the unusual natural gas and is stored in shales,which are rich in organic matter.Shale gas exploitation in China just started recently.Consequently,the systematic researches on the environmental effects and pollution mechanisms of shale gas exploitation are still required.In this study,the hydrogeochemical interactions between fracturing liquid and surrounding media and reactions during the infiltration of flowback liquid from ground surface to aquifer were simulated,based on the available data and the actual conditions of a shale gas exploitation area in Sichuan.Finally,leachability and pollution risk of typical organic compounds in flowback liquid were evaluated.The main results are as follows:（1）The simulation results of hydrogeochemical processes in shale gas exploitation layer showed that mineral dissolution rate from fast to slow in the water-rock interaction follows the order: carbonate minerals（calcite and dolomite）>feldspar mineral（potash feldspar and albite）>pyrite>quartz>clay minerals（illite,montmorillonite,chlorite and kaolinite）.As a result,the concentrations of Ca²⁺,Mg²⁺,HCO3-,K⁺,Fe（t）and Al³⁺ in the solution increased,and the TDS of solution increased by nearly 3 times.The amount of mineral dissolved after water-rock interaction in different shale minerals is proportional to the content of brittle minerals（the sum of quartz,feldspar and carbonate minerals）.（2）The study on the influence of flowback liquid with high salinity and complex components on groundwater environment indicated that the concentrations of Na⁺,K⁺,Ba²⁺and Al³⁺in liquid decreased,due to the cation exchange during infiltration of flowback liquid in unsaturated zone.When it reached aquifer,the interaction（mainly mixing）changed the original water chemistry type of groundwater from HCO3 · Cl-Ca to Cl-Na.The water-rock interaction resulted in the increased concentrations of Na⁺,K⁺ and HCO3-,but the decreased concentration of Ca²⁺.The CO₂ in reinjected flowback liquid can enhance the dissolution of calcite in reinjection layer.（3）The leachability assessment results showed that among 75 target organic compounds in flowback fracturing liquid,3 compounds（1,1-cyclopropanedimethanol,2-methyl-1-butene and 2-methyl-2-butene,4% of the total content）have high leachability and risk to groundwater environment,15 organic compound shave a medium leachability,and the other 57 compounds have a weak leachability.For the studied organic compounds,the relationship between leachability index GUS and log Koc（organic carbon-water distribution coefficient）is: GUS =-1.62 log Koc + 6.42.This indicates that leachability is mainly affected by the sorption process,and decreases with increasing sorption capacity.