Research On Solute Transport Of Groundwater Pollutants Caused By Shale Gas Exploitation

The success of the"shale gas revolution"in the United States has made countries around the world aware of the potential of shale gas,and they began to explore shale gas one after another.At the end of the 20th century,our country began to study shale gas.With the breakthrough of drilling technology and fracturing technology,our country's shale gas industry developed rapidly.In 2015,the Fuling shale gas field announced that it had entered commercial development.So far,our country has become the third country in the world to achieve commercial development of shale gas.Although shale gas is a clean energy source,the environmental impact caused by shale gas extraction is greater than that of conventional oil and gas resources.In order to explore the impact of shale gas mining on the groundwater environment,this study took the hydrogeological unit of the X platform in the Ning 209 well area in the Changning shale gas exploration and development zone is used as the research area.Through the identification of groundwater environmental impacts of the shale gas mining process,and exploring the ways of groundwater pollution caused by shale gas mining,and using the single factor pollution index method to analyze the pollution index of each pollution factor in different working fluids,the characteristic groundwater pollutants of the X platform in the Ning 209 well area caused by shale gas extraction have been determined.This study is based on digital elevation data,Stratum boundary data and rock formation data,through the geological modeling software Geo Modeller to establish a three-dimensional geological model of the study area,and use Fe Flow software to carry out numerical simulation of groundwater flow and solute transport.Through the study of groundwater pollution pathways in shale gas mining,characteristic pollutants of shale gas mining groundwater,and numerical simulation of groundwater flow and solute transport,the main conclusions of this paper are as follows:(1)There are many ways of groundwater pollution caused by shale gas mining.Leaks such as domestic sewage,drilling fluid,and drilling wastewater will cause groundwater pollution to a certain extent,but the main ways can be summarized into two.The first is leakage,that is,when the surface anti-seepage measures fail,pollutants can infiltrate with rainfall and enter the saturated water zone through the vadose zone.The second is lost circulation,that is,when there are large cracks or caves due to drilling encounters,cementing defects,or formation dislocation,the working fluid in the well directly enters the aquifer under the action of the pressure difference.(2)It is calculated by the single-factor pollution index method,and the factor with the higher pollution index is selected as the characteristic pollutant.The pollution index of organic pollutants and Cl^-in polymer solid-free drilling fluid is higher;the pollution index of heavy metals(Al³⁺,Zn²⁺)and Cl^-in KCl-polymer drilling fluid is higher;the pollution index of organic pollutants and Na⁺in oil-based drilling fluid Higher;the pollution index of heavy metal ions(Cd²⁺,Al³⁺,Fe²⁺,Pb²⁺)and NO₃^-in the fracturing fluid is higher.(3)Using Geo Modeller,a three-dimensional geological model established based on data such as digital elevation,stratigraphic boundary and rock occurrence,and corrected by borehole data,can accurately generalize the hydrogeological conditions of the study area.(4)Through the numerical simulation of solute transport,it is found that when the damaged pollutants at the bottom of the sump pool infiltrate from the surface,the volume of groundwater and rock and soil affected reaches the maximum after 365 days of migration,which are 2.55×10⁴m³ and 1.09×10⁵m³,respectively.After 2920 days,the Cl^-content in groundwater and rock and soil completely reached the standard.When the pollutants from damaged well pipes directly enter the aquifer,the volume of groundwater and rock and soil mass affected reaches the maximum after 431 days of migration,which are 1.85×10⁴m³ and6.17×10⁴m³,respectively,and the Cl^-content fully meets the standard after 2014 days of migration.