Numerical Investigation Of Potential Impacts Of Hydraulic Fracturing On Groundwater Environment

In recent years,the attention has gradually attracted by the problem of groundwater pollution caused by hydraulic fracturing technology that extracts shale gas.Specially,whether high salinity groundwater and fracturing fluid injected into the deep(referred to as "deep potential pollutants")migrate upward through natural or man-made geological channels,and cause shallow groundwater pollution is the focus of people's attentionFirstly,this paper surveyed the migrating ways and driving forces of deep potential pollutants that may affect shallow groundwater,summarized the results of domestic and foreign studies on using numerical simulation to evaluate this problem,and analyzed the existing problems in previous studies.Then,a shale gas mining block in Guizhou Province is used as the research area,a typical hydrogeological profile is selected and a two-dimensional vertical,heterogeneous,single-phase,two-component solute transport model is constructed by using FEFLOW.The model set two simulation period that include hydraulic fracturing period and the100-year long period,simulated migration behaviors of contaminants in three scenarios that are the complete caprock and fracturing fissure connecting fault,and shale gas wells are deployed in regional groundwater discharge,the possibility of the impact of hydraulic fracturing on groundwater environment was evaluated.Finally,the sensitivity analysis of the parameters,such as formation permeability coefficient,hydraulic gradient,fault permeability coefficient in the model,was carried out,and the influencing factors affecting the migration of potential pollutants in the deep were discussed.Based on the simulation results,the following conclusions were mainly drawn in this paper :(1)under the condition of complete capping,Whether under the effect of fracturing pressure in the hydraulic fracturing period or regional groundwater flow in the long period of 100 years,deep potential pollutants are impossible to affect shallow groundwater in the study area;On the time scale of tens of thousands of years,the possibility of affecting shallow groundwater is also small;(2)Permeable fault contribute to the upward migration of potential pollutants;in addition,when the fault is connected with fracturing fissure and there is a flow in the fault,the deep potential pollutants reach the shallow aquifer within 6 hours at a fracturing pressure of 50Mpa;(3)Groundwater in the regional groundwater discharge area can promote the upward migration of deep pollutants,but in the hydraulic fracturing period and in the long period of 100 years,the pollutants will not reach the shallow groundwater;(4)fault permeability coefficient,formation permeability coefficient,regional groundwater hydraulic gradient,fault dip angle and direction are all factors influencing the migration of deep potential pollutants.The fault permeability coefficient,formation permeability coefficient,and flow hydraulic gradient of regional groundwater have a great influence on the migration distance of pollutants,which is a positive correlation among them.