Tracing Test And Numerical Simulation Of Geothermal Water Pumping And Irrigation System In Sanqiao Area Of Xi'an City

This paper takes the Xi'an Sanqiao area irrigation system as an example.Under the premise of geothermal geology and hydrogeology in the study area,a three-dimensional numerical model of the study area was established in FEFLOW.The model was validated by using the measured water level and temperature data of the geothermal well in the study area to determine the reliability of the model.On this basis.The optimal well spacing of the pumping and irrigation system in the presence of hydraulic linkages without thermal breakthrough is studied separately;In order to solve the problem of continuous decline of groundwater geothermal water level,the water level drop of pumping wells under four different pumping and irrigation schemes was studied for many years,and the optimal pumping and irrigation scheme was proposed.At the same time,the operating parameters of the irrigation system(thermal reservoir depth,pumping and irrigation amount,inlet temperature)and hydraulic parameters(permeability coefficient,specific storage),thermal parameters(longitudinal dispersivity,thermal conductivity,volume heat specific,parameters(longitudinal dispersivity,thermal conductivity,volume heat specific,porosity)were studied.The sensitivity to the thermal reservoir temperature field and flow field during the operation of the irrigation system and its primary and secondary relationship.By using the thiocyanate as the tracer in the heating season,the connectivity tracing test of the well is carried out,and the connectivity of the pumping well is determined.The main findings are as follows:(1)The results of the tracer test showed that the tracer concentration in the water sample of the HS1 pumping well appeared at 89 days during the 110 days monitoring period in the tracer test.The peak value,although the peak due to excessive human interference is not ideal,can still represent the actual flow velocity of the flow field under the artificial flow field disturbance of the on-site irrigation process,and the flow rate is 0.12 m/h.(2)The simulated tracer test results showed that the tracer arrived at the pumping well in110 days,and verified with the field results,which proved the connectivity between the pumping and irrigation wells.Among the factors affecting the results of the tracer test,pumping and irrigation quantity plays a decisive role in the success of the tracer test.(3)During the heating period of the Sanqiao area,the pumping capacity is 2400m~3/d,100%recharge rate,and the rest of the time is in the intermittent operation mode,there is not only hydraulic connection,but also the reasonable well spacing without thermal breakthrough is200~650m.(4)Under the four different irrigation schemes,the water level of the pumping wells decreased the most in the group pumping no-irrigation scheme,and the water level dropped the least in the single pumping single irrigation scheme.Whether in the single pumping scheme or the group pumping scheme,the groundwater level has risen as the percentage of recharge increases.In all kinds of different irrigation schemes,although the groundwater level in the intermittent irrigation period will always recover,with the continuous exploitation for several decades,the regional underground hot water level still continues to decline.(5)The results show that the sensitivity of each parameter to the aquifer temperature field and its optimal well spacing is as follows:pumping and irrigation quantity>volume heat specific>inlet temperature>thermal reservoir depth>permeability coefficient>specific storage>thermal conductivity>longitudinal dispersivity>porosity.Among many parameters,pumping and irrigation quantity,permeability coefficient,thermal conductivity,longitudinal dispersivity are proportional to the temperature field influence range and the optimal well spacing,thermal reservoir depth,inlet temperatur,specific storage,volume heat specific and porosity are inversely proportional to them.(6)The sensitivity of each parameter to the aquifer flow field is as follows:pumping and irrigation quantity>permeability coefficient>thermal reservoir depth>inlet temperature>specific storage>thermal conductivity=longitudinal dispersivity=volume heat specific=porosity.The pumping and irrigation quantity and specific storage are proportional to the hydraulic gradient.The thermal reservoir depth,inlet temperature and thermal conductivity are inversely proportional to it.The tracer test of the irrigation system selected ammonia thiocyanate as the tracer.