Changes of Temperature and Driving Force during Phase Change in High Temperature Hydrothermal System
The theory of groundwater flow system based on gravity is the main tool to describe the seepage characteristics of groundwater system.The gravity potential generated by water head difference is the main driving force of groundwater migration.However,in the hydrothermal system,there is a deep heat source other than geothermal gradient,which will provide additional energy to the groundwater system and generate new driving force.In the convective hydrothermal system,the temperature of groundwater is lower in the recharge section and higher in the discharge section.The high temperature groundwater in the discharge section will produce changes in density,salinity and viscosity,which will lead to changes in the pressure head of geothermal water and form geothermal driving force.In the high temperature hydrothermal system,the temperature of groundwater in the discharge section is higher,and there may be a phase transition process from liquid water to gaseous water,which causes the ground water temperature to change abruptly,thus causing the change of geothermal driving force.In this paper,the Yangbajing geothermal field in Tibet is taken as an example.Using SiO2 geothermometer,it is found that there is a large difference in water temperature at the same depth between the geothermal reservoir and the geothermal well.By comparing the saturated evaporation line,it is determined that the phase change process causing the temperature drop occurred at 43.9 m from the wellhead.Combined with the first law of thermodynamics,the temperature difference before and after the phase transition is calculated to be about 23.6 ℃,and the resulting change in geothermal driving force is-1.02 m.The results show that the geothermal driving force in the hydrothermal system only exists in the discharge section,and the phase change of geothermal water in the discharge section will cause the change of geothermal driving force.
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