高温水热系统中相变时温度和驱动力的变化

Changes of Temperature and Driving Force during Phase Change in High Temperature Hydrothermal System

基于重力驱动的地下水流系统理论是描述地下水系统渗流特征的主要工具,由水头差产生的重力势是地下水运移的主要驱动力.但在水热系统中,存在除大地增温以外的深部热源,会给地下水系统提供额外的能量,产生新的驱动力.对流型水热系统中,地下水在补给段温度较低而在排泄段温度较高,排泄段的高温地下水会产生密度、盐度、粘度等变化,导致地热水的压力水头发生变化,形成地热驱动力.在高温水热系统中,排泄段地下水的温度更高,可能存在液态水转变为气态水的相变过程,使地下水温度发生突变,从而引起地热驱动力的改变.以西藏羊八井地热田为例,利用SiO_(2)地温计评估发现,热储层与地热井同一深度位置的水温存在较大差异,通过对比饱和蒸发线确定在距井口43.9 m处发生了引起温度骤降的相变过程.结合热力学第一定律,计算得出相变前后的温差约为23.6℃,由此造成的地热驱动力的变化值为-1.02 m.结果表明,水热系统中的地热驱动力仅存在于排泄段,排泄段地热水发生相变会引起地热驱动力的变化.

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 groundwater 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 SiO_(2)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.