增强地热系统停止运行后温度恢复过程的数值模拟

Numerical simulation of temperature recovery process after enhanced geothermal system shutdown

在增强地热系统(EGS)运行过程中,干热岩(HDR)人工储层的温度会迅速下降,外界的热量补充速度远远跟不上系统热量提取的速度,同时也会对人工储层的流体压力产生巨大的影响,EGS停止运行以后人工储层温度和压力场会逐渐的恢复到初始状态。前人对干热岩开采过程中的温度场和流体压力场变化做了大量研究,但很少关注停止开采以后人工储层的温度恢复过程和流体压力场变化,而停止开采以后的温度场恢复过程对于新的地热井选址和废弃地热井的重新利用具有重要意义。利用数值模拟软件COMSOL Multiphysics建立双井EGS运行过程和停止运行之后两个阶段的二维热-孔隙流体耦合数值模型,模型考虑了放射性生热的贡献和不同热导率的影响。研究表明:在EGS运行期间,热对流是热量交换的主要方式,人工储层温度下降过程很快,19 a后EGS已经到达运行寿命,68 a后HDR人工储层已经失去开采价值。而地热开采停止以后,人工储层温度的自然恢复过程长达万年,热传导是主要的热量传递和温度恢复方式,热导率越高,人工储层温度恢复越快。

During the operation of the enhanced geothermal system(EGS),the temperature of the artificial reservoir in the hot dry rock(HDR)will rapidly decline,and the external heat supply will be unable to keep up with the system’s rate of heat extraction.At the same time,it will also have a significant impact on the artificial reservoir’s fluid pressure.The predecessors conducted extensive research on the temperature field and fluid pressure field changes during HDR exploitation process,but little attention has been paid to the temperature recovery process and fluid pressure field changes of artificial reservoirs after production has ceased,this process is critical for the site selection of new geothermal wells and the reuse of abandoned geothermal wells.The numerical simulation software COMSOL Multiphysics is used in this paper to establish a two-dimensional thermal-hydraulic coupling numerical model of the two-well EGS during operation and after shutdown,considering the contribution of radioactive heat generation and the influence of different thermal conductivities.According to the findings,heat convection is the primary mode of heat exchange during EGS operation,and the temperature of the artificial reservoir declines rapidly.The EGS has reached the end of its operational life after 19 years,while the artificial reservoir has lost its exploitation value after 68 years.The natural recovery process of artificial reservoir temperature lasts 10000 years after geothermal exploitation is terminated.The primary technique of heat transfer and temperature recovery is conduction.The higher the thermal conductivity,the sooner the artificial reservoir temperature recovers.