Changes of Temperature and Driving Force during Phase Change in High Temperature Hydrothermal System
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摘要: 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SiO2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.Abstract: 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 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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Key words:
- hydrothermal system /
- phase change /
- temperature /
- geothermal driving force /
- Yangbajing /
- hydrogeology
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图 1 羊八井地热田水文地质简图及采样位置
Fig. 1. Hydrogeological sketch and sampling location of Yangbajing geothermal field
表 1 羊八井地热田地热井基本数据资料
Table 1. Basic data of geothermal wells in Yangbajing geothermal field
井号 类型 深度(m) 井口温度(℃) 井底温度(℃) 开采量(t/h) 勘测时间 井底水压力(kPa) YBJT3 浅层地热水 193 104.8 171 54.8 2006年6月 1 891.4 YBJT4 浅层地热水 454 110.4 173 54.5 2006年6月 4 449.2 YBJT5 浅层地热水 270 110.4 167 132.5 2006年6月 2 646.0 YBJT6 浅层地热水 240 108.8 163 63.2 2006年6月 2 352.0 YBJT7 浅层地热水 300 108.8 172 140.5 2006年6月 2 940.0 ZK4001 深层地热水 1495 159.3 255 242.7 2006年6月 14 651.0 表 2 羊八井地热田水化学数据资料(mg/L)
Table 2. Data of hydrochemistry in Yangbajing geothermal field (mg/L)
井号 类型 温度(℃) pH HCO$ {}_{3}^{-} $ SO$ {}_{4}^{2-} $ Cl- F- Na+ K+ Ca2+ Mg2+ SiO2 水类型 YBJT3 浅层地热水 104.8 8.21 178.0 56.3 518.30 17.90 281.8 36.60 4.2 0.22 160.50 Cl-Na YBJT4 浅层地热水 110.4 8.36 184.2 60.0 547.10 19.40 306.4 41.80 3.9 0.20 169.40 Cl-Na YBJT5 浅层地热水 110.4 9.57 160.1 59.0 512.90 19.20 308.1 42.70 4.6 0.28 172.00 Cl-Na YBJT6 浅层地热水 108.8 9.72 174.5 58.2 507.40 18.50 293.5 39.70 4.1 0.21 166.00 Cl-Na YBJT7 浅层地热水 108.8 9.49 151.1 60.3 559.80 18.50 303.2 41.60 3.6 0.20 171.40 Cl-Na ZK4001 深层地热水 159.3 8.40 363.0 27.0 1 020.00 18.00 709.0 135.00 2.1 0.13 581.50 Cl-Na YBJC1 冷地下水 14.0 7.23 26.5 6.1 1.16 0.48 1.6 1.10 11.8 0.72 10.03 HCO3-Ca YBJC2 冷地表水 8.0 8.15 19.1 10.3 1.27 0.33 2.2 1.37 8.7 1.33 11.50 HCO3-Ca YBJC3 冷地表水 8.0 7.93 24.0 5.0 1.06 0.34 1.4 0.82 9.4 1.03 8.00 HCO3-Ca 表 3 热储层温度(℃)
Table 3. Thermal reservoir temperature (℃)
井号 地热水类型 二氧化硅
(无蒸汽损失)地温计二氧化硅
(最大蒸汽损失)地温计Na-K
地温计Na-K-Ca
(β=1/3)地温计YBJT3 浅层地热水 192.6 156.2 223.3 411.3 YBJT4 浅层地热水 196.3 159.1 229.0 422.4 YBJT5 浅层地热水 197.3 159.9 230.8 421.2 YBJT6 浅层地热水 194.9 158.0 228.0 418.5 YBJT7 浅层地热水 197.1 159.7 229.6 424.4 ZK4001 深层地热水 299.9 236.7 271.4 527.0 表 4 羊八井地热田不同位置的水温
Table 4. Water temperature at different locations of Yangbajing geothermal field
编号 类型 深度(m) 温度(℃) 1 地温计估算的浅热储层 180~280 m,平均230.0 m 158.6 2 代表浅层地热水的地热井底部 平均291.4 m 169.2 3 地温计估算的深热储层 950~1 850 m,平均1 400.0 m 236.7 4 代表深层地热水的ZK4001底部 1 495.0 m 255.0 表 5 饱和水蒸汽压力
Table 5. Saturated water vapor pressure
温度
(℃)绝对压强
(kPa)水蒸汽的密度
(kg·m-3)焓(kJ·kg-1) 汽化热(kJ·kg-1) 液体 水蒸汽 水蒸汽 0 0.61 0 0 2 491.10 2 491.10 50 12.34 0.08 209.34 2 587.40 2 378.10 100 101.33 0.60 418.68 2 677.00 2 258.40 150 476.24 2.54 632.21 2 750.70 2 118.50 200 1 554.77 7.84 852.01 2 795.50 1 943.50 250 3 977.67 20.01 1 081.45 2 790.10 1 708.60 300 8 592.94 46.93 1 325.54 2 708.00 1 382.50 350 16 538.50 113.20 1 636.20 2 516.70 880.50 370 21 040.90 171.00 1 888.25 2 301.90 411.10 374 22 070.90 322.60 2 098.00 2 098.00 0 -
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