Numerical Modelling of Stable Isotope Transport Processes in a Hydrogeothermal System of Kangding-Laoyuling Area
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摘要: 以康定老榆林地区地热系统为研究对象，利用TOUGH-Isotope程序进行水-热-同位素耦合数值模拟，并鉴于研究区大气降水氢氧同位素季节性明显、地震活动活跃，探讨了补给水同位素特征、热储层渗透性变化对地热系统氢氧同位素迁移过程的影响.研究结果表明氢氧同位素模拟值与研究区ZK3钻孔流出水测试值基本拟合，高温地热系统的对流-弥散作用对氢氧同位素迁移过程影响明显；研究区地热水循环条件较好，水-岩作用程度较低，导致氧同位素重化现象不明显；补给水同位素特征、热储层渗透性两个因素对地热水循环过程中的氢氧同位素分布具有明显的影响.开展地热系统流体氢氧同位素迁移过程研究，有助于提高对地热系统动态演化的定量化认识，为地热开发提供支持.Abstract: In Laoyulin geothermal system of Kangding, we uses TOUGH-Isotope program to conduct the numerical simulation of water-heat-isotope coupling. In view of the obvious seasonality of stable isotopes in precipitation and active seismic activity in the research area, the impact that the isotope characteristics of recharge water and thermal reservoir permeability variation on the transport of hydrogen and oxygen isotopes in the geothermal system also are explored in this paper. The results indicate that the calculate values of hydrogen and oxygen isotopes of the hot water are generally with the measured values in ZK3 borehole, and the convection-dispersion effect of the high-temperature geothermal system has a significant effect on the migration process of hydrogen and oxygen isotopes. The geothermal water circulation conditions in the study area are strong, which causes the phenomenon of oxygen isotope enrichment to be slight. In the geothermal water circulation, the two factors-isotope characteristics of recharge water and thermal reservoir permeability have significant effects on the distribution of hydrogen and oxygen isotopes. Therefore, the numerical modelling of stable isotope transport processes in the geothermal system fluid can help to improve the quantitative understanding of the dynamic evolution of the geothermal system, which will provide support for geothermal development.
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图 2 老榆林地热系统地质概况
Fig. 2. Conceptual model of geothermal flow along the Laoyulin geothermal system
图 5 模型达到稳定时温度场情况(℃)
Fig. 5. Temperature distribution in the model as the simulation progresses to a steady-state
图 6 模型排泄区氢氧同位素分布的计算
Fig. 6. Variation of calculated (a) δD and (b) δ18O in the discharge zone, which is comparable to the measurements in geothermal water collected in well ZK3
图 7 地热系统运行到稳定状态的氢氧同位素分布特征
a. 地热系统运行8 000 a δD值分布；b. 地热系统运行30 000 a δD值分布；c. 地热系统运行8 000 a δ18O值分布；d. 地热系统运行30 000 a δ18O值分布
Fig. 7. δD and δ18O evolution in the model as the simulation progresses to a steady-state
图 8 方案1和方案2氢氧同位素分布特征
a. 方案1地热系统δD值分布；b. 方案2地热系统δD值分布；c. 方案1地热系统δ18O值分布；d. 方案2地热系统δ18O值分布
Fig. 8. δD and δ18O distribution in Case 1 and Case 2
图 9 方案1和方案2排泄区氢氧同位素的计算值
Fig. 9. Variation of calculated (a) δD and (b) δ18O in the discharge zone Case 1 and Case 2
图 10 方案3和方案4温度、氢氧同位素分布特征
Fig. 10. Distribution of temperature, δD and δ18O in Case 3 and Case4
图 11 方案3和方案4排泄区氢氧同位素的计算值
Fig. 11. Variation of calculated (a) δD and (b) δ18O in the discharge zone Case 3 and Case 4
表 1 实验获取的模型方程及相关常数设置
Table 1. Experimental equations and parameter setting in the model
实验获取的方程 相关系数取值 主要参数计算值(K)
278.15 < T < 573.15参考文献 $ \begin{array}{l}{10}^{3}\mathrm{l}\mathrm{n}{\alpha }_{l-v}=\frac{a{T}^{3}}{{10}^{9}}+\frac{b{T}^{2}}{{10}^{6}}+\frac{cT}{{10}^{3}}\\ +d+\frac{e{10}^{9}}{{T}^{3}}\end{array} $ a=1 158.8, b=-1 620.1, c=794.84, d=-161.04, e=2.999 2
for HDO0.905 7~0.996 6, - Horita and Wesolowski(1994) $ {10}^{3}\mathrm{l}\mathrm{n}{\alpha }_{l-v}=\frac{{10}^{9}a}{{T}^{3}}+\frac{{10}^{6}b}{{T}^{2}}+\frac{{10}^{3}c}{T}+d $ a=0.350 41, b=-1.66 64, c=6.712 3, d=-7.685 0
for H218O0.988 8~0.997 6, - $ D={10}^{-9}{a}_{i}\mathrm{e}\mathrm{x}\mathrm{p}(\frac{a}{{T}^{2}}+\frac{b}{T}+c) $ a=-535 400, b=1 393.3,
c=2.187 6,
ai=0.983 3
for HDO
ai=0.966 9
for H218O1.29×10-9~19.50×10-9 for HDO, +
1.27×10-9~19.20×10-9 for H218O, +Braud et al.(2005); Merlivat(1978); Shurbajia et al.(1995) $ \mathrm{l}\mathrm{o}\mathrm{g}\left({k}_{rl}\right)=\mathrm{l}\mathrm{o}\mathrm{g}\left({A}_{0}\right)+{E}_{a}/2.3RT $ A0=9.0×10-15 1/s,
Ea=200 kJ /mol,
R=8.314 56 kJ/(mol·K)
for H218O and granite6.78×10-13~7.09×10-13, - Cole and Chakraborty(2001) 
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