Numerical simulation of hydrothermal cycling process and lithium isotope fractionation in a typical high-temperature geothermal system

doi:10.13745/j.esf.sf.2024.7.12

Abstract

Abstract:

Multifield coupling interactions have significant effects on hydrothermal cycles and geothermal fluid chemistry in hydrothermal systems. In this paper, a hydraulic-thermal-chemical (isotope) multifield numerical simulation model is developed using COMSOL-Multiphysics, and the simulation method for lithium isotope fractionation is validated by a simplified profile model. On this basis, a multifield coupling model of the hydrothermal cycle in a typical profile of Yangbajing is established based on the understanding of the hydrothermal cycling process in the Yangbajing geothermal field. Futher, the hydrothermal cycling of the Yangbajing geothermal system and lithium isotope fractionation under water-rock reactions are reproduced, and the influence of the main model parameters on the effect of thermal energy convergence is discussed. The results indicated that high fracture-zone permeability accelerated temperature decline in wall rock near the deep fracture zone, while low permeability limited near-surface hydrothermal activity. After constraining the fracture-zone permeability by surface drainage, it was found that long-lived (nearly 150 ka) high-temperature geothermal features could form near the surface, but only when the fracture zone made direct contact with the deep melt. Provided that such contact occurred and the temperature of the melt heat source remained constant, the depth of the melt had little effect on hydrothermal activity. Prolonged water-rock interactions could lead to significant lithium depletion in the fracture zone, and only when the deep melts provided a continuous source of material for the fracture system could it guarrantee sustained high lithium concentration in geothermal fluids. Based on the lithium isotope fractionation process, the estimated mass fraction of lithium in the involved rocks was ~25—35 mg/kg, and the value of δ⁷Li_rock was ~-2.0‰—0.5‰. The research results contribute to the further understanding of the formation of typical high-temperature geothermal systems.

Key words: typical high-temperature geothermal system, hydrothermal cycle, water-rock reaction, multifield coupling model, lithium isotope

CLC Number:

P314.1
P597

SHI Honglei, WANG Wanli, WANG Guiling, XING Linxiao, LU Chuan, ZHAO Jiayi, LIU Lu, SONG Jiajia. Numerical simulation of hydrothermal cycling process and lithium isotope fractionation in a typical high-temperature geothermal system[J]. Earth Science Frontiers, 2024, 31(6): 104-119.

Figures/Tables 19

Fig.1 Locations of the study area and typical profiles

Fig.2 Tectonic setting of the Nyenching Tanggula granite intrusion and extension. Modified after [18-19].

Fig.3 Simplified model geometry and grids in the validation of isotopic fractionation

Fig.4 Variation of lithium concentrations under different pre-exponential factors

Fig.5 Variation of lithium concentrations in rocks and fluids at the model exit

Fig.6 Lithium isotopic compositions in fluids and rocks at the model exit

Fig.7 Typical Yangbajing profile model geometry and grids

Table 1 Geodetic heat flow data for the study area and adjacent area

测点位置	井号	线性增温段深度/m	K/(W·m^-1·K^-1)	Q/(mW·m^-2)	热流类型
拉萨北郊	Geoth-1	240~600	2.711	66	传导型
马区	Geoth-2	460~530	2.972	106	传导型
羊八井	ZK308	1 030~1 410	2.320	108	传导型
羊应乡	ZK201	140~190	1.578	364	对流传导型
拉多岗	ZK203	70~140	2.712	338	对流传导型

Table 2 Model parameters

参数	基底孔隙率/%	断裂带孔隙率/%	流体密度/ (kg·m^-3)	岩石密度/ (kg·m^-3)	流体黏度/(Pa·s)
取值范围及计算函数	1	10~30	ρ(T,TDS)	2 600	μ(T,TDS)
参数	基底渗透率/m²	断裂带渗透率/m²	岩石热导率/ (W·m^-1·K^-1)	水的热导率/ (W·m^-1·K^-1)	水的比热容/ (J·kg^-1·K^-1)
取值范围及计算函数	k(z)	10^-15~10^-13	2.5	0.56	4 200
参数	岩石比热容/ (J·kg^-1·K^-1)	放射性生热率/ (μW·m^-3)	熔融体温度/℃	熔融体埋深/km	底部边界热流/ (mW·m^-2)
取值范围	750	1.6	600	6~10	66
参数	反应活化能/ (kJ·mol^-1)	化学反应常数/s^-1	摩尔气体常数/ (kJ·mol^-1·K^-1)	岩石锂元素质量分数/ (mg·kg^-1)	岩石锂同位素组成 δ⁷Li/‰
取值范围	80	0.01	8.314 56	20~35	-3.5~0.5

Fig.8 Baseline geothermal field in the study area by modeling

图a Deep hydrothermal cycling processes under different fracture-zone permeability

Fig.10 Geothermal reservoir temperature variations under different fracture-zone permeability

Fig.11 Variations in surface discharge under different fracture-zone permeability

Fig.12 Deep hydrothermal cycling processes under different fracture depths

Fig.13 Geothermal reservoir temperature variations under different fracture depths

Fig.14 Hydrothermal cycling processes under different melt burial depths

Fig.15 Geothermal reservoir temperature variations under different melt depths

Fig.16 Variations of lithium concentrations in the discharge area under different porosity

Fig.17 Lithium concentrations and δ7Li values in geothermal fluids in the discharge area

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