典型高温地热系统水热循环及锂同位素分馏过程模拟研究

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

水热系统中的多场耦合相互作用对水热循环过程和地热流体化学成分具有显著的影响。本文利用COMSOL-Multiphysics建立水热化学(同位素)多场耦合数值模拟模型,通过简化模型对锂同位素分馏模拟方法进行了验证。在此基础上,基于对羊八井地热田水热循环过程的认识,建立了羊八井典型剖面水热循环多场耦合模型;再现了羊八井地热系统的水热循环过程和水岩反应过程中的锂同位素分馏过程,并且讨论了主要的参数对热能聚敛效果的影响。结果表明:较高的断裂带渗透率将加快深部断裂带附近围岩温度的下降,而较低的渗透率则无法在近地表形成一定规模的水热活动;通过地表排泄量对断裂带渗透率进行约束后,发现当断裂带与深部熔融体直接沟通时,才可在近地表形成长期存在(近150 ka)的高温地热显示;在断裂带沟通了深部熔融体且熔融体热源温度不变的前提下,熔融体的具体埋深对水热活动强烈程度的影响不大;长期的水岩活动会使断裂带内锂元素大量消耗,只有当深部熔融体为断裂系统提供持续不断的物质来源时,才能保证地热流体中长期保持较高的锂元素浓度水平;基于锂同位素分馏过程估算出参与水岩反应的岩体中锂元素质量分数为25~35 mg/kg,δ^(7) Li_(rock)为-2.0‰~0.5‰。研究成果有助于进一步认识典型高温地热系统成因机制模式。

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~2535 mg/kg,and the value ofδ^(7) Li_(rock) was~-2.0‰0.5‰.The research results contribute to the further understanding of the formation of typical high-temperature geothermal systems.