FixAl及同位素方法在EGS返排液研究中的应用

The application of FixAl and isotopic methods in the study of flowback fluids from Enhanced Geothermal Systems(EGS)

增强地热系统(EGS)是开采低渗透率热岩体中热能的技术,属于广义的地热储工程。其中,作为换热介质被注入岩体并在换热后返回地表的返排液,不仅是岩体地球化学特征的信息载体,而且其物理化学行为直接影响着EGS系统的运行效果。FixAl化学热力学模拟和水同位素十三线图解在天然水热系统评价中得到了广泛应用,对返排液研究的实用性则是文章的核心问题。文中收集了全球主要EGS项目的返排液资料,基于FixAl方法分析矿物与返排液的化学平衡状态,并计算了流体在深部的热交换温度,用同位素模型验证了EGS系统中原生卤水的驱替过程。研究结果表明,上述方法在EGS返排液研究中是适用的。此外,返排液的化学特征对EGS的指示意义还包括厘定原生卤水在返排液中所占比例,识别岩浆挥发分溶解及储层改造时的添加剂残留,预测结垢趋势和流体腐蚀性等。未来需要通过更多的实验和模拟方法深入研究返排液的化学特征,建立EGS的热-水-力-化学(THMC)耦合模型,为科学开发深层地热能提供依据。

Enhanced Geothermal Systems(EGS)is an emerging technology for exploiting the geothermal resource in hot rocks with low permeability.The flowback fluid injected into the deep reservoir to extract heat,reflects not only the physico-chemical properties of the reservoir but also efficiency of the geothermal resource exploitation.FixAl method and oxygen isotope thermometry have been used widely in natural geothermal systems.In this study,we focused on the applicability of these two methods in EGS flowback experiment.Based on flowback fluids data collected from typical EGS around the world,we evaluated chemical equilibrium states of minerals and flowback fluids and calculated underground fluids thermal exchange temperatures using FixAl method.Using isotope model we verified the mixing and displacement processes of brine.Our studies validated the usefulness of these two methods in EGS research.Moreover,we showed that the chemical characteristics of flowback fluid can be useful in determining brine content of flowback fluids,identifying dissolution of magmatic volatiles and additive residues after reservoir renovation,and predicting scaling trend or corrosiveness of fluids.Future work should focus on in-depth studies of chemical properties of flowback fluids through experiments and modeling,so as to establish a scientific evaluation model for deep geothermal resource extraction.