增强型地热系统中液-岩化学作用数值模拟研究

A Numerical Study on the Effect of Fluid-Rock Reaction during Enhanced Geothermal System Heat Extraction Processes

增强型地热系统(Enhanced Geothermal System,EGS)利用深层岩石中连通的裂隙网络进行流体工质循环,从而实现地热能的持续开采。EGS运行时循环流体工质会与深层岩石产生化学反应,引起岩石中矿物的溶解/沉积,使热储中的裂隙网络形貌产生动态变化,对地下流动与传热过程造成影响。本文分析了EGS中液–岩化学作用特点,详细阐述了在多孔介质热流动模型中耦合入液–岩化学反应的方法,基于已开发成功的EGS传热传质数值模型初步建立了传热–流动–化学(Thermal-Hydraulic-Chemical,THC)多场耦合数值模型,并使用该模型对五井布局EGS的长期运行过程进行了模拟分析,模拟时仅考虑方解石在水流体中溶解和沉积。模拟结果显示,循环流体的注入温度以及注入流体中的矿物离子浓度的设定十分重要。如果二者没有达到较为合适的"平衡",就会导致注入井附近渗透率和孔隙率的持续变化,对EGS的导流能力造成极大影响。

The enhanced geothermal system(EGS) circulates heat transfer fluid in the fracture network in the heat reservoir to extract heat from earth-deep hot dry rocks. During EGS operation, the heat transfer fluid may react with the rock in the heat reservoir. The dissolution/deposition of rock minerals dynamically changes the structure and morphology of fracture network in the reservoir, affecting the heat extraction process. This paper analyzes the characteristics of fluid-rock reaction in EGS reservoir, and details the method of coupling the fluid-rock reaction with thermo-fluid flow in porous medium, an equivalent model to the fractured rock reservoir. A Thermal-Hydraulic-Chemical(THC) model is established upon a previous numerical model developed by our group for heat and mass transport during EGS heat extraction processes. The new model is employed to study the long-term heat extraction process with respect to a quintuplet EGS(one injection well plus four production wells). Only interactions between water and calcite are considered in the simulation. The simulation result indicates the setting of injection temperature and mineral concentrations is crucial. If the two conditions cannot reach equilibrium state, the permeability and porosity around injection well will continually change, which will significantly affect the flow resistance of EGS reservoir.