工质变物性对EGS热开采过程影响的数值模拟

Effects of Variable Properties of Heat Transmission Fluid on EGS Heat Extraction:A Numerical Study

增强型地热系统(EGS)热开采过程中循环工质的温度和压力会经历较大范围的变化,这会造成循环工质的热物性变化,从而影响流体工质的输运和岩石-流体热交换;数值模拟EGS热开采过程,预测EGS的寿命、出力等性能指标有必要考虑循环工质的热物性变化。笔者在EGS热开采过程三维数值模拟中考虑水和超临界二氧化碳的变物性,实现了热流双向耦合。针对水EGS分析了各物性变化对EGS采热性能的影响,并对变物性条件的水和超临界二氧化碳EGS的采热性能进行了对比研究。结果表明:工质在密度影响下开采寿命为9.0a在密度和比定压热容共同影响下的开采寿命为7.5a,说明密度和比定压热容越大则EGS开采寿命越短;在黏度系数影响下的开采寿命为18.0a,说明黏度系数越大则EGS开采寿命越长;导热系数则对EGS采热性能无明显影响。注入压力一定的条件下以水为工质的EGS具有较长寿命,但相同时刻的质量流率和热开采率低于以临界二氧化碳为工质的EGS。

The large changes to the temperature and pressure associated with EGS heat exploitation will lead to pronounced changes to the thermo-physical properties of the heat transmission fluid,which will in turn affect the fluid flow and heat transportation inside the EGS subsurface system.It is necessary to establish a variable thermo-physical property EGS model to simulate the EGS heat extraction process and to predict the performance of EGS including its lifetime and capacity.The present work is extended to a previously developed three dimensional EGS heat extraction model with considering the local thermal non-equilibrium between the rock matrix and fluid flowing in the fractures in the porous reservoir,by introducing a module modeling with the property variation of water and supercritical carbon dioxide（SCCO2）.The model with fully coupled thermal and hydraulic actions isused to investigate the impacts of thermo-physical properties on the water-EGS heat extraction.It is found that the lifetime of the EGS is 9.0aunder the density effects and 7.5aunder the specific heat capacity effects,indicating that the larger the density and the specific heat capacity of the working fluid possessed are,the shorter the EGS＇s lifetime is.Under the viscosity effects,the lifetime of the EGS extends to 18.0a,meaning that the larger the viscosity of the working fluid is,the longer the EGS can be operated.However,the thermal conductivity of working fluid hardly has any effect on the EGS performance.Specially,we compare the heat extraction performance of water-EGS and SCCO2-EGS.Under a fixed injection pressure,the lifetime of water-EGS is longer than that of SCCO2-EGS;but the extraction ratio of the former is lower than the latter at the same time instant mainly due to the much higher mass flow rate of the latter in the EGS operation.