工质密度和储层渗透率演化对EGS系统传热效能的影响

Evolution Effects of Working Fluid Density and Reservoir Permeability on Heat Transfer Efficiency of EGS System

工质进入地热储层后的流动和采热问题是典型的热流固(THM)耦合问题。工质的密度、地热储层的渗透率在热开采过程中受THM耦合作用影响,导致地热储层传热效能的变化。为了探究这一变化,基于等效多孔介质模型建立了THM完全耦合的二维有限元模型和计算程序,模拟了增强地热系统(EGS)长期热开采过程,分析了岩石温度、孔压、储层变形的演化规律,研究了密度、渗透率演化对EGS采热性能的影响。结果表明:在温度、地应力、孔压等因素中,温度是稳定开采阶段对流体密度演化和渗透率演化影响最大的因素。但对于最终储层的传热效能而言,密度演化延缓了储层出口温度的冷却速度,提高了净热提取率;而渗透率演化虽然加速了出口温度的冷却,也提高了净热提取率。研究丰富了斜压流体THM耦合模型的应用,为EGS地热开采和储层管理提供了重要参考。

Flow and thermal extraction problem of working fluid entering geothermal reservoirs is a typical thermo-hydro-mechanical(THM)coupled problem.The density of working fluid and the permeability of geothermal reservoirs have evolved under the influence of THM coupling during flow and heat extraction in the thermal reservoir,which leads to changes in the heat transfer efficiency of geothermal reservoirs.In this work,a two-dimensional fully THM coupled FEM model is established to simulate a long term enhanced geothermal system(EGS)thermal extraction process.The evolution pattern of reservoir displacement,pore pressure,temperature is analyzed,and the evolution effect of working fluid density and reservoir permeability on heat transfer efficiency of EGS system is also investigated.The results show that among the factors of temperature,confining pressure and pore pressure,temperature is the most important influential factor on the evolution of fluid density and permeability during stable extraction stage.However,for the final heat transfer efficiency of reservoir,density evolution delays the cooling rate of the reservoir outlet temperature and improves the net heat extraction rate.While permeability evolution accelerates the cooling of the outlet temperature,it also improves the net heat extraction rate.