增强型地热系统采热性能评价——以共和盆地恰卜恰地区干热岩储层为例

Evaluation on heat extraction performance of enhanced geothermal system:A case study of hot-dry rock reservoirs in the Qiabuqia area of the Gonghe Basin

增强型地热系统(EGS)是从干热岩储层中提取热能的有效手段,准确评价其采热性能对于提高系统的取热效率、延长取热寿命等具有重要意义。为此,以共和盆地恰卜恰地区干热岩储层为研究对象,基于COMSOL Multiphysics建立了表征EGS水热耦合的数值模型,采用整体间接耦合的方式,分别对注入井井筒、含裸眼段裂隙网络热储层及采出井井筒进行瞬态流动传热耦合模拟,并对比分析了裂隙渗透率等参数对EGS采热性能的影响。研究结果表明:(1)不考虑采出井井筒部分热损失,将导致开采初前期采出温度被高估4%~5%,不考虑裂隙网络储层流量与热量损失,将导致对采出流量与热提取速率值的评价过高;(2)增大注入流量,会使EGS维持一段时间高温高热提取率开采,但将导致储层寿命降低,提高生产压力则会降低热提取速率并增大流动阻抗,不利于热开采;(3)增大裂隙渗透率虽然能提高热提取速率,但会降低储层开采寿命;(4)当裂隙开度处于细微变化时,对系统采热效果的影响较小。结论认为,采用分段式间接耦合模拟的方式可以实现对EGS的完整性评价,进而为高温热储层商业化开采提供理论及实践依据。

An enhanced geothermal system(EGS) is an effective means to extract heat from hot-dry rock(HDR) reservoirs,and the accurate evaluation of its heat extraction performance is of great significance in improving its heat extraction efficiency and extending its heat extraction life.Taking the HDR reservoirs in the Qiabuqia area of the Gonghe Basin as the research object,this paper establishes a numerical model based on COMSOL Multiphysics for characterizing EGS hydrothermal coupling.Then,transient flow and heat transfer coupling simulation is carried out on the injection wellbore,the fracture network thermal reservoir with open hole and the production wellbore,respectively,by means of overall indirect coupling.Finally,the influence of permeability and other parameters on the heat extraction performance of EGS is comparatively analyzed.And the following research results are obtained.First,if the heat loss in the production wellbore is not considered,the production temperature will be overestimated by 4% to 5% in the initial and early stages of production.Disregarding the flow and heat loss in fracture network reservoirs will lead to overestimation of production flow rate and heat extraction rate.Second,increasing the injection flow rate will keep EGS operating at high-temperature and high-heat extraction rates for a period of time,but it will reduce the reservoir life.When the production pressure is increased,the heat extraction rate will decrease and the flow resistance will increase,which is not conducive to heat extraction.Third,the increase of fracture permeability can improve the heat extraction rate,but it will reduce the reservoir production life.Fourth,when being slightly changed,fracture opening has less influence on the heat extraction effect of the system.In conclusion,the segmented indirect coupling simulation can achieve the integrity evaluation of EGS,so as to provide a theoretical and practical basis for the commercial extraction of high-temperature thermal reservoirs.