CO2单井增强地热系统取热性能研究

为提高闭式单井系统取热性能，提出一种CO2单井增强地热系统（CO2-SEGS）。建立井筒流动换热和储层热-流-固耦合的数学模型，考虑CO2可压缩性以及井纵向压力传递特性，对比分析了水和CO2在SEGS中的取热性能，研究系统取热性能与封隔间距、井筒保温的关系。结果表明：（1）额定循环流量下，井口生产温度从134.09℃降低至116.06℃；CO2在采出过程中降压膨胀做功，产生明显的温降效应，中心管井口温度比底部低约57℃。（2）井筒不同位置处CO2的密度、热容差异很大，当循环流量小于50 kg/s时，依靠浮升力作用，SEGS可实现自主循环运行，无需额外泵功。（3）当水和CO2的流量分别为15 kg/s和40 kg/s时，两者年均取热速率近似相等，CO2的采出温度比水低约40℃，而压力损耗远小于水。（4）SEGS取热性能与封隔间距以及中心管保温性能呈正相关。研究结果可为SEGS系统的开发提供参考。

Heat Extraction Performance of CO2 Single-Well Enhanced Geothermal System

A CO₂ single-well enhanced geothermal system (CO₂-SEGS) was proposed to improve the heat transfer capacity. A mathematical model of a coupling of the wellbore and reservoir was developed, and CO₂ compressibility and longitudinal pressure transfer were considered. The heat extraction performance of water and CO₂ in SEGS was compared and analyzed to study the relationship between heat extraction performance and sealing spacing and well tube insulation. The following research results were obtained. First, the wellhead production temperature decreased from 134.09^oC to 116.06^oC under the rated injection flow, and CO₂ expanded to do work with the drop of its pressure in the extraction process so as to generate a remarkable temperature drop effect with a temperature drop of about 57^oC. Second, the density and heat capacity of CO₂ varied greatly at different locations of the well tube. When the injection flow was less than 50 kg/s, SEGS can achieve independent circulation through buoyancy without additional pump power. Third, when the flow rates of water and CO₂ were 15 kg/s and 40 kg/s, respectively, their average output power was approximately equal, the production temperature of CO₂ was about 40^oC lower than that of water, while the pressure loss was much less than that of water. Fourth, SEGS heat extraction performance was positively correlated with the sealing spacing and the insulation performance of the central pipe. The results can be used to guide the development of SEGS.