干热岩热能的热管开采方案及其技术可行性研究

Mining Hot Dry Rock Geothermal Energy by Heat Pipe: Conceptual Design and Technical Feasibility Study

常规增强型地热系统(EGS)通过流体工质在裂隙热储中的循环流动来开采岩石中的热能,需要消耗大量的泵功,存在工质流失、管道腐蚀结垢等问题,而且常常由于裂隙网络的井下连通性不够造成EGS建设的失败。为避免这些问题,本文首次提出采用热管来提取干热岩热能的技术方案。为强化热管蒸发段与周围岩石的传热,特别提出在目标热储内充填CO_2流体工质、借助其自然对流强化干热岩热能开采,进一步通过数值仿真及理论分析探讨该方案的技术可行性。数值模型考虑了超临界CO_2的变物性,研究了其在目标热储内的自然对流对热管采热的影响,模拟并比较了在不同条件下热管的采热速率。结果表明,调低热管的采热温度可以显著提高热管采热速率,并且当热储渗透率大于1×10^(-9)m^2时CO_2的自然对流作用会明显提升热管采热速率。本文还比较了热管加热段水平布置与竖直热管的性能差异,发现前者可以提高1.5~2.3倍的采热速率。另外,对该系统中水工质重力热管的携带极限以及管内蒸汽流阻进行了分析探讨。

In conventional enhanced geothermal systems （EGS）, heat is extracted from earth-deep by circulating fluid through fractured-rock heat reservoir. This process generally consumes large amount of pump work, and has problems like fluid loss and pipe scaling etc. Moreover, it often encounters failure due to the bad downhole connectivity of fracture network in the reservoir. To circumvent these problems, conceptual design about mining heat from earth-deep hot dry rock by heat pipe is presented for the first time. Hoping to have strong natural convection of CO2 in the reservoir to enhance the heat transfer between rock and heat pipe, filling the reservoir with CO2 fluid is specially proposed. Further, numerical modeling and theoretical analysis are performed to explore the technical feasibility of the new HDR heat extraction concept. The numerical model considers CO2 of temperature- and pressure- dependent thermophysical properties. Effects of natural convection of CO2 in the target reservoir on the heat extraction by heat pipe are studied; heat extraction rates by different conditioned heat pipes are compared. The results indicate that lowering the working temperature of heat pipe can significantly increase the heat extraction rate and notable increase at the heat extraction rate will be caused by the natural convection of CO2 if the reservoir permeability is higher than 1 × 10 9 m2. It is found from numerical results also that an increase of 1.5 - 3 times at the heat extraction rate can be achieved if the heating section of the heat pipe is arranged horizontally compared to all vertically aligned heat pipe. Additionally, the carrying limit and steam flow resistance for water thermosyphon are analyzed numerically under realistic geothermal conditions.