Convection - heat transfer coupling mechanism for closed-loop heat extraction from hydrothermal resources using horizontal wells
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摘要: [目的]闭式取热技术只利用地层热量，不抽采地热水，近年来成为地热开发研究的重点领域之一，但关注储层对流对传热影响的研究相对缺乏。 [方法]为了提高单井取热量，提出水热型储层水平井闭式取热工艺，增大井筒与储层的接触面积，且充分利用储层对流。建立水平井、循环水、储层互相耦合的流动传热模型，分储层自然对流、强制对流条件开展取热性能预测，揭示储层对流场、温度场演变特征。 [结果和结论]结果表明：（1）储层发育与水平段垂直的强制对流可有效缓解热突破，对取热提升较为明显；与水平段平行的强制对流与自然对流取热量无明显差异。（2）与上覆地层相比，储层段对循环水温的提升占主导地位，平均每米温度提升是上覆地层的 2.4 倍。（3）储层对流场和温度场演变具有高度耦合特性。当储层只存在自然对流时，温度场变化范围与达西流动活跃区局限于井筒邻近区域，沿径向方向扩展；当储层发育强制对流时，温度场和对流场在水平井筒两侧呈非对称分布，低温区和低达西流速区位于对流下游方向。（4）储层温度动态恢复特征受对流条件控制。强制对流可加速对井筒周围区域热量补给，促进温度恢复，具备长年运行的条件。研究成果为水平井取热系统研究与设计提供借鉴。Abstract: [Objective] Closed-loop heat extraction technology, extracting geothermal energy without draining geothermal water, has emerged as a research focus in geothermal energy exploitation. However, there is a lack of studies on the impacts of convection in reservoirs on heat transfer. [Methods] To improve single-well heat extraction, this study proposed a technology of closed-loop heat extraction from hydrothermal reservoirs using horizontal wells. This technology aims to increase the contact area between wellbores and reservoirs while fully leveraging the convection in reservoirs. A flow and heat transfer model characterized by the coupling of horizontal wells, circulating water and reservoirs was established to predict the heat extraction performance under natural/forced convection in hydrothermal reservoirs and reveal the evolutionary characteristics of the convective and temperature fields in reservoirs. [Results and Conclusions] Key findings are as follows: (1) The forced convection perpendicular to the horizontal section in the reservoirs can effectively alleviate the thermal breakthrough, significantly enhancing the heat extraction efficiency. Minor differences were observed in heat extraction under natural convection and forced convection parallel to the horizontal section; (2) Compared to the overburden, the reservoirs exhibited a predominant temperature increase of circulating water, with the temperature increment per meter being 2.4 times higher than that in the overburden; (3) The evolutionary processes of convective and temperature fields in the reservoirs were highly coupled. In the presence of only natural convection in the reservoirs, the temperature field variation range and the Darcy flow active zone were limited to areas near the wellbore, spreading along the radial direction. In the case where forced convection occurred, the temperature and convective fields were asymmetrically distributed on both sides of the horizontal wellbore, with the lower temperature zone and the lower Darcy velocity zone located in the downstream direction of the convection; (4) The dynamic temperature recovery of the reservoirs were governed by convection conditions. Forced convection can promote temperature recovery by accelerating the heat supply to areas around the wellbore, creating favorable conditions for long-term operation. The results of this study provide a reference for the research and design of a heat extraction system using horizontal wells.
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