Objective Closed-loop heat extraction technology, which extracts 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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