Demonstrating the Technical Feasibility of Deep Borehole Heat Exchange in High-Salinity Geothermal Resources: A 3000 m Field Case in the Xining Basin

The development of high-salinity geothermal resources is hindered by corrosion, scaling, and high water treatment costs, posing severe challenges for conventional hydrothermal systems. Deep borehole heat exchanger (DBHE) technology, which extracts heat without water production and avoids contact with high-salinity fluids, offers a promising alternative. In this study, field experiments on a coaxial DBHE were conducted in well SQ-1 (over 3000 m deep) in the Xining Basin, targeting its dual structure: a shallow high-salinity aquifer and a deep high-temperature low-permeability basement. Results show that the deep Proterozoic metamorphic basement has extremely low permeability (10−8 cm/s), with a bottom hole temperature of 113 °C and an average geothermal gradient of 3.39 °C/100 m, confirming it as a stable solid heat source. Under steady-state operation, the heat extraction rate averaged 150 W/m (144–155 W/m, with an uncertainty of approximately ±3 W/m). A marginal effect of flow rate was observed: increasing flow from 40 to 50 m3/h increased heat extraction by only 1.5%, indicating a threshold flow-rate range beyond which continued flow increases yield diminishing returns. No groundwater extraction, corrosion, or scaling was observed during the 13-day test, confirming the short-term operational reliability of the system under the tested conditions. This study validates shifting heat extraction to deep low-permeability basements to avoid high-salinity issues, providing a scientific basis for clean geothermal utilization in similar regions globally.