Sustainable geothermal yield through effective reinjection: reservoir engineering case study of the Dezhou sandstone geothermal reservoir, North Shandong Plain, China

Abstract Sustainable utilization of sandstone geothermal resources is a worldwide challenge due to limited natural recharge and rapid aquifer clogging associated with reinjection of return water back into reservoirs. In the Dezhou area of the North Shandong Plain (NSP), China, technical solutions for successful reinjection have been developed and applied based on nearly 30 years of production experience from the Guantao geothermal reservoir. These solutions include drilling large-diameter reinjection wells, filtration systems, oxygen-free configuration, back-pumping, and acidification. To further investigate the reservoir properties, a pumping test, three reinjection tests, and a tracer test were carried out in a dedicated experimental field setup in Dezhou, which included one regular diameter production well and one large diameter reinjection well. This study integrates effective reinjection technologies with long-term sustainability assessment, providing a comprehensive framework for managing low-temperature sandstone geothermal systems. Based on long-term water-level monitoring data and lumped parameter modelling, a sustainable yield assessment of the reservoir in the urban Dezhou area has been performed, with the deepest permissible water level at 150 m below ground surface over a 100-year time frame. The results suggest that reinjection is the dominant factor influencing sustainability. When the reinjection rate approaches 90%, the sustainable yield corresponds to an average value of 1300 L/s during the space heating period and 500 L/s annually. Using volumetric energy balance calculations, the average thermal energy loss over 100 years of reinjecting cooled return water is estimated to constitute 3% and 8% of the total energy stored relative to the volumes of the closed and open lumped parameter models, respectively. This indicates that the cooling assessment of reinjection should further concentrate on the cooling distribution in the reservoir and the decrease in reservoir temperature in particular locations. This study emphasizes the importance of integrating reinjection strategies with sustainability assessments and highlights the need for continued long-term geothermal resource management research for porous sandstone reservoirs.