Geothermal reservoir reinjection: An experimental and numerical study using a large-scale 3D sand tank

• A large-scale sand tank model is applied to mimic a sandstone geothermal reservoir. • Reinjection is conducted for investigate the hydraulic and hydrothermal mechanisms. • The numerical model is built for the experimental sand tank model. • Optimizing of production considering well spacing, porosity, and permeability. Long-term and continuous exploitation of hydrothermal systems will cause a drop in groundwater level and ground subsidence. Successful reinjection is important to maintain the hydraulic head of the geothermal reservoir and to yield a sustainable utilization of geothermal energy. This study aims to investigate the reinjection mechanism by combining hydraulic and hydrothermal tests in a large-scale three-dimensional sand tank model (12 m × 6 m × 6 m). We conducted the experiments to observe the hydraulic head and temperature evolution during water reinjection at normal and heated temperatures. The experiment results are compared with the numerical results by using the SHEMAT-Suite software. The trends of hydraulic head and temperature changes match well in general, whereas heat loss due to ambient temperature and physical mechanisms such as particle rearrangement may cause differences between the experimental and numerical results. We optimize reservoir output with respect to the well spacing, porosity, and permeability of the model. Increasing porosity and permeability results in a higher geothermal energy output and better economic benefits. Meanwhile, for the well spacing, it is preferable to have a short spacing in low porosity and permeability models and a long spacing in high porosity and permeability models. The study helps understand the mechanism of reinjection in geothermal reservoirs and optimize the sandstone geothermal exploitation management strategy.