Evolution and response mechanism of geo-temperature field in vertically heterogeneous sandstone geothermal reservoirs to reinjection of cooled geothermal water

Accurately characterizing the vertical heterogeneity of sandstone geothermal reservoirs and investigating the evolution of the geo-temperature field under doublet technology, which consists of a closed loop with one production well and one reinjection well in a sandstone geothermal reservoir, are of great significance for the sustainable development and utilization of geothermal resources. In this paper, a vertically heterogeneous hydro-thermal coupling numerical model is established based on long-term dynamic monitoring data of water level depth and long-term, full-wellbore temperature dynamic monitoring. The evolution and trends of the geo-temperature field are studied, and the production and reinjection parameters of the doublet technology are optimized. The influences of the stratification characteristics and vertical heterogeneity of the permeability of the sandstone geothermal reservoir on the results are discussed. The results show that the maximum depth of the funnel-shaped regions reaches 102 m below the reservoir floor and 85 m above the roof. When the production and reinjection flow rate (Q) of the doublet system is set to the conventional value of 70 m3/h, the optimal distance (R) between the production and reinjection wells should be at least 350 m. The thermal breakthrough time (t) of the production well increases as a power function with increasing distance (R), following the regression equation tQ = aR2.6. The analysis further indicates that the outlet temperature of the production well increases with the number of the sandstone aquifers. However, when the number of the sandstone aquifers exceeds three, the impact of additional strata (aquifers) on the thermal breakthrough time of the production well becomes relatively small. An increase in the vertical heterogeneity of the permeability causes the thermal breakthrough time to decrease rapidly. For heterogeneous reservoirs, the thermal breakthrough time can be estimated by applying a reduction factor to the results derived for homogeneous reservoirs; this factor follows a power function of the vertical permeability variance (σ2).