To study the evolution rules and behaviors of heat transport in a sandstone geothermal reservoir caused by cooled water reinjection, this research focuses on the quantitative relationship among reinjection parameters and the thermal breakthrough time of production wells. The permeation, tracer, and reinjection tests were conducted in a simulation model using a large sand tank in conjunction with the numerical simulation method based on COMSOL Multiphysics. Subsequently, sensitivity analysis and nonlinear fitting were performed to investigate the effects of fluid viscosity and density on the reinjection process, and to analyze the impact of reinjection parameters on the thermal breakthrough time of production wells, along with their underlying mechanisms and law. The results indicate that the migration velocity of reinjection water is greater in coarse sand layer compared to that in medium sand layer, and the thermal breakthrough time t is linearly correlated with reinjection rate (Q) raised to the power of − 0.85, temperature difference (ΔT) raised to the power of − 0.21, and spacing between the production and reinjection wells (R) raised to the power of 1.4. The correlation equation and analysis show that when the temperature difference between production and reinjection ΔT is more than 30 ℃, the influence of ΔT on the thermal breakthrough time of production well becomes weak, because ΔT exerts an effect on the thermal breakthrough time of production well t by influencing the relative position of the 18.5 ℃ isotherm in the temperature transition region. The error in reinjecting high-temperature fluid into low-temperature fluid may be corrected by introducing a viscosity correction coefficient α_μ.

There are 1,000 geothermal wells with depths of 1500–2000 m drilled into the upper Tertiary sandstone geothermal reservoir in the Guantao Formation in the North Shandong Plain, which support about 61 million square meters of domestic space heating. Concerning the large-scale exploitation of the Guantao Formation geothermal reservoir and its important role in clean space heating, it is essential to fully understand the characteristics of the geotemperature, geothermal water flow, and hydrochemical features, especially the heat accumulation and geothermal water enrichment mechanism. Based on the data from pumping tests, temperature logging, water level measurements, and hydrochemistry and isotope sampling and analysis, in this study, the heat sources and accumulation, water sources, and enrichment mechanism of the sandstone geothermal reservoir in the Guantao Formation were explored. The geothermal gradient of the cap rocks and the reservoir’s temperature revealed that the geothermal temperature fields are controlled by fault structures, and the relative high temperature zones are distributed in the area where the uplifts and deep heat-controlled faults converge. The geothermal water-rich area is located in the ancient river channel zone. The closer the water-rich area is to the ancient river channel belt, the deeper and coarser the buried conglomerate aquifer is, and the better the water-rich condition is. The best water-rich areas are found in the alluvial fan of the ancient channel belt.