Effects of Well Layout and Injection Rate on Long-Term Heat Extraction Performance in Fractured Geothermal Reservoirs: A DFN-Based Thermo-Hydraulic Study

Efficient heat extraction from fractured geothermal reservoirs is strongly controlled by fracture network heterogeneity, well-layout design, and injection operations. In this study, a two-dimensional discrete fracture network (DFN) model incorporating thermo-hydraulic processes was established to investigate the effects of well layout and injection rate on the long-term heat extraction performance in fractured geothermal reservoirs. Two well-layout orientations, specifically 45°/135° and 0°/90°, were designed according to their geometric relationship with the dominant fracture orientations. For each orientation, four well configurations were considered, including one-injection–one-production, one-injection–two-production, one-injection–three-production, and one-injection–four-production schemes. Three injection rates of 1.0, 1.5, and 2.0 kg/s were then assigned to each well layout, resulting in 24 simulation cases. The spatiotemporal evolution of the temperature field, average outlet temperature, total production mass flow rate, and heat-output power were systematically analyzed. The results show that injected cold water preferentially migrates along connected fractures, and that cooling-front propagation is jointly controlled by fracture connectivity, well spacing, and injection–production alignment. When the well alignment is consistent with the dominant fracture orientation, nearly direct preferential flow channels are more likely to form, leading to a faster outlet-temperature decline and a higher thermal-breakthrough risk. Multi-production-well layouts can activate more fracture pathways and improve the total production mass flow rate, but their enhancement effect is limited by competitive flow diversion and local fracture connectivity. Increasing the injection rate enhances early-stage heat-output power but shortens fluid residence time and accelerates thermal breakthrough, thereby reducing long-term thermal stability. Overall, the 0°/90° multi-production-well layouts exhibit better long-term heat-output performance, while a lower injection rate is more favorable for maintaining a stable outlet temperature and heat-output power. These findings provide useful guidance for well-pattern optimization and injection-scheme selection in fractured geothermal reservoirs.