Long-term hydrothermal evolution and sustainable operation of a deep carbonate geothermal reservoir: A field-calibrated 3D framework for the lower Friulian Plain, Italy

• Field-calibrated 3D TH model anchored to the Grado-2 pumping test. • Regime mapping indicates a conduction–advection shift near Q≈100 L/s. • Moderate discharge and spacing sustain high efficiency over decades. • Higher discharge boosts early power but accelerates thermal depletion. • Permeability anisotropy strongly affects plume migration and breakthrough. Geothermal energy is becoming increasingly important for low-carbon heating, particularly in low-enthalpy carbonate reservoirs. However, the long-term performance of geothermal doublets in such systems is controlled by the competition between advective heat extraction and conductive thermal recharge. Here, we develop a fully coupled three-dimensional thermo-hydraulic model and conduct a systematic design-space exploration using the Grado carbonate system (northeastern Italy) as a field-calibrated reference case. The model is calibrated against the Grado-2 step-drawdown pumping test, reproducing artesian conditions and approximately 23 m of drawdown, and is used to assess long-term operational performance. By varying discharge rate, well spacing, and horizontal permeability anisotropy, the simulations identify two distinct regimes: a conduction-dominated regime and an advection-dominated regime. Moderate discharge rates (35–42 L/s) maintain thermal stability (less than 1 °C decline over 50 years) with transfer efficiency greater than 95%, whereas high extraction rates (100–200 L/s) increase initial thermal output (up to approximately 25 MW) but cause rapid cooling (2–7 °C) and efficiency losses up to 45%. In the advection-dominated regime, permeability anisotropy and well alignment become first-order controls on breakthrough timing. A clear operational threshold is identified: well spacing of at least 700 m is required to suppress thermal interference. The results are synthesized into regime maps and a conservative Sustainability Index that identifies an optimal operational window of 35–42 L/s and 600–800 m spacing. Although calibrated on Grado, the proposed workflow provides a transferable, risk-aware framework for low-enthalpy geothermal doublets in fractured carbonates.