Evolution of karst reservoir geothermal fields driven by production-injection well group layout and its sustainable exploitation threshold

The Chenjiazhuang buried hill geothermal field is an important carbonate karst geothermal reservoir area in Shandong Province. The evolution of the geothermal field under large-scale production-reinjection conditions remains unclear, and there is a particular lack of criteria for determining the sustainable exploitation resource threshold. Numerical simulation is performed to determine sustainable exploitation thresholds under the dual constraints of thermal breakthrough and water-level safety, covering critical production-reinjection rate, critical well spacing, and recoverable resource amount. Based on regional structural and stratigraphic conditions, measured geothermal gradients, terrestrial heat flow, and full-hole temperature logging data, the main controlling factors of geothermal anomalies are systematically analyzed. A three-dimensional hydrothermal coupling numerical model is established using COMSOL Multiphysics. Under balanced production-reinjection constraints, multiple groups of production-reinjection parameters and well layout schemes are designed to simulate the long-term evolution of temperature and hydrodynamic fields in both doublet and multi-well systems, and the sustainable recoverable geothermal resources are quantitatively calculated to determine the threshold. The results show that: Low-temperature reinjected geothermal water accumulates around reinjection wells and forms a cold front that migrates toward production wells driven by density differences, water-level potential differences, and hydrothermal convection within the reservoir. In the doublet system, the thermal breakthrough time t of production wells decreases in a power function with the increase of production-reinjection rate Q, expressed as t=4819Q−1.06, R2=0.9997. It increases exponentially with the rise of well spacing R0, with the fitting formula \begin{document}${t_{{R_0}}} = a{{\rm{e}}^{b{R_0}}} $\end{document}, where a and b are constants. R2 > 0.97. Within a 100-year production-reinjection period, under the dual constraints of thermal breakthrough (average reservoir temperature drop ≤1 ℃) and maximum water table burial depth (Dmax≤170 m), the sustainable exploitation threshold is determined as a well spacing of 600 m and a single-well production-reinjection rate of 80 m3/h, which are the critical safe production-reinjection parameters. Multi-well simulations show that the checkerboard well pattern forms a relatively uniform flow field, with an average reservoir temperature drop of only 0.47 ℃ and a maximum water table burial depth of 159.08–169.41 m over 100 years, meeting both constraints; the orbital pattern results in an uneven flow field and a higher thermal breakthrough risk; the centralized pattern causes severe imbalance of the geothermal field and has the poorest sustainability. Based on the checkerboard layout, a total of 144 production-reinjection well pairs can be theoretically deployed in the whole area. The calculated recoverable geothermal water yield is 27.65×104 m3/d, the annual recoverable geothermal energy is 57.11×1014 J/a, equivalent to 19.49×104 t/a of standard coal, which represents the resource threshold for sustainable exploitation. The findings provide a theoretical basis for the design of production-reinjection well networks and resource assessment in similar karst reservoirs with buried hill settings, and also offer a reference method for determining sustainable exploitation thresholds in comparable geothermal fields.