Long-term operation performance analysis and layout optimization of direct geothermal heating system using multiple deep borehole heat exchangers for natural gas preheating

Direct geothermal heating systems (DGHS) with deep borehole heat exchanger (DBHE) arrays show great promise for natural gas preheating at city gate stations (CGS). However, their large-scale preheating performance, operational stability, and optimal array layout strategies remain unclear. In this study, a practical CGS is selected and a three-dimensional numerical model of a DGHS with a multi-DBHE array, explicitly accounting for groundwater flow, is developed. The preheating performance and long-term stability of array layouts at different burial depths are systematically investigated over 15 preheating cycles. To quantitatively assess and optimize inter-well thermal interaction, a novel index, Space Influence Factor (Fj), is also introduced. The results show that pronounced inter-well thermal interaction occurs among DBHEs arranged parallel to groundwater flow, reducing performance and stability for downstream DBHEs, whereas perpendicular arrangements minimize such effects. Notably, the preheating stability of DBHE arrays is positively correlated with the proportion of permeable strata with groundwater flow. Then, the Fj index is used to determine the optimal multi-DBHE array layouts at burial depths of 1500, 2000, and 2500 m. Under these optimal layouts, the maximum outlet temperature difference between adjacent DBHEs always remains below 1 °C and the occupied strata area is minimized. Finally, implementing these optimized layouts not only improves overall preheating stability, but also ensures that, for burial depths exceeding 2000 m, the CGS can reliably achieve long-term preheating targets. These findings provide robust guidance for the design and deployment of DGHS with multi-DBHE arrays in large-scale natural gas preheating applications.