Abstract

Elevated downhole temperatures in ultra-deep wells (>8000 m) accelerate thermal degradation of drilling fluids and tools, reducing operational safety and efficiency. The reduction of wellbore temperature is an important issue. In this paper, a cooling system model for a drilling fluid was designed. Additionally, a comprehensive analysis of the heat transfer behavior within ultra-deep wells was conducted. A miniaturized heat exchange cooling device was used to simulate various conditions, including fluid media, flow rate, drilling fluid/coolant temperature, and heat exchanger structure. This analysis elucidates the impact of various factors on cooling efficiency. The experimental results show that the cooling effect is best at a medium flow rate, with a refrigerant temperature of−10°C reducing the temperature of pure water from 60°C to 32°C. The experiment also found that the higher the temperature of the pure water and the lower the temperature of the coolant, the better the heat transfer efficiency. For water-based drilling fluid, the optimum cooling flow rate is around 0.52 m/s, with an average Reynolds number of 4966, and the maximum cooling range can exceed 30°C. Furthermore, the coil heat exchanger significantly improves the cooling rate compared to the straight-tube heat exchanger, although the pressure difference also increases. The cooling rate of oil-based drilling fluid at high flow rates is greater than that of water-based drilling fluid, and the pressure difference in the coil heat exchanger for oil-based drilling fluid, which has higher viscosity, increases significantly. This research provides an experimental basis for the design and optimization of drilling fluid surface cooling systems, which is crucial for improving the safety and efficiency of deep and ultra-deep wells drilling.OPEN ACCESS Received: 10/06/2025 Accepted: 15/08/2025

Abstract

Elevated downhole temperatures in ultra-deep wells (>8000 m) accelerate thermal degradation of drilling fluids and tools, reducing operational safety and efficiency. The reduction of wellbore temperature is an important issue. In this paper, a cooling system model for a drilling fluid was designed. Additionally, a comprehensive analysis of the heat transfer behavior within ultra-deep wells was conducted. A miniaturized heat exchange cooling device was used to simulate various conditions, including fluid media, flow rate, drilling fluid/coolant temperature, and heat exchanger structure. This analysis elucidates the impact of various factors on cooling efficiency. The experimental results show that the cooling effect is best at a medium flow rate, with a refrigerant temperature of−10°C reducing the temperature of pure water from 60°C to 32°C. The experiment also found that the higher the temperature of the pure water and the lower the temperature of the coolant, the better the heat transfer efficiency. For water-based drilling fluid, the optimum cooling flow rate is around 0.52 m/s, with an average Reynolds number of 4966, and the maximum cooling range can exceed 30°C. Furthermore, the coil heat exchanger significantly improves the cooling rate compared to the straight-tube heat exchanger, although the pressure difference also increases. The cooling rate of oil-based drilling fluid at high flow rates is greater than that of water-based drilling fluid, and the pressure difference in the coil heat exchanger for oil-based drilling fluid, which has higher viscosity, increases significantly. This research provides an experimental basis for the design and optimization of drilling fluid surface cooling systems, which is crucial for improving the safety and efficiency of deep and ultra-deep wells drilling.OPEN ACCESS Received: 10/06/2025 Accepted: 15/08/2025 Published: 27/11/2025