Heat Transfer Characteristics of Borehole Briquette Samples in Liquid CO₂ High-Temperature Steam Synergistic Impact Process

To enhance the effective range of thermally induced fracturing in coal under alternating temperature effects using liquid CO2 fracturing technology, this paper proposes an improved technique based on traditional liquid CO2 fracturing: the liquid CO2 high-temperature steam synergistic fracturing permeability enhancement technology. This study employs an infrared thermal imaging detector to investigate the temperature evolution characteristics of borehole-like coal bodies during thermal shock processes. It quantitatively analyzes the temperature evolution patterns in the x, y, and z directions in the borehole wall and bottom regions during the liquid CO2 high-temperature steam impact. The results show that during thermal shock, the temperature diffusion in dry coal samples is concentrated and intense, whereas in saturated coal samples, it is uniform and slow. This indicates that fissure water absorbs and stores heat, promoting more uniform temperature diffusion, but also obstructing the migration channels of low-temperature media, thereby reducing the efficiency of temperature diffusion. According to the temperature evolution curves in the x, y, and z directions in the borehole wall and bottom regions, it is found that the temperature of the coal body is negatively correlated with the distance to the impact point. During cold shock, the low-temperature field diffusion is concentrated below the central axis of the coal body, while during heat shock, the high-temperature field is concentrated above the axis. The temperature evolution amplitude is greatest in the middle region between the liquid CO2 and high-temperature steam injection points, indicating that the migration direction of thermal media significantly affects temperature diffusion in the coal body. This research provides a theoretical basis for the study of liquid CO2 high-temperature steam synergistic fracturing technology in coal seams.