Effect of Fracture Aperture and Water Saturation on the In Situ Conversion of Tar-Rich Coal

Abstract

With abundant coal resources globally, in situ conversion technology, an emerging unconventional energy development method, is a crucial pathway for achieving efficient and clean utilization of tar-rich coal. However, its application is challenged by the heterogeneity of fractures, which limits the sweep efficiency of the heating medium, and by high water saturation, which reduces heating efficiency, leading to the formation of a water–oil-gas two-phase flow system and exacerbating the oil/gas retention effect. This study investigates the impact mechanism of fracture aperture and water saturation on the development effectiveness of in situ conversion of tar-rich coal, using the Taiyuan Formation of the Carboniferous System as a case study. Key parameters, including heating method, well type, and well spacing, are optimized through numerical simulation to evaluate energy efficiency and hydrocarbon production. The results indicate that the convective-heating horizontal well with a 20 m spacing is identified as the optimal development scheme, characterized by the highest peak energy efficiency (1.22) and notable time efficiency. An increase in the fracture aperture positively promotes the development effectiveness. When the aperture increases from 0.25 m to 1 m, the reduction in injection pressure results in a decrease in cumulative compression energy, thereby increasing the peak energy efficiency to 1.4. Conversely, an increase in water saturation enhances heat loss and prolongs the development cycle. When water saturation increases from 1.9% to 50%, the energy efficiency decreases by 10.8%, and the cumulative oil production drops by 5%. Under low water saturation conditions, fracture evolution is more stable, and heat is concentrated in the coal matrix, promoting efficient pyrolysis reactions. As a result, the original sample group (1.9%) achieves optimal energy efficiency (peak of 1.3). These findings and key parameters provide theoretical and technical support for the design of pilot-scale in situ conversion projects.