Effect of Proppant on Seepage and Heat Transfer Characteristics in Fractured Granite of Hot Dry Rock Geothermal Systems

Abstract

The seepage and heat transfer characteristics of fractured rock are critical issues in hot dry rock exploitation. This paper investigates the effects of proppant on the seepage characteristics under different conditions through seepage experiments on split granite fractures. Subsequently, seepage–heat transfer coupling mechanisms in fractured granite are elucidated through numerical simulations. The results indicate that the flow rate increases in fractures with large crack widths as temperature rises, while in fractures with small crack widths, an increase in temperature reduces the flow rate. Additionally, normal constraint force increases with rising confining pressure, consequently reducing the flow rate. Extension of seepage paths is favored by higher injection pressures, thereby improving flow rates. The inclusion of proppant effectively supports the fracture, expanding its width and significantly increasing the flow rate. Furthermore, injection into the reservoir forms a low-temperature cooling zone, which gradually advances towards the outlet over time. Initially, the outlet temperature and extract heat rate remain stable before decreasing almost linearly. Proppant filling accelerates the heat transfer rate and significantly boosts the initial extract heat rate; however, it also leads to a faster decline in reservoir heat quantity, resulting in a subsequent extract heat rate lower than that of unfilled fractures. These findings underscore the importance of balanced extract heat efficiency and enhanced geothermal system reservoir longevity for the sustained exploitation of geothermal energy.