Experimental study of enhanced oil recovery by CO2 huff-n-puff in shales and tight sandstones with fractures

Abstract The fractures and kerogen, which generally exist in the shale, are significant to the CO 2 huff-n-puff in the shale reservoir. It is important to study the effects of fractures and kerogen on oil recovery during CO 2 huff-n-puff operations in the fracture–matrix system. In this study, a modified CO 2 huff-n-puff experiment method is developed to estimate the recovery factors and the CO 2 injectivity in the fractured organic-rich shales and tight sandstones. The effects of rock properties, injection pressure, and injection time on the recovery factors and CO 2 usage efficiency in shales and sandstones are discussed, respectively. The results show that although the CO 2 injectivity in the shale is higher than that in the sandstone with the same porosity; besides, the recovery factors of two shale samples are much lower than that of two sandstone samples. This demonstrates that compared with the tight sandstone, more cycles are needed for the shale to reach a higher recovery factor. Furthermore, there are optimal injection pressures (close to the minimum miscible pressure) and CO 2 injection volumes for CO 2 huff-n-puff in the shale. Since the optimal CO 2 injection volume in the shale is higher than that in the sandstone, more injection time is needed to enhance the oil recovery in the shale. There is a reference sense for CO 2 huff-n-puff in the fractured shale oil reservoir for enhanced oil recovery (EOR) purposes.