Abstract
The fractures and kerogen, which generally exist in the shale, are significant to the CO2 huff-n-puff in the shale reservoir. It is important to study the effects of fractures and kerogen on oil recovery during CO2 huff-n-puff operations in the fracture–matrix system. In this study, a modified CO2 huff-n-puff experiment method is developed to estimate the recovery factors and the CO2 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 CO2 usage efficiency in shales and sandstones are discussed, respectively. The results show that although the CO2 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 CO2 injection volumes for CO2 huff-n-puff in the shale. Since the optimal CO2 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 CO2 huff-n-puff in the fractured shale oil reservoir for enhanced oil recovery (EOR) purposes.
Highlights
IntroductionThe majority of newly discovered oil reservoirs are unconventional reservoirs
In recent years, the majority of newly discovered oil reservoirs are unconventional reservoirs
A series of experiments and simulations have been conducted on the Wolfcamp shale to study the effect of different gases on the gas huff-n-puff in the shale, and the results show that the recovery factor of CO2 huff-n-puff is better than C H4 and nitrogen huff-n-puff (Li et al 2017a)
Summary
The majority of newly discovered oil reservoirs are unconventional reservoirs. A series of experiments and simulations have been conducted on the Wolfcamp shale to study the effect of different gases on the gas huff-n-puff in the shale, and the results show that the recovery factor of CO2 huff-n-puff is better than C H4 and nitrogen huff-n-puff (Li et al 2017a). The diffusion coefficient of CO2 in oil is higher than nitrogen and water (Zheng and Yang 2017; Zhu et al 2018c), and the adsorptive properties of C O2 in kerogen are greater than those of other gases (Mitchell et al 2004; Kurniawan et al 2006; Ottiger et al 2008; Pollastro et al 2008). The combined effects of fracture impaction, diffusion, and nanopore result in an additional 3.8% of oil production during CO2 huff-n-puff (Zhang et al 2018). CO2 injection is a good EOR method for shale oil production
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