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 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.

Huff-n-Puff Technology for Enhanced Oil Recovery in Shale/Tight Oil Reservoirs: Progress, Gaps, and Perspectives

“…However, according to Zhu et al, CO 2 injectivity in shale is greater than in sandstone with about the same porosity; furthermore, the oil RFs of two shale cores are substantially lower than the recovery factors of two sandstone plugs. 146 This illustrates that, in comparison to the tight oil reservoir, the shale oil reservoir need longer cycles to attain a higher oil RF. Besides, for the CO 2 H-n-P procedure in the shale layer, there are appropriate injection pressures (in close proximity to the MMP) and CO 2 injection amounts.…”

“…Since a short injection duration would not considerably boost oil recovery, a lengthy injection duration (upward of 150 days) may not be economically possible. However, according to Zhu et al, CO 2 injectivity in shale is greater than in sandstone with about the same porosity; furthermore, the oil RFs of two shale cores are substantially lower than the recovery factors of two sandstone plugs . This illustrates that, in comparison to the tight oil reservoir, the shale oil reservoir need longer cycles to attain a higher oil RF.…”

“…With the technology breakthroughs and worldwide application of simulated fractures, the tight reservoir has accelerated global oil production massively since early 2000. However, the current technology based on fractured horizontal wells has a recovery rate of ∼10% for liquids and 20% for gas. , The fractures largely occur in the shale and are essential to the CO 2 H-n-P process . The fracture network, whether natural or induced, determines the success of the gas cyclic process.…”

“…As predicted, the approach to analysis varies significantly, based on core properties and the core holder method used. For example, experiments using confined core H-n-P injections ,,,, yield different recoveries than experiments using an unconfined core system (free-path holder), the core is soaked in high-pressure steam, allowing the injected gas to circulate across the core container, maximizing the sample’s surface area (Figure d). The unconfined core H-n-P extraction systems, on the other hand, are designed to simulate a high-permeability fracture.…”

“…144,145 The fractures largely occur in the shale and are essential to the CO 2 H-n-P process. 146 The fracture network, whether natural or induced, determines the success of the gas cyclic process. Because of the elevated injection pressures at the wellhead, injected gas is allowed to flow swiftly across the system of fissures to the matrix/fracture interface.…”