Enhanced Oil Recovery of Low-Permeable, Tight, and Shale Reservoirs by CO<sub>2</sub> Injection: A Minireview
Zhichao Zhang, Mingxing Bai, Erlong Yang, Guangshe Zhu, Junhui Bai
Abstract
Water injection into low-permeable, tight, and shale reservoirs is difficult due to the higher seepage resistance, leading to low oil recovery. CO 2 flooding is considered an important technology for enhancing oil recovery (EOR) and achieving CO 2 sequestration within these reservoirs. However, the efficiency of oil extraction is impacted by many variables, such as the CO 2 injection methods, reservoir temperature and pressure, reservoir permeability, reservoir water saturation, and CO 2 -induced asphaltene precipitation, etc. Previous study results reveal that oil recovery for the low-permeable, tight, and shale reservoirs can be gradually enhanced as the transitions of CO 2 injection methods from immiscible to near-miscible to miscible injection. The rise in CO 2 injection pressure and reservoir temperature improves oil recovery by facilitating mass transfer and reducing the interfacial tension (IFT) between CO 2 and oil. However, oil recovery of reservoirs is also diminished due to the increased reservoir damage from more pronounced asphaltene precipitation within the reservoir pores with the elevated pressure and temperature. Therefore, the oil recovery effect by CO 2 injection should be comprehensively analyzed based on the effect of EOR and reservoir damage induced by asphaltene precipitation. The EOR effect deteriorates and the reservoir damage intensifies by CO 2 injection with the reduction of reservoir permeability. The oil recovery of low-permeable reservoirs after water flooding is improved by tertiary CO 2 flooding, due to the improved mobility ratio between CO 2 and the mixed fluids. To reduce the unfavorable impact of CO 2 -induced asphaltene precipitation on oil recovery of reservoirs, CO 2 near-miscible injection achieves an oil recovery of 85%–90% of that by CO 2 miscible injection. The reservoir damage induced by asphaltene precipitation is also reduced with the lower pressure of near-miscible CO 2 injection. To enhance oil recovery of low-permeable reservoirs, CO 2 miscible injection is suitable if the asphaltene content in crude oil is low. However, CO 2 huff and puff injection are more suitable for tight and shale reservoirs with ultralow reservoir permeability.