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Experimental Investigation of Oil Recovery Performance and Permeability Damage in Multilayer Reservoirs after CO₂ and Water–Alternating-CO₂ (CO₂–WAG) Flooding at Miscible Pressures

Wang, Qian, Yang, Shenglai, Lorinczi, Piroska, Glover, Paul W. J., Lei, Hao
Energy & fuels 2019 v.34 no.1 pp. 624-636
asphaltenes, carbon dioxide, control methods, oils, permeability, petroleum, sandstone
Blockage in reservoirs caused by asphaltene deposits and inorganic interactions is a serious problem that may exacerbate the complexity of displacement characteristics in heterogeneous multilayer sandstone reservoirs and affect crude oil recovery performance during CO₂ and CO₂–WAG flooding. In this study, experiments of both CO₂ and CO₂–WAG flooding were carried out on the same multilayer systems under miscible conditions (70 °C, 18 MPa). The two flooding methods were evaluated for oil production performance and reservoir damage. The experimental results indicate that, after CO₂ flooding, the entire system has a low oil recovery factor (RF) of 27.6%, and oil is produced mainly from the high permeability layer (91.4%), while the residual oil remains predominantly in the medium and low permeability layers. The injection pressure of CO₂–WAG flooding is high, but the timing of CO₂ breakthrough (BT) is late, and the oil RF of the entire system reaches 44.5%. The contribution rate of oil production in medium and low permeability layers is improved to 3.8% and 17.1%, respectively. Furthermore, the permeability of the high permeability layer decreases by 16.8% after CO₂ flooding, which is mainly due to asphaltene precipitation. However, after CO₂–WAG flooding, the permeability of each layer is significantly reduced, namely by 29.4%, 16.8%, and 6.9%, respectively. Asphaltene precipitation is still the main factor, but permeability decline caused by CO₂–brine–rock interactions cannot be ignored, especially in the high permeability layer (6.1%). Therefore, for multilayer reservoirs with high heterogeneity, CO₂–WAG flooding provides the better oil displacement performance, but prevention and control measures for asphaltene precipitation are more necessary.