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

Abstract

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℃, 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%), whilst 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.

Effect of Pore-Throat Microstructures on Formation Damage during Miscible CO2Flooding of Tight Sandstone Reservoirs

“…The brine containing Mn 2+ was used to screen the water signal during NMR tests in order to obtain only the oil distribution in the core [14] . Uncertainties in the x and y directions are smaller than the symbol size in all cases [17] . 6 24 hours and were measured by NMR apparatus to obtain a T2 distribution for each core.…”

Mini Review of Miscible Condition Evaluation and Experimental Methods of Gas Miscible Injection in Conventional and Fractured Reservoirs

“…Huang et al 114 slim tube displacement Changqing oilfield of China CO 2 Wang et al 115 vanishing interfacial tension Changqing oilfield, western China CO 2 Hawthorne et al 116 vanishing interfacial tension Bakken crude oil in North Dakota (U.S.A.) CO 2 and hydrocarbon gases…”

Effect of a Pore Throat Microstructure on Miscible CO2 Soaking Alternating Gas Flooding of Tight Sandstone Reservoirs

“…There is a good linear relationship (R 2 >0.98) between the total RFs and the fractal dimension of the core pore-throat structure (Figure 4). The gradients of the relationships are negative, indicating that higher fractal dimensions, which are associated with a greater heterogeneity in the pore microstructure provide lower recovery factors, primarily because more homogeneous rocks exhibit greater CO2 flooding efficiency and larger CO2 sweep volume [12] .…”

“…The main mechanisms of CO2-EOR techniques include (i) the oil-swelling effect, (ii) viscosity reduction, (iii) light-hydrocarbon extraction, and (iv) the reduction of interfacial tension (IFT). They can achieve higher efficiency if carried out at miscible conditions [11][12] . However, CO2-EOR techniques suffer from asphaltene precipitation, which can lead to significant reductions in formation permeability and water wettability.…”