2-D Pore-Scale Experimental Investigations of Asphaltene Deposition and Heavy Oil Recovery by CO2 Flooding

To understand the pore-scale processes of asphaltene deposition during heavy oil recovery by CO 2 flooding, a series of microscopic flooding experiments were conducted in two-dimensional models under high-temperature (80 °C) and high-pressure (2, 6, and 10 MPa) conditions. Two different heavy oils, AZ-4 and KD-9 (with viscosities of 7754 and 19 290 mPa·s, respectively, at 80 °C), were used. In these samples, up to 95% and 85% of oil viscosity reduction, respectively, could be achieved by CO 2 dissolution. During CO 2 flooding in the microscopic models, with increasing fluid pressure (6 and 10 MPa) and CO 2 concentrations (60–80 mol %), both the covered area (17–38%) and the sizes of the deposited particles (up to 200 μm) became larger. At 6 MPa, a dynamic between between dissolution and precipitation was observed in large pores (with diameters of >350 μm). According to the solvent dissolution test (heptane and toluene), the deposited solids comprise both asphaltene and alkane components. Between AZ-4 and KD-9, higher resin/asphaltene ratios tend to reduce asphaltene deposition during the dissolution of CO 2 by stabilizing the solubility of asphaltene in crude oil. In addition, although permeability decreased (up to 15%) with increasing pressure (2–10 MPa) and CO 2 concentrations (25–70 mol %), the oil recovery mostly increased (up to 88.6% with AZ-4). The only decrease in oil recovery occurred with KD-9, which decreased from 64.7% (6 MPa, with a CO 2 concentration of 69.7%) to 56.1% under 10 MPa and considerably high CO 2 concentrations (79.7 mol %). Compared with the light oil system, the tested heavy oil shows considerably improved recovery and less asphaltene deposition during high-concentration CO 2 flooding. Therefore, although CO 2 miscibility can hardly be achieved in heavy oil reservoirs, CO 2 is of great interest due to its high solubility in heavy oil and significant oil viscosity reduction.