Optimum Timing for Miscible CO 2 -EOR after Waterflooding in a Tight Sandstone Formation

In this paper, CO 2 dissolution into a light oil/reservoir brine and optimum timing for CO 2 enhanced oil recovery (CO 2 -EOR) after waterflooding in a tight sandstone formation are experimentally studied. First, the gas–oil ratio (GOR) of CO 2 -saturated light oil and the gas–water ratio (GWR) of CO 2 -saturated reservoir brine are measured by using a PVT system. The measured GOR is found to be approximately six times of the measured GWR at the same pressure and temperature. Second, the equilibrium interfacial tensions (IFTs) between the light oil/reservoir brine and CO 2 are measured at different equilibrium pressures and the actual reservoir temperature by applying the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case. The measured equilibrium IFT between the light oil and CO 2 is about one-third to one tenth of that between the reservoir brine and CO 2 under the same test conditions. Third, the minimum miscibility pressure (MMP) between the light oil and CO 2 is determined to be 11.0 MPa from the measured equilibrium IFTs by using the vanishing interfacial tension (VIT) technique. Lastly, a total of five coreflood tests are performed to determine the optimum timing for miscible CO 2 -EOR after waterflooding. All five tests are conducted at the same production pressure of 12.0 MPa to ensure that CO 2 flooding is miscible. In comparison with CO 2 secondary flooding, CO 2 tertiary flooding has a much delayed CO 2 breakthrough (BT) because the initially injected water substantially reduces the mobility of the subsequently injected CO 2 . Accordingly, the oil recovery factor (RF) of CO 2 flooding in terms of the residual-oil-in-place (ROIP) is much higher. By comparing the total oil RF of waterflooding and CO 2 flooding in terms of the original-oil-in-place (OOIP), the oil RF of CO 2 flooding in terms of the ROIP, and the pore volume (PV) of CO 2 BT from the beginning of CO 2 flooding, it is found that the optimum timing for starting miscible CO 2 tertiary flooding is when waterflooding reaches half of its maximum secondary oil RF.