Optimum Timing for Miscible CO2-EOR after Waterflooding in a Tight Sandstone Formation

Abstract

In this paper, CO2 dissolution into a light oil/reservoir brine and optimum timing for CO2 enhanced oil recovery (CO2-EOR) after waterflooding in a tight sandstone formation are experimentally studied. First, the gas–oil ratio (GOR) of CO2-saturated light oil and the gas–water ratio (GWR) of CO2-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 CO2 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 CO2 is about one-third to one tenth of that between the reservoir brine and CO2 under the same test conditions. Third, the minimum miscibility pressure (MMP) between the light oil and CO2 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 CO2-EOR after waterflooding. All five tests are conducted at the same production pressure of 12.0 MPa to ensure that CO2 flooding is miscible. In comparison with CO2 secondary flooding, CO2 tertiary flooding has a much delayed CO2 breakthrough (BT) because the initially injected water substantially reduces the mobility of the subsequently injected CO2. Accordingly, the oil recovery factor (RF) of CO2 flooding in terms of the residual-oil-in-place (ROIP) is much higher. By comparing the total oil RF of waterflooding and CO2 flooding in terms of the original-oil-in-place (OOIP), the oil RF of CO2 flooding in terms of the ROIP, and the pore volume (PV) of CO2 BT from the beginning of CO2 flooding, it is found that the optimum timing for starting miscible CO2 tertiary flooding is when waterflooding reaches half of its maximum secondary oil RF.