(559p) Optimized Immiscible CO2 Injections for Enhancing Tight Oil Recovery

Kaiqiang Zhang, Songyan Li, Na Jia

Keywords: CO₂ enhanced oil recovery; Optimized injection schemes; Tight oil; Immiscible flooding.

In this study, four different injection schemes are specifically optimized through the high-temperature and pressure CO₂ immiscible flooding in tight formations. More specifically, first, several important physical properties of the light crude oil‒CO₂ systems are measured, which include the oil density and viscosity as well as the oil‒CO₂ IFTs. Then, A total of 10 immiscible coreflood tests are conducted at the injection pressure of P_inj = 20.0 MPa and reservoir temperature of T_res = 130.0 °C by means of the four different injection schemes, i.e., CO₂ continuous gas injection (CGI), CGI + soaking, pulse injection, and gas-alternating-soaking (GAS). In addition to measured oil recovery factors (ORFs), the pressure difference in each test is monitored and analyzed to achieve a better understanding of the aforementioned four CO₂ injection schemes in the tight formation. Finally, the asphaltene and wax contents in the produced oil are measured at different production periods for each test.

The measured oil viscosity and density are found to be reduced with the temperature increase. Also, the minimum miscibility pressure is measured to be 30.0 MPa at T_res = 130.0 °C from the vanishing interfacial tension (VIT) technique. A total of 10 immiscible coreflood tests are conducted to study four different CO₂ injection schemes at the injection pressure of P_inj = 20.0 MPa and reservoir temperature of T_res = 130.0 °C, i.e., CO₂ CGI, CGI + soaking, pulse injection, and GAS. Moreover, soaking is found to be an important step for the CO₂ enhanced oil recovery at the immiscible conditions. Among all tests with the same experimental conditions, the measured ORF of Test #9, which is applied with the CO₂ GAS at the injection rates of q_CO2 = 0.5 cm³/min, is the highest. The CO₂ CGI + soaking at the injection rates of q_CO2 = 0.5 cm³/min has the second highest ORF in Test #4. The CO₂ CGI performs better at a small injection rate, whereas the CO₂ pulse injection always gives a low ORF no matter at small or larger injection rate. A small injection rate is suggested to be used for any CO₂ injection scheme. Tests #1, #4, #7, and #9 at the injection rate of q_CO2 = 0.5 cm³/min outperform other tests by using the same injection scheme under the same experimental conditions in terms of the ORF but larger injection rates. The smaller injection rate also contributes to delay the CO₂ BT so that a higher ORF can be reached with a lower cost. Last but not least, the asphaltene and wax contents are found to be decreased in the produced oil with the progressing oil production. In the final period of oil production, the asphaltene and wax contents reduce to be 33-51% and 14-25% less in comparison with those at the beginning, respectively.

In comparison with the miscible conditions, most CO₂ enhanced oil recovery (EOR) projects in the shale/tight formations are conducted in the immiscible conditions. However, few studies regarding the evaluations of the CO₂ injection schemes under the immiscible conditions have been conducted through the tight coreflood tests. A series of optimum conditions are determined for different CO₂ injection schemes, which are of paramount importance and can be directly applied for various CO₂ EOR projects in shale/tight formations worldwide.