CO2 injection is one of the most promising enhanced oil recovery (EOR) techniques because it can also reduce greenhouse gas through CO2 sequestration. However, effectiveness of CO2 injection dependent on CO2 miscibility. If reservoir is depleted, reservoir pressure is usually not high enough to achieve miscible or near-miscible conditions during CO2 injection in oil reservoirs. In this paper we investigate whether the required miscibility pressure can be reduced by substituting CO2 with alcohol-treated CO2.

Experimental measurements of interfacial tension (IFT) and swelling behavior were made using a high-pressure and high temperature visual cell at 70 °C. Two sets of fluid pairs were used (i) pure CO2 and oil, and (ii) alcohol-treated CO2 and oil. Two types of oil were used: a mixture of 35% hexane and 65% decane (C6-C10 mixture), and pure decane (C10). Ethanol and methanol alcohols were used to prepare alcohol-treated CO2 (a mixture of alcohol and CO2 equilibrated at experimental pressure and temperature). The experiments were matched using an equation of state to provide a quantitative measurement of the miscibility pressures. This study has identified that capillary rise method can be used for IFT measurements down to 0.04 mN/m. The conventional pendant drop method can only be used for IFT values greater than 1 mN/m. For IFT values greater than 1 mN/m, both methods are in close agreement. Validation of IFT simulated model with experimental IFT results showed a good agreement. Results showed that for pure C10, ethanol-treated CO2 cause 0.4 mN/m to 1.5 mN/m IFT reduction leading to 1.1 MPa minimum miscibility pressure (MMP) reduction and methanol-treated CO2 cause 0.4 mN/m to 2.2 mN/m IFT reduction leading to 0.2 MPa MMP reduction. While for C6-C10 mixture, ethanol cause 0.02 to 1.42 mN/m reduction in IFT and methanol cause 0.02 to 0.2 mN/m. Both alcohol have similar reduction of 1.1 MPa in MMP. Results also suggest that alcohol-treated CO2 leads to a better oil swelling behavior for both type of oil which helps to release oil from rock pore. However, ethanol presence in alcohol-treated CO2 shows more oil swelling than methanol. Experimental observations reveal that alcohol-treated CO2 is effective than pure CO2 for IFT reduction and enhancing oil swelling. Furthermore, experiments should be performed using real field crude oil.

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