Effect of CO2-miscibility in EOR

摘要

Success of EOR depends on the interaction of the flooding fluids with the hydrocarbon liquid - gas mixtures under reservoir conditions especially accounting for interfacial and flow behavior. CO2 is becoming increasingly involved in miscible flooding procedures for which there is a growing interest in the respective system properties. In the current work, experimental data on interfacial tension are presented for the binary systems n-heptane, n-decane and crude oil - CO2 and the ternary systems n-heptane, n-decane and crude oil - brine -CO2 at pressures up to 30 MPa and temperatures of up to 400 K. In case of a crude oil in contact with CO2, the interfacial tension is significantly decreased as the pressure rises. Above 20 MPa the interfacial tension maintains a low but relatively constant level at around 2 mN/m. The pressure dependant influence of CO2 on the interfacial tension is compared to nitrogen and methane. For liquid - liquid systems in presence of a fluid like CO2 that is partly miscible with both adjacent liquid phases, there is a reducing effect on the interfacial tension which can be correlated with the diffusion and subsequent accumulation of the gas at the liquid - liquid interface. In the presence of hydrocarbon gases like methane this effect is negligible because of merely non-polar interactions. While the interfacial tension is decisive regarding capillary action especially within the micro pores, the viscosity determines the resistance to flow of the bulk phase, i.e. of the crude oil through the main ducts. As a consequence, reduction in viscosity is essential for improving oil recovery by use of a displacing fluid. Further, the viscosity ratio between the flooding fluid and the oil determines efficiency of the recovery method. In miscible flooding using CO2, the low viscosity ratio represents a challenge. On the other hand, high solubility of the gas within the liquid phase leads to reducing its viscosity by orders of magnitude. This effect is determined by a procedure recently developed and shown in this paper using n-alkanes for calibration and giving some exemplary results for the crude oil - CO2 system. A main difficulty is related to working close to gas saturation within the liquid phase.