Co-injection of a low concentration surfactant and polymer solution, termed Low Tension Polymer Flood (LTPF), has been suggested to improve the efficiency of chemical flooding. The multiphase behavior of a live crude oil from the North Sea containing a single-component anionic surfactant, n-C12-o-xylene sulfonate and NaCl brine, has been studied at different temperatures and pressures with and without the presence of biopolymer. Both systems showed the usual types of phase behavior, i. e., a lower microemulsion, a middle phase microemulsion, and an upper microemulsion termed II(-), III and II(+) respectively. No optimal pressure or temperature was observed by conducting pressure and temperature scans. The solubilization parameter of water is always higher than the solubilization parameter of the oil, S(w)>S(o).

The effect of temperature on the phase behavior is complicated. Equilibrium between two microemulsion phases at temperatures close to the critical temperature for phase transitions, is suggested. The system moved into the II(+) state without reaching optimal conditions for oil recovery as the temperature is increased. The phase behavior is discussed in terms of the oil composition.

The addition of xanthan resulted in an increasing ratio S(w)/S(o) and the system is moved toward the II(-) state. The pressure for the phase transition III/II(-) is lowered, while the pressure for the phase transition II(+)/III is nearly unaffected.

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