**3. Laboratory determination of phase behavior**

Phase behavior laboratory test is conducted to assess surfactant performance in generating optimal microemulsion. The test involves the use of graduated cylinders with stopper and an explosion proof oven. A typical range of surfactant concentration and salinity for the phase behavior test are 0.25 to 2.5 wt.% and 5000 to 40,000 ppm respectively. The procedure starts with adding 50% of surfactant solution in brine into the graduated cylinder with 50% of crude oil of interest. The cylinder is then mixed vigorously and aged in the explosion oven for 14 days at reservoir temperature. The phase type and volume of the mixture are observed and measured on day-14 to identify the optimum microemulsion formulation and condition. The complete phase type descriptions are given in **Table 1**.

Type III is considered to have the best probability of recovering additional oil. Type II is considered to have the poorest chance to recover additional oil. Type II˗ is considered to have the second-best chance to recover additional oil because it shows interaction between the aqueous phase and crude oil. Even though Type II+ demonstrates interaction between the crude oil and the aqueous phase, it is considered to have poorer oil recovery potential than Type II˗.

**Figure 3** illustrates the relationships among phase behavior laboratory test observation, microemulsion structure and Winsor's R ratio.
