The paper addresses problems related to application of a single-well surfactant test (SWST) for a North Sea oil reservoir. Waterflooding an oil production well prior to a SWST by injection of seawater will change the oil properties in the wellbore region. Injection of unsaturated water with respect to hydrocarbon gas in a formation containing a volatile oil with GOR in the range of 100-1000, will cause stripping of the oil.

The injection of cold seawater may alter the temperature in the injection zone and this effect combined with the variation of GOR with distance from the wellbore will result in gradients both in temperature and oil properties. The effect of these variations will influence the phase behavior and the performance of the surfactants as well as the tracers.

Static multiple water-oil contact experiments at reservoir conditions have been performed. These experiments were compared to dynamic core displacements in- order to quantify the stripping of the oil phase by water. The tracer partitioning and hydrolysis were measured both in static experiments and reservoir cores. The effect of GOR on tracer partitioning was also investigated.

The results reveals that the GOR of the specific oil used was reduced from 110 to 10 and the formation volume factor (Bo) from 1.34 to 1.05. The decreasing gas content in the oil changes the phase behavior of the optimized surfactant system towards a lower phase microemulsion.

The surfactant system had to be re-optimized against the stripped oil both by static phase behavior and dynamic core displacements. The effect of stripping leads to a significant change in surfactant and tracer phase behavior. In addition, due to the changes in BO, the saturation values as determined by the tracer test yields low waterflood residual oil saturations compared to expected results of a large scale waterflood.

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