Abstract: The influence of pre contact time on the interfacial tension of alkali/surfactants/crude oil system is studied. The result shows that the value of the minimum IFT rises with the increase of the pre contact time, when the pre contact time lasts from 20 minutes to 1440 minutes, the minimum IFT will rise from 1.3×10^-3mN/m to 4.8×10^-2mN/m,the curve between IFT and precontact time is regular. The equilibrium value of IFT increase smoothly with the lasting of precontact time, it is a constant during the first 120 minutes, and then past 0.01mN/m and keeps increasing smoothly after the 120 minutes. At the same time, it validates that the active substance in the oil has a important function in decreasing the interfacial tension.

Key words: pre contact time, heavy alkylbenzene sulfonate, alkali, ionic strength, interfacial tension, Daqing crude oil