Mechanism of High Stability of Water-in-Oil Emulsions at High Temperature

Thermal flooding by steam injection was a traditional method for exploiting heavy oil. The produced liquid was a highly stable water-in-oil or oil-in-water emulsion in several oilfields. In this work, we focused on studying the effect of high temperature on the stability of an emulsion system involving two typical crude oils (heavy crude oil and light crude oil) and brine. It was impossible to directly measure the interfacial viscoelastic modulus because of the high viscosity of the heavy oil. In order to solve this problem and analyze the contribution of those fractions to the formation of stable emulsions, the heavy crude oil was divided into three cuts: remaining fraction, resin, and asphaltene. The model oils were prepared from the mixture of several heavy crude oil fractions with kerosene:xylene (1:1 v/v) to investigate the high-temperature behaviors of their emulsions. The stability was evaluated through a high-temperature high-pressure (HTHP) visual pressure volume temperature cell, and a temperature up to 200 degrees C was achieved. The automatic pendant drop technique was used to analyze the interfacial rheology of model oils/brine system under HTHP conditions. With increasing formation temperature, the kinetically stable heavy oil emulsion had a much higher viscosity. The stabilities of heavy crude oil and light crude oil emulsions had opposite trends as the formation temperature increased. The stabilities of different model oils indicated that the mass fraction of asphaltene was responsible for the stability of emulsions at high temperature. With increasing temperature, the interfacial viscoelastic properties stabilized by 4.0 wt % asphaltenes had an increasing trend, but the stability of remaining fraction, resin, and 0.4 wt % asphaltene had a decreasing trend. The mechanism of formation of a stable and more rigid interfacial film was revealed by the effect of high temperature on asphaltene aggregation and water molecules.