(183d) Relationship Between Emulsification Behaviors of Crude Oil-Water Systems and Crude Oil Composition

Abstract: Because of the existence of natural surfactants such as asphaltenes, resins, and naphthenic acids in crude oil, oil and water tend to form emulsions in the process of production, which may significantly affect the flow characteristics of the multiphase flow in gathering lines. The emulsification behavior of crude oil-water system is closely related to the composition of crude oil, and the effect of crude oil composition on emulsification behaviors has been well studied. However, the conclusions of previous studies were essentially qualitative, there was lack of quantitative study on the relationship between emulsification behaviors and crude oil composition.

Obviously, the fraction of the emulsified water in a flowing crude oil-water system may be higher than that of a quiescent system. Therefore, it is the emulsified water fraction under flowing conditions that acts as the essential factor affecting the characteristics of multiphase flow. Based on this fact, the present study focuses on the emulsified water fraction under flowing conditions, which is used to characterize the emulsification behaviors of crude oil-water system. And furthermore，the quantitative relationship between emulsification behaviors of crude oil-water systems and crude oil composition is studied.

In this work, the emulsification experiments were performed by using a stirred vessel under various conditions of shear intensity, water fraction and temperature, and eight crude oils with different compositions were used in the experiments. The emulsified water fraction under flowing conditions was determined by extrapolation of the separated water volume to the moment when stirring was just terminated. The mean shear rate in the stirred vessel was used to characterize the shear intensity, and it was calculated by the relationship between energy dissipation rate and shear rate. The experimental results showed that full emulsification of water was observed in a specific range of shear rate and below a critical water fraction, and only partial emulsification might occur above this critical water fraction. The critical water fraction and the range of shear rate for full emulsification differ from oil to oil, implying that they are related to the crude oil composition. For partial emulsification, the emulsified water fraction under flowing conditions was found to well correlate with the entropy production rate of viscous flow regardless of the differences in water fraction, temperature and shear rate, and a power-law equation might well fit the data, i.e. φ_E= k₁S_g^k2 . The undetermined parameters k₁ and k₂ are also dependent on the crude oil composition.

On the basis of the experimental results, further investigation is carried out to get insight into the quantitative relationships between the critical water fraction, the range of shear rate for full emulsification and the values of k₁, k₂ and crude oil composition. A total of 6 parameters were determined to be the representative parameters of oil composition, i.e. the content of asphaltenes and resins, the resin-to-asphaltene ratio, the sum of the concentration of precipitated wax at studied temperature and the content of mechanical impurities, acid value, the average carbon number of crude oil, the viscosity of the liquid continuous phase at 50 ^oC. Further, several correlations were developed by using stepwise regression between the critical water fraction, the range of shear rate for full emulsification, the values of k₁, k₂ and the highly significant parameters of oil composition. These correlations help to quantitatively understand the relationship between emulsification behaviors and crude oil composition. Through the quantitative correlation φ_E= k₁S_g^k2 , the emulsified water fraction under different shearing conditions can be predicted.