(60ad) Determination of Phase Content in Oil-Water Emulsion By Electrochemical Impedance Spectroscopy

The detection of phase content is a key technique in the storage and transport process of petroleum production . Especially, the formation of oil-water emulsion has great effects on the physic properties of the two phase flow in the pipeline. So it needs a simple and effective method to realize the determination of the phase content in oil-water emulsions.

Many investigations have studied the water contents of oil-water two-phase systems in recent years. Such as the distillation, titration, capacitive, microwave-based, ultrasonic techniques. But the determination of phase content of two phase mixture still need more improvements to get satisfied accuracy and practicality. In recent years the electrochemical impedance spectroscopy(EIS) has been proved that has a key contribution on the measurement of the phase content in the multiphase system. The impedance characteristics in frequency-domain always contain abundant physical and chemical information of material. In this way the phase content of mixture exhibits a dominating influence on the impedance characteristics over a frequency range. Some previous works have successfully applied the EIS to the composition measurement in the mixture system. However, there also are some developments need to be done to complete the corresponding theory of this method.

In this study, the characteristic frequency – diffusion frequency in EIS of oil-water emulsions was measured and studied under a frequency range of 5×10⁵ Hz AC field. The Beijiang crude oil was mixed into different volume distilled water and to obtain the oil-water mixtures with a water content of 3.0 ~ 45.0 v/v%. Then the oil-water mixtures were emulsified by stirring for 10 min. In this way, the stable water in oil (W/O) emulsion formed. The measurement was performed by a coaxial double cylindrical electrode connecting to a PSM1700 frequency response analyzer. A frequency sweep from 1 to 5×105 Hz was selected because the impedance spectroscopy of oil-water emulsion in the frequency range contained the electrical characteristics of oil-water emulsion based on the observation of the preliminary experiments.

Basing on the Nyquist diagram of EIS, one semicircle and a straight line with a slope of π/2 in terms of Z’-Z” curve of each emulsions. In this way, the diffusion frequency fd can be defined as the frequency where a semicircle turns into a straight line. The value of f_d is extracted from the EIS. It is obvious that there is a linear relationship between the diffusion frequency and phase content. The increased water content leads to a increasing diffusion frequency. This correlation can be explained by an Randles equivalent circuit basing on the ionic double-layer theory. The equivalent circuit is composed of the resistance of emulsion(R_s), double-layer capacitance(C_dl), and charge transfer resistance(R_ct). The capacitance of double-layer generally related to the diffusion process which dominated by the thickness of the ions double-layer.

In the oil-water emulsion system, the water content has a key effect on the distribution of charged ions which decide the diffusion time. By a polarization microscope, we studied the micro-structure of the oil-water emulsions to explore the distributions of droplets. It is observed that the water content increases the average size and number of the water droplets. Previous work has proved that Log-normal distribution function can characterize the the probability density distribution of the droplets. In this function, the u which points to the mean size of the droplets is calculated. The important thing is that the u increases in a significant linear trend with the increasing water content. Thus the concentration of charged ions in the diffusion layer by the increased size of droplets. As a result, the diffusion frequency strongly depends on the phase content in the oil-water emulsion system.

In a summary, our work proposed the linear correlation between the diffusion frequency and water content. A theoretical equivalent circuit basing on the ions double-layer is provided to explain of the linear relationship. The phase content increase the mean size of the droplets in a linear trend, which leads to a concentration of ions in the double-layer. It is indicates that the EIS method can directly and effectively determine the water content of oil-water emulsions.