Emulsion of waxy crude oil is one of the common states in the subsea pipeline. At low temperatures in offshore environment, waxy crude oils with water could form the crude oil emulsion gel of oil-in-water emulsion. Thus, the waxy crude oil emulsion viscoelastic behavior for deep sea transportation and restarting pipeline safety is particularly important. By means of MASIII HAAKE rheometer which is produced by German company, waxy crude oil emulsion viscoelastic behavior is explored at different volumetric water contents and different shear stresses. By analyzing the rate of change of shear rate in the initial stage, the influence rules of viscoelastic properties were summarized, with the change of volumetric water content and the applied shear stress and based on the experimental results, the law of emulsion is explained from the micro level. It is proposed that brittle fracture exists between wax crystals, and flexible fracture was found in the interaction between water droplets and wax crystals.
With the global strategic oil basis to the deep sea, oil and water mixed transportation technology has great application value in the submarine pipeline. In oil-water flow, waxy crude oil and water form water-in-oil emulsion, compared with waxy crude oil, under low temperature conditions, oil production is complex in deep sea, because waxy crude oil emulsion more easily forms gel structure. Similarly, if the pipe is stopped because of the plan or accident, crude oil or emulsion will form a gel structure in the process of conveying emulsion or oil to the ground with pipeline. To restart, we must make the pump start-up pressure can overcome the yield stress of the waxy crude oil or emulsion gel in the pipeline, in order to ensure the smooth start of the pipeline [
The rheological properties of waxy crude oil itself are complex. There is an interaction between the emulsion liquid system of waxy crude oil, for example, interaction between inner phases and outer phases, droplets and droplets [
Experimental instruments are mainly MARSIII rheometer produced by German HAAKE companies, and used its Z41 coaxial cylinder measurement system. Rheometer equipped with A40 controlled water bath with temperature controlling precision of 0.1˚C produced by German HAAKE companies. Used for the preparation of emulsion instrument is RW20 strong and digital mixer produced by the German IKA company, its speed range is from 0 r/min to 2400 r/min.
The properties of matter of waxy crude oils used in the experiments are shown in
In this paper, the crude oil is high wax oil, which is characterized in that the temperature dropped to below the temperature of wax precipitation point will appear wax crystal. If the temperature of making emulsion is below the temperature of wax precipitation point, it will affect its comparability of the result of viscoelasticity measurement [
Physical parameters | Parameter value |
---|---|
Density of 20˚C (kg/m3) | 865.5 |
Condensation point (˚C) | 33.4 |
Wax precipitation point (˚C) | 40 |
Wax content (wt%) | 15.5 |
10 minutes. Due to the type of impeller affect physical properties of emulsion directly [
The viscoelastic properties of waxy crude oil emulsion are studied by the method of applying constant shear stress in this paper. The viscoelastic-yield behavior of waxy crude oil has been studied. Compared with the crude oil, emulsion viscoelastic properties are more complex. Because of the existence of water-drops, the viscoelastic of waxy crude emulsion gels are not only related to the wax crystal structure, but also to the strength of interfacial film between water droplets and crude oil. It is proposed that brittle fracture exists between wax crystals, and flexible fracture was found in the interaction between droplets and wax crystals, through the changing law of the change rate of shear rate in this paper.
The prepared fresh emulsion was quickly and accurately poured into rheometer measuring tube, then reduce the temperature to 35˚C at the cooling rate of 0.5˚C/min. Before the experiment measuring the time scan is required, which this can measure the time of the sufficient formation of the gel structure of crude oil, it is 45 minutes. Changing new emulsion and repeating the above operation, then 45 minutes later, the measurements were carried out with the method of applying constant shear stress.
Under the above conditions, different volumetric water content of emulsion of 10%, 20%, 30%, 40%, 50% need to be prepared. The emulsions of these water contents are carried out with applying different constant shear stresses. These constant shear stresses are 80 Pa, 90 Pa, 100 Pa, 110 Pa and 120 Pa separately.
At different volumetric water content, the shear rate will go through the following stages: 1) Rapid increase in shear rate; 2) The shear rate is increased by a rapid increase to a gentle increase; 3) Gentle increase stage. With increasing volumetric water content, shear rate value of gentle stage is low. The change of shear rate at the initial stage also changes with the change of water content. Specific changes will be analyzed in the following. Under the constant shear stress of 80 Pa, the variation of shear rate with different water content is shown in
In order to explore the viscoelastic properties of wax crude oil emulsion, the initial stage of the experimental data will be analysed carefully. The initial stage means that the main properties are viscoelastic. After repeated several experiments, the first 20 seconds of the experimental dates are analysed in this paper. By calculating the change rate of shear rate of the first 20 s, the changing law of the change rate of shear rate at the same shear stress and different volumetric water content are showed in
Under the constant shear stress of 80 Pa, the curve of change rate of shear rate―time is shown in
and the balance, the gentler the change rate of shear rate rise and decline. Wherein, the water content of 20% is the transition stage, which curve is between the water content of 10% and the water content of 30%. As shown in
When the water content is 10%, the changing law of the curve is shown in
Investigate its reason, Visintin has been proposed that water droplets are gradually covered with wax crystals, due to the adsorption of wax crystals on the droplet interface, then the gel structure is formed with the droplets wrapped in three-dimensional network of wax crystals. Recently, by studying the creep-recovery characteristics of mock oil emulsion, Haj-shafiei et al. [
data, at the same time, it is also found that the three-dimensional network structure of space between the wax crystals are prone to brittle fracture, wax crystals attached to the water droplets surface are prone to flexible fracture by applying shear stress, because of elastic effect of interfacial film between wax crystals and water droplets.
As shown in
The viscoelastic properties of waxy crude water-in-oil emulsion are studied by the method of applying constant
shear stress in this paper. The change rate of shear rate is analyzed at the initial stage in each of experimental data. The main conclusions are as follows:
1) Each shear stress corresponds to a critical water content. When it is less than the critical water content, the change rate of shear rate showed a trend that first step decreases rapidly and then slowly. When it is more than the critical water content, the change rate of shear rate showed a trend of increasing firstly and then decreasing; and peak value with the increasing of water content decreases; the trend of increasing firstly and then decreasing also becomes more slowly with the water content increasing.
2) Similarly, each water content corresponds to a critical shear stress. When it is greater than the critical shear stress, the change rate of shear rate presents decreasing rapidly at first and then decreasing slowly, and finally tends to zero; when it is less than the critical shear stress, the change rate of shear rate presents increasing at first and then decreasing, finally tends to zero, and the peak value decreases with the decreasing of shear stress, and the trend of increasing firstly and then decreasing with the increasing of shear stress is becoming more and more slow.
The National Natural Science Fund (No 51404072, No 51534004).
Liping Guo,Shuang Shi,Yu Wang, (2016) Study on Viscoelastic Behaviors of Waxy Crude Water-in-Oil Emulsion. Advances in Chemical Engineering and Science,06,209-215. doi: 10.4236/aces.2016.62022