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Remarkable Decrease in Viscosity of Waxy Crude Oil under Electric Field

Name: Ankita Jain

Department: Chemical engineering

Program: Ph D. (4th year)

Name of supervisor: Prof. Jyoti R. Seth and Prof. Vinay A. Juvekar

 

Remarkable Decrease in Viscosity of Waxy Crude Oil under Electric Field

Fig1 depicts the liquefaction of crude oil and the reduction in viscosity by the application of electric field.

Fig2  indicates the reduction in storage (G' ) and loss (G'' ) moduli with the electric field strength and the reduction in viscosity with electric field in the presence of shear. The field is applied for 30 mins in both the cases.

Crude oil is a complex mixture of many components, in which paraffin waxes, asphaltenes, and resins are predominantly responsible for the problem of flow assurance. The waxes which are non-polar compounds, crystallize when the ambient temperature falls below pour point of crude oil, to form a solid-like structure. Asphaltenes and resins are relatively high molecular weight polar compounds. These polar particles act as the nuclei for the crystallization of waxes and help forming a strong network, which can impede the flow of crude oil in pipelines. Crystallization also increases the viscosity of the crude oil due to the gel-forming tendency of the wax crystallites. Mitigation of these problems involve a significant spending of capital by petroleum industries.

Presently two methods are used predominantly for flow assurance: (1) Heating the pipeline and (2) Addition of chemical additives. Both of these have their limitations and are not cost effective. Addition of Chemical additives is mainly used to reduce the pour point of the crude oil. These additives are known as pour point depressants. They are not effective at temperatures below the pour point.  Heating of the pipelines is very expensive. Here, we have used a novel technique of application of DC electric field to crude oil below its pour point in order to break the wax network and reduce the viscosity of the crude oil.

Application of electric field, which precludes use of any additive or heating of pipeline, has been claimed as a promising technique to reduce the viscosity of crude oil. But, all the studies available are confined to temperatures above the pour point, at which the crude oil is a viscous liquid. Also the field is applied along the direction of flow. In our work, the field is applied in the direction perpendicular to the direction of flow and below the pour point of the crude oil at which it is a solid gel. All the experiments for rheological measurements were performed with the electrorheological module of rheometer. For viscosity measurements, a constant shear rate of 10 s-1 was used, and the measurements were performed in three stages:

  1. Crude oil was sheared without applying any electric field (typically for 30 min)
  2. Electric field was switched on and shear was continued (typically for 30 -60 min)
  3. Electric field was switched off and shear was continued (typically for 60-90 min)

In the first stage, the viscosity reduces initially for 5-10 min and then becomes constant, in the second stage, viscosity reduced up to 2 orders in magnitude, and in the third stage, viscosity starts increasing slowly (see the graph in Fig1). During the second stage, i.e., application of electric field, three regimes are observed: induction regime, intermediate regime, and the final regime. During the induction regime, no significant change in the viscosity is observed. Viscosity reduces drastically during the intermediate and the final regimes and this decrease in viscosity follows first order kinetics. The rate constant of intermediate regime varies linearly with the strength of electric field while that of final regime varies with the square of electric field strength. Moreover, from microscopic study we have observed that the network of crude oil breaks during the intermediate regime and the fine fragments of broken network gets aggregated during the final regime. After cessation of field, i.e., during third stage, the rate and extent of recovery depend only on the viscosity of the crude oil at the point of cessation of field.

Wax aggregates being non-polar, are expected to have lower dielectric constant and conductivity than the crude oil. During the application of electric field, the difference in dielectric constants would cause Maxwell stress at the solid-liquid interface and this stress would deform and break the crude oil network into smaller and disconnected particles. With continued application of electric field, the suspended polar particles get aligned and aggregated in the electric field direction. The network breakage and subsequent alignment and aggregation of polar particles results in the reduction of viscosity and the liquefaction of solid crude oil. Dielectric measurements were also performed for crude oil and its solid and liquid components. From these measurements it was concluded that the network experiences compressive Maxwell stress, which is dominated by electric field within the wax fragments and leads to breakdown of the network.

The viscosity measurements were performed in the presence of shear so, to see the effect of only electric field, small amplitude oscillatory measurements were performed. For these measurements, different intensities of electric field were applied under quiescent conditions for 30 min. After 30 min the measurements were performed and the storage (G’) and loss (G’’) moduli in the linear viscoelastic regime were measured. The reduction in both the moduli was observed even for the lower electric field (Fig 2).

Conclusion: The effect of DC electric field on crude oil has been studied in both, the presence and the absence of shear. Experiments were performed on crude oil below its pour point temperature, and it was seen that a solid-like gel gets converted to more liquid-like suspension. The reduction of two orders in magnitude of viscosity as well as moduli have been observed. The rise in viscosity was observed after the cessation electric field but at a very small rate; it is being expected that the crude will never regain its original viscosity.

 

It can be concluded that under the condition of non-polar particles dispersed in a polar medium, application of electric field has the opposite effect from that for an electrorheological fluid. In electrorheology, application of field leads to transformation of a low viscosity fluid to a very high viscosity fluid. But in this case, a high viscosity almost solid-like gel transforms to a very low viscosity fluid. Difference in the states of the crude before and after the application of the electric field is evident from Fig 1.

To access full Paper: Soft Matter, 2020, DOI: 10.1039/D0SM01404D