Elsevier

Fuel

Volume 90, Issue 6, June 2011, Pages 2113-2117
Fuel

Yield behavior of gelled waxy oil in water-in-oil emulsion at temperatures below ice formation

https://doi.org/10.1016/j.fuel.2011.02.030Get rights and content

Abstract

Paraffinic waxes precipitate from bulk oil when oil temperatures are lower than the oil wax appearance temperature. The oil can form a gel if the temperature goes below the pour point, especially under quiescent conditions. The strength of the gelled waxy oil increases as temperature decreases further. Application of a mechanical shear deforms and fractures the gel. It is shown that this strength reduction in the gel is irreversible under isothermal conditions. In subsequent cooling, the prior fractured gel even showed much less yield stress than the gel from the shear-free condition at measured temperature. This study explored the gel strength behavior in water-in-oil (w/o) emulsion state. Three different model oils, water-free oil, 10 wt.% w/o and 30 wt.% w/o, were used to determine the yield stress using vane method. Both emulsified oils showed less yield stress values at temperatures between the pour points and ice temperature. Compared to water-free oil at temperatures below ice formation, the higher yield stresses were observed in 10 wt.% w/o oil; however, the lower yield stresses in 30 wt.% w/o oil. Subsequent cooling option after prior gel breakage was also examined.

Research highlights

► The viscosities and the yield stress values of emulsified oils were compared with a water-free waxy model oil at temperatures below pour point, with some measurements well below the ice point. In viscosity measurements, water-free and emulsified oils showed the shear thinning behavior at temperatures below their WAT, as shown in previous literature. ► In water-free oil, the yield stresses increased more or less linearly as the temperature decreased. Lower yield stress values compared to the water-free oil were measured for the 10 wt.% w/o and 30 wt.% w/o at temperatures between pour point and ice point. This trend is opposite to the one reported in the literature, most likely due to the much lower amount of wax in the model oil. ► The yield stress values of the 10% w/o emulsion crossed over to values higher than those of the water-free oil below the ice point, while those of the 30% w/o emulsion continued to increase but stay below the values for the water-free oil. It is possible that the small amount of water in the 10% w/o emulsion strengthens the gels at temperatures below the ice point while the larger amount of water in the 30% w/o emulsion disrupts networks, effectively acting as an inhibitor. ► Yield stress of the gel once broken and subsequently cooled never did reach the initial yield stress values for the same samples at the same temperature. ► This study shows that the yield behavior of water oil emulsions depends on a number of factors including but not limited to wax and water content, and temperature and shear history.

Introduction

Highly paraffinic crude oils can form gels in pipelines leading to serious flow blockages. The waxes are soluble in the oil above the wax appearance temperature (WAT), and the oil is Newtonian. The oil exhibits shear thinning behavior below the wax appearance temperature as the wax precipitates [1], [2], [3], [4]. It has been shown previously that the emulsified waxy oils also show shear thinning [5], [6], [7]. Water–oil emulsions may form during oil production since water is also produced along with oil in most oil production operations. Natural surfactants in the crude oil, mainly asphaltenes and resins, are known to stabilize the water-in-oil (w/o) emulsions because they carry both the hydrophobic and the hydrophilic functional groups. Hydrophobic wax particles can adsorb on the surface of emulsified water droplets by interacting with natural surfactants at temperatures below WAT in the w/o emulsion. During cooling, newly generated wax particles can cover the surface of water droplets and build the gel network in the oil phase [5] impeding the free movement of water droplets. Emulsified water droplets act like dispersed wax particles and increase the bulk viscosity of the oil as well as yield stress [5], [6].

Three-dimensional gel networks are formed in pipelines on rapid cooling under quiescent conditions. Once the gel develops and the flow stops, certain level of upstream pressure needs to be applied to overcome the yield stress of the gel along the pipeline for restart [8]. A force balance on the gelled length of the pipeline gives the pressure requirement as shown in following equation [9], [10].ΔP=4τLDHere, ΔP, τ, L, and D represent the applied pressure drop, yield stress of the gel, pipeline length, and diameter, respectively.

The strength of wax gel network depends on its temperature and shear history. Variations in rheological properties have been studied as function of temperature, cooling rate, and shear rate [4], [11], [12], [13], [14]. The yield stress is higher at lower temperatures and under quiescent slow rate of cooling [11]. When mechanical shear is applied at temperatures below the pour point, and the gel subsequently cooled, gel formation can be suppressed, even when the magnitude of the shear stress is small [4]. It should be noted that the pour point is not the sole indicator of complete gel network creation and flow blockage. The pour point (ASTM D97 method [15]) is determined when about two weight percent of the soluble waxes come out of solution causing the oil to not flow “freely”. The oil with low pour point may demonstrate a high yield stress after a brief cooling period. It is important to know how the yield strength develops at temperatures below the pour point to help determine restart pressure requirement.

Rheological studies of w/o emulsion have shown that the oil viscosity increases at higher water content to about 70% water-cut, as does the yield stress of the emulsified oils [6]. The yield behavior of emulsified oils at temperatures below the pour point and further cooling to ice formation is of significant interest. We can hypothesize two different types of yield behavior below the ice point. One possibility is an increase in yield stress with water droplets acting as dispersed wax particles. Additional gel strength is expected to be generated as wax film entraps ice, even though majority of the gel strength is usually attributed to the wax gel network. This scenario could be envisioned if there is adequate wax to account for all of the water/ice in the system. In the second scenario, the water droplets may hinder the continuity of the wax network causing a reduction in yield stress, essentially acting as a wax inhibitor. Wax inhibitors and asphaltenes have been shown to decrease the gel strength [12], [16], [17]. In this study, we compared the yield strength of emulsified oils to water-free oil at temperatures below the pour point of the oil. Temperatures below ice formation were also explored. The water-free and emulsified oils were prepared with excess surfactant to ensure formation and sustenance of emulsion. The yield stress was measured after breaking the gel and cooling the samples further to study the evolution of yield stress under secondary cooling.

Section snippets

Emulsion preparation

Oil phase was prepared by mixing a food-grade wax, white mineral oil (Superla-7) and Span-80 (HLB = 4.3). Deionized water was added to the oil to prepare 10 wt.% w/o and 30 wt.% w/o emulsions at 50 °C. The emulsion constituents are shown in Table 1. The wax composition was measured using high temperature gas chromatography, and the carbon number distributions of wax are shown in Fig. 1. Magnetic stirring was used to make the emulsion. The increase in mixture opaqueness was significant. Formation of

Viscosity of w/o emulsions

Fig. 3 shows the viscosities of three samples: water-free oil, 10 wt.% w/o emulsion, and 30 wt.% w/o emulsion. The viscosities are plotted as function of shear rates and temperatures. At temperatures higher than 25 °C, the viscosity values of the three samples were constant regardless of shear rates employed. At temperatures below 25 °C, significant shear thinning is observed for all the three samples. The viscosity values are higher for both the 10 wt.% and 30 wt.% emulsions across the board with

Conclusions

The viscosities and the yield stress values of emulsified oils were compared with a water-free waxy model oil at temperatures below pour point, with some measurements well below the ice point. In viscosity measurements, water-free and emulsified oils showed the shear thinning behavior at temperatures below their WAT, as shown in previous literature.

In water-free oil, the yield stresses increased more or less linearly as the temperature decreased. Lower yield stress values compared to the

Acknowledgement

The authors acknowledge the assistance of Professor Zhigang Fang in Metallurgical Engineering Department.

References (21)

  • R.F.G. Visintin et al.

    Structure of waxy crude oil emulsion gels

    J Non-Newton Fluid Mech

    (2008)
  • M.A. Farah et al.

    Viscosity of water-in-oil emulsions: variation with temperature and water volume fraction

    J Pet Sci Eng

    (2005)
  • R. Venkatesan et al.

    The strength of paraffin gels formed under static and flow conditions

    Chem Eng Sci

    (2005)
  • H.P. Rønningsen

    Rheological behaviour of gelled, waxy North Sea crude oils

    J Pet Sci Eng

    (1992)
  • R.F.G. Visintin et al.

    Rheological behavior and structural interpretation of waxy crude oil gels

    Langmuir

    (2005)
  • Ahn S, Wang KS, Shuler PJ, Creek JL. Paraffin crystal and deposition control by emulsification, SPE 93357 presented at...
  • P. Singh et al.

    Prediction of the wax content of the incipient wax-oil gel in a pipeline: an application of the controlled-stress rheometer

    J Rheol

    (1999)
  • K. Paso et al.

    Characterization of the formation flowability and resolution of Brazilian crude oil emulsion

    Energy Fuels

    (2009)
  • T.S. Golczynski et al.

    Understanding wax problems leads to deepwater flow assurance solutions

    World Oil

    (2006)
  • A. Uhde et al.

    Pipeline problems resulting from the handling of waxy crudes

    J Inst Petrol

    (1971)
There are more references available in the full text version of this article.

Cited by (0)

View full text