Wax-Deposition Experiments Decouple Hydrodynamic Parameters To Aid Scaleup

Topics: Flow assurance
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Oil- and gas-production pipelines typically operate at high Reynolds number and low wall shear stress. Current wax-deposition-prediction models, however, were developed on the basis of laboratory flow-loop experimental data obtained at high shear stress and low Reynolds number. In this study, the effects of the hydrodynamic parameters are decoupled with specially designed flow-loop experiments. The results enhance understanding of the deposition behavior at various hydrodynamic conditions and aid in scaling up from laboratory to field conditions.

Introduction

Subsea production faces both fluid- and flow-based challenges, which eventually can lead to shutdowns, safety issues, and intermittency in production. Among these challenges, the deposition of paraffin, or wax, in the pipelines has gained attention as a flow-assurance problem. The severity can be realized in terms of lost production from reduced flow area, large changes in the pressure drops across pipelines, and changes in fluid properties such as an increase in viscosity of oil with wax precipitation. Accuracy in determining wax buildup across a pipe is critical for designing and applying remediation techniques. For example, accurate prediction of deposition parameters is necessary to manage the pigging process in production lines—including pigging frequencies and mechanical designs of pigs—and treatment of deposits with inhibitors. The assessment of the overall mass flux from the bulk to the interface is a prerequisite for accurate prediction of wax deposition, followed by the prediction of aging and growth individually of the deposit. 

The existing models predicting paraffin deposition use nonrepresentative parameters that cannot be scaled up to field conditions because of the empiricism associated with the parameters. The failure of the parameters may partly be because of the application of incorrect variables used in development of empirical relationships for the parameters. These models show inconsistency in predicting field data. Selecting the correct hydrodynamic parameter for scaleup studies, therefore, is important. 

Experimental Program

Fluid Characterization. The experimental fluids used in this study are laboratory-synthesized model oil containing 5 wt% food-grade wax and Garden Banks condensate.

Experimental Facility. A small-scale facility is used to conduct flow-loop deposition experiments. This facility has three 8-ft pipeline test sections with different diameters. This enables effectively decoupling the effect of Reynolds number and shear stress at the wall on wax deposition. The test sections are configured as pipe-in-pipe to allow for countercurrent flow of oil and glycol. 

The test sections have 0.5-, 1.0-, and 1.5-in. inner diameters and are removable from the flow loop in order to facilitate pigging of the wax mass deposit. The deposited wax mass is collected in order to measure the wax content in the deposit using differential scanning calorimetry. The thickness of the wax samples was calculated from the mass of the deposit, whereas back-calculated thicknesses from pressure-differential measurements are used for the experiments with condensate.

Results and Discussion

The thickness of the deposit was calculated on the basis of the mass collected after each test. The wax mass deposited is normalized per unit surface area to obtain the wax density in order to eliminate the effect of pipe diameters on the deposit. The wax deposit per unit area is called “wax density.” The wax density is used to investigate the effect of two hydrodynamic parameters, Reynolds number and shear stress at the wall, on the deposition phenomenon. 

From the results, it is evident that, when the Reynolds number is constant, the amount of wax mass collected per unit area does not vary significantly, even though the wall shear stress varies dramatically at different conditions for both model and crude oil. For further investigation, wax density was plotted for the constant initial shear stress at the wall and varying Reynolds number. From the results, wax-density values change significantly for different Reynolds numbers, which, in turn, dictate significant changes in the overall mass flux. 

From the available experimental data, it is plausible to conclude that the Reynolds number has a more dominant effect on the wax density than the wall shear stress at different experimental conditions for both model and crude oil. 

The fitting parameters require the Reynolds number in order to compute the thickness. The usage of Reynolds number does not define any discontinuities in the predictive tool, unlike friction factor, which has discontinuities in the transitional regimes. It is also plausible to conclude that the convective forces in transport of wax molecules are much stronger than the shear forces acting near the wall. Further investigation is required on different oils and at different experimental conditions to verify this behavior.

Conclusions

An experimental analysis of wax mass deposit is conducted to study the effect of hydrodynamic parameters (Reynolds number and shear stress at the wall) on model oil and condensate using the parameter normalized wax density. The wax mass density is an appropriate representation of overall flux of wax from the bulk to the interface instead of thickness or wax content. The change in the wax density is minimal for a significant proportional variation of wall shear stress. These results might suggest that wall shear is not a governing factor in the transportation of wax molecules from the bulk to the deposit. The effect of Reynolds number on deposition is dominant, causing considerable changes in the trends of wax mass density. On comparing the effect of wall shear stress and Reynolds number, Reynolds number has a more dominant effect than wall shear stress on the overall mass transport of wax from bulk to the interface. Detailed quantitative and qualitative comparisons between deposit characteristics at various Reynolds numbers and wall shear stresses at laboratory scale provide insight concerning the correct scaleup parameters. In order to develop more-accurate wax-prediction models for operations, particularly for waxy-crude-oil-production lines, it is necessary to tune the proper scaleup parameters for field applications. 

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 27757, “Effect of Hydrodynamic Parameters on Wax Mass Density: Scaleup From Laboratory Flow Loop to Crude Production Pipelines,” by N. Daraboina, SPE, J. Agarwal, SPE, S. Ravichandran, SPE, and C. Sarica, SPE, The University of Tulsa, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

Wax-Deposition Experiments Decouple Hydrodynamic Parameters To Aid Scaleup

01 November 2017

Volume: 69 | Issue: 11

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