Wax precipitation

This case study is only applicable if your copy of Multiflash includes the Wax option.

Waxes are far more difficult to understand than pure solids because they are complex mixtures of solid hydrocarbons that freeze out of crude oils if the temperature is low enough. Waxes are mainly formed from normal paraffins but isoparaffins and naphthenes are also present. As with hydrates the formation of waxes is a serious concern in oil and gas processing.

Before discussing the modelling of wax deposition it is worth referring to a paper by Erickson et al. SPE 26604, (1983). which compares the results of measuring wax appearance temperatures (WAT) using three different experimental techniques. For twelve oils, where there were measurements made by at least two different techniques, there was only one case of complete agreement between two methods. Otherwise the minimum difference between techniques was 8 °F, the maximum difference was 55 °F, whilst the average difference was 24 °F. It appears that the accuracy of WAT measurements has improved in recent years, but it is still difficult to measure; it is realistic when assessing results to assume that experimental error in WAT values may amount to several degrees. We recommend measurements made by Cross Polar Microscopy (CPM) if available.

We also recommended that positive amounts of deposited wax are used to identify the WAT, rather than the strict thermodynamic interpretation of zero percent. The suggested default values are 0.045 wt% for reproducing CPM measurements and 0.3wt% for DSC. The equivalent defaults for mol% are 0.015 mol% for CPM and 0.1 mol% for DSC but there is no automatic conversion between mass and mol%. The mass or mole% of wax is related to the liquid plus wax phases.

Defining the wax model

The original Multisolid wax model is removed in MF39, the Coutinho wax model ( a solid solution model ) is the available wax model in Multiflash. For the more detail, refer to the Multiflash Manual

The Coutinho wax model is a solid solution model which requires the information of normal paraffins. Predictions from Coutinho’s model are largely governed by the n-paraffin distribution. If no experimental data are available for this it can be estimated from the total wax content. If these data too are lacking then Multiflash has procedures to estimate them.

The n-paraffin distribution may be defined differently from that for the remaining liquid phase but we have found that starting the n-paraffin pseudocomponents from C6 and splitting the plus fraction into 15 pseudo-components is again a useful default.

With a limited experimental data set it is not possible to make any definitive statements concerning the accuracy of the model in predicting WAT. What is clear is that Coutinho's model provides a much improved prediction of the amount of wax precipitated as a function of temperature.

Calculating wax appearance temperature (WAT)

The calculation of the wax appearance temperature (WAT), formerly known as the cloud point, is an example of a fixed phase fraction flash.

To define the wax model, select the Waxes tab in the Select/Model set option. Click the Define Model button to define the wax model in the problem definition. Then Click on Close.

Go to the PVT Analysis form to characterise your fluid. If you have an n-paraffin distribution then you should open the PVT Analysis using the button. However, in this example, input file wax.mfl, there is no n-paraffin distribution only a wax content. In this case you should use the normal button.

As the input file includes the wax content, simply providing this value, or if it is not available ticking the “Estimate wax content box”, removes the option of choosing the Infoanal1 distribution. Enter the fluid composition and set both the pseudocomponents and n-paraffins to be split from C6 (or N6) into fifteen fractions.

If you fail to Enter a wax content or to ask for this to be estimated then you will be able to characterise the fluid but when you try to calculate the WAT you will see a warning message box indicating that you don’t have an n-paraffin distribution.

Providing you have n-paraffins in your fluid characterisation you can calculate the WAT at any pressure, by using the WAT, button.

The pressure will be taken from the Pressure text box in the main window. From a study of many waxy fluids we recommend using a small positive amount of wax to identify the WAT and suggest default values for the most common measurement techniques. The default for CPM is preset but this can be altered to any value, including 0%. Click on Calculate WAT to initiate calculation.

Our particular example is based on a supplied problem set up file called wax.mfl. This particular fluid has a reported experimental WAT based on three different measurement techniques. At 1 bar, using CPM the reported WAT was 53C, using NMR was 45°C and using DSC was 40°C. The predicted WAT for the CPM default is 49.4°C and for the suggested DSC default is 37.7°C.

You may wish to vary the n-paraffin distribution for the Coutinho model and see the effect on the predicted WAT. One suggestion is to extend the heavy end as far as possible. You can do this by setting the start of the n-paraffin distribution to something like N90 and only splitting into 1 n-paraffin. You will be warned that the distribution has been extended as far as possible and the highest n-paraffin will be lower than the N90 set. In this case the heaviest n-paraffin is n-76+ and the WAT for the CPM default is 46.4°C. Extending the n-paraffin distribution does not necessarily increase the WAT as there are competing effects from both the properties of the new heaviest n-paraffin and the solubility of the reduced amount of this fraction.

If you have measured values for the WAT then you can tune the pseudocomponent properties for either model to match these values. This is done using the Tools/Matching/Wax Phase option.

For this particular example we can take the WAT at 1 bar to be 53°C for CPM. Keep the characterisation for n-paraffin at N90 and 1 PF, then enter the value or values for the WAT temperatures and pressure and the phase fraction. The fraction chosen can be zero but should probably reflect the suggested defaults for the technique used for the WAT measurement.

The matching facility will amend the values for the melting temperature and the change of enthalpy on melting of the n-paraffin fractions.

The wax boundary can be plotted using the phase envelope button and choosing the wax phase. In this case it makes sense to plot the boundary for .00045 mass fraction as we have just matched to this value at 1 bar and we have a “dead” oil.

For a live oil the amount of wax will be with respect to the total fluid rather than the liquid. This will vary with pressure so in this case it may be better to choose zero mass fraction for the plot. The wax boundary for a live oil is a distinctly different shape. The D marks the point where the wax boundary crosses the bubble point line.

The full range of flashes is available for the Wax models.

Calculating wax precipitation

As with any other phase the amount and composition of the wax phase is determined as part of any flash calculation. Given the uncertainty of the WAT from some experimental techniques and the sensitivity of WAT calculations to the characterisation of the heaviest fractions, a better picture of wax precipitation can be derived from calculation of the wax precipitated as a function of temperature.

Using the Windows version of Multiflash you can carry out a series of PT flashes to see how the wax builds up as the heavier components solidify with decreasing temperature. However, a simpler way is to use the wax precipitation curve button, . Clicking on this produces a table of the wax mass percentage as a with respect to the liquid plus wax precipitated as a function of temperature. The starting temperature is 0°C, or the equivalent in other units, and the finishing temperature is the calculated WAT for zero percent wax. The maximum number of points is twenty but the actual number of points will depend on the WAT, the units used and a sensible step. The pressure will be taken as that specified in the pressure text box or 1 bar if no pressure is specified. The wax precipitation curve below was generated using wax.mfl as supplied.

Wax Precipitation Curve
Pressure:    	1.            bar

  T (degC)   	Wax mass percent in liquid(+wax)
0.           	4.82273
3.           	4.10166
6.           	3.50294
9.           	2.97737
12.          	2.50438
15.          	2.07878
18.          	1.70059
21.          	1.36922
24.          	1.08342
27.          	0.844998
30.          	0.644937
33.          	0.486142
36.          	0.353304
39.          	0.259425
42.          	0.16824
45.          	0.127922
48.          	0.0767653
51.          	0.0402296
54.          	0.0317146
57.          	0.0150222
58.5942      	0.

If you require additional control of the table configuration then you can use a command entered in the Tools/Command box.

The format of the command is

The wax precipitation curve is also plotted

And you can use the Add Data button to add the measured WAT if you wish.