Combined Solids

The previous three case studies looked at solid formation for hydrates, waxes and asphaltenes as separate problems. However, for some fluids at certain conditions it is possible that any or all of these may form at the same time. The formation of any one will affect the overall composition of the fluid and may therefore affect the formation of the other solids. To examine this possibility we have included a Combined Solids model option. The particular model options for each solid have been chosen to provide the best Infochem can offer whilst ensuring compatibility. The common fluid phase model is RKSA. The hydrate models therefore use RKSAINFO as the fluid model, combined with the Electrolyte salt model. The wax model is Coutinho and there is only one asphaltene model. The Combined Solids option is only designed to look at solid formation, if you want to study the more complex problems such as hydrate inhibition you should still choose the dedicated Hydrates model set. In fact, you will see messages to this effect if you only choose a single solid phase in the Combined Solids option.

Asphaltene flocculation

To understand what happens when more than one solid forms a useful starting point is to examine asphaltene flocculation alone. The example input file provided is combsolid.mfl. This includes an oil composition to C20+ which has a molecular weight of 81, wt% resin of 12.04 and wt% asphaltene of 0.7. The fluid is characterised from C6 with 15 fractions. The resins and asphaltenes are allocated as shown below:

 21R 	R34-39            0.42833656
 22R 	R39-45            3.0323581            	38.820144
 23R 	R45-54            2.6015789            	45.143301
 24R 	R54-66            2.1074825            	53.514291
 25R 	R66-74            0.68430587           	66.023029
 26R 	R74+              0.66055633           	74.
 27AS	ASPHALTENE        0.55317548

The asphaltene model parameters are matched with a bubble point of 120°F and 2650 psia and an asphaltene flocculation point of 120°F and 8750 psia. The predicted ADE is plotted below.

Wax and Asphaltene precipitation

To see the effect of simultaneous wax and asphaltene precipitation we first need to re-characterise the fluid with a n-paraffin distribution in order to apply the Coutinho model. In the PVT form tick the box to estimate wax content as none is known. The n-paraffin distribution is also set to C6 and 15 fractions.

The separation of the n-paraffins from the remainder of the liquid also alters the distribution and properties of the resins and asphaltenes:

 19RI	R34-38            0.28591474
 20RI	R38-43            2.3119605            	38.028733
 21RI	R43-48            2.0563615            	42.714417
 22RI	R48-55            1.782901             	48.24157
 23RI	R55-64            1.5031557            	55.034858
 24RI	R64-73            0.8971822            	64.060083
 25RI	R73               0.016889606          	73.272896
 26RI	R74+              0.66025296           	74.
 27AI	ASPHALTENE        0.55317548

This in itself will alter the resin/asphaltene interaction. Allowing the wax to form will then remove some of the n-paraffins from the fluid again changing the proportion of resins in the remaining fluid.

To see the effect choose the Combined Solids option from Select/Model set and specify wax and asphaltenes as the solid phases. Eliminate Hydrates, water and ice for the time being.

As the model has been re-defined we will need to match the asphaltene parameters again, using the same input data. The new parameters will be slightly different because of the altered distribution.

Now plot the gas and asphaltene boundaries as before then add the wax boundary.

As you can see changing the resin distribution and removing some of the n-paraffins has the effect of stabilising the asphaltene slightly.

Hydrates, Waxes and Asphaltenes

To study the effect of allowing hydrates to form we can retain the fluid characterisation used for wax and asphaltene but need to add water. Do this using Select/Components. Initially set the water composition to zero.

Return to the Combined Solids selection and add the hydrates, water and ice to the list of potential phases.

As we have chosen to re-define the model we also need to re-match the asphaltene parameters. This is best done in the absence of water, which is why the initial water composition was set to zero. Using the same input data the asphaltene parameters will be exactly the same as for the wax and asphaltene study.

Now add the water composition, 10g. Adding too much water may cause difficulties when plotting the ADE.

With water present use the phase envelope plotter to generate all the phase boundaries. Starting point for the asphaltene boundary may have to be changed as it is affected by the presence of the other solids.

The wax boundary is not affected by the addition of water or the formation of hydrate, which occurs at lower temperatures. However, the effect on the lower ADE is significant. As the hydrate is formed the light gas hydrate formers are removed from the fluid. This is in effect the reverse of gas injection and the asphaltene is stabilised with flocculation occurring at higher temperatures.

Of course with water present there is also the possibility of a separate water phase.

If the fluid is flashed at 70°F and 1750 psia Multiflash will predict the formation of 6 phases; gas, hydrocarbon liquid, water, hydrate2, asphaltene and wax. With only 10g of water present reducing the temperature slightly removes the water phase owing to the formation of additional hydrate.

Of course, in practice the formation of so many phases will be affected by kinetics as well as thermodynamics.