Case studies – Combined solids
Introduction............................................................................................................................................... 1
Asphaltene flocculation.......................................................................................................................... 1
Wax and Asphaltene precipitation........................................................................................................ 2
Hydrates, Waxes and Asphaltenes....................................................................................................... 3
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.
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 0.392458
22R R39-46 3.04671
23R R46-54 2.61287
24R R54-67 2.11965
25R R67-75 0.692748
26R R75+ 0.662815
27AS ASPHALTENE 0.553910
The asphaltene model parameters are matched with a bubble point of 120F and 2650 psia and an asphaltene flocculation point of 120F and 8750 psia. The predicted ADE is plotted below.
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:
20RI R37-42 1.77096
21RI R42-47 2.27108
22RI R47-54 1.97523
23RI R54-63 1.67646
24RI R63-74 1.17917
25RI R74 1.682313E-02
26RI R75+ 0.661425
27AI ASPHALTENE 0.555300
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 and thus lowering the upper ADE once wax has formed.
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 upper 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 lower temperatures.
Of course with water present there is also the possibility of a separate water phase
If the fluid is flashed at 70F 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.