PVT Analysis
Many users will receive a PVT Analysis for the composition of an oil or gas from one of the PVT laboratories and wish to use this as input to Multiflash. These reports follow a fairly standard format and the PVT Lab Analysis form endeavours to reproduce this to make entering information as easy as possible. The facility to add or delete components from the generated list is also useful.
The case study we are considering here is based on a problem setup file called pvt_anal2.mfl.
To enter a PVT Analysis when you have no measured n-paraffin distribution either choose the Select/PVT Lab Input menu option or click on the 
icon. The Lab Analysis form will then be displayed.
Initially we will consider a case where you only have a single fluid composition. First select the datasource for your discrete (i.e. well-defined) pure components. This can be Infodata or DIPPR and we have chosen Infodata. Next at the top of the column headed Single fluid choose either mass or mol % as appropriate by clicking on the down arrow. If your PVT report offers a choice of mole or mass %, it is the mass % that is the experimentally measured data and should be given preference for separator oils. Next enter the compositions of the discrete components and the compositions of the petroleum cuts. In the form the pseudocomponents or single carbon number (SCN) cuts are labelled C6, C7 etc. In your PVT Laboratory report they may be referred to as hexanes, heptanes etc., with the heaviest being labelled as a plus fraction such as C20+ or eicosanes+. In our example the heaviest SCN is C20.
The overall percentage will be totalled as you enter the compositions. If the final total is not 100 you will be offered the opportunity to normalise the compositions when you characterise the fluid.
You can enter further information to define the stream, such as the molecular weight of the Stock Tank Oil (STO), the total fluid or the heaviest SCN or the specific gravity of either the heaviest SCN or the STO. We have provided general advice on when such data should be supplied in “Fluid composition” on page 39. As the fluid in question has a heavy end (C6+) which comprises more than 50% of the stream we should supply this information if possible. We have therefore entered the molecular weight of the heaviest SCN but if you have the molecular weight of the total fluid available this may be preferable as this is again the measured quantity.
You are now ready to define the basis of your characterisation by choosing where in your existing analysis you want to start redistributing the remaining fluid into new pseudocomponents and how many pseudocomponents you want to split this heavy end into. We’ve started with the simplest case where we have chosen to start the split at the heaviest SCN and only allocate one pseudocomponent. Effectively we are only allocating physical properties to the existing SCNs. Click on the Do Characterisation button and you will see a message box such as
followed by a screenshot of the experimental data and the fitted distribution
Click on OK and Close to return to the main window where the new fluid composition will be reported
The output lists the components and their composition in the units requested. The additional column indicates the lower boundaries of the particular cut, for example C6 comprises the cut from C5.5 to C6.5.
Properties of the individual pseudocomponents may be viewed using Tools/Pure Component Data as usual and further calculations can be carried out on the basis of this characterisation.
At this point, having successfully characterised the fluid, you can also save the input as an .mfl file.
A useful way of seeing how changing characterisations alter the results of phase calculations is to use the phase envelope generator. For instance, plot the phase envelope of this fluid.
You can investigate various aspects of the characterisation and the sensitivity of the phase envelope to changing these.
You can include a n-paraffin distribution by ticking the Estimate Wax Content box. Set the starting point for the n-paraffin to N6 with 15 n-paraffins. In the this case the names and compositions of the fraction cuts will differ,
If you return to the PVT Lab Analysis form and instead of the heaviest SCN choose total liquid and enter a MW of 68. Do the characterisation and plot the phase envelope. Then see what the effect is of extending the heaviest SCN to further fractions, by leaving C20 as the start of the pseudocomponents but choosing to split it into 5 pseudocomponents. Alternatively you can group the components by starting the pseudocomponent split at C8 and grouping the plus fraction into 15 pseudocomponents. You can see that this alters the cricondenbar but the major effect is on the cricondentherm.
Next, return to the original fluid definition and re-plot the phase envelope, then in the PVT Analysis form enter a watercut. This is defined in terms of the volume percentage of the total fluid that is water. In this case choose 3 %. In the main window plot the new phase envelope and the water phase boundary.
Finally, return to the original fluid analysis again and this time add a separator gas. Here we will look at a simple problem where the gas is 100 % methane added at a GOR of 100 m3/m3. Move to the Liquid + Gas tab and enter 100 next to methane in the left hand column headed separator gas and in the Recombined fluid section of the PVT form set the GOR units to m3/m3 and enter 100. Do the characterisation and return to the main window and plot the new phase envelope.
User defined carbon number cuts
In the previous examples the carbon number cuts, SCN, have been defined by the software. If you wish you can over-ride these by choosing your own SCN.
Starting with the pvt_anal2.mfl file go to the PVT Analysis form and change the starting point for the pseudocomponents to C6 with five pseudocomponents. Do the characterisation and return to the main window; you will see that the plus fraction has been defined with five SCN
10P C6-13 33.108052 5.5 11P C13-18 14.878956 12.607067 12P C18-26 10.563102 18.462075 13P C26-41 7.2496189 26.486447 14P C41+ 3.0202717 40.629215
Now, plot the phase envelope and then return to the PVT Analysis. This time check the box for user defined cuts. The drop down box allows you to set the SCN for the specified number of pseudo components, in this case 5.
The amounts and starting points for the new SCN are reflected in the results window.
10P C6 1.775 5.5 11P C6-10 22.231601 6. 12P C10-20 26.692716 10. 13P C20-30 10.53824 20. 14P C30-40 4.3918323 30. 15P C40+ 3.1906107 40.
For these user defined SCN the phase envelope is identical to the original.
A more distorted distribution, e.g.
10P C6 1.775 5.5 11P C6-30 59.462557 6. 12P C30-35 2.6638432 30. 13P C35-37 0.78189604 35. 14P C37-40 0.94609303 37. 15P C40+ 3.1906107 40.
TBP curves
If your PVT analysis data, instead of a detailed SCN/Composition report, is based on assay data, such as a True Boiling Point (TBP) curve or a D86 analysis, you can still enter this and convert the data to fixed carbon number cuts.
Go to the PVT Analysis and click on the tab marked Distillation curves and enter your data. This case study is based on the TBP.mfl file, which has volume % and boiling point data but no Molecular weight or specific gravity.
Once you have entered this proceed as usual and do the characterisation If this is successful the plot will show the comparison of data and fitted distribution,
and the carbon number distribution will be reported in the results window.
Carbon No.
Components Amounts/mole lower boundary
1P C4 0.00078870364 3.5
2P C5 0.03026437 4.5
3P C6-12 0.15090132 5.5
4P C12-15 0.04937333 11.896606
5P C15-18 0.032669471 15.212469
6P C18-22 0.024022918 18.376471
7P C22-25 0.018497293 21.691225
8P C25-30 0.014572272 25.378529
9P C30-35 0.011598081 29.633245
10P C35-41 0.0092434102 34.677386
11P C41-48 0.0073190586 40.796286
12P C48-58 0.00570802 48.390579
13P C58-70 0.0043335075 58.017872
14P C70-87 0.0031427522 70.465232
15P C87-112 0.0020981293 87.10091
16P C112-158 0.0011719901 111.52395
17P C158+ 0.00034349835 158.05652
Once you have defined a fluid model you can carry out calculations or plot the phase boundary as for any fluid.
Black Oil Analysis
The black oil analysis offers the user an opportunity to take a very limited input specification (known as Black Oil input) for a condensate or oil and from this generate a normal compositional analysis. Our example is based on the blackoil.mfl file.
The minimum required input is the gas gravity(relative to air), the STO specific gravity(relative to water) at 60F and 14.7 psi and the solution GOR. The latter is the volume of gas produced at surface standard conditions divided by the volume of oil entering the stock tank at standard conditions. It is often referred to as Rs.
The remainder of the form is the standard PVT, except that you do not provide molecular weight or specific gravity. You can choose the pseudocomponent distribution as normal, depending on the final application. In this case the split is fifteen fractions from C6+. Clicking on Do Characterisation generates the message that the characterisation has been successfully completed – in this case there is no compositional information to generate the compositional plot. The new composition is echoed in the main window and the phase envelope can be plotted as before.
Additional data can be added such as the Watson K-factor and/or the Gas analysis. Plotting the phase envelopes shows the effect of including this data.
