In Part 1 of this article series, I discussed how some of the calculated variables in flotools are derived. The discussion focused on calculations that were derived from a single source variable.
flotools performs more complex calculations that take multiple variables into account. A dedicated library for calculating rate-related variables is one such example. Before I get into the specifics of how these calculations are done, it is important to understand the conventions used by OLGA.
Rate variables and their relationships
OLGA tracks three phases in six fields:
| Vapor | Droplet | Film/Bulk | |
|---|---|---|---|
| Gas | ✓ | ||
| Oil | ✓ | ✓ | |
| Water | ✓ | ✓ | ✓ |
The rate variables calculated by OLGA include mass flow rate, volume flow rate, velocity, superficial velocity, and mass flux (mass flowrate / area). Not all these variables are available for all the fields. Another point to note is that the gas mass flow (GG) includes all components (hydrocarbons + water vapor) flow in gas phase. Finally, another key variable that is required to tie some of these rates together is the volume fraction of each phase in the different fields, commonly called holdup. The following tables show the OLGA variables for the different rate variables in the different fields.
Mass flow rate
| Vapor | Droplet | Film/Bulk | Total | |
|---|---|---|---|---|
| Gas | GG | |||
| Oil | GDHL | GLHL | GLTHL | |
| Water | GLWV | GDWT | GLWT | GLTWT |
| Liquid | GD | GL | GLT | |
| All phases | GT |
Volume flow rate
| Vapor | Droplet | Film/Bulk | Total | |
|---|---|---|---|---|
| Gas | QG | |||
| Oil | QDHL | QLHL | QLTHL | |
| Water | QDWT | QLWT | QLTWT | |
| Liquid | QD | QL | QLT | |
| All phases | QT |
Velocity
| Vapor | Droplet | Film/Bulk | Total | |
|---|---|---|---|---|
| Gas | UG | |||
| Oil | UDHL | ULHL | ||
| Water | UDWT | ULWT | ||
| Liquid | UD | UL | ||
| All phases |
Superficial velocity
| Vapor | Droplet | Film/Bulk | Total | |
|---|---|---|---|---|
| Gas | USG | |||
| Oil | USDHL | USLHL | USLTHL | |
| Water | USDWT | USLWT | USLTWT | |
| Liquid | USD | USL | USLT | |
| All phases | USTOT |
Mass flux
| Vapor | Droplet | Film/Bulk | Total | |
|---|---|---|---|---|
| Gas | WG | |||
| Oil | WDHL | WLHL | ||
| Water | WDWT | WLWT | ||
| Liquid | WD | WL | ||
| All phases | WTOT |
And finally volume fractions
| Vapor | Droplet | Film/Bulk | Total | |
|---|---|---|---|---|
| Gas | 1-HOL | |||
| Oil | GAHL | BEHL | HOLHL | |
| Water | GAWT | BEWT | HOLWT | |
| Liquid | GA | BE | HOL | |
| All phases |
The relationships
Once the variables are known, some generalized relationships can be defined. Other than the variables listed above, we also need to know the cross-sectional area of the pipe where these variables are plotted (which is obtained from the input file), and the density of the phases which are obtained from ROG, ROHL, ROL, and ROWT variables. The generalized relationships are:
- Volume Rate = Mass Flow Rate / Density
- Velocity = Volume Rate / Area of cross section
- Superficial Velocity = Velocity * Volume Fraction
- Mass flux = Mass Flow Rate / Area of cross section
- For liquid phases, Total = Bulk + Droplet
- For any given field, Liquid = Oil + Water
- For all phases, Total = Liquid + Gas
flotools first analyzes which of the variables listed are available from the output files, and given these relationships, starts to fill out other variables based on the ones available. Notice that all relationships relate 3 variables. When any two are available, flotools automatically calculates the third. This scheme is applied iteratively so that calculated variables can be used as sources to calculate other calculated variables.
In the next part, some of the more complex calculations in flotools are discussed.