OLGA 101 – The (my) Basics by Andrew Amaechi Halim

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I’ve attended a couple of OLGA courses (see here) both when it was owned by SPT and after it was bought by Schlumberger. On my first course I was like the proverbial deer in headlights, I thought the instructor was speaking “magjugunsi” (an imaginary language I used to speak when I was a child).

There was a huge amount of information to assimilate within a few short days, the simulator program was incredibly complicated and some attendees on the course were already familiar with using OLGA, so the instructor couldn’t hold them back on my behalf.

Forward a few years later, I was on another OLGA course and true to form, there was another “deer in headlights” attendee. Thankfully, this time it wasn’t me! An OLGA course is the WRONG place for “deer in headlights” behaviour. They are definitely not cheap and you have to take a few days off work to attend!

This is my little contribution to the reduction of “deer in headlights” occurrences at OLGA training courses. It is by no means a substitute training course. I don’t have the inside-track of how OLGA has been designed (an on-going improvement process), the calculations/correlations used, or the algorithms for calculating all the variables available. Most of this is not publically available. However, there’re quite a number of articles/books that discuss the basic multiphase flow correlations you’d expect to find in a multiphase flow simulator. I also believe training courses run by the software providers like Schlumberger, Kongsberg, Calsep et al. are invaluable.

Whilst I think OLGA is a great tool for multiphase (transient) flow analysis (there are other less complicated and excellent software tools available for single / multi- phase analysis). I believe to get the “best bang for your buck” on any OLGA training course, it is useful to broadly know your way around the tool, broadly understand its capabilities and shortcomings, and to use the training course (even at beginner level) to improve your knowledge and optimise use of the software tool. It’s also a good idea to build / create a rapport with the trainer during the course as he /she could provide invaluable assistance in future.

OLGA is the “The industry-standard tool for dynamic multiphase flow simulation” (Schlumberger’s words not mine!). It is used to model steady-state and time-dependent behaviours via prediction of system dynamics like time-varying changes in flow rates, fluid compositions, temperature, solids deposition and operational changes. It consists of a core simulator with additional modules for analysis of specific scenarios. These include:

• Slugtracking – hydrodynamic slug behaviour;
• FEMTherm – advanced heat transfer;
• Comptracking – tracking of individual components;
• Complex Fluids – non-Newtonian behaviour;
• OLGA HD – advanced modeling of the hydraulics;
• Multiphase pumps;
• Corrosion;
• Single component flows.

OLGA is arguably the pre-eminent software tool within the oil and gas industry for undertaking (typically, slow) transient multiphase simulations. It has some competitors vying to overtake it within this niche, whom are successfully competing for simulation accuracy in certain flow cases (such as Ledaflow). However, most of these competitors claim to be “as accurate” as OLGA, using OLGA as the benchmark rather than to be more accurate than OLGA (they need to be bolder). OLGA can lay claim to the following:

1. Best funded R&D investment programme since inception
2. Largest multiphase flow data bank assembled
3. Large number of major participating companies in verification and improvement program

OLGA also has access to operating company data via OVIP (OLGA Verification and Improvement Program), other JIPs, and data from laboratory facilities such as IFE, SINTEF, Tiller, CSM, University of Tulsa, NTNU, Imperial College.

There are next to zero articles explaining the underlying codes / algorithms used for calculations within OLGA. It’s obviously their intellectual property, but not knowing the flow regime correlation, or empirical / mechanistic models / correlations used to estimate physical properties, doesn’t enable us better understand the shortcomings of the model.

However, one can hazard a guess based on other published texts / articles on the fundamental calculations available within OLGA.

Typically, multiphase pipeline simulators perform calculations along the pipe axis (i.e. are 1-dimensional, 1D, models) on a coarse grid, allowing the prediction of realistic results within a short (-ish) time frame especially for long pipelines. These 1D multiphase models typically consider the necessary calculations under 2 main categories:

1. Conservation equations on the bulk fluids – liquid, aqueous and gas phases;
2. Closure correlations describing relationships between the wall and fluids, between fluid phases, fluid properties.

These 1D models also typically use implicit numerical solution methods due to the extended time frames. Within OLGA HD (for stratified flow), a 2D velocity distribution is combined with the 1D conservation equations to yield a 3D flow description substantially improving model predictive ability.

Closure relationships describe the complex interaction of the fluids at interfaces, resulting in the correct boundary values for the flow within each fluid. The combination of conservation equations, mass transfer between phases and closure relationships completely describe the interaction with the pipe wall, the fluid phases and fluid properties.

The model tracks the velocity of liquid droplets (within the gas phase) and liquid dispersions (within the liquid phase) via slip relations (ratio of the velocity of the gas phase to the velocity of the liquid phase) to account for gravity and drag forces. The conservation of mass equations (given below) are joined through interfacial friction + interfacial mass transfer + dispersions.

The conservation equations consist of mass + momentum equations for each phase and an overall energy conservation equation e.g. a 3-fluid model should solve the following conservation equations:

• Conservation of mass equation for gas;
• Conservation of mass equation for liquids (oil/condensate + water);
• Conservation of mass equation for droplets (oil/condensate + water);
• Conservation of momentum equation for the water continuous phase (with dispersed oil/condensate);
• Conservation of momentum equation for the oil/condensate continuous phase (with dispersed water);
• Conservation of momentum equation for the gas phase + liquid droplets;
• Conservation of energy for the combined mixture of the fluid phases.

Multiphase flow can result in a number of different spatial distributions of the gas/liquid or liquid/liquid interfaces. Therefore, knowledge of the prevailing flow regime is very important for accurate determination of fluid hydrodynamics and design variables including liquid holdup, pressure drop, heat / mass transfer coefficients etc at a given section of pipe based on the flow rates, velocities, pressure, temperature and pipe inclination.

Mechanistically driven flow regime prediction methods are being increasingly utilised. These are based on physical mechanisms, which determine mathematical / analytical expressions for the transition boundaries between the different flow regimes. To determine the correct 2-phase flow regime, these mechanistic models utilise input variables such as:

• Fluid properties (from PVT file)
• Flow rates (gas/liquid/combined)
• Pipe diameter
• Inclination angle

Note that 2-phase flow regimes are still used even if the fluid is actually 3-phase.

In OLGA, the OLGA-S mechanistic model is used to determine the flow regime and other variables. OLGA-S is the steady-state version of the multiphase flow model incorporated in the widely used OLGA transient simulator.

For a steady state simulation, where variables are not varying (couldn’t find a better word!) with time. The pipeline is split into sections as done during discretisation, as the calculations for each section are performed sequentially, with the results of the previous section acting as the input for the next section. Basically, the rough calculation sequence for steady state flow is:

• Boundary conditions are taken as the input data for the first section (pressure, flowrate, temperature, pvt);
• For this section, the gas / liquid mass flow rates, fluid properties, pipe diameter and angle of inclination are used to estimate the prevailing flow regime;
• This is then used to estimate the thermophysical properties, liquid holdup and pressure drop;
• Conservation equations are solved for that section;
• Results can be used as input to the next section for further calculations, or saved if it is the last section.

For the transient case, the iterations are more involved. This is in order to account for changes to fluid properties, flow parameters and boundary conditions with each time increment. The flow equations are also nonlinear necessitating further iterations to reach a conclusion.

Boundary conditions (especially pressure and composition) are necessary to close the system of equations and enable simulation.

References:

• The Dynamic Two-Fluid Model OLGA: Theory and Application – K. H. Bendicksen, D. Malnes, R. Moe, S. Nuland, 1991
• Multiphase Flow Simulation – Optimizing Field Productivity – Intan Aziz et. al., 2015
• Basic flow modelling for long distance transport of wellstream fluids – D Biberg, H Holmås, G Staff SPT Group, Norway T Sira, J Nossen, P Andersson, C Lawrence, B Hu, K Holmås Institute for Energy Technology, Norway
• Accounting for flow model uncertainties in gas-condensate field design using the OLGA High Definition Stratified Flow Model – D. Biberg, G. Staff, N. Hoyer, H. Holm, 2015
• A Model for Predicting Flow Regime Transitions in HorizontaI and Near-Horizontal Gas-liquid Flow – Y. Taitel, A.E. Dukler, 1976.
• Mechanistic Modeling of gas-liquid two-phase flow in pipes – O. Shoham, 2005
• Pipe Flow 2: Multi-phase Flow Assurance – O. Bratland, 2013

by Andrew Amaechi Halim

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