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PIPESIM offers fully compositional fluid modeling as an alternative to theBlack Oil model.

Compositional fluid modeling is generally regarded as more accurate, especially for wet gas, condensate and volatile oil systems. However, detailed compositional data is less frequently available to the production engineer.

PIPESIM currently has access to two compositional PVT Frameworks that provide several PVT flash packages.

Original PIPESIM PVT Framework:

• SIS Flash, developed by Schlumberger. This is the same Equation of State package used by other GeoQuest products, such as ECLIPSE Compositional, PVTi, VFPi, and others.

• Multiflash, a third-party compositional package (InfoChem).

102 PIPESIM Fundamentals, Version 2010.1

New PVT Toolbox Framework (available in PIPESIM 2010.1):

• Eclipse 300 Flash, a new interface to ECLIPSE two-phase flash, allowing additional Equation of States.

• DBR Flash, two-phase flash developed by the Schlumberger DBR Technology Center. It has a more extensive component library than ECLIPSE Flash.

• NIST Refprop Flash, two-phase flash using HelmHoltz Equation of State.

Equations of State (EoS)

Equations of State describe the pressure, volume and

temperature (PVT) behavior of pure components and mixtures.

Most thermodynamic and transport properties are derived from the Equation of State. They are a function of pressure and temperature.

One of the simplest Equations of State for this purpose is the ideal gas law, PV=nRT, which is roughly accurate for gases at low pressures and high temperatures.

NOTE: The Black Oil model uses this equation along with a compressibility factor (z) to account for non-ideal behavior.

However, this equation becomes increasingly inaccurate at higher pressures and temperatures, and it fails to predict condensation from a gas to a liquid. As a result, much more accurate Equations of State have been developed for gases and liquids.

The Equations of State available in PIPESIM include:

SIS Flash 2-Parameter Peng-Robinson 3-Parameter Peng-Robinson

2-Parameter Peng-Robinson (advanced) 3-Parameter Peng-Robinson (advanced).

Viscosity

Compositional fluid models also use Viscosity models based on corresponding state theory. Available Viscosity models include:

• Pederson (default)

• Lohrenz-Bray-Clark (LBC)

• Aasberg-Petersen

Comparative testing has shown the Pedersen method to be the most widely applicable and accurate for oil and gas viscosity predictions. Multiflash uses the Pedersen method as the default viscosity model, though an option is available to choose the LBC model for backward compatibility.

The choice you make of the Equation of State has a large effect on the viscosities predicted by these methods. The LBC method is more sensitive to the Equation of State effects than the Pedersen method.

DBR Flash Peng-Robinson (with/without Volume Shift) Soave-Redlich-Kwong (with/without Volume Shift Correction).

ECLIPSE 300 Flash

Peng-Robinson (with/without Volume Shift + Accentric Factor Correction)

Soave-Redlich-Kwong (with/without Volume Shift Correction).

NIST Refprop Flash

HelmHoltz Equation of State

104 PIPESIM Fundamentals, Version 2010.1 Figure 29 Selecting the default Viscosity option

Binary Interaction Parameter (BIP) Set

Binary interaction parameters (BIPs) are adjustable factors used to alter the predictions from a model until the predictions match experimental data as closely as possible.

BIPs are usually generated by fitting experimental VLE or LLE data to the model in question. BIPs apply between pairs of components, although the fitting procedure can be based on both binary and multi-component phase equilibrium information.

Figure 30 Selecting a BIP in the Compositional Properties window

Emulsion Viscosities

An emulsion is a mixture of two immiscible liquid phases. One phase (the dispersed phase) is carried as droplets in the other (the continuous phase). In oil/water systems at low water cuts, oil is usually the continuous phase.

As water cut is increased, there comes a point at which phase inversion occurs, and water becomes the continuous phase. This is the Critical water cut of Phase Inversion, otherwise called the cutoff, which occurs typically between 55% and 70% water cut.

The viscosity of the mixture is usually highest at, and just below, the cutoff.

Emulsion viscosities can be many times higher than the viscosity of either phase alone.

Three mixing rules have been implemented that are identical to the options currently available in the Black Oil section.

You can choose any of these options (Figure 31):

• Set to oil viscosity

• Volume ratio of oil and water viscosities

• Woelflin, which uses Woelflin correlation at water cut less than, or equal to, CUTOFF, and water viscosity at water cut greater than CUTOFF.

Figure 31 Mixing options

106 PIPESIM Fundamentals, Version 2010.1

Flashing Options

Flash calculations are an integral part of all reservoir and process engineering calculations. They are required whenever you wish to know the amounts (in moles) of hydrocarbon liquid and gas coexisting in a reservoir or a vessel at a given pressure and temperature.

These calculations are also performed to determine the composition of the existing hydrocarbon phases.

Given the overall composition of a hydrocarbon system at a specified pressure and temperature, flash calculations can determine four factors:

• Moles of the gas phase

• Moles of the liquid phase

• Composition of the liquid phase

• Composition of the gas phase

The compositional module uses inline flashing (PVT tables built in memory) as the default mode of compositional simulation. For inline flashing, PIPESIM has three options (Figure 32):

Interpolation, Interpolation when close to phase boundary, and Rigorous.

Figure 32 Flashing options

Exercise 1 Creating a Compositional Fluid