Eos Modeling

PVT modeling of reservoir fluids is a Calsep core expertise.

Over the last 30 years, Calsep’s PVT experts have been engaged in equation of state (EoS) modeling projects for reservoir fluids from the North Sea, Middle East, Gulf of Mexico, South and North America, Africa, and Asia Pacific.

Making use of a superior characterization method and advanced parameter regression facilities in PVTsim Nova, Calsep’s consultants can customize an EoS model for any specific need.










Calsep undertakes EoS modeling projects for all types of petroleum reservoir fluids, including:

  • (Acid) gases
  • Gas condensates
  • Near critical fluids
  • Volatile oils
  • Black oils
  • Heavy oils 

Calsep developed EoS models are currently used all over the world for reservoir, flow assurance, and process studies.

Classical Single Fluid EoS Model

A correct representation of gas and oil phase behavior is a requirement to evaluate oil and gas reserves, to plan and design production, and to optimize field operations.

Using reservoir fluid sampling data and a fluid composition, Calsep can develop a sound and robust EoS model providing a solid and reliable foundation for further project studies. The model may be fine-tuned using routine PVT data:

  • Saturation point data
  • Single or Multi-stage separator tests
  • Constant Mass/Composition Expansion (CME/CCE) experiments
  • Constant Volume Depletion (CVD) experiments
  • Differential Liberation (DL) experiments
  • Viscosity data

Calsep develops EoS models based on the classical cubic equations, SRK-Peneloux and PR-Peneloux/PR78-Peneloux, and also using the more complex PC-SAFT equation. For gases, the GERG-2008 equation can be used when the main target is accurate Z-factors.

For a single fluid system Calsep’s PVT consultants can be expected to deliver an EoS model, where the PVT data match is within:

  • 1% on saturation pressure
  • 2% on separator test total GOR’s
  • 2% on densities
  • 5% on liquid drop-out curves for condensates
  • 2% on oil formation volume factors
  • 5% on solution gas oil ratios (Rsd).
  • 5% on oil viscosities

Advanced EoS model for multiple fluids (Common EoS model)

Calsep's expertise also comprises development of common EoS models valid for multiple fluids. Such models can be advantageous for handling multiple fluids in communicating reservoir zones and/or handling fluids with similar characteristics coming from different reservoirs.

With a Common EoS model, a number of fluid samples will have a common component list and a common set of EoS model parameters.

This makes the common EoS model particularly useful in compositional reservoir simulation studies and in process simulation, where multiple feed streams are mixed. If each fluid had its own component list and model parameters, the total number of fluid components could easily increase to an unmanageable level.

When developing a common EoS model for multiple fluids, Calsep’s PVT consultants will customize the characterization and lumping scheme to ensure all PVT data for the fluid samples are simulated with the highest possible accuracy.

Mud contaminated fluids

Bottom hole reservoir fluid samples are often contaminated by base oil from drilling mud. Calsep provides consulting services in order to estimate the composition of a reservoir fluid from a sample with a certain content of base oil contaminate. Calsep will carry out regression to the experimental PVT data existing for the contaminated fluid and provide a regressed non-contaminated fluid composition. Using data for contaminated fluid samples Calsep can develop EoS models for clean reservoir fluid samples both for single fluid samples and for multiple fluid samples as a common EoS model.

EOR EoS model

EoS models for reservoir fluids undergoing gas injection for EOR purposes must respond correctly to multiple contacts with injection gas. It is of particular importance that the EoS model provides a correct representation of the amount of gas required to shift the reservoir fluid to a critical composition. Calsep’s PVT consultants can develop EoS models that accurately match routine as well as EOR PVT data:

  • Swelling data including critical point on saturation pressure curve
  • Equilibrium Contact and Multi Contact data
  • Minimum Miscibility Pressure in Slim Tube experiment
  • Gas Revaporization Experiment (for gas condensates)

EOR PVT data may exist for more than one injection gas. The gases can be a hydrocarbon mixture, pure CO2 or a gas rich in H2S. Calsep can deliver a single fluid or a common EoS model that matches EOR PVT data for multiple fluids while recognizing the shift from oil to gas condensate type of fluid in the swelling experiment for each injection gas.

Since a common EoS model for multiple fluids is typically used in compositional reservoir simulation studies with a high number of grid blocks, a heavily lumped fluid model is often required.

Evaluation of existing EoS models

An existing EoS model may have to be revisited, either because new experimental information has become available or because the production strategy has changed.

Calsep has developed procedures, which can accurately determine whether an existing EoS model is applicable not only for the purpose it was developed, but also for other purposes. Examples could be whether an EoS model developed for reservoir studies could also be used for flow studies or whether a model developed for natural depletion is also applicable to gas injection EOR.


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