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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Hustad, O.S., Jia, N.J., Pedersen, K.S., Memon, A., Leekumjorn, S., High Pressure Data and Modeling Results for Phase Behavior and Asphaltene Onsets of GoM Oil Mixed with Nitrogen, SPE Reservoir Eval. Eng. 3, 2014, pp. 384-395.
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Kumar, A., Gohary, M.E., Pedersen, K.S, and Azeem, J., Gas Injection as an Enhanced Recovery Technique for Gas Condensates. A comparison of three Injection Gases, SPE 177778-MS, Abu Dhabi International Petroleum Exhibition and Conference. UAE, 9-12 November, 2015.
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Negahban, S., Pedersen, K.S., Baisoni, M.A., Sah, P., and Azeem, J., An EOS Model for a Middle East Reservoir Fluid with an Extensive EOR PVT Data Material, SPE-136530-PP, Abu Dhabi International Petroleum Exhibition & Conference in Abu Dhabi, UAE, November 1-4, 2010.
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Pedersen, K.S., Leekumjorn, S., Krejbjerg, K. and Azeem, J., Modeling of EOR PVT data using PC-SAFT equation, SPE-162346-PP, Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, UAE, 11–14 November, 2012.
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Sah, P., Gurdial, G., Pedersen, K.S., Izwan,H., and Ramli, F., Equation-Of-State Modeling For Reservoir Fluid Samples Contaminated By Oil-Based Drilling Mud Using Contaminated Fluid PVT Data, SPE Reservoir Evaluation & Engineering, 15, 2012, pp. 139-149.
Sah, P., Pedersen, K.S., and Shaikh, J.A., EOS modelling, the first step in detailed EOR potential evaluation - A case study, SPE-144025, EORC, Kuala Lumpur, Malaysia. July 19-21, 2011.
Shaikh, J.A. and Sah, P., Experimental PVT data needed to develop EOS model for EOR projects, SPE-144023, EORC, Kuala Lumpur, Malaysia. July 19-21, 2011.
Sørensen, H., Shaikh J.A., CO2 Gas Injection as an EOR Technique – Phase Behavior Considerations, Cutting-Edge Technology for Carbon Capture, Utilization and Storage, Clermont-Ferrand, France, 24 -27 September, 2017.
Tybjerg, P., Pedersen, K.S. and Krejbjerg, K., Reservoir Fluid Characterization Procedure for PC-SAFT Equation of State, SPE 187170-MS, SPE ATCE, San Antonio, Texas, 9-11 October 2017.