PVT & Reservoir Fluid Phase Behaviour

For reservoir fluid modelling, Hydrafact employs a comprehensive thermodynamic model suite developed over two decades by the Heriot-Watt University Reservoir Fluids Research Group. This model is capable of predicting fluid properties and simulating production scenarios such as:

• Depletion by conventional PVT
• Gas injection, multiple backward and forward contact and swelling
• Minimum miscibility pressure and enrichment
• Near well bore simulation, gas inflow, pressure build-up
• Gas recycling of partially depleted reservoirs
• Fluid sample contamination with oil-based drilling muds

The model suite employs ten well-known equations of state (EoS) to perform conventional as well as unconventional PVT calculations at various conditions, and is capable of describing the phase behaviour of reservoir fluids (particularly gas condensate and volatile oil systems) throughout the whole production process.

Phase and Volumetric Behaviour Prediction

 

Our compositional phase behaviour model is in principle capable of predicting all PVT data using only the composition of the original reservoir fluid. The model does however generally need to be evaluated and/or tuned against some measured PVT data prior to being used in reservoir studies with confidence. Although EoS models can reliably predict the phase and volumetric behaviour of oil systems, predictive capabilities can deteriorate as mixtures approach their critical point. To improve reliably, we can tune parameters to match a set of measured data; the model can then be used to predict phase behaviour over a wide range of pressure, temperature and composition relevant to the recovery process.

Phase behaviour measurement and prediction

 

Density, Viscosity and Interfacial Tension Predictions

IFT measurement and prediction

To accurately predict fluid density, a third parameter known as the volume translation factor is included in the EoS. This third parameter is usually calculated using a constant shift parameter for each component. A new method has been developed in-house to determine shift parameters for light hydrocarbons, which has greatly improved phase density predictions, particularly for gas condensate systems.

Accurate knowledge of the viscosity of hydrocarbon fluids is vital in predicting flow behaviour in pipelines, wellbores, and the reservoir. Various viscosity models such as the Lohrenz-Bray-Clark (LBC), one-reference fluid (CS1) and two-reference fluid (CS2) corresponding states principle are included in our phase behaviour model.
Reliable information on fluid interfacial tension (IFT) is of major importance in both petroleum and chemical engineering. The importance of IFT is magnified when dealing with IOR processes where the relative magnitude of interfacial (capillary), gravitational, and viscous forces can considerably impact the recovery of hydrocarbons. The two most commonly used and reliable methods for predicting interfacial tension - the Parachor method and the scaling law - are included in our in-house phase behaviour modelling suite.

 

Contamination of Reservoir Fluids with Oil-based Muds

 

Sampling is the most important element of any reservoir fluid analysis, yet major challenges are still hampering its efficient and low cost operation. Bottom hole samples of reservoir fluids can be captured using different techniques during well testing, drill stem testing, or wireline formation testing (WFT). If an oil-based mud is used in the drilling process, contamination of reservoir fluid samples with drilling mud filtrate usually deteriorates the information provided by sampling.

The impact of contamination on the behaviour and properties of collected samples has been extensively studied by the Heriot-Watt Reservoir Fluids group as part of a long-running Joint Industry Project (JIP). Various theoretical methodologies have been developed to reliably retrieve the composition of uncontaminated reservoir fluids from the compositional analysis of contaminated samples. These can then be used for the reliable prediction of reservoir fluid phase behaviour.

 

PVT and Reservoir Fluids Modelling Capabilities

• Plus fraction characterisation
• Lumping and de-lumping
• Various mixing rules
• Rapid flash calculation
• Saturation pressure and/or temperature calculation
• Constant composition expansion (CCE) test
• Constant volume depletion (CVD) test
• Multiple forward contact
• Multiple backward contact
• Differential liberation
• Separator test
• Methane injection
• CO2 injection
• Swelling test
• Minimum miscibility pressure and enrichment
• Condensate accumulation near the wellbore test
• Interfacial tension
  • Parachor method
  • Scaling law
• Viscosity calculations
  • Lohrenz-Bray-Clark (LBC)
  • One-reference fluid (CS1)
  • Two-reference fluid (CS2)
  • Heriot-Watt modified LBC (LBC-HW)
• Density prediction
• Contamination of reservoir fluids with mud filtrate