Phase characterization of in-situ fluids

Phase behavior calculations require that all components and their properties be specified. Crude oils, however, typically have hundreds of components, making the equation of state (EOS) procedure for the phase behavior of mixtures computationally intensive. Thus, components are often lumped into pseudocomponents to approximate the in-situ fluid characterization.

EOS calculation for in-situ fluids

The characterization of in-situ fluids with pseudocomponents usually takes the following three steps:

1. The hydrocarbon components in the in-situ fluid are analyzed using analytical techniques, such as chromatography or distillation. New analytical techniques often give a reliable analysis for hydrocarbon components up to C₃₀, instead of the traditional C₇. Properties for hydrocarbon components greater than C₃₀ are reported as a C₃₀₊ fraction.
2. The measured components are separated and lumped into a minimum number of pseudocomponents. The chosen number of pseudocomponents is often a result of the measured fluid characterization and the degree of accuracy required (see step three). The properties and selection of the pseudocomponents are determined using a variety of methods as reported in Whitson and Brule. The required pseudocomponent properties are those needed for the cubic EOS calculations, such as critical temperature, pressure, and acentric factor.
3. The pseudocomponent properties are adjusted to match all available phase behavior data (e.g., PVT reports). This process, which typically uses nonlinear regression, is known as EOS tuning. EOS tuning is needed because of the inherent uncertainty in the properties estimated from step two, especially for the heavier components. Binary interaction parameters are typically the first parameters to be adjusted, although all of the parameters may need some tuning. The number of pseudocomponents may need to be increased from step two to obtain a good fit of the calculated phase behavior with the measured phase behavior data.

The selection of pseudocomponents and their property values are likely not unique, as is often the case when numerous model parameters are estimated by fitting measured data with nonlinear regression. Care should be taken to avoid estimates in the pseudocomponent properties that are unphysical and to reduce the number of parameters. Furthermore, the final EOS characterization is most accurate in the range of the measured phase behavior data. Phase behavior data should be collected that covers, as much as possible, the conditions that occur in the reservoir. The characterization should be updated when new data becomes available.

Finally, fluid characterizations may vary from one location in the reservoir to another. In such cases, multiple EOS characterizations might be required. Compositional variations can occur for a variety of reasons. For example, gravity can cause vertical compositional gradients, where heavier components become more concentrated at greater depths. Several sources provide examples of gravitational concentration gradients. Variations caused by thermal gradients are also discussed in Firoozabadi.

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See also

Thermodynamics and phase behavior

First law of thermodynamics

Second law of thermodynamics

Equations of state

Phase behavior of mixtures

Phase behavior of pure fluids

PEH:Thermodynamics_and_Phase_Behavior