URTeC 551: Field-wide Equation of State Model Development

URTeC is just a week away. Our CEO & Founder, Curtis Hays Whitson, will present our paper “Field-wide Equation of State Model Development” Monday 22 July 2.15 pm in Room 605/607. We hope to see you there!

Authors: Bilal Younus*1, Curtis H. Whitson1,2, Ahmad Alavian1, Mathias L. Carlsen1, Sissel Ø. Martinsen1, Kameshwar Singh1 || 1. Whitson AS 2. NTNU


The objective of this paper is to present a detailed workflow for developing a field-wide (or basin-wide) “common” equation of state (EOS) model to describe PVT properties[1] of all reservoir fluids and wellstream mixtures at all relevant conditions of pressure and temperature. The presented workflow is a result of having developed many field-wide EOS models in conventional reservoirs around the world, and more recently several basin-wide EOS models for North American unconventionals (Eagle Ford, Montney, Bakken, Permian and Scoop/Stack). We address several important considerations in developing a common EOS, as well as when and why a common EOS is needed.

The starting point for developing a common EOS is the use of all measured PVT properties and fluid compositions of surface and reservoir samples. The goal of a common EOS is to provide accurate PVT property estimation of all mixtures found throughout the field/basin – within all reservoir(s), throughout the production system and to final surface products – from discovery to abandonment.

Measured PVT data must be scrutinized for quality using a series of consistency checks that include component and phase material balances, cross plots, and continual comparison with EOS results. Using all PVT data from all samples gives a substantial, statistically significant data set that allows trend analysis and outlier identification.

One key to developing a common EOS model is using a sufficient number of components, and proper characterization of heavy fractions that contain varying proportions of the three hydrocarbon groups (paraffin, naphthene and aromatic compounds – PNA). The heavy fractions single carbon numbers C7, C8, C9… and the remaining “residue”, e.g. C36+ are often given average properties that reflect the relative proportions of PNA compounds – i.e. relative paraffinicity (or relative aromaticity). The determination of single carbon number (SCN) and residue properties is what we refer to as heptanes-plus characterization, and it is this characterization that will differ from field to field, or basin to basin.

Sometimes within a given field or basin, the relative paraffinicity may vary so much that a single, common EOS using SCN description is not possible. Two options remain: developing multiple EOS models, or creating a single EOS with some/all heavy fractions having two subfractions – paraffinic and aromatic (e.g. C7P and C7A, C36+P and C36+A). In this latter approach, the P-A split must be estimated, correlated or measured for each fluid mixture, making the approach complicated and less common, but necessary in some fluid systems[2].

Developing a common EOS for a field/basin is necessary because in-situ reservoir fluids may vary spatially, change in composition during depletion and gas injection, and because of fluid mixing throughout the production system – within reservoirs, wells, and topside facilities.

For unconventional basins, only a small number of the thousands of wells have laboratory PVT data available, despite significant well-to-well fluid variations – e.g. gas oil ratio (GOR) ranging from 300 to 300,000 scf/STB in the Eagle Ford and Montney basins. Simple PVT correlations are not applicable over the entire range of fluid compositions. Many wells produce complex retrograde condensates, near-critical fluids, and volatile oils that require an accurate and consistent EOS model for estimating PVT properties required by geologic, engineering, and marketing professionals.

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Curtis Hays Whitson

Kameshwar Singh

Middle East
Ahmad Alavian

Mathias Lia Carlsen

About whitson
whitson supports energy companies, oil services companies, investors and government organizations with expertise and expansive analysis within PVT, gas condensate reservoirs and gas-based EOR. Our coverage ranges from R&D based industry studies to detailed due diligence, transaction or court case projects. We help our clients find best possible answers to complex questions and assist them in the successful decision-making on technical challenges. We do this through a continuous, transparent dialog with our clients – before, during and after our engagement. The company was founded by Dr. Curtis Hays Whitson in 1988 and is a Norwegian corporation located in Trondheim, Norway, with local presence in USA, Middle East, India and Indonesia

[1] In this paper we use the term “EOS model” or simply “EOS” to represent a model that predicts phase equilibrium and volumetric behavior. A separate, compositionally-consistent viscosity model is needed in reservoir simulation because near-critical conditions may exist where phase consistency is important. Pipe flow and process calculations often exist far from critical conditions and can, therefore, use independent phase-specific correlations for viscosity (and density) that are more accurate over a wide range of pressures and temperatures.

[2] Two examples include (1) vertical compositional gradients showing significant API variation with depth (Schulte 1980) and a saturated gas-oil contact with relatively paraffinic gas cap and relatively aromatic oil zone.