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 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.
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
Mathias Lia Carlsen
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
 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.
 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.