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Field-Wide Equation of State Model Development

Authors
Bilal Younus (Whitson AS) | Curtis Whitson (Whitson AS / NTNU) | Ahmad Alavian (Whitson AS) | Mathias Carlsen (Whitson AS) | Sissel Martinsen (Whitson AS) | Kameshwar Singh (Whitson AS)
DOI
https://doi.org/10.15530/urtec-2019-551
Document ID
URTEC-2019-551-MS
Publisher
Unconventional Resources Technology Conference
Source
SPE/AAPG/SEG Unconventional Resources Technology Conference, 22-24 July, Denver, Colorado, USA
Publication Date
2019
Document Type
Conference Paper
Language
English
Copyright
2019. Unconventional Resources Technology Conference
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14 in the last 30 days
111 since 2007
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Abstract

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 properties1 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 systems2.

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.

File Size  604 KBNumber of Pages   40

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