Abstract

Equations of state (EOS) are perhaps the most abused and mis-used concept in compositional reservoir simulation. It is believed that the EOS are often need adjusting or tuning and the well-tuned EOS can capture all phase behavior in the reservoir simulation. However, the well-tuned EOS can not calculate appropriately in the reservoir simulation because the matching objectives, the existing PVT experiments, are not enough to cover all necessary pressure, temperature, and components.

An EOS for CO2, hydrocarbon gas and sour gas injection is tuned for a reservoir fluid offshore Abu Dhabi. The universal EOS for gas injection, EOS-U8, was originally developed as EOS-H18 for hydrocarbon gas injection (both sour and sweet). After the EOS-H18 was developed, additional PVT experiments, swelling tests with CO2 and other two hydrocarbon gases, were conducted and the EOS-U8 was tuned in order to capture the phase behavior of the reservoir fluid and all candidate injection gases. The new, tuned EOS is available for evaluating gas injection, including direct comparison between injection gases. Our tuning procedure for an EOS, including laboratory data selection, is applicable to other reservoir fluids.

The Flash calculation is compared utilizing the tuned EOS using all or selected laboratory data. Limited laboratory data may mislead the tuning process of the EOS parameters for extensive application of the EOS. The methods that gave the best match are also discussed.

Three commercial software for generating EOS parameters were compared for performance, accuracy, and ease of use. The advantages and disadvantages of three commercial EOS PVT programs are compared. The interface and function of the programs are also evaluated. Three EOS based PVT programs show different results with high concentration of CO2 or H2S using the tuned EOS parameters. Therefore, special cares are required for importing EOS parameters form other EOS programs. The most appropriate ways to use the EOS developed by current software products are discussed. The results from this comparison study of EOS based PVT programs are useful for selecting PVT programs.

Finally three EOS (EOS-H8, C8, U8) were compared in the compositional reservoir simulation. The tuned EOS with the selected laboratory data shows different results in the reservoir simulation using the EOS-U8. It is recommended that EOS should be tuned utilizing as much as PVT experiments.

Introduction

Equations of State (EOS) are one of the most important and sensitive factors in reservoir simulation for gas injection study. In 1998, intensive phase behavior study was conducted for a limestone reservoir in a large field of Abu Dhabi offshore. Special PVT tests were conducted using two hydrocarbon gases and one sour gas. The first EOS, EOSH18, was developed, utilizing Software-A, in order to capture the phase behavior of the reservoir fluid with three different components of injection gases.

In the next year, additional phase behavior study was conducted for the reservoir fluid with sour gas, hydrocarbon gas and CO2. The EOS-H18 was updated as EOS-U8 in order to capture the phase behavior of the reservoir fluid with all six gases. In this updating process, it was found that the original EOS might not be compatible with Software-B. The EOS-U8 was developed with the Software-B, which was selected because of the compatibility with the Compositional Simulation Software-D.

In this study, the matching results of EOS using all or selected data are discussed. Then, comparison results of three commercial software for generating EOS parameters are presented. Finally, influence of misused EOS is discussed in the point of the compositional reservoir simulation.

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