Compositional modeling of reservoir fluid systems with high content of nonhydrocarbon gas at HPHT conditions
Authors: Bolotnik D. , Surguchev L.M. , Norvik H. , Yufin P.A. 
Published: December 2007

Abstract
A compositional simulation model based on a new Equation Of State (EOS) developed in the Russian Academy of Sciences was intensively used and validated by modeling a gas-condensate fields in the Pre-Caspian basin (Tenguiz and Karachaganak fields). The specific features of the new EOS provide improved accuracy in simulating fluid systems at High Pressure High Temperature (HPHT) conditions (up to 1000 bar and 200 C) with high non-hydrocarbon gas content in the reservoir gas.
The commercially available simulators based on Redrich-Kvong, Zudkevich-Jofe, Soave-Redrich-Kvong or Peng-Robinson EOS usually require empirical regression on several EOS parameters or coefficients. The advantage of the new EOS model is that it requires calibration of one parameter only (the boiling temperature of the heavy components fraction). This less empirical approach improves the physical validity of the model, and the dynamic changes of the fluid composition during production life of the field can be modeled accurately without the need for additional adjustments of EOS parameters. In this paper the PVT modeling and reservoir simulation results for the Astrakhanskoye gas-condensate field situated in the Caspian region will be presented. Comparison with the results obtained by using a commercial Western reservoir simulator will be discussed. A procedure to convert PVT experimental data (Differential Condensation, Condensate-Gas factor), typical for the Former Soviet Union, into the format required by the commercial simulators (Constant Volume Depletion, Liquid Saturation) will be also discussed and presented in this paper.

Introduction
Today there are many different software tools for dynamic fluid flow simulation. The commercially available simulators - as a rule based on Redrich-Kvong, Zudkevich-Jofe, Soave-Redrich-Kvong or Peng-Robinson EOS - usually require empirical regression on several EOS parameters or coefficients. These algorithms frequently require long computing time, and not user-friendly. The advantage of the new EOS model developed in the Russian Academy of Sciences (RAS) is that it requires calibration of one parameter only, the boiling temperature of the heavy components fraction. With this less empirical approach, the dynamic changes of the fluid composition during production life of the field can be modeled accurately without the need for additional adjustments of other EOS parameters. A compositional simulation model (MRS) based on the new Equation Of State (EOS) developed in the Russian Academy of Sciences was intensively used and validated by modeling gas-condensate fields in the Pre-Caspian basin (Tenguiz and Karachaganak fields). The specific features of the new EOS provide improved accuracy in simulating fluid systems at High Pressure High Temperature (HPHT) conditions (up to 1000 bar and 200 C) with high non-hydrocarbon gas content in the reservoir gas. Now the software is used in the field development for Astrakhanskoje Gas-Condensate Field in Precaspian. To model the compositional behavior of the fluid the MRS package uses a cubic EOS.
In this paper the PVT modeling and reservoir simulation results for the Astrakhanskoye gas-condensate field situated in the Caspian region will be presented. Comparison with the results obtained by using a Western commercial reservoir simulator (algorithms developed by David Pointing, now with Smedvig Technologies) will be discussed. A procedure to convert PVT experimental data (Differential Condensation, Condensate-Gas factor), typical for the Former Soviet Union, into the format required by the commercial simulators (Constant Volume Depletion, Liquid Saturation) is also discussed and presented in this paper.
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