Efficient application of reduced variable transformation and conditional stability testing in reservoir simulation flash calculations

Abstract

Methods and computer readable media for performing a compositional reservoir simulation of a subterranean hydrocarbon-bearing reservoir is provided. Reduced variables for flash computations are utilized with a methodology of conditional stability testing for achieving optimal efficiency of phase behavior computations in a compositional reservoir simulator. A least abundant phase is selected as primary variables for a primary phase and a secondary phase is selected for a more abundant phase to ensure stability by not dividing by a value near zero due to the selection of the primary phase as being associated with phase which is the least abundant. A bounded interval may be used to limit solution changes in reduced variable algorithms to achieve greater stability of algorithms. Stability tests may be performed during flash computations using reduced variables by employing a direct residual form based on the definition of the reduced variables and the tangent-plane distance condition.

Description

technical field [0001] The present invention relates generally to computer-enabled reservoir simulation of fluid flow in subterranean reservoirs, and more particularly to reservoir simulation of composition. Background technique [0002] A compositional reservoir simulator for simulating flow in a subterranean hydrocarbon reservoir can in principle be viewed as modeling a series of connected pools (cells) of mixed fluids at a given pressure, temperature and composition. As time progresses (as the simulator is taking a time step towards some final moment when a result is found), the conditions in the pool change due to fluid movement, oil wells, and other external factors. For each new setting of pressure, temperature and total fluid composition, flash calculations are necessary to establish the quantities of fluid phases and their total and composition. This calculation essentially consists of finding the minimum of the thermodynamic state function (Gibbs free energy GFE), ...

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Problems solved by technology

[0007] "Fast and Robust Algorithm for Compositional Modeling: Part I-Stability Analysis" by Firoozabadi, A. and Pan, H. in SPE 63083 and "Fast and Robust Algorithm by Firoozabadi, A. and Pan, H. in SPE 71603 Algorithm for Compositional Modeling: PartII-Two-Phase Flash", which discusses the use of reduced variable strategies in stability and decomposition calculations for compositional reservoir simulations; however, the authors do not teach how to avoid stability testing

Also, formulation-specific stability algorithms experience convergence difficulties, especially when encountered as far into the undersaturated region as

Also, the decomposition algorithm is formulated in terms of the gas phase and will experience numerical and / or convergence difficulties near the dew point due to the fact that there is no liquid phase

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