A hybrid-EoS approach for multiphase pressure-based equilibrium calculations of reservoir mixtures with brine
M. Wapperom (TU Delft - Civil Engineering & Geosciences)
J. Heringer (Université de Pau et des Pays de l'Adour)
D. V. Nichita (Université de Pau et des Pays de l'Adour)
D. Voskov (Stanford University, TU Delft - Civil Engineering & Geosciences)
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Abstract
Mixtures containing different gases, liquid hydrocarbons and aqueous brines play a very important role in modern energy transition applications. Cubic equations of state have proven reliable for thermodynamic calculations of nonpolar mixtures, but in their conventional form, they are not adequate to predict the interaction between associating molecules and fail to describe the behaviour close to infinite dilution. In this work, we develop an approach for combining thermodynamic models to represent different phase types thereby overcoming this inaccuracy. A cubic equation of state is used for non-aqueous phases, while a separate thermodynamic model is employed for the brine, combining Henry’s constants for solutes and a fugacity model for the water component. In this way, we can maintain the computational efficiency of cubic EoS while modelling properties of the aqueous phase with good accuracy. We distinguish the vapour- and liquid-like roots of the cubic equation of state to correctly identify the minima of the Gibbs free energy surfaces in water-rich compositions. The use of different thermodynamic models introduces a thermodynamic inconsistency that is most noticeable close to the critical conditions of the water phase. However, the approach is valid and particularly accurate far from critical conditions for brine systems. The validity of the method up to these conditions is proven. Within these ranges, the proposed approach outperforms a setup using an association model (CPA EoS) both in terms of accuracy and performance. For optimal use in simulation, we present a framework for modelling multiphase equilibria involving reservoir mixtures with brine, which can be formulated at any pressure-based state specification. We validate the approach by comparing our phase equilibrium calculations with an experimental dataset of gas mixtures with water, relevant to subsurface energy transition applications.
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