Computational methods for reactive transport modeling: A Gibbs energy minimization approach for multiphase equilibrium calculations

• We present a numerical method for chemical equilibrium calculations based on a Gibbs energy minimization approach.
• The method is capable of determining the stable phases at equilibrium without techniques that are dependent on their types (e.g., pure minerals).
• The chemical equilibrium algorithm is applied in reactive transport simulations to demonstrate its effective use in intensive applications.
• The equilibrium calculations are shown to converge within a few iterations (1–5) with a warm-start approach in reactive transport simulations.

We present a numerical method for multiphase chemical equilibrium calculations based on a Gibbs energy minimization approach. The method can accurately and efficiently determine the stable phase assemblage at equilibrium independently of the type of phases and species that constitute the chemical system. We have successfully applied our chemical equilibrium algorithm in reactive transport simulations to demonstrate its effective use in computationally intensive applications. We used FEniCS to solve the governing partial differential equations of mass transport in porous media using finite element methods in unstructured meshes. Our equilibrium calculations were benchmarked with GEMS3K, the numerical kernel of the geochemical package GEMS. This allowed us to compare our results with a well-established Gibbs energy minimization algorithm, as well as their performance on every mesh node, at every time step of the transport simulation. The benchmark shows that our novel chemical equilibrium algorithm is accurate, robust, and efficient for reactive transport applications, and it is an improvement over the Gibbs energy minimization algorithm used in GEMS3K. The proposed chemical equilibrium method has been implemented in Reaktoro, a unified framework for modeling chemically reactive systems, which is now used as an alternative numerical kernel of GEMS.