Abstract
This work presents a novel framework for approximating multiphase chemical equilibrium in porous media flow simulations. This framework facilitates the development of approximation schemes that, by leveraging equilibrium data from previous time steps, significantly reduce computational costs when integrated within a global mass conservation formulation of reactive transport in porous media. The formulation ensures the conservation of mass for an arbitrary number of chemical species distributed across multiple fluid and solid phases, while rigorously enforcing chemical equilibrium and pore volume conservation, and being compatible with the incorporation of additional kinetic reactions when needed. Comparative tests between the approximate schemes and a fully implicit reference version across three test configurations with increasing complexity demonstrates good performance. Additionally, a criterion is introduced to assess the validity of the proposed approximations, enabling adaptive switching to higher-fidelity equilibrium models when required. This adaptive methodology achieves results comparable to the fully implicit scheme while minimizing computational demands.