RAND-based geochemical equilibrium algorithms with applications to underground geological storage of CO[formula omitted

Abstract

Chemical and phase equilibrum (CPE) calculations are indispensable to the geochemical simulation of natural systems, such as freshwater, groundwater, and mineral formations. In natural environments, many chemical reactions occur, both at the interface between rocks and fluids (e.g., dissolution reactions) and in bulk phases (e.g., aqueous speciation). Moreover, the typical ionic species found in nature require thermodynamic models for electrolytes, increasing the calculation complexity. In this work, we present RAND-based algorithms, which originate from Gibbs energy minimization, for geochemical CPE problems for closed systems and open systems. The open system problems specify fixed chemical potentials for non-charged species, as in a constant partial CO2 pressure problem, or for charged ionic species, as in a constant pH problem. We showcase the robustness and efficiency of these algorithms using the mineral system Mg-Si-Ca-CO2-H2O because of its relevance to CO2 underground geological storage. The algorithms show quadratic convergence and the coupled tangent plane distance analysis can determine the most stable phase assemblage. The algorithms generate results in agreement with PHREEQC, whereas they are much faster than PHREEQC. The described algorithms can be potentially used for reliable and fast simulation of CO2 storage in geological formations, and of other processes in natural environments involving geochemical equilibrium.