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Novel Geochemical Calculation for Underground Geological Storage of CO2: From Multiphase Geochemical Equilibrium to Compositional Reservoir Simulation

Underground geological storage (UGS) of CO 2 is one of the most promising technologies to realize large reduction of CO 2 emission in the near future. It entails the injection of CO 2 -rich streams mostly in deep saline aquifers and depleted petroleum reservoirs. It is by nature a multiphase flow process involving multiple physics. To monitor or to assess the feasibility of such a process, it is crucial to have simulation tools that can accurately describe the involved physics. One distinctive feature in CO 2 storage simulation is multiphase equilibrium (phase equilibrium between all the fluid and solid phases) and geochemical reactions (speciation and mineral dissolution/precipitation), which underlies CO 2 partitioning, CO 2 transfer with fluids, and the ultimate fate of injected CO 2 . Simulation tools for CO 2 storage analysis must rely on robust and fast chemical and phase equilibrium (CPE) calculation involving electrolytes, minerals, and fluid phases. Nevertheless, the highly nonlinear nature of the CPE problem has been a standing challenge to the robustness and efficiency of any CO 2 storage simulator. We have recently developed a novel formulation for multiphase reaction equilibrium and the relevant algorithms that can be used for geochemical calculation in CO 2 storage simulation. The formulation and the relevant algorithms, developed based on the RAND method, are a non-stoichiometric Gibbs energy minimization approach, in contrast to the methods using the law of mass action behind the well-known geochemical simulators PHREEQC or TOUGHREACT. Our RAND-based algorithms handle chemical and phase equilibrium simultaneously, solve a smaller set of equations, provide second-order convergence, and ensure convergence through Gibbs energy minimization. We illustrate here the application of the novel geochemical calculation method using three different examples of importance to CO 2 storage in saline aquifers. The examples require tools with varying complexity from standalone equilibrium calculation to three-dimensional compositional all with the RAND method at their cores: This work presents a novel geochemical calculation method that can handle complex multiphase equilibrium and geochemical reaction equilibrium simultaneously. It provides a promising robust and efficient geochemical module for general-purpose CO 2 storage simulations in both aquifers and depleted petroleum reservoirs.