Development of a reactive transport solver in MATLAB Reservoir Simulation Toolbox using the fully-implicit sequential iterative approach

Different operations dealing with the subsurface, such as subsurface CO₂ disposal, haz-ardous waste disposal, geothermal energy extraction, underground hydrogen storage, etc., can change the fluid/flow system underground. The injection of fluids with thermodynamic and chemical properties different from those of the reservoir fluid can trigger a series of chemical reactions, which may affect the fluid and/or rock properties. Depending on the system under study, these changes may be advantageous or unfavorable. Reactive transport modeling is a choice for investigating how these changes can alter the system. In this study, a reactive transport solver is developed in the MATLAB Reservoir Simulation Toolbox using the sequential fully-implicit approach. The developed reactive transport solver is illustrated using reactions and geometries using reactions and geometries relevant for assessing the sealing capacity of a fractured caprock of a deep saline aquifer used for underground CO₂ disposal, and the limitations and advantages of the approach are stated. Moreover, the results of the simulation for two fracture models, the discrete fracture matrix and embedded discrete fracture matrix models, are compared. The simulations demonstrate that hydrogen ion concentration or pH is the primary parameter affecting the extent of dissolution, while the other aqueous species concentrations are less influential. It is also shown that at higher flow rates, dissolution substantially occurs in the vicinity of the main fracture, along the flow direction, while at lower flow rates, because the injected fluid becomes fully buffered closer to the inlet, dissolution only occurs in the vicinity of the inlet over the course of the simulation. Applying the discrete fracture matrix and embedded discrete fracture matrix models to one of the scenarios demonstrates that both yield equivalent results.