Modeling Supercritical CO₂ Flow and Mineralization in Reactive Host Rocks with PFLOTRAN v7.0

Abstract. Understanding the flow and reactivity of CO₂ injected into geologic reservoirs is important for many subsurface applications including secure geologic carbon storage (GCS), critical mineral extraction, enhanced geothermal systems (EGS), and enhanced oil recovery (EOR). Traditionally, subsurface CO₂ injection for GCS applications has focused on geologic formations with favorable subsurface configurations for CO₂ migration and trapping through non-reactive mechanisms such as structural, solubility, and petrophysical trapping to isolate CO₂ in the subsurface. Recently, CO₂-reactive rocks such as mafic and ultramafic basalts have been investigated for their potential to react with injected CO₂ in situ to simultaneously dissolve host rock minerals and mineralize CO₂ as carbonates. Engineering rapid CO₂ mineralization in the subsurface is attractive because of the increased density of stored CO₂, the additional safety factors associated with solidification, and the potential to extract valuable critical minerals. However, the limited availability of tools that are capable of modeling the associated coupled multiphase flow and reactive transport processes, especially at scale, makes it difficult to predict the long term behavior of a commercial-scale CO₂ injection into a reactive host rock. Here we present recent developments in the parallel flow and reactive transport simulator PFLOTRAN to model coupled CO₂-brine flow and reactive transport for a wide range of injection and production applications involving reactive CO₂-brine systems. These developments are based on the well established and trusted CO₂ flow capabilities in the STOMP-CO2 simulator. New capabilities added to PFLOTRAN include new CO₂-brine equations of state with optional thermal coupling, several new constitutive relationships like capillary pressure smoothing and scanning path hysteresis, a fully implicit well model, and native linkage with PFLOTRAN’s well-established reactive transport libraries. A series of numerical benchmarks between PFLOTRAN and STOMP-CO2 verify the newly developed CO₂-brine flow capabilities, and demonstrations of coupled CO₂-brine flow modeling and reactive transport show how CO₂ mineralization can be engineered in reactive host rocks. Finally, an example use case involving copper leaching by CO₂ and critical mineral extraction is presented to showcase the strengths of this new implementation. Several limitations still remain, including limited availability of field data to parameterize models. Future work should constrain the evolution of mineral surface area during mineralization and the temperature/pH dependence of geochemical reactions for specific systems of interest.