  We present the novel numerical model GeoChemFoam, a multiphase reactive
transport solver for simulations on complex pore geometries, including
microfluidic devices and micro-CT images. The geochemical model includes bulk
and surface equilibrium reactions. Multiphase flow is solved using the
Volume-Of-Fluid method and the transport of species is solved using the
Continuous Species Transfer method. The reactive transport equations are solved
using a sequential Operator Splitting method, with the transport step solved
using our OpenFOAM-based Computational Fluid Dynamics toolbox, and the reaction
step solved using Phreeqc, the US geological survey's geochemical solver. The
model is validated by comparison with analytical solutions in 1D and 2D
geometries. We then applied the model to simulate multiphase reactive transport
in two test pore geometries: a 3D pore cavity and a 3D micro-CT image of
Bentheimer sandstone. In each case, we show the pore-scale simulation results
can be used to develop upscaled models that are significantly more accurate
than standard macro-scale equilibrium models.
