Characterization of mixing and reaction between chemical species during cycles of drainage and imbibition in porous media

Mixing and reaction between chemical species during cycles of drainage and imbibition in porous media are investigated using a coupled lattice Boltzmann model (LBM). This coupled LBM is able to simulate advection-diffusion processes with homogeneous reactions under dynamic (immiscible) multiphase flow conditions. A feature of the model is that there is no need to track the interface specifically for the transport domain, which improves the computational efficiency significantly. Transport simulations of non-reactive tracers in a natural pore domain show that some tracers can be trapped with the resident wetting fluid, with the tracers in these stagnant regions (defined here as regions where Pé < 1) being released slowly during the imbibition phase. Moreover, it is found that the relative amount of tracer diffusing from these stagnant regions increases as the system approaches steady-state flow conditions. For bimolecular reactive transport, the results demonstrate that most of the reaction product generated during mixing in stagnant regions accumulates and remains within these regions, with only some release of the product to infiltrating wetting fluid that bypasses adjacent to these regions.