Pore-scale simulation of mixing-induced calcium carbonate precipitation and dissolution in a microfluidic pore network

摘要

We develop a 2-D pore scale model of coupled fluid flow, reactive transport, and calcium carbonate (CaCO_3) precipitation and dissolution. The model is used to simulate transient experimental results of CaCO_3 precipitation and dissolution under supersaturated conditions in a microfluidic pore network (i.e., micromodel) in order to improve understanding of coupled reactive transport systems perturbed by geological CO_2 injection. In the micromodel, precipitation is induced by transverse mixing along the centerline in pore bodies. The reactive transport model includes the impact of pH upon carbonate speciation and a CaCO_3 reaction rate constant, the effect of changing reactive surface area upon the reaction, and the impact of pore blockage from CaCO_3 precipitation on diffusion and flow. Overall, the pore scale model qualitatively captured the precipitate morphology, precipitation rate, and maximum precipitation area using parameter values from the literature. In particular, we found that proper estimation of the effective diffusion coefficient (D_(eff)) and the reactive surface area is necessary to adequately simulate precipitation and dissolution rates. In order to match the initial phase of fast precipitation, it was necessary to consider the top and bottom of the micromodel as additional reactive surfaces. In order to match a later phase when dissolution occurred, it was necessary to increase the dissolution rate compared to the precipitation rate, but the simulated precipitate area was still higher than the experimental results after ～30 min, highlighting the need for future study. The model presented here allows us to simulate and mechanistically evaluate precipitation and dissolution of CaCO_3 observed in a micromodel pore network. This study leads to improved understanding of the fundamental physicochemical processes of CaCO_3 precipitation and dissolution under far-from-equilibrium conditions.