Modeling Dissolution and Precipitation Dynamics During Dedolomitization

We simulate the process of dedolomitization and the precipitation of calcium carbonate using particle tracking. The study is stimulated by the results of a laboratory experiment of reactive transport of injected CaCl₂/HCl in a constant flow field in a column of sucrosic dolomite particles. The injected fluid basically supplied Ca²⁺ and H⁺ and the dedolomitization is a protonation reaction yielding carbonic acid, which in a deprotonation reaction yields CO_3^2-, and reacting with the abundant Ca²⁺ forms the precipitate CaCO₃. The novelty of the simulation is to treat the dynamics of the rate limiting reactants with particle tracking. At each time step the local concentration of H⁺ determines the probability (assuming local carbonate equilibria) of precipitation and dissolution. The precipitation changes the porosity which in turn changes the local flow field. The particle tracking is governed by spatial and temporal distributions within a continuous time random walk framework. This includes the option of either advective-dispersive (Fickian) transport or the effects of disorder of heterogeneous media --- non-Fickian behavior. The dynamics of dedolomitization are examined for different flow conditions and for different spectra of velocity tails of the reactants. The fluctuations in the local velocity distributions, due to porosity changes, create conditions for positive feedbacks leading to preferential pathways and large-scale nonlinearity and precipitation banding. This feature has been observed in the laboratory experiments and is now accounted for by the simulation results at similar time frames, velocities and pH levels.