A new numerical model for pore scale dissolution of calcite due to CO2 saturated water flow in 3D realistic geometry: Principles and first results

A fully coupled reactive transport model at pore-scale has been developed using finite volumes in order to improve the comprehension of reactive flow-through experiments by CO2-saturated water. Six constituents (H+, OH−, HCO3−, Ca2+, CO2⁎ and CO32−) are considered for reactive transport through the 3D pore network geometry of a limestone sample assumed to be of pure calcite. Three speciation reactions at equilibrium (giving three mass action relations) are involved in the bulk of the fluid phase, the electro-neutrality of the solution is imposed (giving one relation), and two transport equations are solved to compute the concentrations of the six constituents with space and time. Fick's law models diffusion and different diffusion coefficients are used for the different constituents. Calcite dissolution rate at the fluid–mineral interface is written as a function of the activities of all the constituents appearing in the dissolution reactions. The pressure and velocity fields of the one-phase solution circulating through the sample are computed solving Stokes equations. For negative times the circulating solution is in equilibrium with the rock sample, and at t = 0 a disequilibrium is introduced (increase of CO2 pressure and/or decrease of Ca2+ concentration). Then, the non-linear system of equations representing the reactive transport is solved until steady state. Applications on realistic 3D geometry (defined from real media images obtained by X-ray computed micro-tomography) illustrate the possibilities offered by this model. The behaviour of an effective reaction rate has been examined for samples having different geometry, showing that, at the pore scale, calcite dissolution is mainly influenced by the mean pore fluid velocity.