『Abstract
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 microtomography)
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.
Keywords: Water-rock interactions; CO2; Pore-scale
modelling; Calcite; Dissolution』
1. Introduction
2. Geochemical model
3. Flow and transport models
3.1. Fluid flow model
3.2. Transport model
3.3. Boundary conditions
4. Results
4.1. Validation
4.2. Realistic examples
4.3. Effective reaction rate
5. Conclusions and perspectives
Acknowledgements
References