Mineral dissolution in porous media: An experimental and modeling study on kinetics, porosity and surface area evolution

Mineral and rock dissolution was studied experimentally using flow-through reactors and reactive transport modeling. The porous media were forsterite, crystalline basalt, and amorphous basalt, dissolved in HCl solutions at pH ∼2.5 and 25 °C. Solution composition, particle surface area, and porosity were determined as a function of travel distance within the reactor and time, using in situ X-ray computed tomography (XMT) and solution chemical composition. The obtained bulk dissolution rates, normalized to the initial geometric surface area, were: log r+,Si −7.59 ± 0.05 for forsterite, −7.64 ± 0.12 for basaltic glass and −8.12 ± 0.24 (mol/m2/s) for crystalline basalt, at 25 °C and pH ∼2.5, similar to those previously obtained using mixed flow reactors and for conditions far from equilibrium. Mineral and rock dissolution resulted in increased porosity and specific surface area of the solids; these changes were not uniformly distributed along the fluid flow path or with time. Similar trends were predicted by reactive transport modeling, however, the exact values of pore volume and surface area were difficult to predict. The results were found to be independent of the method applied in the surface area calculations: either the simple spherical model or the sugar lump model. Also, in the models, stoichiometric mineral dissolution is commonly assumed, but was not observed to occur for either glassy or crystalline basalt. It shows that accurate prediction capabilities of simple reactive transport modeling may be limited for calculating pore volume, mineral and rock surface area changes, and pore fluid chemistry with time and along flow paths. These, in turn, are key parameters in determining dissolution rates, overall chemical mass movement in the system, and fluid flow paths and velocities.