Evaluation of front morphological development of reactive solute transport using behavior diagrams

While flowing through porous medium, groundwater flow dissolves minerals thereby increasing medium porosity and ultimately permeability. Reactive fluid flows preferentially into highly permeable zones, which are therefore dissolved most rapidly, producing a further preferential permeability enhancement. Accordingly, slight non-uniformities present in porous medium can be amplified and lead to fingering reaction fronts. The objective of this study is to investigate dissolution-induced porosity changes on reaction front morphology in homogeneous porous medium with two non-uniformities. Four controlling parameters, including upstream pressure gradient, reaction rate constant, non-uniformities spacing and non-uniformity strength ratio are comprehensively considered. By using a modified version of the numerical code, NSPCRT, to conduct a series of numerical simulations, front behavior diagrams are constructed to illustrate the morphologies of reaction fronts under various combinations of these four factors. Simulation results indicate that the two non-uniformities are inhibited into a planar front under low upstream pressure gradient, merge into a single-fingering front under intermediate upstream pressure gradient, or grow into a double-fingers front under high upstream pressure gradient. Moreover, the two non-uniformities tend to develop into a double-fingering front as the non-uniformity strength ratio increases from 0.2 to 1.0, and merge into a single-fingering front while the non-uniformity strength ratio increases from 1.0 to 1.8. When the reaction rate constant is small, the two non-uniformities merge into a single front. Reaction rate constant significantly affects front advancing velocity. The front advancing velocity decreases with the reaction rate constant. Based on these results, front behavior diagrams which define the morphologies of the reaction fronts for these four parameters are constructed. Moreover, non-uniformity strength ratio and reaction rate constant are identified as two important factors that govern the interaction of dissolution and solute transport in groundwater systems.