Pore-scale Study of Dissolution-induced Changes in Hydrologic Properties of Rocks with Binary Minerals

Abstract:

In many geologic processes, hydrologic properties, such as porosity and permeability, may change due to various processes. Because these hydrologic properties and constitutive relationships are required in numerical models based on continuum approaches, a better understanding and accurate description of these changes is an issue critical for reducing uncertainties in large-scale numerical modeling. While much progress has been made to understand the effects of the physicochemical processes on hydrologic properties, the composition of the solid phase has received little attention. Natural rocks usually consist of multiple minerals with different dissolution rates. Significant difference in the dissolution rate will greatly affect the geometrical evolutions of the rocks which in turn affect the reactive transport processes and hydraulic properties.

In the present study, a pore-scale numerical model for reactive transport processes based on the Lattice Boltzmann method is used to study the dissolution-induced changes in hydrologic properties of a fractured medium and a porous medium. The solid phase of both media consists of two minerals, and a structure reconstruction method called quartet structure generation set is employed to generate the distributions of both minerals. Emphasis is put on the effects of undissolved minerals on the changes of permeability and porosity under different Peclet and Damkohler numbers. The simulation results show that porous layers formed by the undissolved mineral remain behind the dissolution reaction front. Due to the large flow resistance in these porous layers, the permeability increase is insignificant although the porosity increases by a large amount. Besides, due to the heterogeneous characteristics of the dissolution, the chemical, mechanical and hydraulic apertures are very different from each other. Further, simulations in complex porous structures demonstrate that the existence of the porous layers of the nonreactive mineral suppresses the wormholing phenomena observed in the dissolution of mono-mineralic rocks.