Pore-scale study of dissolution-induced changes in hydrologic properties of rocks with binary minerals

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 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 increases very slowly or even remains at a small value although the porosity increases by a large amount. Besides, due to the heterogeneous characteristic 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.

💡 Research Summary

This paper presents a pore‑scale numerical investigation of how mineral dissolution alters the hydraulic properties of fractured and porous rocks that contain two distinct mineral phases. The authors employ a lattice‑Boltzmann method (LBM) framework to solve coupled fluid flow, solute transport, and surface reaction equations on a three‑dimensional grid. To generate realistic microstructures, they use the Quartet Structure Generation Set (QSGS) algorithm, which allows independent control over the spatial distribution, size, and connectivity of a reactive mineral (which dissolves) and an inert mineral (which remains solid).

A series of simulations are performed over a wide range of Peclet numbers (Pe) and Damköhler numbers (Da) to explore the relative importance of advection versus diffusion (Pe) and reaction kinetics versus mass transport (Da). High Pe–high Da conditions correspond to advection‑dominated flow with fast reaction, whereas low Pe–low Da conditions emphasize diffusion and slow reaction. Two representative geometries are examined: (1) a fractured medium where a single high‑conductivity aperture cuts through a rock matrix, and (2) a homogeneous porous medium composed of randomly distributed grains.

The results reveal several key phenomena. First, as the reactive mineral dissolves, the inert mineral forms a persistent porous layer behind the moving dissolution front. Although the overall porosity of the sample can increase dramatically, the permeability rises only modestly or may even plateau. The porous layer, despite its low solid fraction, presents a high hydraulic resistance because its pore throats are narrow and tortuous. Consequently, the permeability–porosity relationship deviates strongly from the classic Kozeny‑Carman trend, especially under high Pe–high Da conditions where the dissolution front advances quickly but the flow is still constrained by the inert layer.

Second, the authors distinguish three types of aperture: chemical aperture (the geometric width of the dissolved zone), mechanical aperture (the physical opening measured after solid removal), and hydraulic aperture (the effective flow‑controlling width derived from Darcy’s law). These apertures differ substantially because the chemical aperture does not account for the residual inert mineral, whereas the hydraulic aperture is limited by the narrow channels left in the porous layer. This discrepancy highlights the difficulty of inferring dissolution progress from permeability measurements alone.

Third, the study examines the emergence of wormholing—a channelization process where high‑permeability pathways rapidly expand, leading to a sudden jump in overall permeability. In mono‑mineralic systems, wormholing is prominent under high Pe–high Da conditions. However, when an inert mineral is present, the wormhole development is suppressed. The residual inert grains break up the continuity of the high‑conductivity channels, forcing the dissolution to proceed more uniformly across the matrix. This finding suggests that natural heterogeneity in mineral composition can act as a stabilizing factor against catastrophic channelization.

From a practical perspective, the work implies that field‑scale reactive transport models must incorporate multi‑mineral heterogeneity to avoid over‑predicting permeability enhancement during acidizing, CO₂ sequestration, or enhanced oil recovery. The sensitivity of hydraulic response to Pe and Da also indicates that site‑specific flow regimes and reaction rates must be carefully calibrated. Moreover, the formation of low‑permeability porous layers offers a potential engineering lever: by deliberately introducing inert particles or by exploiting existing non‑reactive phases, one could tailor the evolution of permeability to maintain reservoir integrity while still achieving desired mineral dissolution.

In summary, the paper demonstrates that LBM‑based pore‑scale simulations, combined with realistic microstructure generation, provide a powerful tool for dissecting the coupled chemistry‑hydrodynamics of binary‑mineral rocks. The insights into porous‑layer formation, aperture disparity, and wormhole suppression advance our understanding of reactive transport in complex geological media and lay groundwork for more accurate predictive models in subsurface engineering applications.