Applying Thermodynamic Framework to Analyze Transport Self-Organization Due to Dissolution/Precipitation Reaction in Porous Medium: Entropy, Enthalpy, Heterogeneity

Coupled dissolution/precipitation reactive processes in transport in porous media are ubiquitous in a multitude of contexts within the field of Earth sciences, such as geological CO2 and H2 storage, contaminant remediation and acid injection in petroleum reservoirs. In particular, the dynamic interaction between the reaction and solute transport, capable of giving rise to the phenomenon of preferential flow paths, is of a critical importance, as these paths play a dominant role in determining the transport properties of the porous medium; still, the approaches to its characterization remain disputed. The emergence of preferential flow paths in porous media can be considered a manifestation of transport self-organization, as they introduce concentration gradients that distance the system from the state of perfect mixing.

To investigate the dynamic reactive-transport interaction and its influence on transport self-organization within the porous media, we consider a 2D Darcy-scale reactive transport simulation, where dissolution and precipitation of the calcite porous matrix are driven by the injection of a low-pH water. The reactive process alters the transport properties of the porous medium, thus creating the reaction-transport interaction. The coupled reactive-transport process is simulated in a series of computational analyses employing the Lagrangian particle tracking approach, capable of capturing the subtleties of the multiscale heterogeneity phenomena. We employ the thermodynamic framework to investigate the emergence of preferential flow paths as the manifestation of transport self-organization; in particular, we are interested in the relationship between the reaction enthalpy that leads to alteration of the medium's transport properties and the resulting change in the transport self-organization.

For initially homogeneous media, our findings show an increase in transport self-organization with time, along with the emergence of the medium heterogeneity due to interaction between the transport and reactive processes. By studying the influence of the Peclet number on the coupled reactive-transport process, we observe that self-organization is more pronounced in diffusion-dominated flows, characterized by low Peclet values. The hydraulic power, dissipated by the fluid, is shown to increase with the increasing medium heterogeneity, as well as with the mean hydraulic conductivity value. This increase in power, supplied to the fluid, results in an intensification of transport self-organization.

For heterogeneous media, we find again that transport self-organization increases with the evolution of the reactive process, along with an increase in the heterogeneity of the medium; their rates of change depend on the initial heterogeneity of the porous medium. These parameters correlate well with the "useful" reactive enthalpy invested in the reactive process, suggesting the existence of a relation between the energy spent and the transport self-organization gained. The self-organization of the breakthrough times exhibits the opposite tendencies, that can be explained by means of a thermodynamic analogy.

Employing thermodynamic framework to investigate the dynamic reaction-transport interaction in porous media may prove beneficial whenever the need exists to estimate the alteration of the overall transport properties of the medium due to emergence of preferential flow paths due to reactive-transport interaction.