Temporal pattern of entropy production induced by reactive infiltration instability in natural porous media

TL;DR

This paper investigates how entropy production patterns in natural porous media reveal stages of microstructural evolution driven by infiltration instability, using high-fidelity digital models and analyzing the effects of heterogeneity and flowrate.

Contribution

It introduces a numerical scheme utilizing greyscale digital models to analyze entropy production in dissolving porous media, linking microstructural evolution to entropy patterns.

Findings

Entropy production patterns identify three stages of microstructural evolution.

Regional mixing reduces infiltration instability by homogenizing reactant distribution.

Microstructural evolution is highly sensitive to initial heterogeneities at low flowrates.

Abstract

The tendency of irreversible processes to generate entropy is the ultimate driving force for the evolution of nature. In engineering, entropy production is often used as a measure of usable energy losses. In this study we show that the analysis of the entropy production patterns can help understand the vastly diversified experimental observations of water-rock interactions in natural porous media. We first present a numerical scheme for the analysis of entropy production in dissolving porous media. Our scheme uses a greyscale digital model of natural chalk obtained by X-ray nanotomography. Greyscale models preserve structural heterogeneities with very high fidelity, which is essential for simulating a system dominated by infiltration instability. We focus on the coupling between two types of entropy production: the percolative entropy generated by dissipating the kinetic energy of fluid flow and the reactive entropy that originates from the consumption of chemical free energy. Their temporal patterns pinpoint three stages of microstructural evolution. We then show that the regional mixing deteriorates infiltration instability by reducing local variations in reactant distribution. In addition, we show that the microstructural evolution can be particularly sensitive to the initially present transport heterogeneities when the global flowrate is small. This dependence on flowrate indicates that the need to resolve the structural features of a porous system is greater when the residence time of the fluid is long.