Instabilities in the dissolution of a porous matrix

TL;DR

This paper reveals a new instantaneous instability in porous rock dissolution, alongside the known steady-front instability, with significance depending on initial porosity and rock composition.

Contribution

It introduces a novel instantaneous instability mechanism in porous dissolution, expanding understanding beyond the traditional steady-front instability.

Findings

Inlet instability is significant in limestone with low initial porosity.

Steady-state instability dominates in quartz-rich sandstones.

The importance of each mechanism depends on porosity ratios.

Abstract

A reactive fluid dissolving the surrounding rock matrix can trigger an instability in the dissolution front, leading to spontaneous formation of pronounced channels or wormholes. Theoretical investigations of this instability have typically focused on a steadily propagating dissolution front that separates regions of high and low porosity. In this paper we show that this is not the only possible dissolutional instability in porous rocks; there is another instability that operates instantaneously on any initial porosity field, including an entirely uniform one. The relative importance of the two mechanisms depends on the ratio of the porosity increase to the initial porosity. We show that the "inlet" instability is likely to be important in limestone formations where the initial porosity is small and there is the possibility of a large increase in permeability. In quartz-rich sandstones, where the proportion of easily soluble material (e.g. carbonate cements) is small, the instability in the steady-state equations is dominant.