Dissolved CO2 stabilizes dissolution front and increases breakthrough porosity of natural porous materials

TL;DR

This study uses numerical simulations to show that dissolved CO2 in water promotes homogeneous dissolution in porous rocks, increasing breakthrough porosity and potentially affecting reservoir integrity in geologic carbon storage.

Contribution

It reveals that dissolved CO2 stabilizes the dissolution front, leading to larger breakthrough porosity, a novel insight into reactive flow in porous media.

Findings

Dissolved CO2 causes homogeneous dissolution patterns.

CO2-free solutions develop wormholes with smaller breakthrough porosity.

Sample size influences the development of wormholes.

Abstract

When reactive fluids flow through a dissolving porous medium, conductive channels form, leading to fluid breakthrough. This phenomenon is important in geologic carbon storage, where the dissolution of CO2 in water increases the acidity and produce microstructures significantly different from those in an intact reservoir. We demonstrate the controlling mechanism for the dissolution patterns in natural porous materials. This was done using numerical simulations based on high resolution digital models of North Sea chalk. We tested three model scenarios, and found that aqueous CO2 dissolve porous media homogeneously, leading to large breakthrough porosity. In contrast, CO2-free solution develops elongated convective channels in porous media, known as wormholes, and resulting in small breakthrough porosity. We further show that a homogeneous dissolution pattern appears because the sample size is smaller than the theoretical size of a developing wormhole. The result indicates that the presence of dissolved CO2 expands the reactive subvolume of a porous medium, and thus enhances the geochemical alteration of reservoir structures and might undermine the sealing integrity of caprocks when minerals dissolve.