Sharp reaction fronts during diffusion-dominated reactive flow. Experimental and numerical study using cement carbonation as example.

Reactive flow, coupling transport and chemical alteration processes, is a key geological driver, taking place over a wide range of conditions and scales. In addition to natural phenomena (e.g., contact metamorphism; ore formation), many geo-energy technologies are also dependent on, or affected by reactive flow.

One well-studied example of reactive flow is the carbonation, alteration, and potential degradation of a typical wellbore cement exposed to a CO₂-bearing fluid under reservoir conditions. As CO₂ permeates the cement, it reacts with Ca²⁺ from portlandite and other cement gel phases to form CaCO₃, leading to an increase in solid volume, and therefore a decrease in porosity (and permeability). Different experimental methodologies have been applied to better understand the progression of the carbonation front, including static exposure in batch apparatuses, and exposure to forced flow driven by a pressure gradient along the sample axis. However, progression of the carbonation front typically shows a dependence on the square root of time, suggesting that even under high pressure gradients, transport is still controlled by diffusion through the low-permeability matrix, rather than advective flow.

Interestingly, cement carbonation is often associated with a sharp reaction front, with large changes in mineral assemblage and fluid composition and pH across a relatively short distance. In this presentation, we take a closer look at the carbonation fronts in a set of cement samples exposed to wet supercritical CO₂ and CO₂-saturated water, using both batch and forced-flow methodologies. We will combine observations based on laboratory experiments with analytical and numerical modelling to address three closely related aspects of reactive transport. First, by explicitly evaluating the Peclet number, we demonstrate that diffusion dominates over advection under the tested conditions, providing a unified interpretation of batch and flow-through experiments. Second, we distinguish between the true diffusion coefficient of CO₂ in the pore fluid and the effective diffusion coefficient inferred from reaction-front propagation, showing that the latter is an emergent quantity controlled by reaction thermodynamics and concentration contrasts rather than a material property. Third, using a reactive transport model that couples fluid transport, solid-fluid reactions, and porosity evolution, we investigate the conditions under which sharp reaction fronts arise and contrast them with regimes that produce smooth transition zones.