Reactive Transport and Self-Sealing Properties of Cement in CO2-Rich Environments

This thesis evaluates the interaction of CO₂ in the aqueous phase with cementitious materials as it is essential for subsurface applications such as CO₂ storage. The research presented in this thesis combines
the diffusive and advective transport of CO₂ in both intact and fractured cement. This thesis is divided into seven chapters, each addressing a specific aspect:

Chapter 2 describes the diffusive transport of CO₂ within intact cement using the Nernst-Plank equation and considers the overall chemical interactions of CO₂ with G-class sulfate-resistant Portland cement. This chapter also explores the effects of environmental conditions, such as pressure and temperature, on solubility, diffusivity, and the kinetics of dissolution/precipitation reactions. An empirical relation is proposed for estimating the length of carbonation over the long term, with results compared to predictions made using Fick's law and Elovich equations.

Chapter 3 deals with changes in the apparent diffusivity of CO₂ in intact cement during the diffusion process. Using the model that we developed in Chapter 2, we explored the effect of cement composition, especially the CH/CSH volumetric ratio. Furthermore, we evaluated the apparent diffusivity changes during the carbonation process.

Chapter 4 addresses the impact of mechanical damage on the length of carbonation by incorporating mechanical damage into the reactive transport model. The chapter also considers the presence of impurities (e.g., SO₂, H₂S, and NO₂) under the assumption of very low partial pressure. We observed that the presence of impurities enhances progressive precipitation of Ettringite at the CO₂ front and gypsum behind the CO₂ front, resulting in pore space blockage. Consequently, it became necessary to integrate a mechanical damage accumulation model, as progressive precipitation induces the formation of a new micro-fracture network. This integration was subsequently addressed in detail in the remaining sections of the chapter.

Chapter 5 investigates the self-sealing or opening behavior of cement fractures in both 1D and more complex 2D rough-walled fractures. In 1D fractures, a slight increase in pressure gradient causes a rapid transition from sealing to non-sealing. However, 2D heterogeneous fractures demonstrate the ability to remain self-sealed even at elevated pressure gradients. The sealing behavior of fractures is affected by the heterogeneity pattern, available pathways, and the interaction time between CO₂ and cement solid phases. Furthermore, the opening/sealing diagram suggested by Brunet et al. (2016) is revised to illustrate more complex but realistic scenarios.

Chapter 6 addresses CO₂ interactions with an unsaturated cement matrix. The moisture transport model is combined with gas transport and chemical equilibration to consider gas solubility in the aqueous phase and evaluate the chemical interactions between the solution, gas phases and cement solid phases. Due to irregularities in moisture transport, the transport mechanism is validated initially before being applied to estimate the quantity of water in pore spaces for enabling reactions. This study simulates the carbonation process for four different samples derived from earlier research.

Chapter 7 provides conclusions, offering insights that improve the reactive transport modeling of aqueous CO₂ in intact and fractured cement. Through extensive investigation of the complex interactions between CO₂ and cementitious materials, this research reduces the risks of CO₂ leakage, thus enabling more efficient and safer subsurface storage applications.