Sylvia M. Mutisya

SUBATECH (UMR 6457 - Institut Mines-Télécom Atlantique, Université de Nantes, CNRS-IN2P3) - France

            Wellbore cement degradation and the potential migration of fluids to the surface through leakage pathways is a major concern in many subsurface operations, such as geological CO₂ sequestration. While leakage pathways can occur in wells due to faulty construction and other mechanical defects, geochemical reactions induced by the injected fluids could cause cement degradation, resulting in damage of wells and the development of leaks. This work focusses on identifying and quantitatively characterizing on the fundamental molecular scale, possible cement degradation mechanisms and reaction pathways, fluid transport rate and the geochemical variables that affect fluid-cement interactions.
            The interaction of CO₂ with cement is investigated using the two main hydrated cement phases: calcium silicate hydrate (C-S-H) and portlandite. The intercalation potential of CO₂/H₂O fluid mixtures is explored using grand canonical Monte Carlo (GCMC) simulations for Calcium Silicate Hydrate (C-S-H) porous systems in equilibrium with binary CO₂/H₂O bulk mixtures. Increasing the Ca/Si ratio of the confining cement pores decreases the adsorption of CO₂ as water competitively adsorbs on the calcium cations, blocking access of CO₂. Next, we use biased ab-initio molecular dynamics (AIMD) simulations to explore the reactivity of CO₂ with the basal and edge surfaces of the portlandite cement phase in scCO₂ and water-rich conditions. The metadynamics approach is applied to accelerate the dynamics of the rare reaction events and to investigate their mechanisms in detail. Our simulations show that supercritical CO₂ undergoes a rapid barrierless carbonation reaction with the edge surfaces of the portlandite crystals. However, the carbonation reaction soon ceases due to the deposition of (bi)carbonate surface complexes which form a carbonate layer.  On the other hand, the presence of water alters the interaction of CO₂ with the portlandite surfaces as water forms well-structured aqueous surface layers. Thus, the water content within the portlandite pores is the rate limiting step in the carbonation reaction of portlandite with H₂O/CO₂ fluid. As such, CO₂ reactivity for pores with highly structured water surface layers (with no bulk-like water) is expected to be limited due to the attenuated inward diffusion of the CO₂ molecules.
 

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