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Assessing the possibility of CO2 leakage is one of the major challenges for geological carbon sequestration. Injected CO2 can react with wellbore cement, which can potentially change cement composition and transport properties. In this work, we develop a reactive transport model based on experimental observations to understand and predict the property evolution of cement in direct contact with CO2-saturated brine under diffusion-controlled conditions. The model reproduced the observed zones of portlandite depletion and calcite formation. Cement alteration is initially fast and slows down at later times. This work also quantified the role of initial cement properties, in particular the ratio of the initial portlandite content to porosity (defined here as φ), in determining the evolution of cement properties. Portlandite-rich cement with large φ values results in a localized “sharp” reactive diffusive front characterized by calcite precipitation, leading to significant porosity reduction, which eventually clogs the pore space and prevents further acid penetration. Severe degradation occurs at the cement–brine interface with large φ values. This alteration increases effective permeability by orders of magnitude for fluids that preferentially flow through the degraded zone. The significant porosity decrease in the calcite zone also leads to orders of magnitude decrease in effective permeability, where fluids flow through the low-permeability calcite zone. The developed reactive transport model provides a valuable tool to link cement–CO2 reactions with the evolution of porosity and permeability. It can be used to quantify and predict long-term wellbore cement behavior and can facilitate the risk assessment associated with geological CO2 sequestration.

Understanding long-term property evolution of cement fractures is essential for assessing well integrity during geological carbon sequestration (GCS). Cement fractures represent preferential leakage pathways in abandoned wells upon exposure to CO 2-rich fluid. Contrasting self-sealing and fracture opening behavior have been observed while a unifying framework is still missing. Here we developed a processbased reactive transport model that explicitly simulates flow and multi-component reactive transport in fractured cement by reproducing experimental observation of sharp flow rate reduction during exposure to carbonated water. The simulation shows similar reaction network as in diffusion-controlled systems without flow. That is, the CO 2-rich water accelerates the portlandite dissolution, releasing calcium that further reacted with carbonate to form calcite. The calibrated model was used for CO 2-flooding numerical experiments in 250 cement fractures with varying initial hydraulic aperture (b) and residence time (τ) defined as the ratio of fracture volume over flow rate. A long τ leads to slow replenishment of carbonated water, calcite precipitation, and self-sealing. The opposite occurs when τ is small with short fracture and fast flow rates. Simulation results indicate a critical residence time  cthe minimum τ required for selfsealing-divides the conditions that trigger the diverging opening and self-sealing behavior. The  c value depends on the initial aperture size through 42 9.8 10 0.254 c bb       . Among the 250 simulated fracture cases, significant changes in effective permeabilityself-healing or openingtypically occur within hours to a day, thus providing supporting argument for the extrapolation of short-term laboratory observation (hours to months) to long-term prediction at relevant GCS time scales (years to hundreds of years).

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