Fracture opening or self-sealing: Critical residence time as a unifying parameter for cement-CO₂-brine interactions

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 CO₂-rich fluid. Contrasting self-sealing and fracture opening behavior have been observed while a unifying framework is still missing. Here we developed a process-based 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 CO₂-rich water accelerates the portlandite dissolution, releasing calcium that further reacted with carbonate to form calcite. The calibrated model was used for CO₂-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 τ_c - the minimum τ required for self-sealing - divides the conditions that trigger the opening and self-sealing behavior. The τ_c value depends on the initial aperture size through τ_c = 9.8 × 10^-4 × b² + 0.254 × b. Among the 250 numerical experiments, significant changes in effective permeability - self-healing or opening - typically 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).