Cementitious materials for oil-well abandonment and numerical simulations of cement durability at oil well conditions

In this work, we investigate Portland cement hydration and durability at elevated temperatures of 60°C – 80°C and at pressures up to 300 bar in water and in artificial brine solutions that mimic the exposing environments of cement plugs used for abandonment of out-sourced off-shore oil wells. A principal aim is to follow the structural and compositional evolution of the C-S-H phase, and thus, solid-state ²⁷Al and ²⁹Si NMR are used as the main analytical tools. The NMR results indicate that hydration at high pressure (300 bar) has no significant impact on the C-S-H phases formed at 60 and 80 °C, however, it may retard the decomposition of ettringite and the AFm phases at 80 °C. Interestingly, the exposure of the hydrated cement paste to the laboratory-made brine solution significantly impacts the microstructure of the formed C-S-H product as a cross-linked C-S-H phase is formed at a high ratio of solution to solid. The samples are additionally cured in CO₂-injected brine, and the effect of particle size on the carbonation degree is studied. The experimental phase assemblages from the investigations will be used in thermodynamic reactive-transport modeling to estimate the durability and physical properties of the hardened cement on a prolonged time scale.