Closing of micro-cavities in well cement upon exposure to CO₂ brine

Long-lasting cement plugging of wells is crucial for successful CO ₂ storage in underground reservoirs. It requires a profoundly improved understanding of the behavior of fractured cement under realistic subsurface conditions including elevated temperature, high pressure and the presence of CO ₂ saturated brine. Here we report computed X-ray tomography studies on the effects of CO ₂ on cement. More specifically, we have exposed cured Portland G cement samples with pre-made microchannels mimicking fractures to CO ₂ saturated brine at elevated pressure (100 bars) and room temperature. The microchannels were observed to get filled with calcite (CaCO ₃) during the CO ₂ exposure. The extent of this self-healing was dependent on the diameter of the leakage path, with narrower channels more readily getting clogged. Chemical simulations taking into account the cement composition, CO ₂ availability, pH, pressure and temperature gave results consistent with our conceptual understanding of how the differences in dissolution/precipitation profiles in the cement may result from the availability of CO ₂. In particular, the modelling provides an explanation why calcite precipitates preferentially in the channels rather than on the external cement sample surfaces. We conclude that the localized precipitation can be ascribed to higher pH inside the cavities compared to near the external surfaces, owing to long diffusion distances giving a locally limited CO ₂ supply within the voids.