Heterogeneous porosity distribution in Portland cement exposed to CO2-rich fluids

Efficient and safe storage of injected supercritical carbon dioxide (CO2) underground is now one potential solution for reducing CO2 emissions in the atmosphere. Preventing any CO2 leakage through a wellbore annulus after injection is a key to maintaining long-term wellbore integrity. Most wells in depleted oil and gas fields may be re-used to inject CO2. These wells were mostly cemented with conventional Portland cement. It is thus crucial to study how such cement behaves at depth in CO2-rich fluids. Set cement samples are exposed to CO2 fluids under pressure and temperature to simulate downhole conditions. The degraded cement exhibits significant mineralogical changes and heterogeneous porosity distribution. The bulk porosity evolution, as well as local porosity gradients through the samples, is quantified using combined mercury porosimetry and back-scattered electron image analysis. Both techniques show an initial sealing stage related to calcium carbonate precipitation plugging the porosity, followed by a dissolution stage marked by a significant increase of porosity.