Porosity changes in mud-affected rock and cement upon reaction with CO₂

Leakage from CO₂ storage reservoirs is a threat towards safe and cost-efficient Carbon Capture and Storage (CCS), and active and abandoned wells have been identified as the most probable leakage paths. This underlines the importance of ensuring well integrity throughout the entire life-cycle of a well. CO₂ is a reactive fluid, but its reactivity with well barriers such as rock and cement will depend on the other fluids present in the well. Both rock and cement are typically affected by drilling fluids before they are exposed to CO₂ in a downhole situation. In the present study we investigate how the presence of drilling fluids affects the way rock and cement are influenced by CO₂ exposure. This is done by performing a detailed X-ray tomography characterization of clean and mud-affected samples before/after they have been batch exposed to CO₂. This revealed that the porosity of the samples was altered after CO₂ exposure. For Saltwash North sandstone, the porosity at the edge and in the center of the sample was observed to decrease after CO₂ exposure. For Castlegate sandstone, the porosity of the sample at the center was observed to increase, while the porosity at the edge was observed to decrease upon exposure to CO₂. The cement sample with mud was observed to have a higher porosity than the pure cement sample.