Effect of Water with Dissolved CO2 on Rock Properties: From Static Aging to Dynamic Flowing

Deep aquifers have been considered as good candidates for CO2 sequestration due to their massive capacities. They have been evaluated extensively not only for storage capacity but also for integrity since CO2 dissolved in formation-water forming weekly acidic carbonated water which can interact with formation rock matrix, especially carbonates. Many studies conducted in this regard were performed under static conditions, i.e., rock samples were exposed to carbonated water in a sealed vessel for a specific duration. This paper investigates the effects of carbonated water on rock properties under continuous dynamic flowing conditions and compares them with those obtained from a previous study under static ageing. In this study, carbonate rock samples were prepared and loaded in a controlled core flooding system, where carbonated water was continuously injected. The changes in rock properties and mechanical integrity vs. time were monitored by means of nuclear magnetic resonance (NMR), resistivity, and ultrasonic measurements. Changes in rock morphology were also captured through high resolution surface micrographs. This process of injecting carbonated water and monitoring changes in rock properties lasted for a total of 6 months. The effluents were also collected during the experiment and analyzed geochemically, using a capillary electrophoresis system. The experimental results showed that changes in rock properties with time of carbonated water injection was continuous, as indicated by NMR and resistivity measurements. From the NMR results, the rock pore surfaces and networks were likely altered due to exposure to carbonated water. The resistivity results revealed that the pore network is getting more complex with increased Archie porosity exponent. High resolution imaging and image analysis indicate that the rock surface morphology was also altered with observed partial dissolutions. The analyzed water effluents confirmed mineral dissolution with excess of calcium and magnesium cations. The ultrasonic measurements showed changes in rock stiffness, especially for calcite-rich carbonates. The above changes are more pronounced during continuous flowing when compared to those observed during static ageing.