Carbon sequestration is one of the approaches to achieve carbon dioxide reduction in the atmosphere. Underground storage of CO2 requires an understanding of geochemical and geomechanical alteration on the integrity of the injection wellbore. In this study, we investigate the reactivity of wet supercritical CO2, at 150°F and 3000 psi on Portland cement plugs (class G), for exposure of up to 5 weeks.
For nanoporous media, such as cement, diffusion is believed to be the major mass transport mechanism. To quantify the extent of the alteration (mineralization/dissolution) on fluid diffusivity through the cement matrix, a novel approach based on Nuclear Magnetic Resonance (NMR) is employed. Deuterium oxide (D2O) imbibition to replace H2O in cement plugs is monitored over time to derive tortuosity. Comparing pre- and post-CO2 exposure D2O intrusion profiles allows us to determine flow path alteration in the cement plugs. Additional characterizations include Fourier Transform Infrared Spectroscopy (FTIR) to observe the change in cement composition, X-ray computed tomography (XCT), along with Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) to determine invasion extent and microstructure modifications, mercury injection capillary pressure (MICP) for pore throat size distribution and BET N2 isothermal adsorption for surface area and pore size distribution. Finally, nanoindentation is conducted on companion samples to determine changes in mechanical properties.
The results show that exposure to wet supercritical CO2 promotes both carbonate precipitation and dissolution simultaneously, which alters the pore structure of the cement. After 5 weeks of exposure, there is evidence of carbonate dissolution in smaller pores (<20 nm) and precipitation in larger pores (20-200 nm), with the precipitation being more dominant, leading to a decrease in total porosity by up to 10%. The alteration of the cement plugs lead to an increase in matrix tortuosity by 6 and 3 times after 2 and 5 weeks of exposure respectively. Although dissolution and precipitation were observed after CO2 exposure, nanoindentation derived from Young’s modulus shows a modest increase from 22 to 25 ±1.5 GPa, which suggests that the cement maintained its strength.
This work shows a systematic approach to monitor and quantify the risk associated with CO2 exposure on Portland cement due to diffusional leakage. Within our test condition, the observation indicates that carbonate precipitation can help cement retain its mechanical integrity while minimizing the migration of CO2 through its matrix.
