Experimental study on the impact of H2S and H2SO4 in CO2 on five different sealant compositions under conditions relevant for geological CO2-storage

Abstract

The integrity of well sealants is a key challenge to secure geological storage of CO₂. While it has been well-established through experimental research that the exposure of such sealants to CO₂-bearing fluids can lead to carbonation, and potentially degradation during prolonged exposure or flow, the impact of impurities present in injected CO₂ has not received much consideration.

This paper reports exposure of five different sealants to simulated well conditions with supercritical CO₂ containing H₂S or H₂SO₄ as impurities. Three of these sealants are based on Portland Cement, while the other two are based on Calcium Aluminate Cement, and a rock-based geopolymer specifically developed for Geological CO₂ Storage (GCS). The impact of the impurities on these sealants was assessed through scanning electron microscopy with energy-dispersive X-ray spectroscopy, computed tomography scanning, and fluid chemical analysis, and compared to previous research where the same five sealants were exposed to clean CO₂ under otherwise identical conditions.

The results show that during exposure to CO₂-saturated water, the presence of H₂S mostly resulted in enhanced sealant alteration depths, and reduced carbonate precipitation. During exposure to wet supercritical CO₂, the presence of H₂S or H₂SO₄ resulted in reduced carbonate precipitation, and enhanced alteration depths in some (H₂S) or all (H₂SO₄) sealants. Additionally, relatively minor degradation was observed in the outer 100–200 μm of samples exposed in the presence of H₂SO₄. Overall, the impacts of impurities were more pronounced for sealants that were more affected by exposure to clean CO₂.