Corrosion mechanisms of sulfoaluminate cement by highly sour gas

This research investigated the property variation and corrosion mechanisms of sulfoaluminate cement in highly sour services. The variations of the specific area and pore structure of the cement after corrosion were measured by cryogenic nitrogen adsorption tests (the BET and BJH methods). The changes in hydration products after corrosion were tested using the X-ray diffractometer (XRD) and thermal analyser (TG/DTG). Finally, the micro-scale morphological variations of hydration products of cement after corrosion were observed using the scanning electron microscope (SEM). The results showed that SAC presents a desirable strength at 60 ℃. The hydration products are mainly Aft after 14-day corrosion at 60 ℃ and CaSO₄ · 2H₂O after 14-day corrosion at 90 ℃. Notable stratification of the set cement is observed after corrosion by H₂S. The outer layer is first corroded, while the inner layer presents short-term strength growth due to swelling. The mechanism of H₂S corrosion is that C—S—H and CH in the hydration products of cement react with H₂S respectively and produce swelling Aft and CaSO₄ · 2H₂O, which results in cracking of cement and reduction in compressive strengths. The findings of this research provide the experimental and theoretical basis for applications of sulfoaluminate cement in cementing of oil and gas wells with highly sour gas.