Corrosion Behavior and Mechanism Analysis of Oilwell Cement Under CO₂ and H₂S Conditions

In order to explore the corrosion effect of CO₂ and H₂S in oilwell cement, the samples were exposed to a CO₂/H₂S environment. The permeability, microstructure, chemical composition, and tensile strength of the cement samples were measured by a pulse pore-permeability tester, scanning electron microscope (SEM), X-ray diffraction (XRD), and rock tensile compression tester, respectively. The corrosion behavior and mechanism of the cement samples were analyzed. The experimental results show that the corrosion degree increases with the exposure time, and the permeability increases from 0.0003 md on the seventh day to 0.0646 md on the thirtieth day. It can be concluded from the microscopic morphology that the internal structure of the corroded cement sample is compact, without an obvious loose structure, but there are some particles with different particle sizes. The composition phases of the products after corrosion are mainly calcium carbonate and silica, and the content of calcium carbonate in the corrosion area increases obviously. The tensile strength of the tested cement sample is only 9.8 MPa. The reaction of gaseous CO₂ and H₂S with calcium-bearing phases in a cement sample is known to cause a lowering of alkalinity, leading to the corrosion of the cement sample. Due to the existence of carbonation, carbon dioxide will gradually diffuse into the interior of the cement sample and react with hydrogen ions, calcium ions, and bicarbonate ions in the pore fluid of the cement sample to form varieties of calcium carbonate, resulting in the shrinkage of the cement sample, surface cracks, and ultimately affecting the compactness and sealing effect of the cement sample.