Many researchers have conducted study on mechanism of single phase of CO2 or H2S corrosion to cement stone, but the corrosion mechanism of CO2/H2S mixture to cement is seldom involved. The fact is that most of sour gas fields contain CO2/H2S mixture, for example, Puguang gas reservoir in China contains H2S and CO2 as high as 15% and 8% by volume. In this paper, test methods are established under bottomhole condition of Puguang gas reservoir, changes of compressive strength and permeability of corroded cement stone samples by H2S/CO2 mixture are evaluated to study corrosion mechanism of CO2/H2S mixture to cement stone.
Slurry is prepared according to API Spec10B and poured into test mold and cured in HTHP cell for 24 hours. The half of set samples are removed from molds and numbered and putted into the corroding chamber and cured for 21 days, then seal the chamber and inject CO2/H2S mixture and apply heat and pressure according to required temperature and CO2/H2S partial pressure. The rest are cured in HTHP cell as comparative samples. The temperature and pressure in the chamber keep constant during the test.
Compressive strength and permeability cement stone samples were tested to probe the changes before and after corrosion. Microstructure and corroded products of samples were observed by SEM and XRD to investigate changes before and after corrosion.
Result shows the corroded products of CO2/H2S mixture to cement are similar to those by single-component H2S or CO2 gas, except that the amount of expansive crystal produced by H2S is reduced. Combination of H2S and CO2 accelerates the corrosion progress, the recession of strengthen and permeability is more serious than that of single H2S or CO2, but CO2 dominates the whole corrosion process after the long duration. Fly ash and Clay have benefits to resist corrosion of combination of H2S and CO2.
The CO2/H2S mixture will bring serious corrosion on cement ring in wet environments, and damage its' seal ability. The higher the temperature, the more severe of the recession of strengthen and permeability, which presents a complete opposite rule with corrosion by H2S or CO2. The composition of cement slurry is the predominant factor affecting cement corrosion resistance. The introduction of Latex and Fly ash and clay into system will reduce the alkalinity in the cement slurry system and improves the corrosion resistance. The hydration products are crystallized after corrosion under combination of H2S and CO2, and loose arrangement of those crystals is the reason of strength decline and permeability rise.