Corrosion Mechanism Under Highly Acidic Gas Wells Cement Environment

With the discovering of the large marine oil and gas fields in Tarim Basin, Sichuan Basin, marine reservoir exploration and development become the important energy supply in our country. The marine reservoirs of our country have the characteristics of burial depth, high pressure and high content of acid gases(H2S/CO2). Because of the long-term exposure to high acid gas, the property of cement stones will decline and the zonal isolation will fail, which leads to drilling tool and casing damage and induces formation fluid channeling. In this circumstance, it will seriously threaten to natural environment and personal safety and increase the cost of exploration and development. Therefore, the durability of the cement stone improving in acid gas field environment becomes increasingly important and urgent. The corrosion mechanism of cement stone under the high containing CO2/H2S environment must firstly be cleared to solve this problem.In the condition of single CO2or H2S corrosion, H2S&CO2joint corrosion, the compressive strength, corrosion depth, porosity and permeability of different density cement stones are measured before and after corrosion by changing partial pressure of gas, corrosion temperature and corrosion time. The microstructure and composition of cement stones were analyzed by SEM and XRD. Finally, the cement slurry system and the measure of increasing corrosion resistance of cement stones will be obtained. Through the research of this article, the results and conclusions as following:(1)Whether it be single gas corrosion or H2S&CO2corrosion in a wet environment, it will corrode the cement stone seriously. With the gas pressure, corrosion time and temperature increasing, the corrosion rate and corrosion degree of the cement stone increase.(2)The Ca(OH)2and C-S-H are the main hydration products of cement stone, which react with CO2to CaCCb. CaCCb can react with CO2to Ca(HCO3)2further The corrosion reactions decrease the strength and increase porosity of cement. CaSO4.2H2O will be generated when the products reacted with H2S, which increase the internal stress and decrease the compressive strength of the cement.(3) With removing the surface dense layer of cement stones and testing by porosity and permeability, the internal pore structure of cement stones can be reflected. Combined with scanning electron microscopy (SEM), porosity and permeability analysis, the internal pore structure of cement stones were researched in both single CO2/H2S corrosion and CO2&H2S joint corrosion.(4) The order of severity under the three conditions from high to low is the CO2&H2S joint corrosion, the single H2S corrosion and the single CO2corrosion.(5) Adding fly ash or latex into the cement may reduce the content of alkaline components or increase the compaction of cement stones, which can improve the CO2and H2S corrosion resistance of oil well cement stones.