Corrosion Product Scale Analysis in Polymer Drilling Fluid Containing H_2S/CO_2

With the successive development of sour gas fields,the study on corrosion mechanism and the optimization of downhole strings in drilling and oil production process have become the research focus.Through the high temperature and high pressure corrosion test and with the aid of scanning electron microscopy(SEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS),the composition difference and corrosion mechanism of corrosion scales in vapor and liquid phases of S135 drill pipe steel were studied in polymer drilling fluid under H2S/CO2 coexist environment.The results showed that the corrosion rate of S135 steel increased with the temperature and CO2 partial pressure raise both for vapor and liquid phases.For corrosion scale in vapor phase,the corrosion extent was serious than liquid,the corrosion morphology become local corrosion and the pits of corrosion scale surface was larger than liquid.But for liquid phase corrosion,corrosion products were more uniform and dense.Corrosion scales contain C,O,S,Fe,Si elements both for vapor and liquid phases,and the composition of corrosion scales was not very different,mainly composed of Fe3O4,FeS,α-FeOOH,FeS2,and a small amount of Fe7S8 and Fe3S4.Moreover,the surface of corrosion scales was also mixed with silicate material.The corrosion of drill pipe steel came from carbon dioxide corrosion,oxygen corrosion,hydrogen sulfide corrosion,elemental sulfur corrosion,and galvanic corrosion between corrosion product film and steel.