The Study Of Corrosion Mechanism Of Drill Pipe And Pipeline, Inhibition And Mechanism Of The Inhibitor For Drill Pipe And Pipeline

In well-drilling project, corrosion of drill steel and pipeline is ubiquitous phenomena. Corrosion of drill pipe results from O₂ and ions of drilling fluids as well as temperature, and carbon dioxide in the natural gas dissolves in saltwater and results in the formation of a weak carbonic acid often causing severe corrosion in carbon steel pipelines, especially at high temperature and pressure.Ten kinds of salt and mixed salt solutions were selected according to the composition of drilling fluid, to analyze how to effect the corrosion of G105 steel. The results showed that corrosion rate decreased with increase of salt concentration, corrosion rate of G105 steel in inorganic salt were higher than that in organic salt. Corrosion rate of G105 steel in ZnBr₂ solution was highest; corrosion rates in CaBr₂ and NaCl solution were lower. Cathodic reductions were O₂ and H⁺ depolarization reduction in ZnBr₂ and ZnCl₂ solution. Contents of DO decreased linearly with increase in concentration. Instantaneous corrosion rate showed that corrosion products of G105 steel in single salt solution had almost no effect on corrosion rate.Orthogonal experiment of compound salt showed that Cl^- indicated a remarkable effect on corrosion rate, and then the temperature, and Br- had the least impact. Ca²⁺ influenced corrosion rate more obviously than Mg²⁺ did. Corrosion rate was the lowest in mixed saltwater including 5mol/LCl^-,0.04mol/LCa²⁺,0.008mol/LMg²⁺,0.016mol/LBr- at 40°C, and the lowest corrosion rate was 0.0197g/m² ? h(0.022mm/a).Corrosion product of No1,No2,No10 mainly wasβ-FeOOH.Corrosion mechanism of G105 steel in different density mixed saltwater was that corrosion rate decreased, then increased with increase in density, and increased with increase in temperature.Injecting inhibitor is effective and economical anti-corrosion method. Inhibition efficiency of QA in mixed saltwater was 94% at concentration of 200mg/L at 60℃. QA inhibits both the anodic and cathodic reaction, especially anodic reaction, and corrosion potential shifts negatively. QA has obvious effect on anodic slope, but cathodic polarization line changes slightly.Inhibition efficiencies of four-compound and five-compound inhibitors in 23.7%NaCl solution were up to 80%.Four-compound inhibitor had obvious effect on cathodic and anodic slope, and the inhibitor was mix-type inhibitor, which mainly inhibited anodic process. Inhibition mechanism changed at the different concentrations of inhibitor. Five-compound inhibitor also was mix-type inhibitor, which mainly inhibited anodic process, and inhibition efficiency increases with increase in concentration of inhibitor.Two imidazoline inhibitors both inhibited Q235 steel in saltwater saturated with carbon dioxide well, and inhibition efficiency increased with increase in concentration of inhibitor. Inhibitory capability of HSJ-2 was better than HSJ-1. Corrosion potentials had a little change, and two inhibitors influenced anodic and cathodic slope obviously. HSJ-2 was adsorbed on the steel for a longer time than HSJ-1, but HSJ-2 was disrobed from the steel. The inhibition effect of two imidazoline inhibitors was caused by "blocking effect" at 298K and 328K.The adsorption of two imidazoline inhibitors on Q235 steel can be fitted to Frumkin isotherm. k in Frumkin isotherm was about 20000 for HSJ-1, f was between 2.4 and 2.8; And k was between 46000 and 63000, f was about 2 for HSJ-2 by using matlab programming fitting, but results were very different by least squares method fitting: k about 35000, f 2.4～2.9. HSJ-2 seemed more favorable to adsorb at the surface than HSJ-1, so inhibition efficiency of HSJ-2 was higher than HSJ-1. Inhibitors were attracted each other.ΔGoads of two imidazoline inhibitors were negative.Molecular dynamics simulation showed that both the two imidazoline inhibitors can adsorb on the Fe surface through the imidazoline ring and heteroatoms, and the alkyl chain of HSJ-1 approximately perpendicular to the surface, HSJ-2 approximately parallel to the surface.Corrosion product of Q235 in saltwater saturated with carbon dioxide mainly wasγ-FeOOH,FeCO₃,Fe₃O₄ at 298K, and FeCO₃,Fe₃O₄ and Fe₂O₃ at 328K...