Development and Corrosion Inhibition Mechanisms of a Corrosion Inhibitor for Self-Diverting Acids

Self-converting acids are widely used in the acid fracturing of carbonate reservoirs. Conventional corrosion inhibitors used in self-converting acids have, because of the unique molecular structure of the viscoelastic surfactants (VES), poor compatibility with other components, thereby drastically reducing their corrosion inhibitive performance. In this study, two pyridine quaternary ammonium salt corrosion inhibitors, SI-1 and SI-2, are synthesized with 1-chloromethylnaphthalene, 4-ethyl pyridine, benzyl chloride and 2, 3-cyclopentenopyridine. Using corrosion tester and rheometer, the corrosion inhibitive performance of SI-1 and SI-2 in self-diverting acids is studied. Also studied are the effects of SI-1 and SI-2 on the viscosity of the diverting acids. Using SEM, EDS, atomic force microscope and X-ray photoelectron spectroscopy, the surface morphology and chemical components of the steel plates before and after corrosion are analyzed from a microscopic point of view. The corrosion inhibition mechanisms of SI-1 and SI-2 are investigated using molecular dynamic simulation method. The results of these studies show that the two pyridine quaternary ammonium corrosion inhibitors have good corrosion inhibitive performance in self-converting acids and they only very slightly affect the viscosity of the self-converting acids being studied. The two corrosion inhibitors have wide applicability and are cost-effective. Compared with SI-1, SI-2 is a better corrosion inhibitor. At 90 °C, the corrosion rate of the steel plates in 1% corrosion inhibitor is 1.04 g/(m²∙h), at 120 °C, the corrosion rate is 7.43 g/(m²∙h), the final viscosity of the VES residual acids is 190 mPa∙s or higher, and the operation cost can be reduced by 20%. When adding 1% corrosion inhibitor SI-1 or SI-2 into the self-convert acid, no apparent corrosion can be found on the surface of the steel plates, and the Fe content is greatly increased, indicating a significant reduction in surface roughness. When SI-2 is used, the Fe content is increased from 86% to 94%, and the Ra value reduced from 137 nm to 84 nm. In both cases, C-N and carbonyl C=O are detected on the surface of the steel plates, indicating the existence of adsorption membranes formed thereon by the corrosion inhibitors. Molecular dynamic simulation results show that SI-2 has small energy gaps and big adsorption energy, and when the SI-2 molecules are adsorbed on the surface of the metal, a dense adsorption membrane is generated, isolating the contact between the corrosion media and the surfaces of the steel, hence remarkably inhibiting the corrosion of the steel. The application of SI-2 in the Bohai oilfield and the Iraq Missan oilfield has been very successful.