Preparation of Corrosion Inhibitor for Self-Diverting Acid and Mechanisms of Corrosion Inhibition

Self-diverting acids are widely used in the acidification and fracturing of carbonate reservoirs. Corrosion inhibitors, because of the special molecular structures of the viscoelastic surfactants, have poor compatibility with other additives and corrosion control efficiency that is quite low. In this study, two pyridine quaternary ammonium salt corrosion inhibitors, SI-1 and SI-2, are synthesized with 1-chloromethylnaphthalene, 4-ethylpyridine, benzyl chloride and 2,3-cyclopentenopyridine. The corrosion control performance and the effects on the viscosity of self-diverting acids of SI-1 and SI-2 were investigated using corrosion tester and rheometer. The morphology and chemical components of steel plates before and after corrosion by acids were analyzed from the microscopic perspective by means of SEM, EDS, atomic force microscope and X-ray photoelectron spectroscopy. The corrosion mechanisms of the two corrosion inhibitors were studied using molecular dynamics simulation. The results of researches show that SI-1 and SI-2 have good corrosion control performance in self-diverting acids, minor effects on the viscosity of the self-diverting acids and wide applicability, and is cost-efficient. Compared with SI-1, SI-2 has better performance. A self-diverting acid treated with 1% SI-2 has corrosion rate at 90 ℃ of 1.04 g/(m²∙h), corrosion rate at 120 ℃ of 7.43 g/(m²∙h), final viscosity of the VES residual acid stabilized at 190 mPa∙s or higher. The cost of treatment can be reduced by 20% with SI-2. After adding 1% corrosion inhibitor in the acid, the surfaces of the steel plates show no obvious corrosion, the Fe content increases greatly, and the surface roughness decreases significantly. With SI-2, the Fe content increases from 86% to 94%, and R_a decreases from 137 nm to 84 nm. In both cases C—N and organic C=O bonds are detected on the surfaces of the steel plates, indicating the existence of the adsorption membranes of the corrosion inhibitors. Molecular dynamics simulation shows the mechanisms of corrosion inhibition are as follows: SI-2 has small energy gaps and high adsorption energy. After being adsorbed on the surfaces of the steel plates, a dense adsorption membrane is produced, isolating the corrosion media and the surfaces of the steel plates, thereby remarkably inhibiting the corrosion process of the steel plates. SI-2 has been used in the Bohai Oilfield and the Missan Oilfield (Iraq) with excellent operation achievement.