Corrosion Inhibitor Used in Self-diverting Acid

Viscoelastic Surfactant (VES) is a crucial component of diverting acid systems. Due to its unique amphiphilic molecular structure, as the acid-rock reaction progresses and pH changes, the surface charge of the VES molecules alters. This causes the micellar morphology to transition from spherical to rod-like or worm-like structures, resulting in a significant increase in system viscosity. This enables fluid diversion and uniform stimulation of carbonate reservoirs. As diverting acid systems contain no polymer molecules, they cause no residual damage and achieve uniform acid distribution. Consequently, they are now widely applied in carbonate reservoir stimulation.

The interaction between corrosion inhibitors and the surface charge/molecular geometry of VES significantly impacts the rheology and corrosivity of diverting acid systems. Conventional corrosion inhibitors exhibit poor compatibility with diverting acids, primarily manifesting as corrosion rates far exceeding those in regular hydrochloric acid and severe degradation of diverting acid viscosity. Some organic components of the corrosion inhibitor penetrate into the VES micelles, reducing the effective concentration of the inhibitor. This prevents the formation of a protective film blocking H⁺ on the steel surface, thereby lowering inhibition efficiency . The non-polar carbon chains of the corrosion inhibitor extending into the micelle core can cause a reduction in diverting acid viscosity. Higher concentrations of corrosion inhibitor lead to more drastic viscosity loss. Hanafy et al. found no chemical reaction between corrosion inhibitors and VES molecules, identifying electrostatic interactions as the primary cause of VES viscosity reduction. Li et al.and Salar et al.  demonstrated that corrosion inhibitors disrupt the rod-like micellar structure of surfactants, with the destruction of the micellar assembly being the main reason for VES viscosity loss.

Current acidizing corrosion inhibitors are primarily categorized as Mannich bases, imidazolines, and quaternary ammonium salts. They inhibit electrochemical corrosion by forming an adsorbed film on the metal surface via sulfur, nitrogen, and oxygen functional groups in their molecular structures. Mannich base inhibitors typically have large molecular weights and contain lipophilic groups like benzene rings, making them prone to penetrate micelles and disrupt micellar stability. Alkyl quaternary ammonium compounds have been studied as diverting acid corrosion inhibitors but exhibit high corrosion rates. The introduction of aromatic quaternary ammonium compounds enhances molecular anisotropy and strengthens intermolecular attractive forces on the metal surface, substantially reducing corrosivity. Research by Viacheslau et al.  showed good compatibility of quaternary ammonium salt corrosion inhibitors in diverting acids, making them an important research direction for diverting acid corrosion inhibitors. Currently available commercial VES corrosion inhibitors are often highly specific (designed for proprietary VES products) and relatively expensive with high corrosion rates.

Two pyridinium quaternary ammonium salt corrosion inhibitors were synthesized. Their corrosion inhibition performance in diverting acids, their impact on diverting acid viscosity, and other properties were analyzed and evaluated. Their inhibition mechanism was also investigated, aiming to develop VES corrosion inhibitors with good performance, low cost, and broader applicability.

To achieve effective corrosion inhibition and good compatibility, the molecular structure of a diverting acid corrosion inhibitor should meet the following requirements:

1. The molecule should contain a stable planar structure or flexible branched chains (e.g., benzene ring, C-C branches, C-N chains) to enhance surface coverage and inhibition.

2. The molecule should contain atoms (N, P, S, O) or strong adsorbing groups capable of donating lone pair electrons to interact with iron atoms on the tubular surface, forming a dense protective film.

3. The molecule should lack active hydrogen atoms, as these can destabilize micelles and reduce the diverting viscosity of the acid.

4. The inhibitor should be water-soluble. Oil-soluble inhibitors' organic components more readily penetrate the micelles formed by the diverting agent, especially at high temperatures, significantly destabilizing the micelles and reducing acid viscosity.

Summary:

1. The incompatibility of conventional HCl corrosion inhibitors with self-diverting acid systems arises because organic components of the inhibitor penetrate micelles. This both reduces the effective inhibitor concentration (lowering inhibition) and disrupts the micellar structure (reducing viscosity).

2. A corrosion inhibitor suitable for self-diverting acid was developed. It controls static corrosion rates below 5 g/(m²·h) at 90°C and dynamic corrosion rates below 20 g/(m²·h) at 120°C, demonstrating good inhibition without affecting the diverting viscosity.

3. The developed self-diverting acid corrosion inhibitor was successfully applied in 15 wells in the Dagang Oilfield (China) and the Ahdeb Oilfield (Iraq), effectively ensuring the safe execution and success of the acid diversion treatments.