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Enhanced acids are generally prepared by mixing acid with halide salts or concentrated halide solutions (such as sodium chloride, calcium chloride, sodium bromide, calcium bromide, or mixtures of zinc bromide and calcium bromide, etc.). The choice of salts or brines is primarily based on the need to enhance acidity and cost efficiency. Many relevant publications exist on the corrosion of acidic substances and corrosion inhibitors. However, there are very few reports available on research regarding the enhanced corrosion behavior of acids on pipes. Corrosion test data show that the corrosivity of the acid system enhanced by brine is much higher compared to that of the acid system prepared using µ water at the same concentration (conventional acid). When enhanced acids are used, certain corrosion inhibitors or combinations of inhibitors that are effective for conventional acids cannot provide satisfactory corrosion protection for conventional oil field pipelines. Therefore, it is proposed that when evaluating intensifying acids, attention should be paid to their compatibility and efficiency, as well as maintaining their concentration and solubility properties. I. Introduction Over the years, acidizing fluids for enhancing production have been widely used in various operations such as wellbore cleaning, matrix acidization, and fracturing acidization during well completion or subsequent workover tasks. Depending on the type of construction and the nature of formation damage, the acids used can be hydrochloric acid (HCl), hydrochloric acid–hydrofluoric acid (HCl-HF), or organic acids (such as acetic acid, formic acid, or their mixtures). Sometimes, based on engineering requirements, to prevent gas invasion, excessive mixing of formation fluids and density separation, or to maintain hydrostatic pressure, it is necessary to increase the density of the acid solution, for which weighted acid is used. Enhanced acid can be used for reservoir modification, tubing string cleaning, descaling, removing polymer damage, and more. The usage method of intensified acid is the same as that of conventional acid. The oil industry has developed certain corrosion inhibitors or combinations of corrosion inhibitors that can effectively protect metal pipes under downhole conditions. However, there is very little literature available on the effectiveness of these corrosion inhibitors and combinations of corrosion inhibitors in aggravated acid. As with the use of conventional acids, there are also risks associated with using weighted acids when treating high-temperature reservoirs. The results of the corrosion tests clearly show that the corrosivity of enhanced acids is much higher, or even vastly different, from that of conventional acids at the same concentration. Therefore, studying the enhanced corrosion behavior of acids on metal materials has become a key issue. The purpose of this study is to evaluate the enhanced corrosivity of acid on downhole metallic materials at temperatures of 200–230°F. The evaluation focuses primarily on the effect of saline-type corrosion inhibitors (or combinations of inhibitors) on exacerbating the corrosivity of hydrochloric acid (HCl). Based on the data obtained from laboratory evaluations, it will provide basic guidance for designing an effective acidizing fluid system with low corrosivity and good compatibility, which is required for production enhancement operations. II. Corrosion Evaluation 1. Test materials: saline, metal specimens, and concentrated hydrochloric acid. Several types of saline used in the studies on concentrated acid testing were examined in this study, including four solutions with concentrations of 10.0 ppmg of NaCl, 11.6 ppmg of CaCl2, 12.5 ppmg of NaBr, and 14.2 ppmg of CaBr2, as well as a mixed solution containing 19.2 ppmg of ZnBr2 and CaBr2. These brines are easily available; drilling fluid or brine service companies refer to them as high-density brines or industrial brines. The metal specimens used for the corrosion test were N-80 carbon steel and 13 nickel steel. The acids used in the tests included hydrochloric acid with concentrations of 5%, 7.5%, 10%, and 15% on a weighted basis. They were respectively added to NaCl, CaCl2, NaBr, CaBr2 solutions, as well as ZnBr2/CaBr2 mixed solutions to reach a certain concentration. In the tests, all acid solutions will be mixed with a corrosion inhibitor (or a combination of inhibitors) to suppress corrosion. Table 1 lists the types of evaluation for corrosion inhibitors (or combinations of corrosion inhibitors) and their dosages. Tests were conducted on the performance of these corrosion inhibitors (or combinations of inhibitors) in various acids, and the results showed that they can effectively protect N-80 carbon steel and 13 nickel steel materials at high temperatures. 2. Acid corrosion test and evaluation criteria: First, glass beads are used to polish each metal specimen in order to remove surface rust spots. The specimen was weighed before and after the test to determine the corrosion rate. The test was conducted in the corrosion autoclave apparatus at a pressure of 3000 psig, within the expected temperature range. After the test, clean the specimens with acetone, and then gently blow air to remove any remaining corrosion inhibitor (or combination of corrosion inhibitors). Observe the morphology of the erosion spots on the specimen under a microscope. During the acidization period (acid contact time), it is acceptable in the industry for the corrosion rate of the pipe material caused by the acid to be below 0.050 lb/ft2. In addition, the criteria for evaluating pitting also take into account pitting sizes and shapes that are smaller than those currently present. In terms of acidification, ranges where the size and shape of the erosion spots are equal to or greater than that indicated by “2” can be considered unacceptable, although the corrosion rate based on weight loss may fall within the acceptable range. 3. Characterization of pitting size and shape: 0: No pitting on the specimen surface ； 0-1: One or several very small and shallow erosion spots form on the surface of the specimen ； 2: Small, shallow pinhole-like corrosion spots form on the surface of the specimen ； 3: The small pitting formed on the surface of the specimen accounts for 1/32 to 1/16 of the surface area ； 4: The small pitting formed on the surface of the specimen is larger than 1/16 of the surface area ； 5: Large holes or randomly distributed deep erosion spots form on the surface of the specimen. 4. Discussion of test results: Table 2 shows the results of a corrosion test conducted at 200°F for 6 hours, with 7.5% HCl concentration, enhanced by 14.5 ppmg of ZnBr2/CaBr2. After adding corrosion inhibitor A to conventional acids, the corrosion rates of N-80 carbon steel and 13Cr steel remain within the limits permitted by the industry. However, by adding corrosion inhibitor A to the concentrated acid, the resulting corrosion rate was much higher than the 0.050 lb/ft2 limit permitted by the industry. Increasing the concentration of Corrosion Inhibitor A to 100% or adding Corrosion Inhibitor B to enhance the corrosion inhibition performance did not provide effective protection for the surfaces of the two specimens. Furthermore, the morphology of surface pitting on all N-80 and 13Cr materials exceeded the limits permitted by the industry. The addition of ZnBr2/CaBr2 had a decisive effect on the performance of corrosion inhibitors A and B. It can be seen that the acid corrosion severity is significantly increased compared to conventional acids. When corrosion inhibitor combination I was used, the corrosion of both metal specimens was effectively controlled. Therefore, the corrosion inhibitor combination I can effectively suppress the corrosiveness of acid when intensified using a ZnBr2/CaBr2 brine solution. At 230°F, N-80 was tested for 6 hours with an acid solution enhanced by various saline solutions. Under the aggravating effect of CaCl2 saline, adding corrosion inhibitor combination I to 5% HCl can provide protection for N-80 ； However, when the acid concentration rises to 10%, and especially to 15%, its corrosion inhibition effect drops significantly; at these concentrations, the corrosion rate is 5 times the allowable value. When corrosion inhibitor combination I was used in an HCl acid solution containing NaCl, NaBr, CaBr2, and nBr2/CaBr2, the corrosion rate of the N-80 specimen by the acid solution was very low, generally ranging from 0.005 lb/ft2 to 0.013 lb/ft2, which is the same as the corrosion rate caused by conventional acids. The test results clearly show that the addition of NaCl, NaBr, CaBr2, and ZnBr2/CaBr2 does not reduce the performance of corrosion inhibitor combination I, whereas the addition of CaCl2 has a decisive impact on its performance. A corrosion test was conducted on 13Cr specimens at 230°F for 24 hours. All conventional hydrochloric acid systems can effectively control the corrosion of 13Cr respectively. However, when corrosion inhibitor combination I was added to HCl with an increased density due to CaCl2, its corrosivity increased significantly; especially at acid concentrations of 10% and 15%, the corrosion rates were 3.4 times and 6.8 times respectively the allowable corrosion rate. This once again shows that the addition of CaCl2 reduced the performance of the low-corrosion inhibitor combination I. When corrosion inhibitor combination I was used in acid solutions enhanced with NaCl, NaBr, CaBr2, and ZnBr2/CaBr2, the corrosion rates for 13Cr were all below 0.05 lb/ft2, except in the case of 15% HCl enhanced with CaBr2, where the corrosion rate was 0.057 lb/ft2. When Corrosion Inhibitor Combination II was added to this 15% HCl, corrosion was controlled, and the corrosion rate dropped to 0.033 lb/ft2. As mentioned earlier, CaCl2 has an adverse effect on the performance of corrosion inhibitor combination I. Therefore, to determine whether the corrosion behavior of CaCl2 in weighted hydrochloric acid can be controlled, corrosion inhibitors B and synergists C and D were added to the acid solution respectively, and corrosion inhibition efficacy tests on 13Cr were conducted at 230°F for over 24 hours. The test results shown in Table 3 indicate that corrosion inhibition against 5% and 10% hydrochloric acid is possible only when corrosion inhibitor B together with the synergistic components C and D (i.e., Corrosion Inhibitor Combination II) is used. However, when the acid concentration is 15%, the corrosion inhibitor loses its effectiveness. This shows that it is very difficult to suppress the corrosivity of hydrochloric acid exacerbated by CaCl2. When designing acidization operations, the acid concentration is primarily determined based on the mobility or solubility of the acid regarding the formation minerals or foreign substances. The intensified acid, which has the same acid concentration as conventional acid, exhibits a corrosion rate that is significantly different from that of conventional acid due to the addition of brine and corrosion inhibitors. For example, ZnBr2/CaBr2 mixed with corrosion inhibitors A or B makes the corrosivity of concentrated acid much greater than that of conventional acid at the same concentration ； After using corrosion inhibitor combination I, its corrosivity was effectively controlled. In HCl enhanced by CaCl2 solution, the corrosion inhibitor combination I was able to provide protection for N-80 at low acid concentrations (5%), but lost its corrosion-inhibiting effect when the acid concentration increased (10% and 15%). The same was true in all the protection tests for 13Cr. After adding Corrosion Inhibitor Combination II, the corrosion of 13Cr by 5% and 10% hydrochloric acid was effectively controlled, but the corrosion rate of 13Cr by 15% hydrochloric acid still exceeded the allowable value. It is evident that the CaCl2 solution and the ZnBr2/CaBr2 solution have a significant impact on the performance of the corrosion inhibitor (or combination of inhibitors). Furthermore, corrosion inhibitor combination I can suppress the corrosivity of hydrochloric acid N£ 80 and 13Cr exacerbated by NaCl, NaBr, and CaBr2. However, to suppress the corrosion of 13Cr by 15% hydrochloric acid exacerbated by CaBr2, corrosion inhibitor combination II needs to be added. It can be seen that not only the type of saline, but also the corrosion inhibitor (or combination of inhibitors) has a significant impact on enhancing the corrosivity of acid. Further research is needed on the interaction between saltwater and corrosion inhibitor components. The performance evaluation of corrosion inhibitors (or combinations of corrosion inhibitors) shows that those used for conventional acids provide poor inhibition of the corrosivity caused by enhanced acids. Studies have shown that corrosion inhibitors combinations I and II, when combined with inhibitor B, enhancer C, or a mixture of enhancer C and D, can effectively suppress the corrosivity of acid solutions exacerbated by NaCl, NaBr, CaBr2, and ZnBr2/CaBr2. Studies show that adding synergistic components to corrosion inhibitors is an alternative approach to suppressing the corrosivity of enhanced acids under high-temperature or prolonged exposure conditions. Therefore, selecting an appropriate combination of corrosion inhibitors to control the corrosiveness of enhanced acid is crucial for the success of acidizing operations. III. Conclusions 1. The type of saline used has a significant impact on exacerbating acid corrosion. 2. CaCl2 and ZnBr2/CaBr2 solutions have a decisive influence on the performance of the corrosion inhibitor (or combination of inhibitors). The corrosion performance by hydrochloric acid exacerbated by CaCl2 is difficult to suppress. 3. NaCl, NaBr, and CaBr2 have no effect or only a minimal effect on the combined performance of the corrosion inhibitors. 4. Sometimes, the corrosion inhibitors (or combinations of corrosion inhibitors) used in conventional acids are not effective in enhancing acid performance. 5. The combined use of corrosion inhibitors and synergistic components can more effectively control the corrosivity of enhanced acids. 6. When designing acidization projects, it is highly recommended to conduct laboratory tests using acid solutions in order to determine the effectiveness of corrosion inhibitors (or combinations of inhibitors). □