Preventing coiled tubing corrosion by hydrochloric acid in hot wells

Achala Danait, Jay Elliott* and Anupom Sabhapondit, Halliburton; and Surachai Kanchiak, Chevron Thailand Exploration and Production (CTEP)

Various forms of corrosion, internal and external, can occur with the use of coiled tubing (CT) in oilfield operations. One common source of corrosion results from the use of hydrochloric (HCl) acid, which is often pumped through or around the CT as part of a typical treatment. This exposes both internal and external faces of the CT to the acid’s corrosive effects; for this reason, the control and prevention of acid-related corrosion on CT is critical to help prevent costly and dangerous CT failures. However, at this time, little research exists on the prevention of HCl acid corrosion at temperatures greater than 350°F.

Asia-Pacific operators routinely use CT for various acidizing applications, including calcium carbonate scale removal. These treatments require high concentrations of HCl acid (about 15%), at depths where temperatures can reach 425°F. Controlling CT corrosion from HCl acid at these high temperatures is a serious concern, particularly because few high-temperature corrosion inhibitors are available.

Using one of the most widely applied CT alloys, QT-800, researchers tested the effectiveness of various acid-corrosion inhibitors and the resulting HCl acid concentrations required to provide adequate corrosion protection up to 450°F. In most cases, it was found that the performance of a quaternary amine-based corrosion inhibitor was comparable to a propargyl alcohol-based corrosion inhibitor. The addition of an antimony-based intensifier to the acid blend was also necessary to achieve low corrosion losses. Furthermore, as temperatures approached 425°F, reducing the HCl acid concentration to as low as 5% was required to achieve an HCl acid blend capable of dissolving calcium carbonate scale while maintaining the industry-acceptable corrosion-loss values of 0.05 lb/sq ft or less. This research will help ensure continued safe use of the CT through successive HCl acid treatments in high-temperature wells.

INTRODUCTION

 The alloy QT-800 is classified as a modified ASTM A-606, Type 4, high-strength, low-alloy steel. Type 4 steels contain additional alloying elements, along with copper, which provides a level of corrosion resistance substantially better than that of carbon steels with or without copper addition. These steels are therefore a preferred choice for CT.

Electrochemical tests using new and cycled CT-90 and CT-100 in various common oilfield fluids suggest that CT corrosion tendency is not significantly accelerated as a result of cycling at low temperatures of 100°F.1 However, at 295°F, significantly higher corrosion rates were observed for both cycled CT-90 and cycled CT-100. This reduced resistance of fatigued coil underlines the importance of correctly protecting the CT against corrosion at elevated temperatures.

Corrosion inhibitors can be broadly classified into two types: anodic and cathodic. Anodic inhibitors function by sharing electrons with anodic sites at the metal surface, forming a dative bond. Cathodic inhibitors function by forming a protective film on the metal surface by attracting the inhibitor to cathodic areas through electrostatic attraction.2

Arsenic compounds were commonly used as corrosion inhibitors for HCl acid until concerns about toxicity led to their elimination. The next generation of corrosion inhibitors used acetylenic alcohols and coal-tar derivatives blended with solvents and dispersants. Quaternary-ammonium compounds have also demonstrated promising inhibition behavior.

At especially high temperatures (when corrosion is most severe), certain additives are required. These additives, referred to as intensifiers, cannot be considered inhibitors when used alone, but can improve the effectiveness of organic inhibitors when used with them. The commonly used intensifiers are copper salts, potassium iodide, formic acid and antimony compounds. Copper salts as inhibitor aids are not preferred because of environmental concerns. Antimony salts exhibit excellent intensifier properties at HCl acid concentrations below 17% and temperatures above 250°F.3

TESTING

Few acid-corrosion inhibitors and intensifiers have been screened for testing at the high-temperature range from 250 to 450°F. The objective of this study was to design an acid blend that could provide adequate corrosion protection for QT-800 alloy when exposed to high temperatures (up to 450°F) for a period of 4 hr, while at the same time remaining effective in the primary goal of dissolving calcium carbonate scales. The targeted maximum corrosion loss for this testing was 0.05 lb/sq ft with no pitting. The selected inhibitors and intensifiers with their general chemical nature are listed in Table 1.

TABLE 1. Selected corrosion inhibitors and intensifiers

Propargyl alcohol-based materials have been widely used to provide corrosion inhibition at elevated temperatures,4 and so were chosen as the initial inhibitor for testing purposes. Regarding the intensifiers, because of the high bottomhole temperatures of the study and HCl acid concentrations below 17%, antimony salts were preferred for initial testing because of their advantages under similar test conditions.3

Dosages of the corrosion inhibitors and intensifiers are based on the commercial blends available and not on a pure material basis. All inhibitor and intensifier concentrations are reported in volume percentages of the total acid blend. Surfactants and penetrating agents were used as per industry norms and kept constant in all experiments.

Different corrosion behaviors can be obtained under the same test conditions, if the same grade of CT is procured from different sources. To minimize variation, the test specimens were obtained from the actual QT-800 CT used on location, and the same sample source was used for all testing coupons. The typical metallurgy of QT-800 is presented in Table 2.

The QT-800 tubing used for testing had a 1.296-in. ID, 0.102-in. wall thickness and 1.5-in. OD, and was cut into cylindrical coupons with a height of 0.4 in. and a surface area of 4.4 sq in. Care was taken to ensure that the coupon was not heated during the deburring process because this could change the properties of the metal.

Acid-corrosion testing was conducted using small Hastelloy B-2 autoclaves, which test one specimen at a time. The coupons were initially degreased with acetone and then bead-blasted. The initial weight of each coupon was recorded, and then the coupon was suspended in a 125-ml glass cell with the help of  Teflon tape, and 100 ml of acid blend was placed in the cell. After capping the cell, the remaining autoclave volume was filled with mineral oil, which acts as a heat-transfer medium, and the cell contents were pressurized to 1,000 psi with nitrogen. Pressure was maintained using a backpressure regulator assembly, which allows for automatic bleed-off of excess pressure developed during heating and release of corrosion products. The autoclaves were heated using Eurotherm controllers. The contact times included a 75-min. heat-up time and a 15-min. cool-down time.

 After each test, the autoclave was first allowed to cool slowly at room temperature and then was cooled in water. The pressure was released, and the glass cell was carefully removed. The coupon was cleaned with a soft brush using a mildly abrasive cleaning powder, washed with water and rinsed with acetone. The coupon was dried and the final weight was recorded.

Temperature effects. Figure 1 illustrates the static high-pressure/high-temperature (HPHT) corrosion-loss data at temperatures from 250 to 400°F. Acid blends with 15% and 17% HCl acid containing propargyl alcohol-based inhibitor (Inhibitor-P) and antimony-based intensifier (Intensifier-1) were tested for 4 hr.

Up to 325°F, acceptable corrosion-loss values (0.05 lb/ft or less) could be achieved using 17% HCl and 2% each of Inhibitor-P and Intensifier-1. From 325 to 350°F, adequate corrosion protection was obtained by either decreasing the HCl acid concentration to 15% or by increasing the concentrations of Inhibitor-P and Intensifier-1 to 4% each. At 375°F, 15% HCl acid and 4% each of Inhibitor-P and Intensifier-1 were required to achieve acceptable corrosion-loss values.

The corrosion loss increased sharply above 375°F, requiring the use of lower acid concentrations for the acid blends.

HCl concentration effects. Figure 2 presents the static HPHT corrosion-loss data at temperatures from 375 to 400°F. Acid blends with HCl acid concentrations below 15% were tested with Inhibitor-P and Intensifier-1 for 4 hr.

The corrosion loss increased drastically above 375°F, as expected. An HCl acid concentration of 10% was necessary to adequately protect the QT-800 alloy at 400°F, while an HCl acid concentration as low as 5% was required to enable the Inhibitor-P/Intensifier-1 pair to inhibit the acid suitably at 425°F. At these high temperatures, the inhibitor and intensifier concentrations were maintained at 4% each. Because the results achieved were near the limit of this inhibitor-intensifier system, it was decided to additionally investigate the behavior of a quaternary-ammonium salt-based inhibitor (Inhibitor-Q).

TABLE 2. Typical metallurgy and properties of QT-800 coiled tubing

TABLE 3.  HPHT corrosion-loss data with Inhibitor-Q at 400ºF after 4 hr

TABLE 4. Effect of different intensifiers on corrosion loss in 15% HCl acid at 400°F after 4 hr

Fig. 1. Effect of temperature from 250 to 400ºF on corrosion loss for 4 hr.

Fig. 2. Effect of HCl concentration on corrosion loss for 4 hr.

Inhibitor-Q effects at 400°F. Quaternary amines have long been known as excellent corrosion inhibitors for their ability to form a film on the steel surface.5,6 To evaluate the effectiveness of the quaternary-ammonium salt-based inhibitor on corrosion loss at high temperatures, it was decided to use this inhibitor, Inhibitor-Q, in conjunction with Intensifier-1, which was used in earlier tests. HPHT corrosion-loss data are presented in Table 3.

Using 15% HCl acid and the same concentration (4%) of inhibitor as in previous tests, it was observed that at 400°F, Inhibitor-Q provided slightly better corrosion protection than Inhibitor-P. Increasing the Inhibitor-Q concentration to 5% did not result in any improvement in the corrosion-loss values. A reduction in HCl acid concentration below 15% had higher corrosion-loss values when the Inhibitor-Q and Intensifier-1 concentrations were maintained constant at 4% each. This trend was not observed for the Inhibitor-P/Intensifier-1 combination. Further investigations should be conducted to identify the cause of this anomalous behavior.

Intensifier variation effects. Three other commonly available intensifiers were tested at high temperatures with QT-800 alloy: Intensifier-2, an alkali iodide-based material; Intensifier-3, an organic acid-based material; and Intensifier-4, an organic phosphonium chloride-based material. All these intensifiers are generally recommended for high-temperature applications.

As shown in Table 4, Intensifiers-2, -3 and -4 showed much higher corrosion losses than Intensifier-1. Thus, the antimony-based Intensifier-1 is the most effective intensifier when used in combination with either the propargyl Inhibitor-P or the quaternary ammonium-based inhibitor-Q.

CONCLUSIONS

Based on these results, it is now possible to protect the QT-800 alloy from HCl acid corrosion at temperatures up to 425°F. This can be achieved through the combined use of Inhibitor-P and Intensifier-1, along with the reduction in HCl acid concentration down to 5% as the temperature is increased. However, reducing the HCl acid concentration to this degree will also reduce the dissolution power of the resulting HCl acid blend, requiring higher volumes of acid to dissolve the calcium carbonate scales. In situations where pumping high volumes of acid is not practical, an organic acid blend having a dissolving power equivalent to 15% HCl acid can be considered as an alternative to HCl acid when the temperature reaches 425°F or higher.    

ACKNOWLEDGMENTS

This article was prepared from a poster presented at the 15th SPE/ICoTA European Well Intervention Round Table held in Aberdeen, UK, Nov. 18–19, 2009. The authors thank the management of Halliburton and Chevron for permission to present these findings. Efforts taken by Giselle Braganza, Jajati Nanda and Balakrishnan Shanmugaraj for the lab work are greatly appreciated.

LITERATURE CITED

 1 Van Arman, W., McCoy, T., Cassidy, J. and R. Rosine, “The effect of corrosion in coiled tubing and its prevention,” SPE 60744 presented at the SPE/ICoTA Coiled Tubing Roundtable, Houston, Texas, April 5–6, 2000.

 2 Foster, G., Oakes, B. and C. Kucera, “Acetylenic corrosion inhibitors,” Industrial & Engineering Chemistry, 51, No. 7, 1959, pp. 825–828.

 3 Walker, M., “Method and composition for acidising subterranean formations,” US Patent No. 4498997, 1985.

 4 Aramaki, K. and E. Fujioka, “Spectroscopic investigations on the inhibition mechanism of propargyl alcohol for iron corrosion in hydrochloric acid at elevated temperatures,” Corrosion, 53, No. 4, 1997, pp. 319–326.

 5 Tiwari, L., “Design and development of quaternary amine compounds: Corrosion inhibitors with improved environmental profiles,” SPE 95081 presented at the International Symposium on Oilfield Corrosion, Aberdeen, UK, May 13, 2005.

 6 Katheeri, M. and H. Nasr-El-Din, “Application of CE and CE-MS to assay corrosion inhibitors used in well stimulation treatments,” SPE 95112-MS presented at the International Symposium on Oilfield Corrosion, Aberdeen, UK, May 13, 2005.