Mechanisms of Corrosion Inhibitors Used in Acidizing Wells

Abstract

Summary. Two model compounds, n-dodecylpyridinium bromide (n-DDPB) and 1-octyn-3-ol, were tested in HCl acid as inhibitors for J55 oilfield stee. This paper describes the kinetic and chemical analyses conducted to arrive at inhibition mechanisms for these model compounds. These studies show that the pyridinium forms a weak bond with the chloride-covered surface an is sensitive to temperature and [HCl]. Octynol, however, chemisorbs and produces a film that contains a reaction product of the acetylenic alcohol. This film is quite insensitive to changes in temperature and [HCI]. Introduction The stimulation of oil and gas wells with concentrated solutions of HCl acid is an important procedure, performed to increase production and to remove formation damage. Because the acid must be pumped through steel production tubing before entering the formation, it must contain corrosion inhibitors. These inhibitors must function for at least 2 hours while the acid is being pumped into the producing formation. In some cases (e.g., when acid is used as a perforating fluid), the inhibitors must provide protection for as long as 24 hours. Many grades of tubing are used, depending on the temperature and pressure of the well, but API Grade J55 (55,000-psi [380-MPa] minimum tensile strength) is most commonly found in wells with bottomhole static temperatures less than 200deg.F [ less than 95 deg.C]. The commercial inhibitors used in stimulation acids are complex proprietary mixtures but frequently contain alkyl or alkylaryl nitrogen compounds and acetylenic alcohols. These two classes of compounds have been the subject of numerous mechanistic studies of corrosion inhibitors for pure iron or mild steel in HCl, but little fundamental information exists concerning API-grade steels. We chose to look at two substantially different chemicals, nDDPB and 1 -octyn-3-ol, as inhibitors for J55 steel in aqueous HCl. Inhibition isotherms were constructed with corrosion rates determined by electrochemical methods in 4.5 M (15.7%) HCl at 85 and 150deg.F [30 and 65 deg.C]. With these data for guidance, a detailed kinetic study was made with iron dissolution techniques in 0.3 to 6.0 M (I to 20%) HCl at 50 to 210deg.F [10 to 99deg.C). Because mac-rofilms have been observed on the surfaces of steel specimens after contact with HCl containing octynol, several surface and solution analytical methods were used to determine the composition of the films. Theory Corrosion inhibitors can be classified in several ways. Because corrosion of steel in HCl is an electrochemical process, inhibitors may retard the anodic (metal-dissolution) reaction, the cathodic (hydrogen-evolution) reaction, or both processes. While some chemicals, such as arsenic salts, are predominantly anodic inhibitors in HCl, most organic materials form films and retard both half-cell reactions. Inhibitors can operate at the solution/metal interface or in the electrolyte layer interposed between the interface and the bulk of the solution. This layer is often called the "interphase." Finally, inhibitors can be classified as primary or secondary inhibitors, depending on the reactions that take place with the organic molecule itself. A primary inhibitor will not be changed chemically, while a secondary material will react and produce a substance that is the real inhibitor molecule. When selecting the pyridinium bromide and octynol for study, we hoped that the two chemicals would provide examples of different classes of inhibitors and would illustrate different mechanisms of action. Experimental Materials. Rotating-disk electrodes were machined from API Grade J55 steel (Lot A) and polytetrafluoroethylene to expose 0.0465 in.2 [0.3 cm2] of surface. Corrosion coupons of J55 (Lot B) were purchased as 39-ft [12-m] joints, cut into 3.88-in.2 [25-CM2] specimens and grit-blasted to remove mill scale. AISI 1008 (0.08 % C) and 1018 (0.18% C) steels were used for the reaction-product iden-tification test. Table I shows the elemental compositions, while to microstructures of the two steels are depicted in Figs. I and 2. Solutions of HCl were made up from 11.8 M (37%) reagent-grade HCl and doubly deionized water and standardized by titration with NaOH to a phenolphthalein endpoint. DDPB (C 17H30N Br, FW 327.9) was made by refluxing equimotar amounts of 1 -bromododecane and pyridine in a solution containing 20 wt% isopropyl alcohol for 6 hours. Bromide titrations showed the reaction to be >98% complete. The acetylenic inhibitor (98%), I-octyn-3-ol (FW 126.2), was used without purification. Equipment. All of the electrochemical tests were run in a thermostatted cell fitted with a platinum wire counter-electrode, an Ag/AgCl reference, and a gas purge tube. A rotating-disk electrode (RDE) system was used to provide velocity. The polarization data were collected with an ECO Instruments Electrochemoscope IITM system, which consists of an ECO 553 digital potentiostat connected to a Hewlett-Packard 87 computer with disk drive and graphics plotter. The iron-dissolution tests were run in water baths thermostatted at 50 to 210deg.F [ 10 to 99 deg. C]. The acid solutions were not deaerat-ed. The total dissolved Fe was measured with an atomic absorption (AA) spectrophotometer calibrated at 2481 A [248.1 mn] over the range of 1 to 10 ppm Fe. The FT-IR instrument used was a Nicolet Model MX-360deg.-C equipped with a versatile reflectance attachment (VRA) with retromirror accessory (RMA in VRA, 97 1/2 deg.) from Harrick Scientific Corp. The RMA was set at an angle of 68 deg. for maximum detector response. A KRS-5 grid polarizer (Model IGP 225) was placed in the beam just ahead of the reflectance accessory. Other analytical instruments used included a Waters M6000 liquid chromatograph with -Styragel columns (one 500 A [50 nm] and three 100 A [10 MM ), a refractive index detector and an ultraviolet detector at 2540 A [254 nm). Experimental Procedures. The electrochemical tests were run after the electrode tips were degreased in ethanol and ground to 320 grit. A total of 10.0 mL of 4.5 M HCl was placed in the cell and deoxygenated with nitrogen for 15 minutes as the solution reached the test temperature. SPEPE P. 584^