13 Chromium Stainless Steel in Acid Stimulations.pdf

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CORROSION ENGINEERING

555CORROSION–Vol. 50, No. 7

Submitted for publication October 1993; in revised form, March 1994.Presented as paper no. 80 at CORROSION/93, March 1993, NewOrleans, LA.

* Shell Research Ltd., Sittingbourne, Kent, ME9 8AG, England.** Shell Research BV, P.O. Box 40, 6800 AA, Arnhem, The Netherlands.(1) UNS numbers are listed in Metals and Alloys in the Unified Numbering

System, published by the Society of Automotive Engineers (SAE) and

cosponsored by ASTM.

Corrosion Behavior of 13% Chromium Steelin Acid Stimulations

S. Huizinga* and W.E. Liek**

ABSTRACT

In gas production wells, carbon (C) steel tubulars subject to carbon dioxide (CO 2 ) corrosion have been replaced gradually

with corrosion-resistant 13% chromium (Cr) steel of the martensitic type (AISI type 420 [UNS S42000]), eliminating the need for inhibition and possibly extending tubular lifetime.

The effect of matrix acid stimulation treatments on 13% Cr steel tubulars was studied. Laboratory tests using electrochemical techniques were conducted in “live” acid,“spent” acid, and artificial production fluids in the presence of CO 2 . Tests included simulation of a full acid treatment cycle to

study repassivation of the steel. Typical inhibited live acid,based on 15% hydrochloric acid (HCl), was not unacceptably aggressive toward 13% Cr steel. It was shown that severe damage from locally initiated acid attack in back-produced

spent acid, with a reduced inhibitor content but still highly acidic, could be avoided by limiting the contact time with the tubular steel. In typical chloride (Cl – )-containing brines,13% Cr steel repassivated after acidization within some hours. A

risk of pitting corrosion existed only at very high Cl – levels at

elevated temperatures.

KEYWORDS: acid stimulation, carbon dioxide, chloride,chromium, corrosion, gas production, inhibition, passivation,pitting, type 420 steel

INTRODUCTION

Over the last few years, carbon (C) steel tubulars ingas production have been replaced gradually with 13%chromium (Cr) steel of the martensitic type, such asAISI type 420 (UNS S42000).(1) The corrosion rate ofthis corrosion-resistant alloy (CRA) is much lower than

that of C steel in the presence of carbon dioxide (CO2)and chloride (Cl – )-containing water. Replacement isintended to reduce operating costs by eliminating theneed for inhibition and, possibly, by extending tubularlifetime.

In addition to being resistant to attack byCl – -containing acidic water, the 13% Cr steel mustsurvive acid stimulation treatments. In some gasproduction wells, caliper surveys performed after anacid job have indicated corrosive attack. Continuingcorrosion could not be ruled out on the basis of thesurvey results. Later, when the string was pulled,

onshore inspection revealed attack of a localizednature, but unlike pitting of a normal stainless steel(SS). The pits were wider, without undercut, andappeared in streaks. Investigation revealed theexpected martensitic microstructure. The compositionmet the API 5CT specification.1

The present work was devoted to the behavior of13% Cr steel in “live” acid, back-produced “spent” acid,and sweet production brines. Laboratory experimentswere performed in these environments, usingelectrochemical techniques and including a simulationof a full acid treatment cycle to study repassivation of

the steel. The objective was to identify the likely cause0010-9312/94/000127/$5.00+$0.50/0

© 1994, NACE International




556 CORROSION–JULY 1994

of the corrosive attack and prevent it by providing abasis for improved stimulation procedures.

MATRIX ACID STIMULATION

Many hydrocarbon reservoirs are found in

limestone formations or in carbonate-bearingsandstone. To increase production, it sometimes isnecessary to improve the permeability of the formationby using an acid matrix stimulation treatment. The acidpumped down the well commonly is called live acid.When sandstone formations are treated, threeformulations typically are used: the preflush, mainflush, and afterflush. The preflush is based on regular15% hydrochloric acid (HCl). The main-flush, alsocalled mud acid, is based on 12% HCl and 1.5%hydrofluoric acid (HF).2 The afterflush should providerapid formation cleanup. This can be done usingethylene glycol monobutyl ether (EGMBE) or methanol.

The acids used pose a threat to the integrity of theproduction tubing. To prevent severe corrosion andprecipitation, a well-formulated acid contains a range ofadditives. A corrosion inhibitor, often based on aminesor acetylenic alcohols, is used commonly.3-4 Becauseof their organic nature, inhibitor molecules may besensitive to thermal decomposition. Because of theiroften cationic nature, inhibitor molecules may adsorbonto silica surfaces in the formation. Because thesolubility of inhibitor molecules often is limited to themillimolar range, surfactants may be used to solubilizeor disperse excess inhibitor.5 An inhibitor intensifier,

such as a copper (Cu) halogenide, often is added.Sequestering agents, such as citric acid, aid thesolubilization of iron oxides and hydroxides. Citric acidis claimed to be highly corrosive to steel.3 Silt-suspending agents are surface-active compoundssuch as polyacrylates that keep silt released from theformation in suspension. Instead of applying aseparate afterflush, methanol or EGMBE may beadded as a cosolvent to the main flush to removeformation damage. However, these compounds maylower the inhibitor performance because they may aidin solubilizing the protective inhibitor film on the metal

surface. Live acid normally still will contain somedissolved oxygen (O2).

The composition of the spent acid (i.e., the acidthat is produced back from the well after completion ofthe treatment) depends strongly on the nature of thelive acid and the formation. Moreover, the compositionwill change with time after back production has started.Because the acid dissolves rock, the pH goes up andusually reaches a value of ~ 1 in the spent acid.Fluoride will be produced back, either as fluoride ionsor as fluorosilicate. Cl – will be present in large amounts.In general, inhibitor should be prevented from entering

the formation. Therefore, ideally, the composition and

quantity of live acid are such that the inhibitor isdepleted and is not produced back. The inhibitorintensifier will be back-produced and, when theremaining inhibitor effectiveness is low, cause Cuplating on the tubulars. The sequestering agent alsostill will be present, either complexed with metal ions or

free in solution. Silt-suspension agent is thought to bepresent, (partly) engaged in suspending silt. A widerange of other ions – sodium (Na+), magnesium (Mg2+),calcium (Ca2+), iron (Fe2+), and silicate – originate fromthe formation and may be present in the spent acid. Inparticular, the combination of fluorosilicate with metalions such as Na+ may cause precipitation of gelatinouscompounds. Any O2 present in the live acid will beconsumed downhole in corrosion and oxidationreactions, rendering the spent acid essentially O2-free.

EXPERIMENTAL SETUP

AND PROCEDURES

Materials Electrodes for the electrochemical and exposure

studies were prepared from type 420 13% Cr steeltubulars meeting the API 5CT specification.1 Cylindricalspecimens with 6-mm outer diam and 45-mm length(8.6 cm2 surface area) were machined from longi-tudinal sections of the tubing and threaded to alloweasy attachment in the test equipment. Composition isgiven in Table 1. Prior to exposure, the specimenswere polished to 400-grit finish with silicon paper toremove any scales, rinsed in distilled water, degreased

in ethanol, and dried in air.The live acid used in the study consisted of HCl,

methanol, a sequestering agent (citric acid), a silt-suspension agent, and a corrosion inhibitor. Theformulation is given in Table 2.

Some spent acids were obtained from actual gasfield acid stimulation jobs. In addition, artificial spentacid was prepared by dissolving 150 g Cl – as calciumchloride (CaCl2) in HCl with a pH of 1 (type A) or incitric acid with a pH of 1 (type B) (Table 3).

Brines 1 and 2 were prepared by dissolving CaCl2in demineralized water in Cl – concentrations of 150 g/L

and 10 g/L, respectively (Table 4). These Cl – contentsspanned the range for relevant onshore and offshorewells.

In all cases, the CO2 gas used for the gas cap hada very low O2 content (< 10 ppm) to allow simulation ofO2-free downhole conditions.

Equipment Electrochemical experiments on the live and spent

acids were performed in a standard glass corrosion cellaccording to ASTM G 5-87 (Figure 1).6 The cell wasequipped with a condenser for solution reflux.

Electrochemical and exposure tests, simulating a full





acid treatment cycle and performing exposure to brine,were carried out in a double-walled glass autoclaveequipped with a magnetically coupled stirrer (Figure 2).The SS lid was provided with feedthroughs for gas,liquid, and electrical connections. Figure 3 schema-tically depicts the full autoclave system, which could be

operated at pressures up to 6 bar (0.6 MPa). Theautoclave was connected to a preconditioning vessel,from which the liquid was transferred by applying aslight overpressure and using a gas lift system. Thegas lift also provided the driving force for liquidcirculation via a cooler, a combined pH and silver-silverchloride (Ag-AgCl) reference electrode operating atambient temperature, and via a heater back into theautoclave. The test specimens, serving as electrodesfor the electrochemical measurements, were attachedto a probe assembly (Figure 4). O2 levels in both CO2

gas supply and autoclave outlet were monitored with ahigh-temperature, ceramic membrane-type O2 sensor.

Electrochemical Measurements Electrochemical polarization curves were

measured at 0.2 mV/s with a potentiostat, controlled bya personal computer. Corrosion potentials (Ecorr) wererecorded. In the case of passive behavior, the pittingpotential (Epit), defined as the potential at which theanodic current density showed a steep increase(usually at a level of ~ 100 µA/cm2), was determined.

The protection or repassivation potential (Epas),which is the potential at which the curve, afterchanging scan direction, intersects the original curve or

attains the original passive value, was determined. Tocalculate corrosion rates (Vcorr), a Tafel analysis wasapplied over 200 mV around Ecorr in those cases wherethe polarization curve showed exponential behavior.Vcorr also were calculated from the polarizationresistance (Rp), obtained from polarization curvesmeasured from 15 mV cathodic to 15 mV anodic ofEcorr. The Stern-Geary constant needed for thiscalculation was obtained from Tafel analysis or takenas 25 mV.7

Electrochemical impedance measurements wereused to assess the corrosion behavior in brine. A

frequency response analyzer combined with apotentiostat controlled by a computer was used for thispurpose. Rp was calculated from the data using anequivalent circuit approach. To calculate Vcorr, a Stern-Geary constant of 25 mV was used.

Examination of Test Specimens After exposure, test specimens were dried and

examined under the light microscope for localizedattack and scale formation. If corrosion product waspresent on the sample’s surface, the specimens weredescaled with Clarke’s solution (inhibited HCl), dried,

and examined visually again. Weight loss was

TABLE 1Composition of 13% Cr Steel Specimen

Element wt%

Carbon 0.195Chromium 13.1Manganese 0.81

Phosphorus 0.021Sulfur 0.006Silicon 0.27Iron Balance

TABLE 2Composition of Live Acid

Compound Contents

HCl 15 wt%Methanol 30 vol%

Citric acid 2.5 wt%Silt-suspension agent 0.5 vol%Corrosion inhibitor 0.8 vol%

TABLE 3Composition of Artificial Spent Acid

[Cl –]Acid Compound Amount pH (g/L)

A Aqueous HCl 0.1 N 1 154CaCl2 234 g/L

B Aqueous citric acid 2.5 wt% 1 150CaCl2 234 g/L

determined by comparing the weight after descalingwith the weight prior to exposure. This practice allowedcalculation of a time-averaged Vcorr.

RESULTS AND DISCUSSION

Corrosion in Live Acid To study the corrosion behavior of 13% Cr steel

exposed to live acid, short-duration (some hours to

TABLE 4Composition of CaCl 2 Brines

CaCl2 Cl –

Concentration ConcentrationType (g/L) (g/L)

Brine 1 (concentrated) 234 150

Brine 2 (medium) 16 10





some days) experiments were performed in thestandard glass corrosion cell in the acid formulation asgiven in Table 2 and in separate components thatmade up the full formulation. Because of the presenceof dissolved O2 in live acid, air- and CO2-saturatedsolutions were used. Vcorr were determined from

electrochemical polarization curves by Tafel analysisand from Rp (Table 5). Although some inaccuracy in thequoted Vcorr must be accepted, observed trends werereliable, and numbers were correct ± 50%.

Vcorr of 13% Cr steel in pure 15% HCl was stronglydependent upon temperature, varying from ~ 30 mm/yat 20°C to > 300 mm/y at 100°C (Experiments 1through 6). In a separate experiment of 24-h exposureat ambient temperature, Vcorr calculated from weightloss was 20 mm/y.

The effect of purging with CO2 compared to airwas negligible, as was expected for corrosion at lowpH. The addition of 2.5% citric acid (the sequesteringagent) and of methanol did not cause a significanteffect (Experiments 1 through 5). The polarizationcurve in Figure 5 (Experiment 3) shows the typicalbehavior of an actively corroding alloy.

The electrochemical results showed no beneficialeffect was conferred by the addition of methanol(Experiment 3, Figure 5).

A remarkable inhibitive effect of the silt-suspensionagent was found (Experiment 9). Figure 6 shows astrong retardation of the cathodic hydrogen evolutionand the anodic corrosion reaction, resulting in a lowVcorr (~ 2 mm/y) at an almost unchanged Ecorr.

Apparently, the surface activity of this componenthindered electron transfer at the alloy surface, possiblyby forming a film. This effect could prove useful inprotecting steel against attack by spent acid in caseswhere the inhibitor may have been depleted.

Addition of the organic inhibitor (Experiments 7, 8,and 10 through 12) at ambient temperature directlychanged Ecorr of the 13% Cr steel in the anodicdirection. From the Evans or Tafel plots (Experiment 7,Figure 7), it was concluded that the cathodic hydrogenevolution was strongly limited by diffusion, but that,since the potential shift was anodic, the anodic reaction

was inhibited even more strongly. Upon raising thetemperature, the potential became more cathodicagain, but it did not drop < –400 mVSCE (Experiment 8,Figure 8). The smaller shift at elevated temperatureindicated a less pronounced retardation of the anodicreaction. It has been reported that adsorption oforganic cations, such as quaternary ammonium ions orpyridinium ions, to the Fe surface in acid solution canresult in a positive potential jump.8 In principle, ananodic shift could enhance the risk of pitting attack.However, this was not observed in the presentexperiments. The inhibitor provided adequate

protection. At the present low pH, a passive film that

FIGURE 1. Standard glass corrosion cell.

FIGURE 2. Glass-walled autoclave with stirrer and electrode assembly.





might show anodic breakdown was not expected to bepresent on the alloy surface. The presence of theinhibitor reduced Vcorr from ~ 20 mm/y to 1 mm/y. Asynergistic effect was evident when both silt-suspension agent and inhibitor were added to the HClsolution (Experiments 11 and 12). Vcorr was reduced

< 0.3 mm/y.Contrary to the response in uninhibited acid

solution, raising the temperature from ambient to100°C had no effect on Vcorr of 13% Cr steel in inhibitedlive acid (Experiments 8 and 12). According to theEvans plot (Experiment 8, Figure 8) diffusion limitationof the cathodic reaction was no longer present.Although Vcorr was small (0.3 mm/y), the anodic andcathodic branches increased steeply. Therefore, whenthe steel would be applied in a galvanic couple with amore noble alloy, enhanced corrosion would result.

Experiments 11 and 12, in which 13% Cr steel

was exposed to the full formulation of live acid, werefollowed by exposure to uninhibited acid. Inhibitorretention was not observed, and the measured Vcorr

was of the same order of magnitude as in Experiment1.

These results indicated that inhibited live acid wasunlikely to cause severe corrosive attack of 13% Crtubulars.

Corrosion in Spent Acid Actual Spent Acid — To study the corrosion

behavior of 13% Cr steel in spent acids, some acids

were obtained from stimulation jobs of gas wells. The

acid treatments comprised both a preflush (HCl) and amain flush (HCl/HF, mud acid). Some of the spentacids exhibited a tendency to form gelatinous deposits.In general, the gelatinous nature may have beencaused by an imperfect preflush that led to theformation of insoluble alkali fluorosilicate salts. The pH

values ranged from 0 to 2, which were below thedepassivation pH for 13% Cr steel. The steel,therefore, was not protected by a passive chromiumoxide film and had to be protected solely by theinhibitive substances.

The spent acids were analyzed for metal ions, Cl – ,and fluoride ions (F – ) by inductively coupled plasmaatomic spectroscopy (ICP-AS). The recovered Cl –

concentrations ranged from 50 g/L to 270 g/L Cl – . Mostspent acids contained ~ 100 g/L Cl – , whereas muchless F – was present. In all cases, Cu ions could bedetected. An appreciable amount of Fe ions was

found. It was not known what part of the Fe resultedfrom corrosion, but the presence of Cr in the spentacid, assuming it did not originate from the formation,indicated corrosion at least contributed to the metal ioncontent.

The loss of inhibitor in the matrix treatment wasnot known. Therefore, the preferred method toevaluate inhibitor effectiveness in the acids was toinvestigate their corrosiveness on 13% Cr steelspecimens using electrochemical and weight lossmethods.

Electrochemical polarization curves were

measured in the standard glass corrosion cell under a

FIGURE 3. Autoclave setup with preconditioning vessel, liquid and gas circulation, and gas outlet (PR = pressure reading, TR = temperature reading).





CO2 gas cap at temperatures from 20°C to 100°C.When compared with the results in live acid (Table 5),Ecorr in spent acid at ambient temperature was foundmore cathodic. This may have indicated a lack ofcorrosion inhibitor. Highest Vcorr occurred at the higher

temperatures. At ambient temperature, only sometenths of mm/y were recorded. At 80°C to 100°C, rates> 5 mm/y were found. Since recording of thepolarization curves lasted only a couple of hours,pitting was not observed on the specimens. However,the risk of pitting attack could be assessed from theshape of the curves. Typical polarization curves for twospent acids at 80°C are plotted in Figures 9 and 10.Ecorr, Epit, Epas, and Vcorr calculated from the curves aregiven in Table 6. In the case of spent acid X (Figure 9),Epit was anodic of Ecorr, but Epas was cathodic of Ecorr.For spent acid Y (Figure 10), Epit and Epas were anodic

of Ecorr.

In principle, therefore, spontaneous pitting wouldnot be expected in either case, and existing pits wouldbe expected to grow only in acid X. However, since thepotential differences were small, adequate protectionagainst localized corrosion would not be provided bythese spent acids at elevated temperatures.

To investigate the occurrence of localizedcorrosion further, a 24-h exposure test was performedon 13% Cr steel specimens in spent acid Y at 100°Cunder CO2. Severe localized attack resulted. Vcorr,calculated from weight loss, amounted to 21 mm/y, butthe local penetration rate was higher. Vcorr determinedfrom electrochemistry (Table 6) was much lowerbecause it represented the instantaneous rate duringthe first hours of exposure, whereas localized attackedmainly developed after prolonged exposure. On somelocations on the specimens, Cu that originated from theinhibitor intensifier plated out. This was interpreted asanother indication of insufficient inhibition.

Artificial Spent Acid — In an earlier work, it wassuggested that citric acid would cause more severecorrosion than HCl.4 Since citric acid often is added asa sequestering agent to keep Fe ions in solution,corrosion in spent acids might be enhanced.

To assess this effect, 13% Cr steel specimenswere exposed to artificial spent acids with pH = 1 andlacking an inhibitor, based on HCl or on citric acid(A and B in Table 3, respectively) in the standard glasscorrosion cell. Electrochemical polarization curveswere measured (Table 7).

At ambient temperature, the calculated Vcorr

ranged from 2 mm/y to 7 mm/y. Raising thetemperature by 50°C to 100°C caused a markedincrease in Vcorr. The citric acid was much morecorrosive than HCl at 100°C, with Vcorr amounting to100 mm/y and 20 mm/y, respectively. In all cases,attack occurred over the full specimen surface. Typicalpitting damage was not observed. This was inagreement with the general shape of the polarizationcurves, which indicated active corrosion.

This behavior contrasted with that of the actualspent acids, where overall Vcorr were smaller andlocalized attack was observed. The high Vcorr observed

in artificial spent acid, however, could be taken toexemplify the effect that occurs when hot spent acidcomes into contact with 13% Cr steel at locationswhere a lack of inhibitive substances exists. Thepenetration rates at such locations, therefore, could beon the order of 100 mm/y.

Although the formulation of general conclusionswas hampered by the sometimes inhomogeneous andunknown composition of the actual spent acids, it wasevident that such acids may constitute a veryaggressive environment for 13% Cr steel. Duringnormal back-production conditions, exposure of the

13% Cr steel to spent acid is expected to be short

(a)

(b)

FIGURE 4. Electrochemical probe with cylindrical test specimens.





enough to prevent unacceptable damage. However,results indicated prolonged exposure to spent acid

containing insufficient inhibitive species may causesevere pitting or localized acid attack of the 13% Crsteel. Particularly at high temperature, citric acid wasshown to be more aggressive then HCl. Gelatinouscompounds may aggravate corrosion due to the risk ofunderdeposit attack. The plating out of Cu on the steelsurface was interpreted as a sign of insufficientinhibition.

Corrosion in Brine Passivation Behavior of 13% Cr Steel on

Immersion in Brine — Using the standard glass cell,

experiments were performed at 100°C under 1 bar(0.1 MPa) CO2 to study the behavior of 13% Cr steelon immersion in brine, both at 15 x 104 ppm and at1 x 104 ppm Cl – (Brines 1 and 2 in Table 4, simulatingsweet production brines). Changes in passivity wereassessed by recording electrochemical polarizationcurves as a function of time after immersion, takingcare that the potential scan did not irreversibly disturbthe specimen surface.

Figures 11 through 14 show the curves obtaineddirectly after immersion and at 2 h, 7 h, and 31 h afterimmersion, respectively, in Brine 2 (1 x 104 ppm Cl – ).

Ecorr, Epit, and Epas values derived from the plots are

given in Table 8. The overall appearance of the curvesbarely changed with time, but a remarkable shift of Epas

in the anodic direction occurred. From being cathodicrelative to Ecorr upon immersion, it became anodicwithin some hours. The difference between Epit andEcorr also increased with time. This indicated a changefrom a state where spontaneous pitting would beunlikely but existing pits would grow to one whereneither spontaneous pitting nor pit growth would occur.

The experiments described above in Brine 2 alsowere performed in Brine 1 (15 x 104 ppm Cl – ).Qualitatively, the behavior was the same, but the shiftin Epas was much smaller, with Epas almost coincidingwith Ecorr. Moreover, it took > 3 days for Epas to shift

past Ecorr. Although accurate values for Epit were notobtained from all polarization curves, the passive rangebetween corrosion and Epit appeared much smallerthan with 1 x 104 ppm Cl – . It was evident that theimprovement in passivity was much less pronouncedthan in 15 x 104 ppm Cl – . Whether spontaneous pittingand pit growth actually would occur in this case wouldhave to be determined in separate exposureexperiments of longer duration.

Autoclave Exposure Tests in Brine Without Prior Acid Treatment — Cylindrically shaped 13% Crspecimens were exposed to brine at 100°C in the glass

autoclave setup. The brine was presaturated with CO2

TABLE 5Results of Potentiodynamic Polarization Tests on 13% Cr Steel in 5% HCl (wt%) with Additions

Susp. Citric

Exp. Inhibitor Agent Methanol Acid Temp. Ecorr Rp Anodic Cathodic Vcorr

No. Gas (vol%) (vol%) (vol%) (wt%) (°C) (mVSCE) (kΩ-cm2) (mVSCE) (mVSCE) (mm/y) Method

1 Air – – – – Ambient –444 0.0121 184 202 43 TA

20 PR2 CO2 – – – – Ambient –454 0.0178 135 187 20 TA

23 PR3 Air – – 30 – Ambient –411 0.0083 107 140 33 TA

36 PR4 Air – – 30 2.5 Ambient –411 0.0103 94 130 20 TA

26 PR5 CO2 – – – 2.5 Ambient –442 0.0193 133 156 16 TA

20 PR6 CO2 – – – 2.5 100 –427 0.002 8,946 501 390 TA

1,100 PR7 CO2 0.8 – – 2.5 Ambient –285 0.218 53 360 0.3 TA

1.0 PR8 CO2 0.8 – – 2.5 100 –395 0.418 61 99 0.3 TA

0.3 PR9 Air – 0.5 30 – Ambient –411 0.0911 76 126 1.6 TA

2.6 PR10 Air 0.8 – 30 – Ambient –243 0.195 37 408 1.0 TA

1.0 PR11 Air 0.8 0.5 30 2.5 Ambient –230 1.603 38 130 < 0.3 TA

< 0.3 PR12 Air 0.8 0.5 30 2.5 40 –264 0.591 39 170 < 0.3 TA

< 0.3 PR

(A) TA = Tafel analysis, PR = polarization resistance.

Tafel Constant





gas for several hours at ambient temperature in theconditioning vessel and then transferred by use of thegas lift to the autoclave with three test specimensalready mounted. During exposure, the electrolyte waspurged continually with CO2. The time-averaged Vcorr

was calculated from the specimen weight loss.Electrochemical impedance spectroscopy (EIS) was

used to monitor Vcorr during exposure. Results of

FIGURE 6. Electrochemical polarization curve in 15% HCl with 0.5% silt-suspension agent at ambient temperature.

FIGURE 5. Electrochemical polarization curve in 15% HCl at ambient temperature.

exposure in Brine 1 (15 x 104 ppm Cl – , Table 4) with4.5 bar (0.45 MPa) CO2 and in Brine 2 (1 x 104 ppmCl – , Table 4) with 1.7 bar (0.17 MPa) CO2, simulatingtypical offshore and onshore conditions, respectively,are given in Table 9.

In the case of Brine 1, Vcorr calculated from thetotal weight loss was about double that in Brine 2

(Table 9). From EIS results (Figures 15 and 16), it wasevident that Vcorr decreased with time, in Brine 1 to~ 0.14 mm/y and in Brine 2 to ~ 0.06 mm/y, which wasin reasonable agreement with the weight loss results.The appearance of an extra feature in the impedanceplots within a few hours indicated formation of aprotective layer on the specimen surface.

Autoclave Exposure Tests in Brine With Prior Acid Treatment — Similar exposure tests were performed,but now preceded by an acid treatment to simulate thedifferent stages of an actual matrix stimulation job. Vcorr

calculated from the total weight loss (i.e., including the

acid exposure phase of the experiment) are given inTable 9. The time dependence of the corrosion,calculated from EIS data, is depicted in Figures 17and 18.

Specimens first were exposed for 24 h at ambienttemperature to live acid as specified in Table 2. Vcorr,calculated from EIS measurements, dropped to< 0.05 mm/y (Figures 17 and 18, stage A), which wasin agreement with results reported in Table 5. The liveacid then was removed, and the autoclave waswashed several times with deaerated demineralizedwater, keeping the autoclave system under CO2.

Artificial spent acid, based on HCl and CaCl2 (Table 3,

FIGURE 7. Electrochemical polarization curve in 15% HCl with 0.8% inhibitor at ambient temperature.





acid formulation A), was introduced and the tempera-ture raised to 100°C. Exposure was extended to 4 h inExperiments 3 and 4 (Table 9). Figures 17 and 18(stage B) showed a rapid and large increase in Vcorr,indicating inhibitor retention on the specimen surfacewas negligible. The reason for the difference in Vcorr

between Experiments 3 and 4 was not clear, but it was

evident that, in both cases, the specimens wereactively corroding, resulting in a roughened surface.

The autoclave again was washed with deaera-ted water and filled with CO2-saturated Brine 1(15 x 104 ppm Cl – ) in Experiment 3 and with Brine 2(1 x 104 ppm Cl – ) in Experiment 4 (Table 9). Exposureto the brine was carried out for at least 14 days atCO2 pressures of 4.5 bar (0.45 MPa) and 1.7 bar(0.17 MPa), respectively, as in the experiments withoutacid treatment. The temperature was kept at 100°C,except during the first 3 days in Experiment 4, wherea disturbance of the thermostat unit caused the

temperature to drop to 25°C. The time dependence ofVcorr, as calculated from EIS measurements, is shownin Figures 17 and 18, stage C.

In the concentrated Brine 1 (Figure 17), Vcorr

stabilized at 0.9 mm/y, which was in good agreementwith the result obtained without acid pretreatment(Experiment 1, Figure 15). The EIS spectrum showedthe presence of at least two features, with a lowerfrequency one probably deriving from layer formationon the specimen surface. After termination of the test,the specimens were found to be covered with a veryadherent, lead pencil-like surface layer. Apart from the

much rougher surface of the specimens in the present

case, which also was reflected in the shape of the EISplot, the results of exposure to Brine 1 without and withprior acid treatment in Experiments 1 and 3,respectively, were very similar. Figure 18 shows thetime dependence of Vcorr when exposure after acidtreatment was performed in Brine 2. The relatively lowvalue of Vcorr in the brine up to 100 h was a result of the

interruption of the heat supply. As in the case of

FIGURE 9. Electrochemical polarization curve in spent acid X at 80 °C under CO 2 .

FIGURE 8. Electrochemical polarization curve in 15% HCl with 0.8% inhibitor at 100 °C.

FIGURE 10. Electrochemical polarization curve in spent acid Y at 80 °C under CO 2 .





FIGURE 11. Polarization curve in Brine 2 at 100 °C under CO 2

upon immersion.FIGURE 12. Polarization curve in Brine 2 at 100 °C under CO 2

2 h after immersion.

experiments without and with prior acid treatment, itwas concluded that activation of the 13% Cr steel inacid did not prohibit the subsequent passivation of thesteel in CO2-saturated brine. As indicated by thepolarization curves, highly concentrated brines causeda risk of pitting attack, irrespective of prior acidtreatment.

SUMMARY

Corrosion in Live Acid Adequate protection in live acid was provided by a

normal inhibitor formulation, including silt-suspensionagent.

In the absence of the inhibitor, the silt-suspensionagent provided ~ 90% protection on its own.

Raising the temperature from ambient to 100°Cand adding citric acid did not enhance corrosion in theinhibited acid significantly.

Results indicated that, in a normal matrixstimulation treatment, inhibited live acid was unlikely tocause severe corrosive attack of 13% Cr tubulars.

Corrosion in Spent Acid Results indicated spent acids constitute a very

aggressive environment.At elevated temperatures, citric acid was more

aggressive than HCl.Results clearly showed spent acid containing

insufficient inhibitive species could cause severelocalized attack of 13% Cr steel upon prolonged

exposure, particularly at elevated temperatures.

Experiment 2 without acid pretreatment, Vcorr

decreased with time to ~ 0.1 mm/y at 300 h ofexposure and did not fully stabilize.

Vcorr calculated from the total weight loss wasmuch larger when acid pretreatment was applied(Table 9), indicating that the larger part of the lossoccurred during exposure to the artificial spent acid.

From the similarity of the behavior in brine in the









TABLE 6Electrochemical Polarization Measurements

of 13% Cr Steel in Typical Spent Acids at 80 °C Under CO 2

Epit Epas

Spent Ecorr Epit Epas Vcorr –Ecorr –Ecorr

Acid (mVSCE) (mVSCE) (mVSCE) (mm/y) (mVSCE) (mVSCE)

X –497 –469 –518 0.7 28 –2(A)

Y –390 –352 –368 1.2 38 22(B)

(A) Figure 9.(B) Figure 10.


of 13% Cr Steel in Artificial Spent Acids (pH = 1) Under CO 2

Temperature Ecorr Vcorr

(°C) Acid (mVSCE) (mm/y)

20 A, HCl –562 720 B, Citric –521 250 A, HCl –594 17

100 A, HCl –598 22100 B, Citric –592 100


of 13% Cr Steel in Brine 2 (1 x 10

4

ppm Cl

–

)at 100 °C with 1 Bar (0.1 MPa) CO 2

Time After Epit Epas

Immersion Ecorr Epit Epas –Ecorr –Ecorr

(h) (mVSCE) (mVSCE) (mVSCE) (mVSCE) (mVSCE)

0 –609 –476 –679 133 –702 –678 –476 –679 202 –17 –658 –388 –635 270 23

31 –662 –318 –635 344 27

TABLE 9Exposure Tests in Brine at 100 °C With and Without Prior Acid Treatment

ExposureExp. Acid Pre- CO2 Time Vcorr

(A)

No. Brine Treatment (Bar) (Days) (mm/y)

1 1 No 4.5 11 0.082 2 No 1.7 14 0.033 1 Yes 4.5 29 0.234 2 Yes 1.7 14 1.95

(A) Calculated from total weight loss (i.e., including acid pretreatmentfor Experiments 3 and 4); the average for three exposed specimens

is given.

Conditions

Corrosion in Brine When exposed directly to a brine containing

1 x 104 ppm Cl – at 100°C, the passivity of 13% Cr steelimproved within some hours to a stable situation withVcorr < 0.1 mm/y. When a simulated acid stimulationcycle was performed prior to exposure to the brine,the same behavior was observed, although on a

roughened surface. With 15 x 104 ppm Cl – , the behavior was quali-

tatively similar, but the improvement of passivity occur-red much slower, with Vcorr dropping to ~ 0.1 mm/y. Arisk of pitting existed with and without acid pretreat-ment.

Thus, it was shown that repassivation occurs innormal brines, even if 13% Cr steel is activated with acid.

In their study of 13% Cr steel repassivation,Cassidy, et al., also concluded that active corrosionoccurred in live and in spent acid.9 In their aggressivebrine formulation, containing 2% acetic acid, passiva-

tion did not occur, irrespective of acid treatment. 13%Cr steel, however, should not be applied in suchenvironments. The present work showed thatpassivation does occur in normal, less acidic brines.

CONCLUSIONS

Typical inhibited live acid (15% HCl) was notunacceptably aggressive toward 13% Cr steel. Severe damage from locally initiated acid attackcan occur when steel remains in contact with hot spentacid with reduced inhibitor content but still highly acidic

(pH = 1).




566 CORROSION JULY 1994

FIGURE 15. V corr in Brine 1 at 100 °C and 4.5 bar (0.45 MPa)CO 2 vs exposure time.

FIGURE 16. V corr in Brine 2 at 100 °C and 1.7 bar (0.17 MPa)CO 2 vs exposure time.

13% Cr steel is adequately resistant to typicalCl – -containing brines. Only at very high Cl – levels,repassivation after acidifying may take some days, anda risk of pitting exists. Thus, the most critical phase in a matrix acidizingtreatment is the back production of spent acids.Operations must take precautions to minimizeexposure in this environment.

REFERENCES

1. American Petroleum Institute (API) Specification 5CT, 2nd ed.,November 1, 1989 (Washington, DC: API, 1989).

2. J.L. Gidley, J. Pet. Tech. 23 (1971): p. 551.

3. C.F. Smith, F.E. Dollarhide, N.J. Byth, J. Pet. Tech. 30 (1978): p. 737.

4. C.W. Crowe, S.S. Minor, “Effect of Acid Corrosion Inhibitors Upon MatrixStimulation Results,” Society of Petroleum Engineers (SPE) Formation

Damage Control Symposium, Layfayette, LA, March 24-25, 1982, paper

no. 11,119 (Dallas, TX: SPE, 1982).

5. F.B. Growcock, “Surfactants Can Affect Corrosion Inhibition of Oilfield

Steel,” Society of Petroleum Engineers (SPE) Int. Symp. on Oilfield

Chemicals, San Antonio, TX, Feb. 4-6, 1987, paper no. 16,265 (Dallas,

TX: SPE, 1987).

6. ASTM Standard G 5-87, Annual Book of ASTM Standards, Vol. 03.02,

Wear and Erosion (Philadelphia, PA: ASTM, 1988).

7. M. Stern, A.L.J. Geary, J. Electrochem. Soc. 104 (1957): p. 56.

8. L.I. Antropov, I.S. Pogrebova, G.I. Dremova, Prot. Met. 7 (1971): p. 1.

9. J.M. Cassidy, K.R. Lancaster, M.L. Walker, CORROSION/93, paper no.

95 (Houston, TX: NACE, 1993).

FIGURE 17. V corr vs time in simulated acid treatment: A = 24 h in live acid; B = 4 h in spent acid, Table 3, acid A, followed by

exposure to Brine 1 at 100 °C under 4.5 bar (0.45 MPa) CO 2;

C = 600 h.

FIGURE 18. V corr vs time in simulated acid treatment: A = 23 h in live acid; B = 4 h in spent acid, Table 3, acid A, followed by exposure to Brine 2 at 100 °C under 1.7 bar (0.17 MPa) CO 2 ;

C = 275 h.

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