Effect of HCO3- concentration on CO2 corrosion in oil and gas fields Effect of HCO3- concentration on CO2 corrosion in oil and gas fields

Effect of HCO3- concentration on CO2 corrosion in oil and gas fields

  • 期刊名字:北京科技大学学报(英文版)
  • 文件大小:358kb
  • 论文作者:Guoan Zhang,Minxu Lu,Chengwen
  • 作者单位:Corrosion and Protection Center
  • 更新时间:2020-09-15
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Journal of University of Science and Technology BeijingMaterialsVolume I3, Number I, February 2006, Page 44Effect of HCO3 concentration on CO2 corrosion in oil and gas fieldsGuoan Zhang, Minxu Lu, Chengwen Chai, and Yinshun WuCorrosion and Protection Center, University of Science and Technology Bejing, Beiing 100083, China(Received 2004-09-15)Abstract: The effect of HCO;~ concentration on CO2 corrosion was investigated by polarization measurement of potentiodynamicscans and weight-loss method. Under the conditions of high pressure and high temperature, the corrosion rate of steel X65 decreasedwith the increase of HCO3 concentration, while pH of solution increased. SEM, EDS, and XRD results of the corrosion scales indi-cated that the typical FeCO3 crystallite was found at low HCO;~ concentration but Ca(Fe,Mg)(CO3)2 was found at high HCO3~ con-centration. Ca2+ and Mg + are precipitated preferential to Fe2+ at high pH value. Potentiodynamic polarization curves showed that thecathodic current density decreases with the increase of HCO5 concentration at low HCO3 concentration. When the HCO3 concen-tration reaches 0. 126 mol/L, increasing HCO, concentration promotes cathodic reactions. Anodic behavior is an active process atlow HCO3- concentration and the anodic current density decreases with the increase of HCO3- concentration. An evident active- pas-sive behavior is exhibited in anodic process at 0.126 mol/L HCO3.Key words: oil and gas fields; carbon dioxide corrosion; bicarbonate concentration; polarization curves[This work was financially supported by the National Natural Science Foundation of China Key Program (No.50231020) and theNational Key Basic Research and Development Plan Program (No.G1999065004).]1. Introduction2HCO3- +2e -→2CO32-+H2(3)CO2 corrosion leads to the failure of pipelines and2H2O+2e - > 20H+H2(4)equipment and results in great economic loss andFe-→Fe2++ 2e(5)calamitous accidents. Leakage of crude oil due to CO2corrosion induces fire accidents, water resource, andThe anodic reaction is mainly the dissolution ofenvironmental pollution [1]. It has been one of theiron (Eq. (5)), though it maybe through several steps.most common corrosion problems in oil and gas in-During these processes, a corrosion scale (FeCO3) isdustry because of both a high corrosion rate andformed on the surface of the steel (Eqs. (6)-(8)) andsevere localized corrosion [2-4].the properties and morphologies of the scales influ-When CO2 dissolves into water at high temperatureence the corrosion rate significantly.and high pressure carbonic acid is formed, which isFe2+CO32 -→FeCO3(6)more corrosive to carbon steel than strong acid (suchFe2+ + 2HCO3 -→Fe (HCO3)2(7)as HC1) at the same pH value. Several mechanismshave been proposed for the dissolution of carbon steelFe (HCO3)2→FeCO3 + CO2 + H2O(8in CO2 aqueous solution. The main cathodic reactionsThe formation water in oil and gas fields usuallycan be summarized by four reactions [5-6]. At low pH,contains various salt solutions with different concen-H+ reduction is the dominant cathodic process becausetrations. The concentration of bicarbonate ions hasof the high concentration of H+ (Eq. (1)). When pHimportant effects on the CO2 corrosion of pipeline.increases to 4-6, the direct reductions of HCO3- andHowever, there are still many disputes about the effectH2CO3 become important (Eqs. (2) and (3)). At highof HCO3 concentration on CO2 corrosion. The studiesoverpotential, the dominant cathodic reaction changesof Videm et al. [7] indicated that HCO3- could in-to direct reduction of water (Eq. (4)).2H++ 2e→H2(1)crease the pH中国煤化工rate of car-bon steel. TheyYHCNMHGsivityincar-2H2CO3+2e -→2HCO3+H2(2)bon steel in a correct range of carbon dioxide, bicar-Corresponding author: Guoan Zhang, E-mail: zga2003@ 163.com.G.A. Zhang et al, Effect of HCO3 concentration on CO2 corrosion in oil and gas fields45bonate, sodium chloride, and potential. But a break-genated by bubbling super pure CO2 (99.95%) for4 hdown of this passivity would result in pitting. Jasinskiprior to the experiment. Coupons were then quickly[8] revealed that no obvious difference was found inmounted on the holder and the autoclave was closed.the corrosion rate of carbon steel though HCO; inOne more hour CO2 purge was carried out to deaeratecreases the pH of the solution. XU, et al. [9] suggestedoxygen entering in the mounting process. The tem-that when the temperature and HCO3 concentration isperature and pressure were raised to desired values.lower, the corrosion rate decreases due to inhibition ofAfter testing, coupons were removed from autoclavecathodic reactions. When the temperature is high, theand rinsed with deionized water. They were dividedincrease of HCO; concentration could inhibit the cor-into two groups: the coupons in group one were des-rosion of carbon steel at low concentration of HCO3~caled and the weight loss measured using an electronicwhile promoting corrosion at high concentration ofbalance with the precision of 0.1 mg; the coupons inHCO;.group two were not descaled. The morphology, com-The effect of HCO3- concentration on corrosion be-position and microstructure of the scales were ana-havior of carbon steel was investigated by means oflyzed by SEM, EDS, and XRD.polarization measurement of potentiodynamic scansA Solartron 1280 unit was used for electrochemicaland weight loss method. The morphology, composi-measurement in a bath. A three-electrode test cell wastion and microstructure of the scales were investigatedused with a platinum plane as the counter electrode. Aby Scanning Electron Microscope (SEM), Energysaturated calomel electrode (SCE) was chosen as theDispersive Spectroscopy (EDS), and X-Ray Diffusionreference electrode. All the potentials in this articleanalyzer (XRD).are relative to this reference electrode. API X65 steelrods (φ15 mmx5 mm) were employed as the working2. Experimentalelectrode with an exposed area of 1.77 cm2. CouponsAPI X65 pipeline steel was used in this experiment.were polished up to 800-grit silicon carbide paper, rin-The following chemical composition (wt%) of thesed with deionized water, and degreased with acetonesteel is: C, 0.04; Si, 0.2; Mn, 1.5; P, 0.011; S, 0.003;prior to the experiment. The solution was deaerated byMo, 0.02; Fe: balance. High temperature and highpurging CO2 (99.95%) for 4 h. Coupons were thenpressure corrosion testing was carried out in an auto-quickly mounted on the holder. One more hour CO2clave (10 L volume) with magnetic transmission.purge was carried out to deaerate oxygen entering inCoupons were machined into 1/6 cirque with an outerthe mounting process. CO2 gas was allowed to bubblediameter of φ72 mm and inner diameter of φ64 mm,into the solution at low flow rate to ensure entire satu-sting of 35 mm, width of 11 mm. Coupons were pol-ration throughout the measure. Gas exit was sealedished up to 800-grit silicon carbide paper, rinsed withwith water. The potential extent of polarization meas-deionized water and degreased in acetone. Electrolyteurement of potentiodynamic scans was - -800-1000 mVsolution was made from analytical grade reagents andvs Eorr. A sweep rate of 20 mV/min was performed.deionized water, simulating the formation waterAll the electrochemical experiments were performeddrawn out from oil and gas fields. Its chemical com-at 65°C under atmospheric pressure.position is shown in Table 1. The solution was deoxy-Table 1. Composition of formation water drawn out from oil fieldmg/LK++ Na+Mg2Ca2+CTHCO3Total ion concentration256876435804815373543. Results and discussiondirectly and promotes the dissolution of carbon steel.Fig.1 showed the effect of HCO3 concentration on3.1. Effect of HCO; concentration on corrosionweight loss and pH (calculated with a self-compliedrate at high temperature under high pressureprogram) under the condition of the CO2 partial pres-As mentioned earlier, the effect of HCO3- concen-sure 0.3 Mpa, the temperature of 65°C, the static andtration on CO2 corrosion of carbon steel still haveflow rate of 0.5 m/s. Three coupons were weighed af-many disputes. On the one hand, HCO3~ increases theter scales removal for each test. The data symbols rep-pH of the solution, which decreases the reduction ofresent the average value for each testing conditionH+ and the dissolubility of corrosion scales (FeCO3),while the errol中国煤化工t and lowestthus decreases the corrosion rate of carbon steel andvalues. It could| YHC N M H Gecreases andon the other, HCO3 participates in cathodic reductionpH increases with the increase of HCO3 concentration..4(J. Univ. Sci. Technol. Bejing, Vol.13, No.1, Feb 2006The reduction of weight loss decreases with the in-pends on HCO3 concentration in the solution. Thecrease of HCO3 concentration. This may be ascribedcorrosion rate under dynamic conditions (0.5 m/s) isto the low pH at low HCO3 concentration andobviously higher than that of static conditions at lowcathodic process was mainly the reduction of H+. pHHCO3 concentration. However, when the HCO3- con-increases quickly with the increase of HCO3 concen-centration reaches 0. 126 mol/L, no obvious differencetration, which decreases the reduction of H+ corre-of corrosion rate was observed between dynamic andspondingly and therefore, reduces corrosion rate.static conditions. Cathodic process is the reduction ofWhen HCO3 concentration was high, pH of the solu-H+ at low HCO; concentration, primarily. Fluid flowtion increased. The direct reductions of H2CO3 andan accelerate the diffusion of H+ and promote theHCO3~ become dominant in cathodic process, gradu-cathodic process. The corrosion scales, formed at lowally. The reductions of H2CO3, HCO; counteract par-pH, were loose and non-protective, which were easytially the decrease of corrosion rate resulting from in-to peel off and lost protection under the washing outhibition of H+ reduction. Thus corrosion rate decreasesof fluid. On the contrary, cathodic process changed toslowly with the increase of HCO3 concentration.the direct reduction of H2CO3, HCO3 at high HCO3-concentration. The corrosion scales were compact,integrated and lttle influenced by the fluid. Thus the2-difference of corrosion rate between static and dy-e 10一6namic conditions is insignificant.8Flow rate: 0?6Flow rate: 0.5 m/sThe effects of HCO3 concentration on the properti-CalcualdpH |5 喜多4es and morphology of the corrosion scales were in-vestigated by surface analysis measures. SEM andEDS results of scales showed that the typical FeCO3一3crystallite was found at low HCO3- concentration (Fig. .).02 0.020.06 0.10 0.12 ()), while the Ca, Mg compounds are formed atConcentration ofHCO; / (molL-)high HCO3 concentration (Fig. 2 (b) under static con-Fig. 1. Effect of HCO3 concentration of on weight loss fordition. The content of Ca reaches 62.08wt% (shown insteel X65; the CO2 partial pressure is 0.3 MPa, the tempera-Table 2) with a small and compact crystalline grainture is 65°C, the flow rate is 0 or 0.5 m/s, and the exposed(Fig. 3), which is related to a pronounced reduction oftime is 6 days.corrosion rate. This indicated that Ca2+ and Mg2+ areIt could also be seen that the effect of flow rate de-more easy to precipitate than Fe2+ at high pH. X-ray15.0中力(b)中HTE(c(d) L.so 100.1韩norFig.2. SEM surface images of the corrosion scales of steel X65 after testing in中国煤化工ion with dif-ferent concentrations of HCO;~ and flow rates: (a) 0.0047 mol/L, 0; (b) 0.126 moMHCN MH Gls; (d) 0.126mol/L, 0.5 m/s..G.A. Zhang et al, Effect of HCO3 concentration on CO2 corrosion in oil and gas fields47diffraction was used to identify the constituents of theHCO3 are loose and non-protective. Part of scalesscales developed at different HCO3- concentration.peel off due to the washing-out of flow (Fig. 2 ()),Fig. 4 shows XRD patterns of the scales formed withwhich result in a higher corrosion rate compared to0.0047 mol/L and 0. 126 mol/L HCO3 , respectively. Itstatic condition (shown as Fig. 1). However, when theconfirms the presence of FeCO3 at low HCO3 con-HCO3 concentration is increased to 0. 126 mol/L, nocentration and Ca(Fe,Mg)(CO3)2 at high HCO3 con-obvious difference is found in the scales betweencentration. This indicates that Ca2+, Fe2+, Mg2+ andstatic and dynamic conditions (Fig. 2 (b), ()). BothCO3- co-deposites to form carbonate complex. Thethe scales are carbonate complexes with small andcarbonates of Ca, Fe, and Mg are all calcites, and theycompact crystalline grains. These compact, integratedcan replace each other to form carbonate complexesscales can withstand the washing-out of flow and[10-12]. Under dynamic condition (0.5 m/s), the scalestherefore, reduce the corrosion rate. This is consistentformed in the solution containing 0.0047 mol/Lwith the results of weight-loss measurement.Table 2. EDS analytical results of corrosion scales of steel X65 after testing in CO2-containing aqueous solution with differ-ent concentrations of HCO3Content of the elements / wt%Concentration of HCO;_FCaCSNaMn0.0047 mol/L93.790.841.021.932.420.126 mol/L18.725.535.947.27120-(a)F20(b10016080一会60一e 12040一昌804gci20十AIn |re5.56311.12616.689Lnergy / keVFnergy /keVFig. 3. EDS for corrosion scales of steel X65 after testing in CO2-containing aqueous solution with different concentrationsof HCO3: (a) 0.0047mol/L; (b) 0.126mol/L. (CO2 partial pressure: 0.3 MPa, temperature: 65°C, flow rate: 0, exposed time: 6days,)8000” (a●FeCO,3000 F(b)个60002500a Ca(Fe,Mg)(CO),2000400015001000500上2(106(801204(100 120201()201(Fig. 4. XRD patterns of corrosion scales of steel X65 after testing in CO2-containing aqueous solution with different concen-trations of HCO3: (a) 0.0047 molL; (b) 0.126mol/L. (CO2 partial pressure: 0.3 MPa, temperature: 65°C, flow rate: 0, ex-posed time: 6 days.). .3.2. Polarization measurement of potentiodynamicof the solution is low. The reduction of H+ is the .scansdominant cathodic process. The cathodic current den-Fig. 5 shows the polarization curves of API X65 insity decreases with the increase of HCO3 concentra-CO2-saturated aqueous solutions (composition astion due to increasing pH. When the HCO3- concen-shown Table 1) under the conditions of 0.1 MPa (CO2tration reaches 0.126 mol/L, the cathodic current den-partial pressure), 65°C, 0 m/s. The cathodic processsity increases中国煤化工tion becausepossibly includes four reactions (Eqs. (1)-(4)). Whenthe direct reduMHCN M H Gomes domi-the HCO; concentration is less than 0.042 mol/L, pHnant and the incuasung Lu3 wiii aion promotes.48J. Univ. Sci. Technol. Bejing, Vol.13, No.1, Feb 2006the cathodic process. Anodic process exhibits a transi-from -713.68 mV to -787.54 mV with the increase oftion from activation to passivation and the currentHCO3 concentration. This ascribes that HCO3- inhib-density decreases with the increase of HCO3 concen-its the cathodic process at less than 0.042 mol/L andtration. An evident active- passive behavior exhibits inwhen HCO; concentration reaches 0. 126 mol/L, the .anodic process at 0. 126 mol/L HCO; . This indicatesanodic current density increases in active zone. Thethat corrosion scales form preferentially at high pHcorrosion current density (Icor) decreases when theand inhibits the anodic dissolution process.HCO; concentration is less than 0.042 mol/L and in-creases when the HCO3~ concentration reaches 0.126mol/L. This is different from the results of high-0-2↓pressure experiment. Under the conditions of hightemperature and high pressure, corrosion scales al-035ways form on the surface of coupons. The increasing04-20.0047 mol/Lof HCO3 concentration increases the pH of solution,0-0126mlLwhich results in a compact, integrated corrosion scaleand a descension of solubility of corrosion scales. This-1.4 -1.0-0.6-0.20.2effect is more dominant than that of HCO; which in-Potential/ V vs SCEcreases the corrosion rate by promoting the cathodicFig. 5. Polarization curves of steel X65 in CO2-saturatedreactions. However, during electrochemical measure-aqueous solutions with different concentrations of HCO3.ment which is performed at 0.1 MPa CO2 partial pres-Table 3 shows the electrochemical parameters (Ecorsure, a protective corrosion scale is difficult to form.LCorrs β, B) acquired by ftting the measuring data inWhen the HCO3 concentration reaches 0.126 mol/L,feeble polarization zone by means of Gauss-Newtonthe increase of corrosion current due to the direct re-method. The free corrosion potential (Ecor) descendsduction of H2CO, HCO3 becomes dominant.Table 3. Fitting results of the electrochemical parametersHCO;- concentration /Em/ mVIom/B/ mVB。1 mVpH (CO2 saturated)(mol:L-)Vs SCE(102 mA.cm-2)-713.6811.2137.83121.524.50.0047-720.128.0443.3899.050.0141-755.834.9738.9782.206.0.042-774.842.05155.220.126-787.544.5138.14 .86.497.0The measured pH increases with the increase ofcompact, and more protective. Under dynamic condi-HCO3 concentration, which is consistent with the cal-tion (0.5 m/s), the corrosion scales forming at lowculated pH under the conditions of high temperatureHCO3~ concentration are loose and non-protectiveand high pressure. But the pH under the conditions ofwhile there are no obvious differences in the corrosionhigh temperature and high pressure is much lower,scales between dynamic and static conditions at highwhich is ascribed to large solubility at high CO2 par-HCO3 concentration.tial pressure.(3) Potentiodynamic polarization curves show thatthe cathodic current density decreases with the in-4. Conclusionscrease of HCO3- concentration at low HCO;- concen-(1) The results obtained by weight loss testing indi-tration. When the HCO3- concentration reaches 0.126cated that the corrosion rate of carbon steel decreasesmol/L,increasing HCO3 concentration promotesand pH increases with the increase of HCO;- concen-cathodic reactions. Anodic process is an active be-tration. Ca2+ and Mg2+ are easier to precipitate thanhavior at low HCO; concentration and the anodic cur-rent density decreases with the increase of HCO3Fe2+ at high pH.concentration. An evident active- passive behavior ex-(2) Under static condition, the corrosion scale is thehibits in anodic process at 0.126 mol/L HCO3.typical compact FeCO3 crystallite at low HCO; con-centration. When HCO3 concentration is high, theReferences中国煤化工corrosion scale is composed of Ca(Fe,Mg)(CO3)2[1] M.X. Lu, z.MHCN M H Grality and typi-complex, whose crystalline grain is more minute,cal cases for corros1on 1n Ine process oI extraction, gath-.G.A. Zhang et al, Effect of HCO3 concentration on CO2 corrosion in oil and gas fields49ering, storage and transmission for oil and gas, Corros.No.4, p.280.Prot. (in Chinese), 23(2002), No.3, p.105.7] K. Videm and A.M. Koren. Corrosion, passivity, and pit-[2] M.B. Kermani and A. Morshed, Carbon dioxide corrosionting of carbon steel in aqueous solutions of HCO3, CO2,in oil and gas production- A compendium, Corrosion,and Cr, Corrosion, 49(1993), No.9, p.746.59(2003), No.8, p.659.8] R. Jasinski, Corrosion of N80-type steel by CO/water[3] K. 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