Corrosion Studies in Natural Gas Condensate Wells PROTECTIVE LAYERS D. 4, SHOCK
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ai ~iiechanisniis proposed to elplain the absence of local\\-:iter surface. Thr esterit of article poiritetl out (8)that ized attack and the low rate of general attack in a certain spreading n’as determined by type of natural gas condensate well. This postulates the measuring the dinmetei~of tht, all natural gas condensate presence of a naturally occurring inhibitor in the hjdron.ells are potentially corrodlens after spreading had 1W:Wd. carbon phase which reacts with the steel surface to form a ing systems. Tlierc is, howTable I gives the valucs for thin but high13 protectiFe la?er. I n in\estigation was evc’r, a typc, in which corrosivci attack is negligible. the extent, of spreading of the made of the hjdrocarbon phase, particularly from the hydrocarbon from both types This is truc in spite of thc fact point of \iew of its aciditj. This disclosed the presence of of ~vells. For comparison thv that any differences in conisome constituent which was not found in the hydrocarbon spreading of purified mineral position of vapor phase, ivater phase’of a corrosite well. The choice of an inhibitor for oil is includcsd. Rcproduciblc injection into a corrosi\e well is discussed on the basis of phase, or operating contliw u l t s were difficult to obtairi this mechanism and the information obtained 011 the tions seem insufficient ti) .;incc it was found that the eshydrocarbon phase. Naphthenic acid was found to meet account for the great differtent of spreading was a fun wells with results to he es_. occurring. This prubcit)ly iiivolvc,d extraction of’polar compoundpected from such w-ella. Thcx b e t s \\er(% then intcArchanged aiirl the ljy the n-ater. However, qualitatively it wac. shoivn that the* previously corroded coupons were found t’ohave become pasrivc., liquid hydrocarbon from the noncorrosive 11ell sprea9s t o n eve11 t o suhzequcnt esposure to laboratory atmosphere. Thc previously uncorroded coupons eshibitcd effects typicd of the corgreattir extent than does that from the corrosive well. Rogers (I.? I measured the relative \retting p o w r on steel of’ liquid hydrocarrosive well. 1lic.rographic examination has shown that a t i i p i i c o tioils anti ivvxtcr produwd from both t,he corrosive and the nonof attack in the noncorrosive \vel1 is due to the presence of a surface film which acts more or lees as a mechanical tinrrier bctn-ctxn corrosive. \vcll. His r e d t , shox that, although the hydrocarbon* the basis metal and the corrosive agent (sj. do not spread to any great estent on a polished steel surface in thr. l17hero corrosive attack is evident in the.? \v(,ll.s, it is knou n tir prewnce of r a t e r , thi: noncorrosive well condenarttc wets the i n v t 1 1 better than docs the corrosivt, condc>nsatt:. be due to the presence of acidic constituent- (4.f . i ) in th(. g:istream. One method of combating the attack, tlic~refore,woultl The fact t h a t these condensates s h o ; ~a tendency to spread be to reduce the acidit>-by injecting alkaline materials, such a, indicates the prcscncc of a polar group ( 1 2 ) . Since the water soda ash, into the n-cll. This, however, in some instanccs lias thct phase is knon-n to contain the lower fatty acids (7, 1 3 ) , it seemed disadvantage of precipitating salts in the producing 1inc.s and thereprobable that the polar compound.‘ in the hydrocarbon phase were' by plugging the ne11 or a t least reducing the production rat(,. acidic. Another method of abating the corrosive attack n-ould be t o inI t was believed that the acid contc,nt n o u l d be 1 0 and ~ t,herefore duce the formation of surface films which would act in a nianni.r the method of analysis \\-odd h a w to be eensit ive enough to dcsimilar to t h a t of films formed in the noncorrosive n-ell. tect as little as 5 p.p.m. of acid. The acidities of the condensat(,> This investigation is concerned with the source from which the tvere compared by using a 100-ml. sample and a 0.02 S solution of nat’ural passivity develops and also with inhihitors which might sodium hydroside in a microburet. The results in the last column act t o protect the metal in a similar way. of Tablc I are typical and indicate a higher acid content in the noncorrosive \vel1 condensate. NATURAL PASSIVITY By using the L4.S.T.1L neutralization number method (Z), elcctrornetric titration curves of the condensate from the two types of In compariiig the surfaces of coupons after ~x’posurt~ to the corwells w r e obtained. These wcrc compared with the titration r o ~ i v eand noncorrosive nell, it was noted that in general the coupons from the latter were less easily \vet b>-r a t e r than those from the former. Thi.; difference \\-as also evidcwt from observations on wellhead fittings and pip? secli3ns which had been TABLE I. SPREADING OF HYDROCARBOXS csposed to the gas stream. Extent of Acid Spreading, Titration ( I ) , The hydrophobic nature of the passive layer indicated t h a t the Diameter LII. 0.02 iV in hlni. h-;rtOH film was derived from organic materials. Thvrefore, a n investiCorrosive well condensate 12 0.68 gation of the liquid hydrocarbon phase \vas undcrtaki,n to detect Soncorrosive well condensarr 25 3 83 differences in composition and in physical nature. d 0.02-ml. Light niineral oil 5 0 03 drop of the liquid hydrocarbon was placed on a clean, diitillctl PItEVIOCS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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one hand and the noncorrosive condensate and the naphthenic acid-hydrocarbon solution curves on the other. I n the latter “ases there are inflections in the curves at a n “apparent” pH of ibout 7.5. Such inflections do not appear in the former cases The curve for the prepared stearic acid-hydrocarbon solution reGembles none of the others. There is evidently some constituent, -1milar in nature to the naphthenic acids, in the condensate of the noncorrosive well r h i c h either is not a component of the condensate from the corrosive w l l or is present in the latter in considerrbly smaller quantities. Sinw it has already been shown (8) that riaphthenic acids inhibited the attack of acetic acid on steel, it is possible that this material or some constituent of i t or a substancr derived from either (e.g., a n iron salt) is responsible for the paw i t v rlcwploped in thr noncorrosive well.
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/NEUTRALIZATION CURVES IN BENZENE-ALCOHOL I
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NAPHTHENIC ACID A S INHIBITOR
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On the basis of these results, naphthenic acid, dissolved in mineral seal oil, was injected into the corrosive well. A series of coupons were exposed according to the technique previously described (8). Figure 2 shows the surface after 4 w e e k exposure in the inhibited well and after the same length of time in the same well prior to t,he inh,ibitor injection. A comparison of these two photomicrographs indicates clbarly the effect of naphthenic acid in retarding the attack. The value of naphthenic acid as a n inhibitor is further borne out by the change in weight (Figure 3). Curve A represents the weight loss over a period of 4 weeks for the coupons (0.06 X 1 X 3 inch S.1E 1020 steel) exposed in the uninhibited gas stream, and curve B is for a 4-week period during inhibition. Of greater importance is the fact, that there was no evidence of pitting on coupons exposed to the inhibited gas stream for 8 weeks. Pits had appeared on coupons exposed for only 3 or 4 wceks in this well prior to naphthenic acid injection (8). X comparison of the photomicrograph of the inhibited well coupon (Figure 2 ) with a coupon from the noncorrosive wvell (Figure 4)shows the similarity in the type of surface developed. These results provide evidence that the injected naphthenir acid Kas functioning as a n inhibitor. Comparison of the surfaces and laboratory experiments previously cited (8) indicates I hat the naphthenic acids are capable of providing a protective laycr on the steel in anaerobic systems iThich will retard acid type %ttack
Figure 1. Titration Curies for Corrosile Well (aboce) and Noncorrosi5e a ell (below) zurves for the folloxing prepared hydrocarbon solutions: Tht first contained 200 p.p.m. acetic acid; :he secaond, 100 p.p.m naphthenic acid; and the third, 100 p,p.m. stearic acid. As Figure 1 shows, there is a definite similarity betn-een the corrosivc rondensate and acetic acid-hydrocarbon solution rurves on thf
DICHROMATE A S INHIBITOR
dodium dichromate is a widely used inhibitor for steel although has previously been employed almost exclusively in systems to whirh therp rvaq free access of air. Laboratory exposure tests in
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TIME I N WEEKS Figure 6. T e i g h t Change Curve l’or Well Inhibited with Sodium Dichromate ~.k~itraIlj-, no pattern as developed by the noncorrosive well surface layer. Occasionally rings were found which were identified (9, 10) as being due to sodium chloride. The right-hand pattern of Figure 7, obtained from t,he surface layers formed in the corrosive well, was identified as a mixture 0 ’ ferric oxide and sodium chloride. The ferric oxide must have been formed during the period in which the sample was exposed to air. Holmberg (11) reported the presence of FeC03 and FerOl in surf.sce layers formed in a corrosive well, by x-ray diffraction. The Fe203 found in this investigation may have come from the FeC03 by conversion and oxidation. The electron diffraction pattern made from scrapings of the surface of coupons exposed in the inhibited corrosive well are diffuse. It appears, therefore, t h a t the surface films formed on steel in the noncorrosive well, or inhibited well, are amorphous and films of the corrofiive well are not.
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CONCLUSIO~S
The evidence, prcwntcd here can be rcwonatily iriterprc,tcd tis I I .\,11. >w. tor Tehting hIaterialh, Standard Xo. 2335--84T'.Xeushowing that one or more polar organic compounds arc prcwnt i l l tralization Nilxiher for Petroleum Products by Color Indicator. tjhe hydrocarbon phase of the noncorrosive well. This matcrinl L'! Illid., Xo. GCi4--2, Seutralizatioii Sutnber for Petroleuni l'rodprobably reacts n-ith the steel surface to form an iron salt n-hic.11 u c t - by Electrometric Titration. remains on the surfacc as an amorphous, coherent, adliercnt film. Damin, l i a t c , ISD. ESO.CHEM., 38,369 (1916). The iron salt would be oriented xvith the cwhouyl group t o n a r d '41 I;ilei%, C. IC., Y . G ~ . 4 . A('ori.04nn . Research Project Corr~tiiiithe metal surface and the hydrocarbon cwd exposed t i ) thil strcxni. tee, Dallas, Ilin lites, Aug., 1945, 19-30. t S I I.:iierts, Carlson. Smith. Aircher.a n d Barr. Proc. S a t / . G'asoiirrr The film apparently is capable of repairing itself ~ r h c i i Asaoc. A m . . 1916. 51. I J ~w a r i n g occurs. This mechanism n-ould account for tlic l i y [ I r ~ ~ ( t i l t:vana. Cy.11., J . C h r m . Soc.. 1946, 2 O i , phobic nature of the layers, the thinncw of t h r f i l i n , the :il~srniv~ ot 171 Giiffiii. 1%.T., n n r l G r w o . E , C'.. Natl. .J~\OC. Corroiion L L I K ~ ~ . tliflerential attack, and the low general rate of attack. M a y 1'346. Both naphthenic acid and sodium dichroniatc alipt~art o I N , '\I Tlnc-kerninn. S o r n ~ a r i a, i d S1ioc.k.' I). -1.. ISD. I
