AES_and_EDS

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AES and EDS microanalysis of a petroleum

well tubing in cross-section

A. Cosultchi a,b,*, J.R. Vargas a, B. Zeiferta, E. Garciafigueroa b,c,A. Garca-Borquez c, V.H. Lara d, P. Bosch d

aDepartment of Metallurgical Engineering, ESIQIE-IPN, Mexico City, Mexicob

Instituto Mexicano del Petroleo, 152 Eje Central L. Cardenas, 07730 Mexico City, MexicocESFM-IPN, UPALM, Ed. 9, 07738 Mexico City, Mexico

dUAM-I, Av. Michoacan y Purisima, Iztapalapa, 09340 Mexico City,Mexico

Received 15 February 2001; received in revised form 19 October 2001; accepted 25 October 2001

Abstract

Two techniques, Auger electron spectroscopy (AES) and energy dispersive spectroscopy (EDS), were applied to obtain the

composition of the cross-section of a piece of tubing used within a petroleum well. The structure and composition of the steel

wall and of the internal oxide layer were achieved. Physical and chemical changes of the original iron oxide layer induced by

petroleum compounds were discussed. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Auger electron microscopy; EDS; XRD; Microanalysis; Pipe; Iron compounds; Pyrrhotite; Petroleum organic compounds

1. Introduction

Selection of a tubing string used in petroleum

extraction is governed by mechanical properties,

which are based on the well depth and operation

conditions. The pipe furnished to form the coiled

tubing string is normally an API grade, seamless andmanufactured by hot working and, subsequently, cold-

finished [1]. The internal surface of the tubing is

usually covered with a thin iron oxide multi-layer,

which offers protection against atmospheric corrosion

[2,3]. Assuming that light hydrocarbons are natural

corrosion inhibitors [4,5], the initial oxide layer com-

position is expected to prevail on the tubing surface

after its contact with petroleum. However, when car-

bon steel is working in ambient containing sour (H2S)

or sweet (CO2) gases, the surface is damaged by

corrosion or sulfide stress cracking [69]. Addition-

ally, petroleum organic material is often found adheredon the tubing surface. Consequently, the internal

diameter of the tubing is reduced, which contributes

to the declination of the petroleum flow. So far, the

interaction between petroleum and iron oxide scale has

not been considered in the study of the mechanism of

organic deposition in petroleum wells. In this work, the

AES and EDS characterization of the modified iron

oxide scale as a consequence of its contact with

petroleum compounds is presented and the mechanism

of the organic deposit formation is discussed.

0167-577X/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved.P I I : S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 3 8 4 - 1

* Corresponding author. Instituto Mexicano del Petroleo, 152,

Eje Central L. Cardenas, 07730 Mexico City, Mexico. Tel.: +52-

5-3337013; fax: +52-5-5678776.

E-mail address: [email protected] (A. Cosultchi).

www.elsevier.com/locate/matlet

August 2002

Materials Letters 55 (2002) 312317


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Auger electron spectroscopy (AES) provides a

surface microanalysis with a sampling depth on the

order of 1 nm, whereas the characteristic X-ray (EDS)

sampling depth was 0.3 Am (or 300 nm). Therefore,the AES information was assumed as surface compo-

sition, whereas the EDS results were assumed as bulk

composition.

2. Experimental

A tubing string was extracted from a well located in

a southeastern Mexican field, which produces a 29.5j

API crude oil from the upper Jurassic Kimmeridge age

zone reservoir with a bottom hole temperature of 420

K. A tubing piece with a black layer of solid material

of 2 cm thickness adhered on its internal surface was

collected. Coupons of 1.5 1.5 cm2 and 0.3 cm

thickness were lathe-turned at low velocity and one

of the cross-sections was polished. The bulk elemental

composition was obtained using a scanning electron

microscope (SEM) JEOL JSM 6300 fitted with a

NORAN energy dispersive X-ray analysis facility

(EDS) [10]. The operating conditions for the chemical

composition obtained by EDS were 15 keV for the

initial electron energy, Cu Ka

radiation and a working

distance of 39 mm. A scanning electron microscope

JEOL JAMP 30 fitted with Auger electron spectro-

scopy (AES) and using an electron beam of 10 KeV

performed the surface elemental composition. Eacharea was sputtered with Argon during 30 min previous

to the spectrum recording. Additionally, the element

concentrations were calculated using their relative

Auger sensitivity factors from literature [11]. The

quantitative analysis precision is affected by the fac-

tors selection process and, consequently, the expected

errors used to be not less than F 20%. A Siemens

D500 X-ray diffractometer (XRD) with CuKa

radia-

tion and a diffracted beam monochromator was used.

In order to study the coupon surface with adhered

material, the coupon was placed in the diffractometer

sample holder in order to fulfill the Bragg condition.

Thus, information on the preferred orientation of the

crystalline compounds was obtained.

3. Results

A layer of material with a thickness of 8 to 15 Am

covers the surface of the coupon, as previously reported

[12]. The diffraction pattern of this surface layer,

shown in Fig. 1, exhibits intense and sharp peaks

Fig. 1. XRD diffractogram of the L-80 coupon surface with adhered material.

A. Cosultchi et al. / Materials Letters 55 (2002) 312317 313


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corresponding to crystalline phases identified as mag-

netite (JCPDS 11-0614) and pyrrhotite (JCPDS 22-

0358). The relative intensities of the peaks correspond-

ing to magnetite do not reproduce the values of the

JCPDS card; the crystals are, therefore, oriented toward

the direction of the most intense peak (311). Appa-

rently, the steel substrate and its manufacturing con-ditions influenced the growth of the magnetite crystals.

Such is not the case for pyrrhotite crystals, as the peak

heights reproduce the JCPDS reported intensities.

The SEM micrograph in Fig. 2 shows a view of the

polished cross-section of the coupon. The bulk metal,

shown in Fig. 2, apparently exhibits a martensitic

structure [13]. The bulk metaloxide scale interface(Z3) is irregular and exhibits a stripes-like morphol-

ogy penetrating the bulk metal. The oxide scale and

the coupon border also exhibit irregular morpholo-

gies, with valley and edge-like structures (Z4 and Z5,

respectively).

EDS and AES spectra were recorded from five

zones across the tube section, as indicated in Fig. 2.

The elemental composition results obtained by these

two techniques are presented in Table 1 and summar-

ized as follows.

(1) The surface composition of Zone 1 (Z1) indi-

cates that iron is oxidized and there are higher

amounts of manganese, chromium and molybdenum

than the bulk composition shows.

(2) The surface composition of Zone 2 (Z2) evi-

dences an increase of oxygen content as well as the

presence of carbon and silicon. The bulk composition

indicates also an increment of the oxygen content.

(3) The surface composition of Zone 3 (Z3) con-

firms the presence of sulfur, which is associated with

an increase in the oxygen content and a decrease in

the carbon amount. Additionally, the Auger KLL peak

of carbon splits into two, at 272 and 270 eV, whichsuggests that there may be at least two types of carbon

bonds. The bulk composition is similar to the previous

zone with aluminum as an additional element.

Fig. 2. Micrograph of the tubing cross-section showing the position

and the EDS and AES recorded spectra.

Table 1

Surface (AES) and bulk (EDS) elemental compositions (in wt.%)

Element Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

AES EDS AES EDS AES EDS AES EDS AES EDS

Fe 51.6 97.54 18.85 80.35 14.79 81.42 6.66 71.27 20.47 80.11

O 28.74 0.56 57.72 17.94 64.01 16.64 22.56 15.74 41.32 10.76

Mn 6.47 1.52 5.42 1.42 3.40 1.05 1.18 1.36

Si 0.37 2.6 0.29 1.86 0.52 2.26 0.28

S 11.18 38.68 23.61 5.38

C 15.41 4.76 27.21 2.69 14.6 1.07

Cr 8.16 0.23 0.07 0.12

Mo 5.03

Al 0.15 5.38 0.19

Ca 4.89 0.64 0.19

Cl 0.22 0.26

K 0.29

Na 0.55

The error estimation for EDS is F 4%, while for AES is F 20%.

A. Cosultchi et al. / Materials Letters 55 (2002) 312317314


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(4) The surface composition of Zone 4 (Z4) shown

in Fig. 3a indicates high amounts of sulfur and calcium,

while the bulk composition exhibits high amounts of

aluminum. Additionally, the bulk composition regis-tered the same iron oxidation degree as Zone 3. In this

zone, the Auger KLL oxygen peak underwent a change

of shape and the splitting of the signal.

(5) The surface composition of Zone 5 (Z5) shown

in Fig. 3b indicates that the oxygen content has

decreased and the sulfur content has increased. The

bulk composition indicates a less than ideal ironoxidation degree (O/Fe atomic ratio is 0.47). Carbon,

sulfur and other elements were also identified in the

bulk composition. The Auger peaks of Fe at 598 and

652 eV shift to 601 and 655 eV, respectively.

4. Discussion

The AES and EDS spectra registered from different

zones across the tubing wall depict the variation of

concentration of the main elements on the surface and

in the bulk steel, respectively. The steel composition is

nonuniform, as shown in the bulk composition from

Zones 1 to 3. However, this composition corresponds

to an L-80 grade type 1, which is usually produced

from a CMn or CMnMo steel [6].

Most of the petroleum compounds involved in the

formation of the adhered layer of organic deposit are

polar molecules containing N, O and S along with C

and H atoms. Additionally, inorganic compounds are

constantly carried out from the reservoir, together with

the petroleum flow. Furthermore, rupture of the orig-

inal iron oxide layer might be induced within the wellby different events such as the geothermal gradient as

well as by mechanical stress originated by the freely

suspended tubing string weight. The contact between

the iron oxide layer and petroleum increases as the

surface is enlarged, and moreover, the bulk metal is

exposed to petroleum ambient. Therefore, the mor-

phologies and composition of the scale layer, Zones 4

and 5, reveal such events. The coupon border, which

corresponds to the internal surface of the tubing,

exhibits irregular structures, which are associated with

the presence of high amounts of sulfur, carbon,calcium and aluminum.

Thus, the valley-like structures (Z4) are related to

surface composition rich in sulfur, carbon and cal-

cium, although sulfur was not observed in the bulk

composition. Therefore, the source of the surface

sulfur amount must be BaSO4 as determined else-

where [14], being one of the drilling mud compo-

nents. As to carbon, the presence of organic com-

pounds on the coupon surface was established by

FTIR in the reflection mode. Accordingly, the infraredFig. 3. AES and EDS spectra registered from (a) Z4 and (b) Z5

coupon zones.



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spectrum, presented elsewhere [12], revealed the

characteristic vibration bands of methyl and methyl-

ene groups corresponding to saturate and aromatic

hydrocarbons, together with some other minor vibra-tion band assigned to oxygen bearing compounds like

hydroxyl and carbonyl compounds. Changes in the

shape and splitting of the Auger KLL oxygen peak

indicate that, in this zone, there may be at least two

different types of oxygen-bearing compounds, organic

as well as inorganic compounds.

In the edge-like structures (Z5), the Auger Fe peaks

are shifted from their normal positions, which indi-

cates the presence of new iron bonds. Indeed, as

shown elsewhere [14], the Mossbauer Spectroscopyidentified compounds such as magnetite, maghemite,

pyrrhotite, lepidocrocite and goetite in the scrapped

oxide layer. Petroleum sulfur compounds and, espe-

cially, the H2S molecules are adsorbed mainly on non-

coordinative iron sites on the steel surface, thus, sulfur

atoms left on the surface reacts with Fe followed by

the formation of iron sulfide phases [15]. Therefore,

the original iron oxide layer experienced chemical and

physical changes as a consequence of its contact with

petroleum.

Magnetite, which is the main compound of the

scale layer, crystallizes in a cubic inverse spinel

structure. The surface structure of Fe3O4 (111) yields

a strongly relaxed bulk termination with a quarter

monolayer of Fe atoms exposed and raised above a

complete monolayer of oxygen. Maghemite (g-Fe2O3)

is formed by hydrolysis as a passivating layer on the

magnetite surface [9]. Iron cations on the iron oxide

surfaces has Lewis acidic character [16], although

when hydroxyl groups are adsorbed on the oxide

surface, they may act as Bronsted acidic sites, which

may dissociate and protonate adsorbed bases [15,16].

Therefore, specific chemisorption of organic com-pounds on steel or preoxidized steel surface is a very

complex process, in which case, molecules with high

donor orbital energy are strong bonded to a surface

with low acceptor orbital energy [17]. Moreover, the

strength of the surface bond is directly correlated to

the electronegativity of the organic molecule func-

tional groups.

Eventually, the steel and scale wettability by water

or by hydrocarbons is critical in the formation process

of corrosion products or organic deposits on the

tubing surface inside the petroleum wells. Fig. 4shows schematically the structure of the scale layer

before and after petroleum contact.

5. Conclusions

The EDS and AES techniques let us determine the

chemical composition at different depths and from

different zones through the cross-section of the petro-

leum tubing. Thus, a nonuniform composition of theFig. 4. Scheme of the structure of the scale (a) formed at room

temperature and (b) in petroleum ambient.

A. Cosultchi et al. / Materials Letters 55 (2002) 312317316


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steel as well as changes in the structure and compo-

sition of the iron oxide scale are identified. The

surface chemical transformations are related to the

formation of new iron phases as a consequence ofsteel surface contact with petroleum compounds and

organic compounds chemisorption. Likewise, physi-

cal changes of the tubing internal border reflect the

consequences of the scale fracturing and the adher-

ence of organic and mineral compounds as a thick

layer on this surface.

Acknowledgements

This research was supported by Instituto Mexicano

del Petroleo Grand FIES 97-06-I.

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