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Transcript of 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%.
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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.
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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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