66 MATERIALS PERFORMANCE May 2008

FAILURE ANALYSIS

Premium Connection Downhole

Tubing Corrosion

LU SHUANLU, Trim Oil Field, Xinjang, China,

and China University of Petroleum, Beijing, China

XIANG JIANMIN, KANG YANJUN, CHANG ZELIANG, AND DONG XUNCHANG,

Trim Oil Field, Xinjiang, China

ZHAI TONGGUANG, University of Kentucky, Lexington, Kentucky

T is article discusses a fi eld background search,

macro- and micro-corrosion morphology analysis,

and material testing of corroded premium

connection downhole tubing. Tubing failure was

caused by erosion-corrosion from highly disturbed

liquid and high shear stress at the couplings.

After two years and nine months of

service, corrosion was found on

the internal surfaces of premium

connection tubing with couplings

of 2 7/8-in (73-mm) by 6.4-lb/ft P110

tubing. The well depth range of corrosion

was 1,312 to 4,921 ft (40 to 1,500 m) with

temperatures of 183 to 212°F (84 to

100°C). The premium connection tubing,

from 2,428 to 4,265 ft (740 to 1,300 m)

in depth with a temperature of 192 to

205°F (89 to 96°C), was severely cor-

roded. The fi eld pin ends of the tubing

were more severely corroded than the

mill pin ends, and some of the fi eld pin

ends with severe corrosion products were

washed out.

The output oil is 178 ton/d, and car-

bon dioxide (CO2) content is 0.69%. The

temperature is 284°F (140°C) at the bot-

tom, and 167°F (75°C) at the well head.

The pressure is 8,122 psi (56 MPa) at the

bottom, and 4,351 psi (30 MPa) at the

wellhead. The tubing string is 16,732 ft

(5,100 m) long.

Corrosion Analysis

Macro-Corrosion Morphology Analysis

Figure 1 shows the corrosion mor-

phology on the internal surface of the

tubing. Corrosion was limited to the in-

ternal surface throughout the length of

the coupling, and featured axial groove

morphology. The corrosion became in-

creasingly severe from the mill coupling

end to the subsidiary torque shoulder of

the fi eld end. There was corrosion perfo-

ration at the fi eld pin end, and the corro-

sion from the fi eld pin end to 28 mm away

was most severe. The remaining wall

thickness at the fi eld pin end was only

~0.4 mm; the remaining wall thickness

at the mill pin end was 1.5 to 2.2 mm.

The morphology showed erosion-corro-

sion (Figures 1 and 2). There was also

corrosion on the internal surface of the

May 2008 MATERIALS PERFORMANCE 67

M A T E R I A L S S E L E C T I O N & D E S I G N

tubing body, but it revealed no erosion-

corrosion features, and corrosion was

much less than that at the connections.

Micro-Corrosion Morphology Analysis

Scanning electron microscopy of the

corrosion perforation and pits on the in-

ternal surface of the tubing revealed cor-

roded penetration (Figure 3), CO2 corro-

sion (Figures 4 and 5) , and belt

erosion-corrosion (Figure 6) morphology.

The corrosion product contained C, O,

Na, Al, Si, S, Cl, Ca, Mn, and Fe.

Micro-corrosion morphology and cor-

rosion product analysis revealed that

corrosion products were carbonate and

chloride, indicating CO2 and erosion-cor-

rosion. These data show that the tubing

failure was caused by both erosion-corro-

sion and CO2 corrosion.

Material Test and Analysis of Tubing

The test results indicated that the tub-

ing material met API 5CT,1 and that the

tubing experienced erosion-corrosion

and CO2 corrosion. Erosion-corrosion is

the combined effect of fl uid (or fl uid and

corrosive medium) and erosive particles

in the tubing. The fl uid and hard impu-

rity particles together form the mecha-

nism of erosion-corrosion, and are con-

tinuously impinging and ploughing the

metal surface and removing metal fl akes

in some local regions.2 Erosion-corrosion

is primarily affected by the strike angle

and shape of the particles. The extent of

damage and distribution depends on the

geometrical shape of the tubing. In parts

subject to abrupt geometric change, ero-

sion-corrosion is very severe because the

parts are exposed to violent fl uid turbu-

lence and abrasive particle concentra-

tion.3 The presence of a corrosive me-

dium causes corrosion on metal surfaces.

Once the corrosion product is removed

by fl uid and impurities, the bare metal is

FIGURE 1

Corrosion and perforation morphology on internal surface of the premium connec-tion.

FIGURE 2

The corrosion and perforation morphology on internal surface of the pin. (Magnifi -cation of left side of Figure 1.)

68 MATERIALS PERFORMANCE May 2008

M A T E R I A L S S E L E C T I O N & D E S I G N Premium Connection Downhole Tubing Corrrosion

exposed to corrosion. The severity of the

corrosion increases as the corrosion prod-

uct removal process continues.

Discussion

After less than three years of service,

tubing corrosion perforations occurred.

The cause of corrosion is related to the

premium connection confi guration and

the corrosiveness of the fl uid.

Premium Connection Confi guration

The most severe corrosion area was

limited to the internal surface near the

fi eld pin end. This corrosion is related to

the premium connection confi guration.

Although the premium connection is a

fl ush bore, there is a V form notch at the

subsidiary torque shoulder that is subject

to abrupt geometric change (Figure 7).

Turbulence occurs at the V form notch as

liquid fl ows through the premium connec-

tion; erosion-corrosion takes place be-

cause of the shear stress stemming from

the geometric changes in the connection.

At the same time, corrosion occurred on

the internal surface of the tubing because

of CO2 and Cl– in the oil. Erosion acceler-

ated corrosion because low-density corro-

sion products could be easily washed out.

The confi guration of the premium con-

nection is the same at the fi eld and mill

ends. Corrosion at the fi eld end is more

severe than that at the mill end because

of different make up torque between the

two ends. The pin bore of the connection

is reduced because of interference be-

tween the pin and box. If make up torque

is too large at the fi eld end, erosion-cor-

rosion will increase because the geometric

change increases. If make up torque is

insuffi cient at the fi eld end, the shoulders

of the pin and box do not come in contact;

this forms an aperture subject to abrupt

geometric change between the pin shoul-

der and box shoulder. More severe liquid

turbulence and shear stress will occur,

FIGURE 3 FIGURE 4

Corrosion perforation morphology on internal surface of the fi eld pin end. (Magnifi cation of corrosion perforation in Figure 2.)

Corrosion micro-morphology at the areas with corrosion products (left) and without corrosion products (right) on the internal surface of the pin end.

FIGURE 5 FIGURE 6

CO2 corrosion product micro-morphology on internal surface

of the pin.Belt erosion-corrosion micro-morphology on the internal surface of the coupling.

May 2008 MATERIALS PERFORMANCE 69

FIGURE 7

Premium connection confi guration.

M A T E R I A L S S E L E C T I O N & D E S I G N

which leads fi nally to more severe erosion-

corrosion.4 Beside the V form notch at the

subsidiary shoulder, there is a geometric

shape change between the pin and cou-

pling on the internal surface of the pre-

mium connection; the pin inner diameter

is smaller than that of the coupling, and

that can cause liquid turbulence.

The severe corrosion range on the

internal surface of the tubing was limited

to the coupling length, from mill cou-

pling end to field coupling end. The

distribution of corrosion on the internal

surface may be related to the pin bore

reduction and stress change after make

up. As the joints are made up, the cou-

pling swells and reduces the pin bore; this

causes micro-structure and stress changes

on the internal surface of the section,

which permits corrosion to take place as

the liquid flows through the tubing

bore.

Corrosion Medium and

Corrosion Process

Corrosion deposits were carbonates

formed from CO2, and chlorides formed

from Cl– and water, so the tubing expe-

rienced corrosion resulting from CO2,

Cl–, and water in the well.

Research indicates that at ~90°C, the

iron carbonate (FeCO3) product film

formed by corrosion is thick, but loose,

and not dense, and does not prevent

further corrosion.5 The FeCO3 product

fi lm formed at temperatures >90°C is

thin, but dense, and minimizes further

corrosion. The most severe area of tubing

corrosion was located at 740 to 1,300 m

depth, with temperatures of 89 to 96°C.

This is in accordance with the research

results.

Test results have shown that CO2 cor-

rosion increases with velocity of fl ow.5

The rate of corrosion increases ~68% as

the velocity of fl ow increases from 0.1 to

1.0 m/s. The daily output was 178 tons

and the velocity of fl ow was 0.1 m/s.

Conclusions and

Recommendations

• The tubing failure was caused by

erosion-corrosion and CO2.

• The cause of the premium connec-

tion corrosion was the V form notch

at the subsidiary shoulder, causing

an abrupt geometric change. This

notch led to fl uid turbulence and

erosion-corrosion. Also contributing

to corrosion were the CO2 and Cl–

in the fl uid.

• The use of properly constructed

premium connection tubing is rec-

ommended.

References

1 API 5CT, “Specifi cation for Casing and Tubing” (Washington, DC: API).

2 J. Postlethwaite, S. Nesic, “Erosion in Disturbed Liquid/Particle Pipe Flow: Effects of Flow Geometry and Particle Surface Roughness,” Corrosion 49, 10 (1993): p. 850.

3 A.V. Levy, “Erosion and Erosion-Corrosion of Metals,” Corrosion 51, 11 (1995): p. 872.

4 Z. Guoxian, “The Research of Effect on CO

2 Corrosion Velocity,” Oil Field

Equipment.

5 L. Shuanlu, Z. Guozheng, L. Mingxu, “Analysis of N80 BTC Downhole Tub-ing Corrosion,” MP 43, 10 (2004): p. 46.

LU SHUANLU is a professor and works at Trim Oil

Field, PO Box 78, Korla, Xinjiang, 84100, China. He

graduated with a metal material specialty from

Xi’an Jiao Tong University in 1993. He specializes

in failure analysis, with 25 years of experience in

inspection and research of oil country tubular

goods.

XIANG JIANMIN is a professor and works at Trim

Oil Field. He graduated with an oil extraction

engineering specialty from the Southwest Institute

of Petroleum in 1978. He has 30 years of

experience on various aspects of oil production and

management.

KANG YANJUN is a senior engineer at Trim Oil Field.

He graduated with a drilling engineering specialty

from Jianghan Institute of Petroleum in 1983. He

has 25 years of experience in drilling, engineering,

and management.

CHANG ZELIANG is a professor and works at Trim Oil

Field. He graduated with an oil extraction

engineering specialty from the Daqing Institute of

Petroleum in 1991. He has 17 years of experience

in corrosion science and protection technology for

OCTG and oilfi eld equipment.

DONG XUNCHANG is a senior engineer at Trim Oil

Field. He graduated with an oilfi eld equipment

specialty from the Jianghan Institute of Petroleum

in 1991. He has 17 years of experience in corrosion

science and protection technologies for oil

pipelines.

ZHAI TONGGUANG is an associate professor in the

Department of Chemical and Materials Engineering,

University of Kentucky, 177 F. Paul Anderson

Tower, Lexington, KY 40506. He has more than 20

years of experience in materials research and

failure analysis. He has a D.Phil. degree in

materials science from the University of Oxford, is

a recipient of the NSF CAREER Award, and has

published more than 50 technical articles.