Research Article The Application of Mechanical-Chemical...
7
Research Article The Application of Mechanical-Chemical Corrosion Theory in Downhole Tubing CO 2 Corrosion Research Peike Zhu, Wei Yan, Liyu Deng, and Jingen Deng State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China Correspondence should be addressed to Wei Yan; [email protected] Received 29 September 2014; Revised 29 January 2015; Accepted 30 January 2015 Academic Editor: Zhimin Liu Copyright © 2015 Peike Zhu et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Indoor simulating experiment is a main method for oil field CO 2 corrosion research. Experimental parameters are very important for an accurate simulation. Based on the mechanical-chemical corrosion theory, the external load may be possible to accelerate the corrosion rate. However, the influence of N 2 pressure on CO 2 corrosion during the simulating experiment is negligible. Because the coupon stress induced by additional N 2 pressure is very low, therefore, the N 2 adding procedure can be cancelled and a more safety working space for researchers will be created. However, it does not mean that mechanical-chemical corrosion influence can be ignored. For downhole tubing, the hoop stress generated from the formation compress or liquid column internal pressure is remarkable; stress effect on corrosion has to be taken into consideration. When pit cavity especially occurred on the internal tubing surface, the stress concentration effect will induce a much higher local stress. Mechanical-chemical corrosion will become significant and more study should be performed on this topic. 1. Introduction Downhole casing and tubing CO 2 corrosion, a complicated metal degradation process, is a comprehensive topic related to several research areas, such as electrochemical corro- sion theories, fluid mechanics, oil and gas separating the- ory, and downhole string mechanics [1]. Indoor simulating experiment is a main method for CO 2 corrosion research. Experiment parameters are very important for accurate simulation. As most researchers have done, N 2 gas is added in the container (e.g., autoclave) to a certain formation value in order to simulate the real downhole condition. e complementary pressure, usually very high, is defined by wellbore depth or reservoir pressure. It will result in a higher pressure in the container which is dangerous to the researchers and other people. is paper is trying to investigate if the complementary N 2 will have a significant effect on CO 2 corrosion results and how much the effect will be. 2. Theory Basis It is considered that the total pressure may have an effect on simulating CO 2 corrosion probably because the external pressure will influence the metal’s corrosion process and accelerate corrosion rate especially [2–5]. is theory is related to mechanical-chemical phenomenon reported by Gutman. From the perspective of thermodynamic, if the system of identical positive ions is subjected simultaneously to the effect of two external factors–mechanical and electrical, the expression for the reversible potential is as follows: = 0 + ln + + Δ . (1) And the decrease of equilibrium electrical potential of the system of positive ions as the result of presence of a mechan- ical action (Δ) is determined by the expression: Δ = − Δ , (2) Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2015, Article ID 296278, 6 pages http://dx.doi.org/10.1155/2015/296278
Transcript of Research Article The Application of Mechanical-Chemical...
Research ArticleThe Application of Mechanical-Chemical Corrosion Theory inDownhole Tubing CO2 Corrosion Research
Peike Zhu Wei Yan Liyu Deng and Jingen Deng
State Key Laboratory of Petroleum Resource and Prospecting China University of Petroleum Beijing 102249 China
Correspondence should be addressed to Wei Yan yanwei289126com
Received 29 September 2014 Revised 29 January 2015 Accepted 30 January 2015
Academic Editor Zhimin Liu
Copyright copy 2015 Peike Zhu et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
Indoor simulating experiment is a main method for oil field CO2corrosion research Experimental parameters are very important
for an accurate simulation Based on the mechanical-chemical corrosion theory the external load may be possible to accelerate thecorrosion rate However the influence of N
2pressure on CO
2corrosion during the simulating experiment is negligible Because
the coupon stress induced by additional N2pressure is very low therefore the N
2adding procedure can be cancelled and a more
safety working space for researchers will be created However it does not mean that mechanical-chemical corrosion influencecan be ignored For downhole tubing the hoop stress generated from the formation compress or liquid column internal pressureis remarkable stress effect on corrosion has to be taken into consideration When pit cavity especially occurred on the internaltubing surface the stress concentration effect will induce a much higher local stress Mechanical-chemical corrosion will becomesignificant and more study should be performed on this topic
1 Introduction
Downhole casing and tubing CO2corrosion a complicated
metal degradation process is a comprehensive topic relatedto several research areas such as electrochemical corro-sion theories fluid mechanics oil and gas separating the-ory and downhole string mechanics [1] Indoor simulatingexperiment is a main method for CO
2corrosion research
Experiment parameters are very important for accuratesimulation As most researchers have done N
2gas is added
in the container (eg autoclave) to a certain formationvalue in order to simulate the real downhole conditionThe complementary pressure usually very high is definedby wellbore depth or reservoir pressure It will result ina higher pressure in the container which is dangerous tothe researchers and other people This paper is trying toinvestigate if the complementary N
2will have a significant
effect on CO2corrosion results and how much the effect will
be
2 Theory Basis
It is considered that the total pressure may have an effecton simulating CO
2corrosion probably because the external
pressure will influence the metalrsquos corrosion process andaccelerate corrosion rate especially [2ndash5] This theory isrelated to mechanical-chemical phenomenon reported byGutman From the perspective of thermodynamic if thesystem of identical positive ions is subjected simultaneouslyto the effect of two external factorsndashmechanical and electricalthe expression for the reversible potential is as follows
120583 = 120583
0+ 119877119879 ln 119886 + 119899119865120593 + Δ119875119881
119898 (1)
And the decrease of equilibrium electrical potential of thesystem of positive ions as the result of presence of a mechan-ical action (Δ119875) is determined by the expression
Δ120593 = minus
Δ119875119881
119898
119911119865
(2)
Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2015 Article ID 296278 6 pageshttpdxdoiorg1011552015296278
2 Advances in Materials Science and Engineering
where 120583
0is the chemical potential of the substance (here
refers to metal) in standard state (119886 = 1) 119877 is the universalgas constant 119879 is absolute temperature of a system 119886 isthermodynamic (ordinary) activityΔ119875 is excess pressure119881
119898
is molar volume of the substance 119899 is charge transfer valency119865 is faraday number 120593 is electrical potential
From electrochemical corrosion kinetic view given thereaction is (119872 rarr 119872
119899++ 119899119890) the electrochemical activation
free energy equals the sum of chemical activation free energyand electrochemical potential As shown in (3)
Δ
119866
119872
119883
= Δ119866
119883
119872+ 120573119899119865ΔΦ
(3)
The electrochemical reaction rate is
V119890= 119888
119872
119896119879
ℎ
exp(minus
Δ119866
119883
119872+ 120573119899119865ΔΦ
119877119879
)
V119890= 119888
119872
119896119879
ℎ
exp(
minusΔ119866
119883
119872
119877119879
) exp(minus
120573119899119865ΔΦ
119877119879
)
(4)
Given the reaction rate constant 119896
119888= (119896119879ℎ) exp(minusΔ119866119883
119872
119877119879) then
V119890= 119896
119888119888
119872exp(minus
120573119899119865ΔΦ
119877119879
) (5)
For a reversible electrochemical reaction
997888rarr
V119890=
997888rarr
119896
119888119888
119872exp(minus
120573119899119865ΔΦ
119877119879
)
larr997888
V119890=
larr997888
119896
119888119888
119872+exp(
(1 minus 120573) 119899119865ΔΦ
119877119879
)
(6)
where 119888
119872is concentration of 119872 119896 is Boltzmann constant
119879 is absolute temperature of a system ℎ is Planck constantΔΦ is overpotential 119899 is charge transfer valency 120573 is fractionnumber (less than 1)
Multiply faraday constant 119865 for (6) and given exchangecurrent 119868
0= 119865
997888rarr
119896
119888119888
119872= 119865
larr997888
119896
119888119888
119872+ then the metal dissolution and
deposition current can be expressed as (7) and (8) separately
119868 = 119868
0exp(minus
120573119899119865ΔΦ
119877119879
) (7)
119868 = 119868
0exp(
(1 minus 120573) 119899119865ΔΦ
119877119879
) (8)
When external force is applied on a metal which isimmersed in an acidic solution the mechano-electrochem-ical activation free energy equaled the sum of chemicalactivation free energy electrochemical potential and surfacepotential elevated by the stress as shown in (9)
As compared to nonstress condition the external forceonly changes the free energy of solid state 119872 and has noinfluence on deposition process (red solid curve)
Δ
119866
119872
119883
= Δ
119866
119872
119883
minus Δ119875119881
119898= Δ119866
119883
119872+ 120573119865ΔΦ minus Δ119875119881
119898
(9)
x
y
z
120590x
120590y
120590z
Mixed gas pressure
Figure 1 Stress analysis of specimens
The dissolution current under stress condition is
119868 = 119868
0exp(
minus120573119899119865ΔΦ + Δ119875119881
119898
119877119879
) (10)
Stress effect is an influence factor on metal dissolution
119868 =
119868 times exp (
Δ119875119881
119898
119877119879
) (11)
When specimens are immersed into solution sealedwith a high-temperature and high-pressure autoclave [6]regardless of the difference of fluid column pressure in heightliquid pressures on specimen from all directions are equaland unit stress state of specimen is as shown in Figure 1
It is assumed that the pressure medium in autoclave ispressurized to 20MPa byN
2during the experiment and120590
119909=
120590
119910= 120590
119911= 20MPa hydrostatic pressure is
120590
119909+ 120590
119910+ 120590
119911
3
= 20MPa (12)
And given the temperature is 100∘C (37315 K) molar vol-ume of steel is 712 times 10minus6m3mol and gas constant 119877 is8314 Jsdotmolminus1sdotKminus1 according to the discussion in Section 2(11) the influence coefficient of corrosion rate caused bysystem pressure is
exp 119881Δ119875
119877119879
= exp 712 times 10
minus6times 20 times 10
6
8314 times 37315
= 1047
(13)
Under the system pressure corrosion rate increases lessthan 5 However the repeated test error rate of corro-sion simulation experiment is always between 5 and 10sometimes even higher Thus it can be considered thatpressurization to 20MPa by nitrogen has little or no influenceon the corrosion test result
3 Verification Experiments
A series of experiments were conducted to verify themechanical-chemical corrosion theory The materials usedin this study are tubing steels N80 1Cr and 3Cr80 Thechemical compositions of the materials are shown in Table 1The solution used in this study is simulated oil field water
Advances in Materials Science and Engineering 3
Table 1 Chemical compositions of N80 1Cr80 and 3Cr80 (mass fraction wt)
Material C Si Mn P S Cr Mo NiN80 024 022 119 0013 0004 0036 0021 00281Cr80 020 027 042 00089 000082 103 024 0103Cr80 019 032 047 00089 000082 293 039 017
Table 2 Oil field water ions concentration (mgL)
K+Na+ Ca2+ Mg2+ CLminus SO4
2minus HCO3
minus CO3
2minus
8350 3885 57 18975 850 337 0
The pH value is 68 and the ion concentrations are shown inTable 2 CO
2partial pressure is 04MPa and the flow velocity
is 15ms Experiments were conducted in three autoclaves atthe same time and the N
2pressure filling in three autoclaves
is 0MPa 4MPa and 15MPa respectivelyThe equipment forthis study is CWYF-1 high-temperature and high-pressuredynamic corrosion autoclave
The specimens of 50mm in length 10mm in widthand 3mm in thickness were polished with silicon carbidenumber 320 and number 600 papers rinsed with distilledwater degreased in acetone and then weighed after dryingThe specimens were mounted on a specimen holder madeof polytetrafluoroethylene (PTFE) The holder was placedin the autoclave filled with corrosive medium and then theautoclave was closed
The solution was deoxygenated by pure nitrogen for 2hours at 45∘C Then the system was given required partialpressure (the partial pressure of CO
2is 04MPamaintenance
purging CO2for 1 hour until solution in the autoclave gets
saturated and controlled the design pressure by nitrogen)and temperature (90∘C) After 72 hours the specimen sur-faces were washed with distilled water and the corrosionproducts were pickled in 10 hydrochloric acid solutionto get removed and the solution contained an inhibitorto protect the base Then the specimens were rinsed withdistilled water degreased in acetone and dried after that thespecimens were photographed to record the corrosion statusFinally the residual masses of the specimens were weighedon an electronic balance (accuracy 01mg) and the averagecorrosion rate was calculated according to NACE RP0775-2005
As shown in Table 3 under the three pressure conditionsthere is little difference in the corrosion of the same materialSevere pitting corrosion appeared on the N80 specimens but1Cr80 and 3Cr80 suffered general corrosion As shown inFigure 2 the corrosion rates of N80 under three pressureconditions are basically consistent 1Cr80 and 3Cr80 aresimilar to N80 and the corrosion rate did not increase withsystempressureThus it can be seen thatwhen theCO
2partial
pressure is constant (ie the content of CO2in the solution is
constant) the system pressure (ie hydrostatic pressure) haslittle impact on the corrosion rate and corrosionmorphology
According to the results of indoor simulation experi-ments andmechanical-chemical corrosion theory additionalN2did not have obvious impact on the coupons surface in the
samematerial and the general corrosion rates aremuch closer
30
20
10
00C
orro
sion
rate
(mm
y)
N80
CO2 = 04MPaCO2 = 04MPa N2 = 40MPaCO2 = 04MPa N2 = 150MPa
15620
14496
15410 20383
20917
19025
15863
15546
14907
1Cr80 3Cr80
Figure 2 Corrosion rates contrast under different total pressures
for each material Thus adding N2can be omitted during
the simulation experimentWithout adding highN2pressure
the test result is still reliable enough Skip this process canreduce the experiment cost and make it more efficient Whatis more important is that skipping this high-pressure processcan avoid potential security threats to the researchers
4 Stress Analysis in AnticorrosionDesign for Downhole String
Equation (13) can be regarded as an influence coefficientof corrosion process Under different temperature the rela-tionship between influence coefficient and stress is shown inFigure 3
Figure 3 demonstrates that stress influence coefficientof corrosion is very small under the low stress conditionswhen stress is greater than 100MPa the impact of stress oncorrosion is gradually increased and becomes nonignorableIn addition the promotion of the stress in low temperaturecorrosion is greater than that in high-temperature corrosionThis needs to be stressed that corrosion rate is different fromcoefficient of corrosion but the corrosion has positive corre-lation with coefficient of corrosion and a higher coefficientwill induce a higher corrosion rate
Under the internal fluid pressure formation (or cement)external extrusion pressure axial force and bending the wallstress of downhole string often can reach a much higher
4 Advances in Materials Science and Engineering
Table 3 Appearance of specimen and average corrosion rate
Material Photos of specimen Corrosion System pressure Corrosion ratemma
N80
Pitting 04MPa0MPa 15620
Pitting 04MPa4MPa 14496
Pitting 04MPa15MPa 15410
1Cr
Generalcorrosion 04MPa0MPa 20383
Generalcorrosion 04MPa4MPa 20917
Generalcorrosion 04MPa15MPa 19025
3Cr
Generalcorrosion 04MPa0MPa 15863
Generalcorrosion 04MPa4MPa 15546
Generalcorrosion 04MPa15MPa 14907
Stress (MPa)6005004003002001000
Coe
ffici
ent o
f cor
rosio
n 4
3
2
1
0
60∘C
100∘C
150∘C
Figure 3 The relationship between corrosion coefficient and aver-age normal stress
stress level (greater than 100Mpa) In the process of downholetubing anti-corrosion design the hoop stress on the casingwall under the internal pressure has obvious promotingeffect on corrosion rate For the larger diameter-thickness
ratio pipe circumferential stress on the wall under internalpressure can be calculated by [7]
120590 =
pr119900
119905
(14)
It is assumed that casing outer diameter is 9-5810158401015840 wallthickness is 1199mm and inner pressure is 20MPa Thewall stress calculated by above equation is 205MPa (elasticmodulus is 208GPa and Poissonrsquos ratio is 028)
According to the curve in Figure 3 when the stress is200MPa influence coefficient of corrosion rate is about 15which will have great influence on the corrosion
Another more serious situation is stress concentrationaround pits in internal surface of downhole tubingThe finiteelement model of two spherical pits is shown in Figure 4
Model parameters are as follows outer diameter2445mm (9-5810158401015840) wall thickness 1199mm casing gradeN80 elastic modulus 208Gpa Poissonrsquos ratio 028 hemi-sphere pits diameter 80mm (119877 = 40mm) pit cavity depth40mm
Advances in Materials Science and Engineering 5
x
y
z
Figure 4 Finite element models of two spherical pits in casing internal surface
+3653e + 02
+3356e + 02
+3058e + 02
+2761e + 02
+2464e + 02
+2167e + 02
+1869e + 02
+1572e + 02
+1275e + 02
+9775e + 01
+6802e + 01
+3830e + 01
+8570e + 00
Figure 5 Cross section stress distribution of double cavities (119871119877 =
22)
When the ratios of 119871119877 (119871 is the distance between twohemisphere centers) are equal to 22 and 10 respectively(center distance between the two pits and radius) andinternal pressure is 10MPa the stress distribution around thepits is shown in Figures 5 and 6
Tensile stress appears at the intersecting line of theconcave of pit and 119883-119885 plane and the maximum stress is inthe double pits intersection Figures 6 and 7 are the profilesalong the axis of casing and through the center of pit
When 119871119877 = 22 (Figure 5) the stress concentrationfactor is 365MPa100MPa = 365 when 119871119877 = 10 (Figure 6)the stress concentration factor is 292MPa100MPa = 292
When 119871119877 = 22 as shown in Figure 7 stress at the outeredge of pit is 209MPa stress at the inner edge of pit is365MPa which is maximum bottom stress is 197MPa thedistance between the inner edges of the two pits is 08mmand midpoint stress at the inner edge of double pits is325MPa When 119871119877 = 10 as shown in Figure 8 the stress
+2918e + 02
+2687e + 02
+2455e + 02
224e+2 + 02
+1993e + 02
+1762e + 02
+1530e + 02
1299e+ + 02
+1068e + 02
+8368e + 01
+6056e + 01
744e+3 + 01
432e+1 + 01
Figure 6 Cross section stress distribution of single cavity (119871119877 =
10)
0
0
2 4 6 8 10 12 14 16 18 20
Distance (mm)
Stre
ss (M
Pa)
400
350
300
250
200
150
100
50
Figure 7 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 22
at the outer edge of pit is 214MPa stress at the inner edge ofpit is 292MPa which is maximum bottom stress is 203MPatwo-pit-intersect and the maximum stress at the intersectionis 292MPa
6 Advances in Materials Science and Engineering
0
0
2 4 6 8 10 12 14
Distance (mm)
Stre
ss (M
Pa)
350
300
250
200
150
100
50
Figure 8 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 10
According to mechano-chemical corrosion theory stressinfluence coefficient can reach 25 Therefore when selectinganti-corrosion materials for downhole string and evaluatingservice life of the tubing string for the corrosive gas fields it isnecessary to consider wall stress influence on electrochemicalcorrosion
5 Conclusion
(1) If the stress is not very high (less than 50MPa) theinfluence of stress on corrosion process is negligible
(2) The influence of system pressure on corrosion ratecan be ignored and the procedure of adding inert gas(eg Nitrogen) pressure can be skipped In this waythe experimental efficiency will be increased and thesecurity threats from the high pressure autoclave canbe avoided
(3) Downhole tubing hoop stress generated due to theformation compress or liquid column pressure isremarkable therefore stress effect has to be takeninto consideration Especially when pit corrosionoccurred on the internal tubing surface the stressconcentration effect will raise the local stress to amuch higher value
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
The research work was supported by China University ofpetroleum (Beijing) Scientific Research Fund Project (YJRC-2013-19)
References
[1] W Yan J Deng X Dong et al ldquoInvestigation of calculationmethod of CO
2partial pressure in oil and gas wellrdquo Drilling amp
Production Technology vol 34 no 5 pp 19ndash22 2011 (Chinese)[2] E M Gutman Mechanochemistry of Materials Cambridge
International Science 1998
[3] T G Movchan N E Esipova P V Eryukin N B Uriev and AI Rusanov ldquoMechanochemical effects in processes of corrosionof metalsrdquo Russian Journal of General Chemistry vol 75 no 11pp 1681ndash1686 2005
[4] X F Liu J Zhan andQ J Liu ldquoThe influence of tensile stress onelectrochemical noise from aluminum alloy in chloride mediardquoCorrosion Science vol 51 no 6 pp 1460ndash1466 2009
[5] S Rao L Zhu D Li Z Zhang and Q Zhong ldquoEffects ofmechanochemistry to the pitting behaviour of LY12CZ alu-minum alloyrdquo Journal of the Chinese Society of Corrosion andProtection vol 27 no 4 pp 228ndash232 2007
[6] W Yan and J Deng ldquoInvestigation of mechanochemical corro-sion of downhole tubing and casingrdquo China Offshore Oil andGas vol 26 no 1-2 pp 87ndash91 2014
[7] H Liu Material Mechanics Higher Education Press BeijingChina 2006
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2 Advances in Materials Science and Engineering
where 120583
0is the chemical potential of the substance (here
refers to metal) in standard state (119886 = 1) 119877 is the universalgas constant 119879 is absolute temperature of a system 119886 isthermodynamic (ordinary) activityΔ119875 is excess pressure119881
119898
is molar volume of the substance 119899 is charge transfer valency119865 is faraday number 120593 is electrical potential
From electrochemical corrosion kinetic view given thereaction is (119872 rarr 119872
119899++ 119899119890) the electrochemical activation
free energy equals the sum of chemical activation free energyand electrochemical potential As shown in (3)
Δ
119866
119872
119883
= Δ119866
119883
119872+ 120573119899119865ΔΦ
(3)
The electrochemical reaction rate is
V119890= 119888
119872
119896119879
ℎ
exp(minus
Δ119866
119883
119872+ 120573119899119865ΔΦ
119877119879
)
V119890= 119888
119872
119896119879
ℎ
exp(
minusΔ119866
119883
119872
119877119879
) exp(minus
120573119899119865ΔΦ
119877119879
)
(4)
Given the reaction rate constant 119896
119888= (119896119879ℎ) exp(minusΔ119866119883
119872
119877119879) then
V119890= 119896
119888119888
119872exp(minus
120573119899119865ΔΦ
119877119879
) (5)
For a reversible electrochemical reaction
997888rarr
V119890=
997888rarr
119896
119888119888
119872exp(minus
120573119899119865ΔΦ
119877119879
)
larr997888
V119890=
larr997888
119896
119888119888
119872+exp(
(1 minus 120573) 119899119865ΔΦ
119877119879
)
(6)
where 119888
119872is concentration of 119872 119896 is Boltzmann constant
119879 is absolute temperature of a system ℎ is Planck constantΔΦ is overpotential 119899 is charge transfer valency 120573 is fractionnumber (less than 1)
Multiply faraday constant 119865 for (6) and given exchangecurrent 119868
0= 119865
997888rarr
119896
119888119888
119872= 119865
larr997888
119896
119888119888
119872+ then the metal dissolution and
deposition current can be expressed as (7) and (8) separately
119868 = 119868
0exp(minus
120573119899119865ΔΦ
119877119879
) (7)
119868 = 119868
0exp(
(1 minus 120573) 119899119865ΔΦ
119877119879
) (8)
When external force is applied on a metal which isimmersed in an acidic solution the mechano-electrochem-ical activation free energy equaled the sum of chemicalactivation free energy electrochemical potential and surfacepotential elevated by the stress as shown in (9)
As compared to nonstress condition the external forceonly changes the free energy of solid state 119872 and has noinfluence on deposition process (red solid curve)
Δ
119866
119872
119883
= Δ
119866
119872
119883
minus Δ119875119881
119898= Δ119866
119883
119872+ 120573119865ΔΦ minus Δ119875119881
119898
(9)
x
y
z
120590x
120590y
120590z
Mixed gas pressure
Figure 1 Stress analysis of specimens
The dissolution current under stress condition is
119868 = 119868
0exp(
minus120573119899119865ΔΦ + Δ119875119881
119898
119877119879
) (10)
Stress effect is an influence factor on metal dissolution
119868 =
119868 times exp (
Δ119875119881
119898
119877119879
) (11)
When specimens are immersed into solution sealedwith a high-temperature and high-pressure autoclave [6]regardless of the difference of fluid column pressure in heightliquid pressures on specimen from all directions are equaland unit stress state of specimen is as shown in Figure 1
It is assumed that the pressure medium in autoclave ispressurized to 20MPa byN
2during the experiment and120590
119909=
120590
119910= 120590
119911= 20MPa hydrostatic pressure is
120590
119909+ 120590
119910+ 120590
119911
3
= 20MPa (12)
And given the temperature is 100∘C (37315 K) molar vol-ume of steel is 712 times 10minus6m3mol and gas constant 119877 is8314 Jsdotmolminus1sdotKminus1 according to the discussion in Section 2(11) the influence coefficient of corrosion rate caused bysystem pressure is
exp 119881Δ119875
119877119879
= exp 712 times 10
minus6times 20 times 10
6
8314 times 37315
= 1047
(13)
Under the system pressure corrosion rate increases lessthan 5 However the repeated test error rate of corro-sion simulation experiment is always between 5 and 10sometimes even higher Thus it can be considered thatpressurization to 20MPa by nitrogen has little or no influenceon the corrosion test result
3 Verification Experiments
A series of experiments were conducted to verify themechanical-chemical corrosion theory The materials usedin this study are tubing steels N80 1Cr and 3Cr80 Thechemical compositions of the materials are shown in Table 1The solution used in this study is simulated oil field water
Advances in Materials Science and Engineering 3
Table 1 Chemical compositions of N80 1Cr80 and 3Cr80 (mass fraction wt)
Material C Si Mn P S Cr Mo NiN80 024 022 119 0013 0004 0036 0021 00281Cr80 020 027 042 00089 000082 103 024 0103Cr80 019 032 047 00089 000082 293 039 017
Table 2 Oil field water ions concentration (mgL)
K+Na+ Ca2+ Mg2+ CLminus SO4
2minus HCO3
minus CO3
2minus
8350 3885 57 18975 850 337 0
The pH value is 68 and the ion concentrations are shown inTable 2 CO
2partial pressure is 04MPa and the flow velocity
is 15ms Experiments were conducted in three autoclaves atthe same time and the N
2pressure filling in three autoclaves
is 0MPa 4MPa and 15MPa respectivelyThe equipment forthis study is CWYF-1 high-temperature and high-pressuredynamic corrosion autoclave
The specimens of 50mm in length 10mm in widthand 3mm in thickness were polished with silicon carbidenumber 320 and number 600 papers rinsed with distilledwater degreased in acetone and then weighed after dryingThe specimens were mounted on a specimen holder madeof polytetrafluoroethylene (PTFE) The holder was placedin the autoclave filled with corrosive medium and then theautoclave was closed
The solution was deoxygenated by pure nitrogen for 2hours at 45∘C Then the system was given required partialpressure (the partial pressure of CO
2is 04MPamaintenance
purging CO2for 1 hour until solution in the autoclave gets
saturated and controlled the design pressure by nitrogen)and temperature (90∘C) After 72 hours the specimen sur-faces were washed with distilled water and the corrosionproducts were pickled in 10 hydrochloric acid solutionto get removed and the solution contained an inhibitorto protect the base Then the specimens were rinsed withdistilled water degreased in acetone and dried after that thespecimens were photographed to record the corrosion statusFinally the residual masses of the specimens were weighedon an electronic balance (accuracy 01mg) and the averagecorrosion rate was calculated according to NACE RP0775-2005
As shown in Table 3 under the three pressure conditionsthere is little difference in the corrosion of the same materialSevere pitting corrosion appeared on the N80 specimens but1Cr80 and 3Cr80 suffered general corrosion As shown inFigure 2 the corrosion rates of N80 under three pressureconditions are basically consistent 1Cr80 and 3Cr80 aresimilar to N80 and the corrosion rate did not increase withsystempressureThus it can be seen thatwhen theCO
2partial
pressure is constant (ie the content of CO2in the solution is
constant) the system pressure (ie hydrostatic pressure) haslittle impact on the corrosion rate and corrosionmorphology
According to the results of indoor simulation experi-ments andmechanical-chemical corrosion theory additionalN2did not have obvious impact on the coupons surface in the
samematerial and the general corrosion rates aremuch closer
30
20
10
00C
orro
sion
rate
(mm
y)
N80
CO2 = 04MPaCO2 = 04MPa N2 = 40MPaCO2 = 04MPa N2 = 150MPa
15620
14496
15410 20383
20917
19025
15863
15546
14907
1Cr80 3Cr80
Figure 2 Corrosion rates contrast under different total pressures
for each material Thus adding N2can be omitted during
the simulation experimentWithout adding highN2pressure
the test result is still reliable enough Skip this process canreduce the experiment cost and make it more efficient Whatis more important is that skipping this high-pressure processcan avoid potential security threats to the researchers
4 Stress Analysis in AnticorrosionDesign for Downhole String
Equation (13) can be regarded as an influence coefficientof corrosion process Under different temperature the rela-tionship between influence coefficient and stress is shown inFigure 3
Figure 3 demonstrates that stress influence coefficientof corrosion is very small under the low stress conditionswhen stress is greater than 100MPa the impact of stress oncorrosion is gradually increased and becomes nonignorableIn addition the promotion of the stress in low temperaturecorrosion is greater than that in high-temperature corrosionThis needs to be stressed that corrosion rate is different fromcoefficient of corrosion but the corrosion has positive corre-lation with coefficient of corrosion and a higher coefficientwill induce a higher corrosion rate
Under the internal fluid pressure formation (or cement)external extrusion pressure axial force and bending the wallstress of downhole string often can reach a much higher
4 Advances in Materials Science and Engineering
Table 3 Appearance of specimen and average corrosion rate
Material Photos of specimen Corrosion System pressure Corrosion ratemma
N80
Pitting 04MPa0MPa 15620
Pitting 04MPa4MPa 14496
Pitting 04MPa15MPa 15410
1Cr
Generalcorrosion 04MPa0MPa 20383
Generalcorrosion 04MPa4MPa 20917
Generalcorrosion 04MPa15MPa 19025
3Cr
Generalcorrosion 04MPa0MPa 15863
Generalcorrosion 04MPa4MPa 15546
Generalcorrosion 04MPa15MPa 14907
Stress (MPa)6005004003002001000
Coe
ffici
ent o
f cor
rosio
n 4
3
2
1
0
60∘C
100∘C
150∘C
Figure 3 The relationship between corrosion coefficient and aver-age normal stress
stress level (greater than 100Mpa) In the process of downholetubing anti-corrosion design the hoop stress on the casingwall under the internal pressure has obvious promotingeffect on corrosion rate For the larger diameter-thickness
ratio pipe circumferential stress on the wall under internalpressure can be calculated by [7]
120590 =
pr119900
119905
(14)
It is assumed that casing outer diameter is 9-5810158401015840 wallthickness is 1199mm and inner pressure is 20MPa Thewall stress calculated by above equation is 205MPa (elasticmodulus is 208GPa and Poissonrsquos ratio is 028)
According to the curve in Figure 3 when the stress is200MPa influence coefficient of corrosion rate is about 15which will have great influence on the corrosion
Another more serious situation is stress concentrationaround pits in internal surface of downhole tubingThe finiteelement model of two spherical pits is shown in Figure 4
Model parameters are as follows outer diameter2445mm (9-5810158401015840) wall thickness 1199mm casing gradeN80 elastic modulus 208Gpa Poissonrsquos ratio 028 hemi-sphere pits diameter 80mm (119877 = 40mm) pit cavity depth40mm
Advances in Materials Science and Engineering 5
x
y
z
Figure 4 Finite element models of two spherical pits in casing internal surface
+3653e + 02
+3356e + 02
+3058e + 02
+2761e + 02
+2464e + 02
+2167e + 02
+1869e + 02
+1572e + 02
+1275e + 02
+9775e + 01
+6802e + 01
+3830e + 01
+8570e + 00
Figure 5 Cross section stress distribution of double cavities (119871119877 =
22)
When the ratios of 119871119877 (119871 is the distance between twohemisphere centers) are equal to 22 and 10 respectively(center distance between the two pits and radius) andinternal pressure is 10MPa the stress distribution around thepits is shown in Figures 5 and 6
Tensile stress appears at the intersecting line of theconcave of pit and 119883-119885 plane and the maximum stress is inthe double pits intersection Figures 6 and 7 are the profilesalong the axis of casing and through the center of pit
When 119871119877 = 22 (Figure 5) the stress concentrationfactor is 365MPa100MPa = 365 when 119871119877 = 10 (Figure 6)the stress concentration factor is 292MPa100MPa = 292
When 119871119877 = 22 as shown in Figure 7 stress at the outeredge of pit is 209MPa stress at the inner edge of pit is365MPa which is maximum bottom stress is 197MPa thedistance between the inner edges of the two pits is 08mmand midpoint stress at the inner edge of double pits is325MPa When 119871119877 = 10 as shown in Figure 8 the stress
+2918e + 02
+2687e + 02
+2455e + 02
224e+2 + 02
+1993e + 02
+1762e + 02
+1530e + 02
1299e+ + 02
+1068e + 02
+8368e + 01
+6056e + 01
744e+3 + 01
432e+1 + 01
Figure 6 Cross section stress distribution of single cavity (119871119877 =
10)
0
0
2 4 6 8 10 12 14 16 18 20
Distance (mm)
Stre
ss (M
Pa)
400
350
300
250
200
150
100
50
Figure 7 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 22
at the outer edge of pit is 214MPa stress at the inner edge ofpit is 292MPa which is maximum bottom stress is 203MPatwo-pit-intersect and the maximum stress at the intersectionis 292MPa
6 Advances in Materials Science and Engineering
0
0
2 4 6 8 10 12 14
Distance (mm)
Stre
ss (M
Pa)
350
300
250
200
150
100
50
Figure 8 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 10
According to mechano-chemical corrosion theory stressinfluence coefficient can reach 25 Therefore when selectinganti-corrosion materials for downhole string and evaluatingservice life of the tubing string for the corrosive gas fields it isnecessary to consider wall stress influence on electrochemicalcorrosion
5 Conclusion
(1) If the stress is not very high (less than 50MPa) theinfluence of stress on corrosion process is negligible
(2) The influence of system pressure on corrosion ratecan be ignored and the procedure of adding inert gas(eg Nitrogen) pressure can be skipped In this waythe experimental efficiency will be increased and thesecurity threats from the high pressure autoclave canbe avoided
(3) Downhole tubing hoop stress generated due to theformation compress or liquid column pressure isremarkable therefore stress effect has to be takeninto consideration Especially when pit corrosionoccurred on the internal tubing surface the stressconcentration effect will raise the local stress to amuch higher value
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
The research work was supported by China University ofpetroleum (Beijing) Scientific Research Fund Project (YJRC-2013-19)
References
[1] W Yan J Deng X Dong et al ldquoInvestigation of calculationmethod of CO
2partial pressure in oil and gas wellrdquo Drilling amp
Production Technology vol 34 no 5 pp 19ndash22 2011 (Chinese)[2] E M Gutman Mechanochemistry of Materials Cambridge
International Science 1998
[3] T G Movchan N E Esipova P V Eryukin N B Uriev and AI Rusanov ldquoMechanochemical effects in processes of corrosionof metalsrdquo Russian Journal of General Chemistry vol 75 no 11pp 1681ndash1686 2005
[4] X F Liu J Zhan andQ J Liu ldquoThe influence of tensile stress onelectrochemical noise from aluminum alloy in chloride mediardquoCorrosion Science vol 51 no 6 pp 1460ndash1466 2009
[5] S Rao L Zhu D Li Z Zhang and Q Zhong ldquoEffects ofmechanochemistry to the pitting behaviour of LY12CZ alu-minum alloyrdquo Journal of the Chinese Society of Corrosion andProtection vol 27 no 4 pp 228ndash232 2007
[6] W Yan and J Deng ldquoInvestigation of mechanochemical corro-sion of downhole tubing and casingrdquo China Offshore Oil andGas vol 26 no 1-2 pp 87ndash91 2014
[7] H Liu Material Mechanics Higher Education Press BeijingChina 2006
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
Advances in Materials Science and Engineering 3
Table 1 Chemical compositions of N80 1Cr80 and 3Cr80 (mass fraction wt)
Material C Si Mn P S Cr Mo NiN80 024 022 119 0013 0004 0036 0021 00281Cr80 020 027 042 00089 000082 103 024 0103Cr80 019 032 047 00089 000082 293 039 017
Table 2 Oil field water ions concentration (mgL)
K+Na+ Ca2+ Mg2+ CLminus SO4
2minus HCO3
minus CO3
2minus
8350 3885 57 18975 850 337 0
The pH value is 68 and the ion concentrations are shown inTable 2 CO
2partial pressure is 04MPa and the flow velocity
is 15ms Experiments were conducted in three autoclaves atthe same time and the N
2pressure filling in three autoclaves
is 0MPa 4MPa and 15MPa respectivelyThe equipment forthis study is CWYF-1 high-temperature and high-pressuredynamic corrosion autoclave
The specimens of 50mm in length 10mm in widthand 3mm in thickness were polished with silicon carbidenumber 320 and number 600 papers rinsed with distilledwater degreased in acetone and then weighed after dryingThe specimens were mounted on a specimen holder madeof polytetrafluoroethylene (PTFE) The holder was placedin the autoclave filled with corrosive medium and then theautoclave was closed
The solution was deoxygenated by pure nitrogen for 2hours at 45∘C Then the system was given required partialpressure (the partial pressure of CO
2is 04MPamaintenance
purging CO2for 1 hour until solution in the autoclave gets
saturated and controlled the design pressure by nitrogen)and temperature (90∘C) After 72 hours the specimen sur-faces were washed with distilled water and the corrosionproducts were pickled in 10 hydrochloric acid solutionto get removed and the solution contained an inhibitorto protect the base Then the specimens were rinsed withdistilled water degreased in acetone and dried after that thespecimens were photographed to record the corrosion statusFinally the residual masses of the specimens were weighedon an electronic balance (accuracy 01mg) and the averagecorrosion rate was calculated according to NACE RP0775-2005
As shown in Table 3 under the three pressure conditionsthere is little difference in the corrosion of the same materialSevere pitting corrosion appeared on the N80 specimens but1Cr80 and 3Cr80 suffered general corrosion As shown inFigure 2 the corrosion rates of N80 under three pressureconditions are basically consistent 1Cr80 and 3Cr80 aresimilar to N80 and the corrosion rate did not increase withsystempressureThus it can be seen thatwhen theCO
2partial
pressure is constant (ie the content of CO2in the solution is
constant) the system pressure (ie hydrostatic pressure) haslittle impact on the corrosion rate and corrosionmorphology
According to the results of indoor simulation experi-ments andmechanical-chemical corrosion theory additionalN2did not have obvious impact on the coupons surface in the
samematerial and the general corrosion rates aremuch closer
30
20
10
00C
orro
sion
rate
(mm
y)
N80
CO2 = 04MPaCO2 = 04MPa N2 = 40MPaCO2 = 04MPa N2 = 150MPa
15620
14496
15410 20383
20917
19025
15863
15546
14907
1Cr80 3Cr80
Figure 2 Corrosion rates contrast under different total pressures
for each material Thus adding N2can be omitted during
the simulation experimentWithout adding highN2pressure
the test result is still reliable enough Skip this process canreduce the experiment cost and make it more efficient Whatis more important is that skipping this high-pressure processcan avoid potential security threats to the researchers
4 Stress Analysis in AnticorrosionDesign for Downhole String
Equation (13) can be regarded as an influence coefficientof corrosion process Under different temperature the rela-tionship between influence coefficient and stress is shown inFigure 3
Figure 3 demonstrates that stress influence coefficientof corrosion is very small under the low stress conditionswhen stress is greater than 100MPa the impact of stress oncorrosion is gradually increased and becomes nonignorableIn addition the promotion of the stress in low temperaturecorrosion is greater than that in high-temperature corrosionThis needs to be stressed that corrosion rate is different fromcoefficient of corrosion but the corrosion has positive corre-lation with coefficient of corrosion and a higher coefficientwill induce a higher corrosion rate
Under the internal fluid pressure formation (or cement)external extrusion pressure axial force and bending the wallstress of downhole string often can reach a much higher
4 Advances in Materials Science and Engineering
Table 3 Appearance of specimen and average corrosion rate
Material Photos of specimen Corrosion System pressure Corrosion ratemma
N80
Pitting 04MPa0MPa 15620
Pitting 04MPa4MPa 14496
Pitting 04MPa15MPa 15410
1Cr
Generalcorrosion 04MPa0MPa 20383
Generalcorrosion 04MPa4MPa 20917
Generalcorrosion 04MPa15MPa 19025
3Cr
Generalcorrosion 04MPa0MPa 15863
Generalcorrosion 04MPa4MPa 15546
Generalcorrosion 04MPa15MPa 14907
Stress (MPa)6005004003002001000
Coe
ffici
ent o
f cor
rosio
n 4
3
2
1
0
60∘C
100∘C
150∘C
Figure 3 The relationship between corrosion coefficient and aver-age normal stress
stress level (greater than 100Mpa) In the process of downholetubing anti-corrosion design the hoop stress on the casingwall under the internal pressure has obvious promotingeffect on corrosion rate For the larger diameter-thickness
ratio pipe circumferential stress on the wall under internalpressure can be calculated by [7]
120590 =
pr119900
119905
(14)
It is assumed that casing outer diameter is 9-5810158401015840 wallthickness is 1199mm and inner pressure is 20MPa Thewall stress calculated by above equation is 205MPa (elasticmodulus is 208GPa and Poissonrsquos ratio is 028)
According to the curve in Figure 3 when the stress is200MPa influence coefficient of corrosion rate is about 15which will have great influence on the corrosion
Another more serious situation is stress concentrationaround pits in internal surface of downhole tubingThe finiteelement model of two spherical pits is shown in Figure 4
Model parameters are as follows outer diameter2445mm (9-5810158401015840) wall thickness 1199mm casing gradeN80 elastic modulus 208Gpa Poissonrsquos ratio 028 hemi-sphere pits diameter 80mm (119877 = 40mm) pit cavity depth40mm
Advances in Materials Science and Engineering 5
x
y
z
Figure 4 Finite element models of two spherical pits in casing internal surface
+3653e + 02
+3356e + 02
+3058e + 02
+2761e + 02
+2464e + 02
+2167e + 02
+1869e + 02
+1572e + 02
+1275e + 02
+9775e + 01
+6802e + 01
+3830e + 01
+8570e + 00
Figure 5 Cross section stress distribution of double cavities (119871119877 =
22)
When the ratios of 119871119877 (119871 is the distance between twohemisphere centers) are equal to 22 and 10 respectively(center distance between the two pits and radius) andinternal pressure is 10MPa the stress distribution around thepits is shown in Figures 5 and 6
Tensile stress appears at the intersecting line of theconcave of pit and 119883-119885 plane and the maximum stress is inthe double pits intersection Figures 6 and 7 are the profilesalong the axis of casing and through the center of pit
When 119871119877 = 22 (Figure 5) the stress concentrationfactor is 365MPa100MPa = 365 when 119871119877 = 10 (Figure 6)the stress concentration factor is 292MPa100MPa = 292
When 119871119877 = 22 as shown in Figure 7 stress at the outeredge of pit is 209MPa stress at the inner edge of pit is365MPa which is maximum bottom stress is 197MPa thedistance between the inner edges of the two pits is 08mmand midpoint stress at the inner edge of double pits is325MPa When 119871119877 = 10 as shown in Figure 8 the stress
+2918e + 02
+2687e + 02
+2455e + 02
224e+2 + 02
+1993e + 02
+1762e + 02
+1530e + 02
1299e+ + 02
+1068e + 02
+8368e + 01
+6056e + 01
744e+3 + 01
432e+1 + 01
Figure 6 Cross section stress distribution of single cavity (119871119877 =
10)
0
0
2 4 6 8 10 12 14 16 18 20
Distance (mm)
Stre
ss (M
Pa)
400
350
300
250
200
150
100
50
Figure 7 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 22
at the outer edge of pit is 214MPa stress at the inner edge ofpit is 292MPa which is maximum bottom stress is 203MPatwo-pit-intersect and the maximum stress at the intersectionis 292MPa
6 Advances in Materials Science and Engineering
0
0
2 4 6 8 10 12 14
Distance (mm)
Stre
ss (M
Pa)
350
300
250
200
150
100
50
Figure 8 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 10
According to mechano-chemical corrosion theory stressinfluence coefficient can reach 25 Therefore when selectinganti-corrosion materials for downhole string and evaluatingservice life of the tubing string for the corrosive gas fields it isnecessary to consider wall stress influence on electrochemicalcorrosion
5 Conclusion
(1) If the stress is not very high (less than 50MPa) theinfluence of stress on corrosion process is negligible
(2) The influence of system pressure on corrosion ratecan be ignored and the procedure of adding inert gas(eg Nitrogen) pressure can be skipped In this waythe experimental efficiency will be increased and thesecurity threats from the high pressure autoclave canbe avoided
(3) Downhole tubing hoop stress generated due to theformation compress or liquid column pressure isremarkable therefore stress effect has to be takeninto consideration Especially when pit corrosionoccurred on the internal tubing surface the stressconcentration effect will raise the local stress to amuch higher value
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
The research work was supported by China University ofpetroleum (Beijing) Scientific Research Fund Project (YJRC-2013-19)
References
[1] W Yan J Deng X Dong et al ldquoInvestigation of calculationmethod of CO
2partial pressure in oil and gas wellrdquo Drilling amp
Production Technology vol 34 no 5 pp 19ndash22 2011 (Chinese)[2] E M Gutman Mechanochemistry of Materials Cambridge
International Science 1998
[3] T G Movchan N E Esipova P V Eryukin N B Uriev and AI Rusanov ldquoMechanochemical effects in processes of corrosionof metalsrdquo Russian Journal of General Chemistry vol 75 no 11pp 1681ndash1686 2005
[4] X F Liu J Zhan andQ J Liu ldquoThe influence of tensile stress onelectrochemical noise from aluminum alloy in chloride mediardquoCorrosion Science vol 51 no 6 pp 1460ndash1466 2009
[5] S Rao L Zhu D Li Z Zhang and Q Zhong ldquoEffects ofmechanochemistry to the pitting behaviour of LY12CZ alu-minum alloyrdquo Journal of the Chinese Society of Corrosion andProtection vol 27 no 4 pp 228ndash232 2007
[6] W Yan and J Deng ldquoInvestigation of mechanochemical corro-sion of downhole tubing and casingrdquo China Offshore Oil andGas vol 26 no 1-2 pp 87ndash91 2014
[7] H Liu Material Mechanics Higher Education Press BeijingChina 2006
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
4 Advances in Materials Science and Engineering
Table 3 Appearance of specimen and average corrosion rate
Material Photos of specimen Corrosion System pressure Corrosion ratemma
N80
Pitting 04MPa0MPa 15620
Pitting 04MPa4MPa 14496
Pitting 04MPa15MPa 15410
1Cr
Generalcorrosion 04MPa0MPa 20383
Generalcorrosion 04MPa4MPa 20917
Generalcorrosion 04MPa15MPa 19025
3Cr
Generalcorrosion 04MPa0MPa 15863
Generalcorrosion 04MPa4MPa 15546
Generalcorrosion 04MPa15MPa 14907
Stress (MPa)6005004003002001000
Coe
ffici
ent o
f cor
rosio
n 4
3
2
1
0
60∘C
100∘C
150∘C
Figure 3 The relationship between corrosion coefficient and aver-age normal stress
stress level (greater than 100Mpa) In the process of downholetubing anti-corrosion design the hoop stress on the casingwall under the internal pressure has obvious promotingeffect on corrosion rate For the larger diameter-thickness
ratio pipe circumferential stress on the wall under internalpressure can be calculated by [7]
120590 =
pr119900
119905
(14)
It is assumed that casing outer diameter is 9-5810158401015840 wallthickness is 1199mm and inner pressure is 20MPa Thewall stress calculated by above equation is 205MPa (elasticmodulus is 208GPa and Poissonrsquos ratio is 028)
According to the curve in Figure 3 when the stress is200MPa influence coefficient of corrosion rate is about 15which will have great influence on the corrosion
Another more serious situation is stress concentrationaround pits in internal surface of downhole tubingThe finiteelement model of two spherical pits is shown in Figure 4
Model parameters are as follows outer diameter2445mm (9-5810158401015840) wall thickness 1199mm casing gradeN80 elastic modulus 208Gpa Poissonrsquos ratio 028 hemi-sphere pits diameter 80mm (119877 = 40mm) pit cavity depth40mm
Advances in Materials Science and Engineering 5
x
y
z
Figure 4 Finite element models of two spherical pits in casing internal surface
+3653e + 02
+3356e + 02
+3058e + 02
+2761e + 02
+2464e + 02
+2167e + 02
+1869e + 02
+1572e + 02
+1275e + 02
+9775e + 01
+6802e + 01
+3830e + 01
+8570e + 00
Figure 5 Cross section stress distribution of double cavities (119871119877 =
22)
When the ratios of 119871119877 (119871 is the distance between twohemisphere centers) are equal to 22 and 10 respectively(center distance between the two pits and radius) andinternal pressure is 10MPa the stress distribution around thepits is shown in Figures 5 and 6
Tensile stress appears at the intersecting line of theconcave of pit and 119883-119885 plane and the maximum stress is inthe double pits intersection Figures 6 and 7 are the profilesalong the axis of casing and through the center of pit
When 119871119877 = 22 (Figure 5) the stress concentrationfactor is 365MPa100MPa = 365 when 119871119877 = 10 (Figure 6)the stress concentration factor is 292MPa100MPa = 292
When 119871119877 = 22 as shown in Figure 7 stress at the outeredge of pit is 209MPa stress at the inner edge of pit is365MPa which is maximum bottom stress is 197MPa thedistance between the inner edges of the two pits is 08mmand midpoint stress at the inner edge of double pits is325MPa When 119871119877 = 10 as shown in Figure 8 the stress
+2918e + 02
+2687e + 02
+2455e + 02
224e+2 + 02
+1993e + 02
+1762e + 02
+1530e + 02
1299e+ + 02
+1068e + 02
+8368e + 01
+6056e + 01
744e+3 + 01
432e+1 + 01
Figure 6 Cross section stress distribution of single cavity (119871119877 =
10)
0
0
2 4 6 8 10 12 14 16 18 20
Distance (mm)
Stre
ss (M
Pa)
400
350
300
250
200
150
100
50
Figure 7 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 22
at the outer edge of pit is 214MPa stress at the inner edge ofpit is 292MPa which is maximum bottom stress is 203MPatwo-pit-intersect and the maximum stress at the intersectionis 292MPa
6 Advances in Materials Science and Engineering
0
0
2 4 6 8 10 12 14
Distance (mm)
Stre
ss (M
Pa)
350
300
250
200
150
100
50
Figure 8 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 10
According to mechano-chemical corrosion theory stressinfluence coefficient can reach 25 Therefore when selectinganti-corrosion materials for downhole string and evaluatingservice life of the tubing string for the corrosive gas fields it isnecessary to consider wall stress influence on electrochemicalcorrosion
5 Conclusion
(1) If the stress is not very high (less than 50MPa) theinfluence of stress on corrosion process is negligible
(2) The influence of system pressure on corrosion ratecan be ignored and the procedure of adding inert gas(eg Nitrogen) pressure can be skipped In this waythe experimental efficiency will be increased and thesecurity threats from the high pressure autoclave canbe avoided
(3) Downhole tubing hoop stress generated due to theformation compress or liquid column pressure isremarkable therefore stress effect has to be takeninto consideration Especially when pit corrosionoccurred on the internal tubing surface the stressconcentration effect will raise the local stress to amuch higher value
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
The research work was supported by China University ofpetroleum (Beijing) Scientific Research Fund Project (YJRC-2013-19)
References
[1] W Yan J Deng X Dong et al ldquoInvestigation of calculationmethod of CO
2partial pressure in oil and gas wellrdquo Drilling amp
Production Technology vol 34 no 5 pp 19ndash22 2011 (Chinese)[2] E M Gutman Mechanochemistry of Materials Cambridge
International Science 1998
[3] T G Movchan N E Esipova P V Eryukin N B Uriev and AI Rusanov ldquoMechanochemical effects in processes of corrosionof metalsrdquo Russian Journal of General Chemistry vol 75 no 11pp 1681ndash1686 2005
[4] X F Liu J Zhan andQ J Liu ldquoThe influence of tensile stress onelectrochemical noise from aluminum alloy in chloride mediardquoCorrosion Science vol 51 no 6 pp 1460ndash1466 2009
[5] S Rao L Zhu D Li Z Zhang and Q Zhong ldquoEffects ofmechanochemistry to the pitting behaviour of LY12CZ alu-minum alloyrdquo Journal of the Chinese Society of Corrosion andProtection vol 27 no 4 pp 228ndash232 2007
[6] W Yan and J Deng ldquoInvestigation of mechanochemical corro-sion of downhole tubing and casingrdquo China Offshore Oil andGas vol 26 no 1-2 pp 87ndash91 2014
[7] H Liu Material Mechanics Higher Education Press BeijingChina 2006
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
Advances in Materials Science and Engineering 5
x
y
z
Figure 4 Finite element models of two spherical pits in casing internal surface
+3653e + 02
+3356e + 02
+3058e + 02
+2761e + 02
+2464e + 02
+2167e + 02
+1869e + 02
+1572e + 02
+1275e + 02
+9775e + 01
+6802e + 01
+3830e + 01
+8570e + 00
Figure 5 Cross section stress distribution of double cavities (119871119877 =
22)
When the ratios of 119871119877 (119871 is the distance between twohemisphere centers) are equal to 22 and 10 respectively(center distance between the two pits and radius) andinternal pressure is 10MPa the stress distribution around thepits is shown in Figures 5 and 6
Tensile stress appears at the intersecting line of theconcave of pit and 119883-119885 plane and the maximum stress is inthe double pits intersection Figures 6 and 7 are the profilesalong the axis of casing and through the center of pit
When 119871119877 = 22 (Figure 5) the stress concentrationfactor is 365MPa100MPa = 365 when 119871119877 = 10 (Figure 6)the stress concentration factor is 292MPa100MPa = 292
When 119871119877 = 22 as shown in Figure 7 stress at the outeredge of pit is 209MPa stress at the inner edge of pit is365MPa which is maximum bottom stress is 197MPa thedistance between the inner edges of the two pits is 08mmand midpoint stress at the inner edge of double pits is325MPa When 119871119877 = 10 as shown in Figure 8 the stress
+2918e + 02
+2687e + 02
+2455e + 02
224e+2 + 02
+1993e + 02
+1762e + 02
+1530e + 02
1299e+ + 02
+1068e + 02
+8368e + 01
+6056e + 01
744e+3 + 01
432e+1 + 01
Figure 6 Cross section stress distribution of single cavity (119871119877 =
10)
0
0
2 4 6 8 10 12 14 16 18 20
Distance (mm)
Stre
ss (M
Pa)
400
350
300
250
200
150
100
50
Figure 7 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 22
at the outer edge of pit is 214MPa stress at the inner edge ofpit is 292MPa which is maximum bottom stress is 203MPatwo-pit-intersect and the maximum stress at the intersectionis 292MPa
6 Advances in Materials Science and Engineering
0
0
2 4 6 8 10 12 14
Distance (mm)
Stre
ss (M
Pa)
350
300
250
200
150
100
50
Figure 8 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 10
According to mechano-chemical corrosion theory stressinfluence coefficient can reach 25 Therefore when selectinganti-corrosion materials for downhole string and evaluatingservice life of the tubing string for the corrosive gas fields it isnecessary to consider wall stress influence on electrochemicalcorrosion
5 Conclusion
(1) If the stress is not very high (less than 50MPa) theinfluence of stress on corrosion process is negligible
(2) The influence of system pressure on corrosion ratecan be ignored and the procedure of adding inert gas(eg Nitrogen) pressure can be skipped In this waythe experimental efficiency will be increased and thesecurity threats from the high pressure autoclave canbe avoided
(3) Downhole tubing hoop stress generated due to theformation compress or liquid column pressure isremarkable therefore stress effect has to be takeninto consideration Especially when pit corrosionoccurred on the internal tubing surface the stressconcentration effect will raise the local stress to amuch higher value
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
The research work was supported by China University ofpetroleum (Beijing) Scientific Research Fund Project (YJRC-2013-19)
References
[1] W Yan J Deng X Dong et al ldquoInvestigation of calculationmethod of CO
2partial pressure in oil and gas wellrdquo Drilling amp
Production Technology vol 34 no 5 pp 19ndash22 2011 (Chinese)[2] E M Gutman Mechanochemistry of Materials Cambridge
International Science 1998
[3] T G Movchan N E Esipova P V Eryukin N B Uriev and AI Rusanov ldquoMechanochemical effects in processes of corrosionof metalsrdquo Russian Journal of General Chemistry vol 75 no 11pp 1681ndash1686 2005
[4] X F Liu J Zhan andQ J Liu ldquoThe influence of tensile stress onelectrochemical noise from aluminum alloy in chloride mediardquoCorrosion Science vol 51 no 6 pp 1460ndash1466 2009
[5] S Rao L Zhu D Li Z Zhang and Q Zhong ldquoEffects ofmechanochemistry to the pitting behaviour of LY12CZ alu-minum alloyrdquo Journal of the Chinese Society of Corrosion andProtection vol 27 no 4 pp 228ndash232 2007
[6] W Yan and J Deng ldquoInvestigation of mechanochemical corro-sion of downhole tubing and casingrdquo China Offshore Oil andGas vol 26 no 1-2 pp 87ndash91 2014
[7] H Liu Material Mechanics Higher Education Press BeijingChina 2006
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
6 Advances in Materials Science and Engineering
0
0
2 4 6 8 10 12 14
Distance (mm)
Stre
ss (M
Pa)
350
300
250
200
150
100
50
Figure 8 The stress of node on the intersecting lines between 119883-119885plane and cavity 119871119877 = 10
According to mechano-chemical corrosion theory stressinfluence coefficient can reach 25 Therefore when selectinganti-corrosion materials for downhole string and evaluatingservice life of the tubing string for the corrosive gas fields it isnecessary to consider wall stress influence on electrochemicalcorrosion
5 Conclusion
(1) If the stress is not very high (less than 50MPa) theinfluence of stress on corrosion process is negligible
(2) The influence of system pressure on corrosion ratecan be ignored and the procedure of adding inert gas(eg Nitrogen) pressure can be skipped In this waythe experimental efficiency will be increased and thesecurity threats from the high pressure autoclave canbe avoided
(3) Downhole tubing hoop stress generated due to theformation compress or liquid column pressure isremarkable therefore stress effect has to be takeninto consideration Especially when pit corrosionoccurred on the internal tubing surface the stressconcentration effect will raise the local stress to amuch higher value
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
The research work was supported by China University ofpetroleum (Beijing) Scientific Research Fund Project (YJRC-2013-19)
References
[1] W Yan J Deng X Dong et al ldquoInvestigation of calculationmethod of CO
2partial pressure in oil and gas wellrdquo Drilling amp
Production Technology vol 34 no 5 pp 19ndash22 2011 (Chinese)[2] E M Gutman Mechanochemistry of Materials Cambridge
International Science 1998
[3] T G Movchan N E Esipova P V Eryukin N B Uriev and AI Rusanov ldquoMechanochemical effects in processes of corrosionof metalsrdquo Russian Journal of General Chemistry vol 75 no 11pp 1681ndash1686 2005
[4] X F Liu J Zhan andQ J Liu ldquoThe influence of tensile stress onelectrochemical noise from aluminum alloy in chloride mediardquoCorrosion Science vol 51 no 6 pp 1460ndash1466 2009
[5] S Rao L Zhu D Li Z Zhang and Q Zhong ldquoEffects ofmechanochemistry to the pitting behaviour of LY12CZ alu-minum alloyrdquo Journal of the Chinese Society of Corrosion andProtection vol 27 no 4 pp 228ndash232 2007
[6] W Yan and J Deng ldquoInvestigation of mechanochemical corro-sion of downhole tubing and casingrdquo China Offshore Oil andGas vol 26 no 1-2 pp 87ndash91 2014
[7] H Liu Material Mechanics Higher Education Press BeijingChina 2006
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
