Presentation to the Royal Society of Chemistry's "Chemicals in the Oilfield" conference, November 2005 The paper describes how formate brines protect steels against CO2 corrosion. It also shows the results of stress corrosion cracking tests on CRA steel samples exposed to high-density completion brines containing oxygen at 160 deg C. The 13Cr, 22Cr and 25Cr steels all cracked in the presence of calcium bromide brine containing oxygen.

1. Inhibition of Carbon Dioxide
Corrosion by Formate Brines in High
Temperature Environments
John D. Downs, Alan Carnegie and William Benton,
Cabot Specialty Fluids Ltd
Håkon Leth-Olsen,
Norsk Hydro ASA, Corporate Research Centre
Chemistry in the Oil Industry IX, EOSCA,
Royal Society of Chemistry
Manchester 1th November 2005

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Gas-injection tubing
with NaCl completion
fluid. CO2 leakage
Chlorine SCC
of AISI 316L pipe
What worries a corrosion engineer
Localised corrosion Stress Corrosion Cracking (SCC)

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Outline
 General corrosivity of halide and formate brines
 CO2 corrosion test in K/Cs-formate and CaBr2
 SCC tests in K/Cs-formate and CaBr2 brines with O2 influx
 Summary

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Corrosivity in halide brines
 Halide well brines:
 NaCl Max SG: 1.2 NaCl-NaBr Max SG: 1.5
 CaCl2 Max SG: 1.39 CaBr2 Max SG: 1.70
 CaCl2–CaBr2 Max SG: 1.8 ZnBr2 Max SG: 2.3
 Mixes
 NaCl, CaCl2, CaBr2 basically non-corrosive
 Neutral salts
 Not oxidising
 Disadvantageous:
 ZnBr2 : Acid salt. Corrosive!
 SCC of CRAs in chloride brines if oxygen or CO2 influx
 Localised corrosion in CaBr2:
– If acidified with acid gas? (no buffer effect)
– If oxygen influx?

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Corrosivity in formate brines (without CO2 influx)
 Three basic formate well brines:
 Na formate: HCOONa Max SG: 1.3
 K formate: HCOOK Max SG: 1.57
 Cs formate: HCOOCs Max SG: 2.3
 Mixes of the three
 Basically low corrosivity against all relevant well materials
 Not oxidising
 High pH (9-10)
 Low halide contents (< 3000 ppm)
 Highly buffered (soluble carbonate and bicarbonate)
– Maintains a high pH - usually not acidified with acid gas
 No reported failures in the field due to corrosion in formate brines

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Concerns with formate brines
 Corrosivity if acidified due to influx of large amounts of acid gass
 Not likely under drilling and short term use
 Theoretical possibility in packer fluid
 SCC of CRAs if formate brine contaminated with chloride?
 With O2 influx?
 With CO2 influx?
 Concerns addressed in recent and ongoing test programs

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Relevant materials
Material Specification Cr (%) Ni (%) Mo (%) Mn (%) C (%)
Other
(%)
C-steel ST-37-2 - - - - < 0.20 -
13Cr L80 13.0 0.4 0.09 1.67
Co: 0.8
V: 0.38
S13Cr
(Hypo II)
NT-CRSS-
110
12.2 5.73 2.1 0.44 0.02
Cu: 1.4
Si: 0.3
22Cr 22.0 5.0 3.2 - - N : 0.18
25Cr 25.0 7.0 4.0 - - N: 0.3

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CO2 corrosion test in K-formate [1]
[1] H. Leth-Olsen, SPE 95072, Aberdeen , UK, 13 May 2005
C-steel Super 13%CrStandard 13%Cr
1.53 SG
K-formate
10 bar CO2
120 – 180°C
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 200 400 600 800 1000 1200
Time (hours)
Corrosionrate(mm/y)
0
50
100
150
200
Corrosionrate(mpy)
C-steel 13% Cr
120°C
140°C
160°C
180°C
150°C
120°C
Carbon steel
Iron carbonate

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Results from earlier tests in CaBr2 [1]
[1] H. Leth-Olsen, SPE 95072, Aberdeen , UK, 13 May 2005
C-steel Standard 13%Cr Super 13%Cr
LPR corrosion rates for C-Steel in CaBr2 , 10 bar CO2
0
0.5
1
1.5
2
0 200 400 600 800 1000 1200 1400 1600
Time (hours)
Corrosionrate
(mm/year)
0
10
20
30
40
50
60
70
80
Uninhibited
Inhibited
120
o
C
140
o
C
160
o
C
180
o
C
150
o
C
120
o
C
Localised
corrosion
1.53 SG CaBr2
10 bar CO2
120 – 180°C

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CO2 corrosion tests - experimental
 Test solutions:
 K/Cs formate, 1.7 SG, fully buffered, pH 10.5
 CaBr2, 1.7 SG , pH 5
 Temperatures: 150°C and 175°C
 Exposure times: Until stable LPR values (6 and 4 weeks)
 Materials: C-steel, Standard 13Cr, Super 13Cr, Duplex 22Cr, Duplex 25Cr
 CO2 pressure: 10 bar (autoclaves)

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CO2 corrosion tests - experimental
 Test methods:
 Visual, measurements of local corrosion depths
 Weight losses
 Linear Polarisation Resistance (LPR) – Real-time corr. measurements
 Scanning Electron Microscopy (SEM) of scales
 Severe conditions – simulating the worst-case situation of buffer
overwhelmed .
 Unlikely in service for the buffered formate brines
 More likely for the halide brines (no carbonate buffer)

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Example LPR curve, K/Cs formate, 150°C, 10 bar CO2

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Specimens, K/Cs-formate, 175°C, 10 bar CO2

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Scales formed in K/Cs-formate, 175°C, 10 bar CO2
Super 13CrC-steel

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Specimens, CaBr2, 175°C, 10 bar CO2

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Result Example 2: CaBr2, 175°C, 10 bar CO2
Super 13CrC-steel

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Long-term (LPR) corrosion rates – 150°C

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Localised corrosion – 150°C

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Long-term (LPR) corrosion rates – 175°C

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Localised corrosion – 175°C

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Summary CO2 corrosion tests
 C-steel:
 High initial corrosion rates in both brines followed by build-up of protective scale
 Relatively low long-term rates at 150°C and 175°C in both brines
 Severe localised corrosion in CaBr2 at 175 °C (up to 5 mm/year)
 Standard 13Cr
 Relatively low general corr. rates in both brines– Protective scale
 Localised corrosion in CaBr2 at 150°C
 Super 13Cr
 Increasing general corrosion rates with temperature in K/Cs formate
 Low rates in bromide brine
 Duplex 22Cr and 25Cr
 Low rates in both brines

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Stress Corrosion Cracking tests
 Test solutions:
 K/Cs formate, 1.7 SG, pH 10.5 , 10.000 ppm Cl-
 CaBr2, 1.7 SG , pH 5, no Cl-
 CaBr2, 1.7 SG , pH 5, 10.000 ppm Cl-
 Materials: Super 13Cr, Duplex 22Cr, Duplex 25Cr, AISI 316L (ref.)
 Exposure time: 3 months (inspection every 30 days)
 Temperature: 160°C
 Gas: 0.2 bar 02, 10 bar N2
 Test methods: U-bends and C-rings, pre-stressed to yield

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1 month
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr OK SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC? SCC (1/3)
Duplex 25Cr OK OK OK
AISI 316L OK OK? OK
2 months
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr SCC? (2/3) SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC (3/3) SCC (3/3)
Duplex 25Cr OK SCC (1/3) SCC (1/3)
AISI 316L SCC (1/1) SCC (1/1) SCC (1/1)
3 months
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr SCC (2/3) SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC (3/3) SCC (3/3)
Duplex 25Cr OK SCC (2/3) SCC (2/3)
AISI 316L SCC (1/1) SCC (1/1) SCC (1/1)
SCC results
1 month
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr OK SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC? SCC (1/3)
Duplex 25Cr OK OK OK
AISI 316L OK OK? OK
2 months
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr SCC? (2/3) SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC (3/3) SCC (3/3)
Duplex 25Cr OK SCC (1/3) SCC (1/3)
AISI 316L SCC (1/1) SCC (1/1) SCC (1/1)
3 months
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr SCC (2/3) SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC (3/3) SCC (3/3)
Duplex 25Cr OK SCC (2/3) SCC (2/3)
AISI 316L SCC (1/1) SCC (1/1) SCC (1/1)
1 month
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr OK SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC? SCC (1/3)
Duplex 25Cr OK OK OK
AISI 316L OK OK? OK
2 months
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr SCC? (2/3) SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC (3/3) SCC (3/3)
Duplex 25Cr OK SCC (1/3) SCC (1/3)
AISI 316L SCC (1/1) SCC (1/1) SCC (1/1)
3 months
Formate + Chloride CaBr2 no Chloride CaBr2 + Chloride
Super 13Cr SCC (2/3) SCC (3/3) SCC (3/3)
Duplex 22Cr OK SCC (3/3) SCC (3/3)
Duplex 25Cr OK SCC (2/3) SCC (2/3)
AISI 316L SCC (1/1) SCC (1/1) SCC (1/1)

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SCC results – K/Cs formate
with 10000 ppm Cl-
Super 13Cr, 3 months
Duplex 22Cr
3 months
Duplex 25Cr
3 months

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SCC results – K/Cs formate with 10000 ppm Cl-
Super 13Cr, 3 months

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SCC results – CaBr2 without chlorides
Super 13Cr, 1 month Duplex 22Cr, 2 months Duplex 25Cr, 2 months

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SCC results – CaBr2 without chlorides
Duplex 25Cr, 2 months

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SCC results – CaBr2 with 1000 ppm Cl-
Super 13Cr, 1 month Duplex 22Cr, 2 months Duplex 25Cr, 2 months

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Summary SCC test (160°C, oxygen present)
 K/Cs formate with 10000 ppm chloride:
 SCC initiation of Super 13Cr after 3 months
 SCC of AISI 316L
 No SCC of duplex 22Cr and 25 Cr
 CaBr2 without chlorides:
 Severe cracking of all CRAs (S13Cr, AISI 316L, Duplex 22Cr and Duplex 25Cr)
 CaBr2 with 10000 ppm chlorides:
 Severe cracking of all CRAs (S13Cr, AISI 316L, Duplex 22Cr and Duplex 25Cr)
 Ongoing work:
 K/Cs formate with 3000 ppm chloride, O2, 160°C
 K/Cs formate with 10000 ppm chloride, 10 bar CO2, oxygen free, 160°C
 CaBr2 with 10000 ppm chlorides, 10 bar CO2, oxygen free, 160°C

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Summary
 K/Cs formate with CO2 influx:
 Iron carbonate scale formed on C-steel and Standard 13Cr
 Increased corrosion of Super 13Cr at 175°C (0.4 mm/y)
 Duplex CRAs practically resistant
 K/Cs formate + 10000 ppm Cl-
with O2 influx:
 Signs of SCC cracks of Super 13Cr after 3 months
 No signs of SCC cracking of duplex CRAs
 CaBr2 with CO2 influx:
 Relatively low long term general corrosion rates on all materials
 Localised corrosion on C-steel at 175°C and Standard 13Cr at 150°C
 CaBr2 + 10000 ppm Cl-
with O2 influx:
 Severe SCC cracking of all CRAs

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Acknowledgements
Cabot Speciality Fluids Ltd. is acknowledged for financing the work and
Gro Skåum and Ingmarie Liberger, Hydro Corporate Research Centre, is
acknowledged for performing the experimental work.