11

Fe - CO2 CorrosionFe - CO2 CorrosionRahmad Budi ArmanRahmad Budi ArmanMuhammad PribadiMuhammad Pribadi

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Internal CorrosionInternal Corrosion

The internal corrosion of vessels, equipment and piping in The internal corrosion of vessels, equipment and piping in the offshore / onshore facilities depends on internal process the offshore / onshore facilities depends on internal process fluids and operating conditions. The process fluid may vary fluids and operating conditions. The process fluid may vary from corrosive liquid or gaseous hydrocarbon, corrosive from corrosive liquid or gaseous hydrocarbon, corrosive chemicals, produced water and brine. Based on this, the chemicals, produced water and brine. Based on this, the corrosion occurring under these conditions can be broadly corrosion occurring under these conditions can be broadly classified as:classified as:Sweet corrosion ( CO2 present )Sweet corrosion ( CO2 present ) Sour corrosion (H2S or combination of H2S and CO2 Sour corrosion (H2S or combination of H2S and CO2 present);present);Amine corrosion and cracking.Amine corrosion and cracking.Chloride corrosion.Chloride corrosion.

33

Internal CorrosionInternal Corrosion

44

COCO22 Corrosion Corrosion

Wet CO2 corrosion is generally referred to in Wet CO2 corrosion is generally referred to in the oil and gas industry as ‘sweet corrosion’ the oil and gas industry as ‘sweet corrosion’ when H2S is absent. When CO2 is present when H2S is absent. When CO2 is present in the gas phase, any water in contact with in the gas phase, any water in contact with this gas will dissolve CO2 to a concentration this gas will dissolve CO2 to a concentration proportional to the partial pressure PCO2, of proportional to the partial pressure PCO2, of the CO2 in the gas.the CO2 in the gas.

55

COCO22 Corrosion Corrosion

In the well fluid, CO2 will be in equilibrium In the well fluid, CO2 will be in equilibrium between the three phases, water, oil and between the three phases, water, oil and gas. The quantities of CO2 present in each gas. The quantities of CO2 present in each of these phases are therefore interrelated, of these phases are therefore interrelated, with differences in concentration and activity with differences in concentration and activity coefficient. In multiphase effluents, which coefficient. In multiphase effluents, which are generally highly turbulent, even if the are generally highly turbulent, even if the CO2 is rarely in perfect equilibrium between CO2 is rarely in perfect equilibrium between the three phases, the deviations can never the three phases, the deviations can never be very large. be very large.

66

COCO22 Corrosion Corrosion

Hence,Hence, The CO2 is at its solubility equilibrium in The CO2 is at its solubility equilibrium in

the water and hydrocarbon phases.the water and hydrocarbon phases. The CO2 content of the water is The CO2 content of the water is

determined by its fugacity in the gas determined by its fugacity in the gas phase in contact with the water, or by phase in contact with the water, or by default, the last gas phase. default, the last gas phase.

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COCO22 Corrosion Corrosion

Based on CO2 concentration proportion, Based on CO2 concentration proportion, partial pressure and temperature, the CO2 partial pressure and temperature, the CO2 corrosion rate will vary. The type of corrosion rate will vary. The type of corrosion that takes place in the topside corrosion that takes place in the topside facilities due to CO2 corrosion is usually in facilities due to CO2 corrosion is usually in the form of localized attack (Mesa) pitting the form of localized attack (Mesa) pitting and preferential weld corrosion. and preferential weld corrosion.

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COCO22 Corrosion Corrosion

The overall reaction by which CO2 corrodes The overall reaction by which CO2 corrodes steel is as follows:steel is as follows:

CO2 + H2O → H2CO3 (Carbonic acid)CO2 + H2O → H2CO3 (Carbonic acid)Fe + H2CO3 → Fe CO3 + H2Fe + H2CO3 → Fe CO3 + H2

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COCO22 Corrosion Corrosion

Based on this a monogram, which provides Based on this a monogram, which provides a relationship between corrosion rate of a relationship between corrosion rate of steel, temperature and CO2 partial pressure steel, temperature and CO2 partial pressure was developed by deWaard and Milliams of was developed by deWaard and Milliams of Shell International. This was later modified Shell International. This was later modified to take care of various parameters, such as to take care of various parameters, such as high system pressure, velocity, presence of high system pressure, velocity, presence of crude, pH, glycol in the system and crude, pH, glycol in the system and corrosion protective film formation at high corrosion protective film formation at high temperatures.temperatures.

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COCO22 Corrosion CorrosionThe empirical relationship developed The empirical relationship developed between PCO2 and the corrosion rate is: between PCO2 and the corrosion rate is: Log υ =Log υ = 5.8 – 5.8 – 17101710 + 0.67 log (ƒ CO2) + 0.67 log (ƒ CO2) TTΥ =Υ = predicted corrosion rate for CS (mm/yr)predicted corrosion rate for CS (mm/yr)T =T = Temperature (°K)Temperature (°K)ƒ CO2ƒ CO2 == a x PCO2a x PCO2aa == Fugacity Coefficient Fugacity Coefficient

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COCO22 Corrosion CorrosionThe use of fugacity, rather than partial The use of fugacity, rather than partial pressure, allows for the non-ideality of the pressure, allows for the non-ideality of the gas with increasing pressure and gas with increasing pressure and temperature.temperature.Starting with the worst-case corrosion rate Starting with the worst-case corrosion rate prediction by the deWaard-Milliams prediction by the deWaard-Milliams equation, correction factors can be applied equation, correction factors can be applied to quantify the influence of environmental to quantify the influence of environmental parameters and of corrosion product scales parameters and of corrosion product scales formed under various conditions.formed under various conditions.

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COCO22 Corrosion CorrosionFrom field studies, it has been found that From field studies, it has been found that carbonic acid can form protective scale above carbonic acid can form protective scale above 60°C.60°C.The correction factor Fscale for the basic The correction factor Fscale for the basic corrosion rate equation can be calculated:corrosion rate equation can be calculated:Log Fscale =Log Fscale = 2400 – 2400 – 0.60.6 Log (ƒ CO2) – 6.7 Log (ƒ CO2) – 6.7

TTWith a maximum value of Fscale of 1.With a maximum value of Fscale of 1.

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COCO22 Corrosion CorrosionFrom field studies, it has been found that From field studies, it has been found that carbonic acid can form protective scale above carbonic acid can form protective scale above 60°C.60°C.The correction factor Fscale for the basic The correction factor Fscale for the basic corrosion rate equation can be calculated:corrosion rate equation can be calculated:Log Fscale =Log Fscale = 2400 – 2400 – 0.60.6 Log (ƒ CO2) – 6.7 Log (ƒ CO2) – 6.7

TTWith a maximum value of Fscale of 1.With a maximum value of Fscale of 1.

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COCO22 Corrosion CorrosionSimilarly, pH level has an influence on the Similarly, pH level has an influence on the corrosion rate. With increase in pH the corrosion rate. With increase in pH the corrosion rate decreases and this is particularly corrosion rate decreases and this is particularly significant in cases where bicarbonate is present significant in cases where bicarbonate is present in produced water.in produced water.Wet CO2 corrosion may be mitigated by the use Wet CO2 corrosion may be mitigated by the use of inhibitors. The effect of inhibitors varies of inhibitors. The effect of inhibitors varies depending on the type and operating conditions. depending on the type and operating conditions. Usually inhibitors can provide about 90- 95% Usually inhibitors can provide about 90- 95% efficiency in reducing the corrosion rate.efficiency in reducing the corrosion rate.

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COCO22 Corrosion CorrosionTaking into account all the above factors the Corrected Taking into account all the above factors the Corrected Corrosion Rate (CCR) is as follows:Corrosion Rate (CCR) is as follows:CCRCCR== UCR x Fc x Ft x Fi x FPHUCR x Fc x Ft x Fi x FPH mm/yrmm/yr

UCRUCR== Uninhibited corrosion rateUninhibited corrosion rateFtFt == temperature factor (scaling)temperature factor (scaling)FcFc == Water Condensation factorWater Condensation factorFiFi == Inhibitor factorInhibitor factor

(0.05 for 95% inhibitor efficiency)(0.05 for 95% inhibitor efficiency)FPHFPH == pH factor pH factor

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COCO22 Corrosion Corrosion

This approach to carbon steel corrosion due This approach to carbon steel corrosion due to wet CO2 has been incorporated into to wet CO2 has been incorporated into software known as ‘The Electronic Corrosion software known as ‘The Electronic Corrosion Engineer’ and this program has been used Engineer’ and this program has been used for corrosion rate calculations for corrosion rate calculations

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COCO22 Corrosion Corrosion

The field contains capacity of approximately 75 The field contains capacity of approximately 75 MMSCFD gas, 2500 BWPD Water, 600 BOPD Oil MMSCFD gas, 2500 BWPD Water, 600 BOPD Oil ( 10000 ppm H2S, Chloride contain approximately ( 10000 ppm H2S, Chloride contain approximately 19,000 ppm and 25 % mol CO2 ) an expected field 19,000 ppm and 25 % mol CO2 ) an expected field life of approximately 20 years. life of approximately 20 years.

PT.PERTAMINAPT.PERTAMINA plans to develop Gundih Block plans to develop Gundih Block gas reserves located at Kedung Tuban, gas reserves located at Kedung Tuban, Randublatung, Kradenan and Kedung Lusi in Blora Randublatung, Kradenan and Kedung Lusi in Blora Region-Central of Jawa, Region-Central of Jawa,

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COCO22 Corrosion CorrosionCORROSION RATE CALCULATION PROGRAM  

input lifetime (yearr) 20

input P (psig) 150.00

P (psia) 164.70

input t (oF) 80.00

input t (oC) 26.67

input CO2 %mole 0.2500

pCO2 (psia) 0.4118

CRuninhib (mpy) 4.28

CRuninhib (mm/yr) 0.1088

CA (mm) 2.18