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The Microstructure Effect on Fracture Toughness of Ferritic Ni-alloyed Steels for CO2 Reinjection

Ehanced oil recovery working pressure can reach up to 500 bars and in case of sudden depressurization local temperature can decrease down to -60°C. The aim of this work is to evaluate fracture toughness of two nickel containing steels as an alternative material.

Product Number: 51317--9204-SG
ISBN: 9204 2017 CP
Author: Carlos Kwietniewski
Publication Date: 2017
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Production of oil and gas in the Brazilian pre-salt faces several technical challenges and one of them that is a major concern is the presence of CO2 in high concentration. After separation this gas is used for re-injection increasing the reservoir productivity which is known as Enhanced Oil Recovering (EOR). This operation involves the transportation of CO2 at pressures up to 500bars. The pipelines used for re-injection must operate safely even at low temperatures which in case of leakage can reach temperatures down to -60°C. The aim of this work is to evaluate the fracture toughness of two nickel-containing steels as an alternative material to manufacture low-temperature toughness improved CO2 transporting pipelines for EOR. Optical and scanning electron microscopies were employed to characterize the steels microstructures. Electron backscattered diffraction was used to estimate the effective grain size and the density of high-angle grain boundaries. Fracture toughness was determined by the use of the crack tip opening displacement methodology. The results indicated that for the as-rolled condition the large islands of the microconstituent M/A in the 5¹/²Ni steel had a detrimental effect on fracture toughness at -100ºC while finer M/A particles and lower effective grain size with higher density of high-angle grain boundaries in the 9 Ni steel turned its fracture toughness practically temperature independent. Additionally heat treatment (quenching and tempering) has the potential to dissolve the M/A hard particles and consequently improve fracture toughness at low temperature.

Key words: Fracture toughness, CTOD, CO2, M/A hard particles, EBSD

Production of oil and gas in the Brazilian pre-salt faces several technical challenges and one of them that is a major concern is the presence of CO2 in high concentration. After separation this gas is used for re-injection increasing the reservoir productivity which is known as Enhanced Oil Recovering (EOR). This operation involves the transportation of CO2 at pressures up to 500bars. The pipelines used for re-injection must operate safely even at low temperatures which in case of leakage can reach temperatures down to -60°C. The aim of this work is to evaluate the fracture toughness of two nickel-containing steels as an alternative material to manufacture low-temperature toughness improved CO2 transporting pipelines for EOR. Optical and scanning electron microscopies were employed to characterize the steels microstructures. Electron backscattered diffraction was used to estimate the effective grain size and the density of high-angle grain boundaries. Fracture toughness was determined by the use of the crack tip opening displacement methodology. The results indicated that for the as-rolled condition the large islands of the microconstituent M/A in the 5¹/²Ni steel had a detrimental effect on fracture toughness at -100ºC while finer M/A particles and lower effective grain size with higher density of high-angle grain boundaries in the 9 Ni steel turned its fracture toughness practically temperature independent. Additionally heat treatment (quenching and tempering) has the potential to dissolve the M/A hard particles and consequently improve fracture toughness at low temperature.

Key words: Fracture toughness, CTOD, CO2, M/A hard particles, EBSD

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