Stability of Iron Sulfide Scales for Long Term Corrosion Protection in RasGas Tubulars

Abstract

In sour wet gas wells, an iron sulfide scale develops on the carbon steel production tubing wall which significantly retards the rate of metal loss due to internal corrosion. During late field life as reservoir pressure depletion occurs, high flow velocities in the wellbore generate high shear stresses on production tubing walls that can potentially damage the existing corrosion scale and expose the metal to higher corrosion rates. The modeling work done to estimate late field life shear stresses was reported in IPTC-17293-MS and presented in 2014 IPTC in Doha, Qatar. This paper discusses the findings from lab tests conducted in modified autoclaves and a customized flow loop to evaluate the stability of the iron sulfide coating at the predicted late life shear stresses. The iron sulfide scale in the well tubulars was carefully characterized from historical scale composition data as well as analysis of freshly collected samples. This scale was then successfully generated in the lab under carefully controlled conditions. The stability of the generated scale was then tested in High Pressure High Temperature (HPHT) autoclaves and in a customized flow loop by subjecting it to a range of shear stresses. The results provide the first insights into stability of iron sulfide scale under different shear conditions. Findings from this testing program are being utilized to: Develop effective erosion/corrosion management strategy Develop corrosion monitoring and wellbore integrity surveillance programs Define potential future constraints on well production rates Guide materials selection for future wells in the field