Posted by Chris Craft on July 02, 1996 at 14:25:11:
I would agree with the main conclusions:
One or two observations,
I have found in the past that sour service (1000-5000ppm) lines tend to have
lower overall corrosion levels but are significantly worse for localised
pitting attack. This is thought to be due to FeS scaling,
which generally coats the inside of the tubing/line internally, where this is eroded,localised
attack occurs. FeCO3 also causes a similar effect but forms a stronger scale(?).
Although the CO2 services you have examined are at the upper end
of our experience range, the poor performance of corrosion
inhibitors would suggest poor persistence, this would suggest two
problems:
1.Inhibitor dosing rate problems - this is very often the case
with downhole batch inhibitor dosing due to the way it works.
2. High water production - particularly for condensate soluble
inhibitors - where the produced water phase may prevent the formation
of a good inhibitor film on the tubing.
Finally, the use of inhibitor efficiency can be missleading,
the mechanics of (filming) corrosion inhibition means that there is
either a film (and protection) or there isn't (no protection)-
I prefer to think of corrosion inhibitor performance to be
gauged by a residual corrosion rate, left over after application of
the inhibitor. Corrosion rates in the region of 0-0.3mm/yr indicate
satisfactory performance.
regards,
Chris Craft