Produced water oxygen ingress 1

[rscosta1976]

Gentlemen,

I'm involved on a discussion regarding material selection for a gas well produced water system. Some colleagues are defending the idea that the produced water system should be considered oxygen free. I'm not sure I agree with that.

Water characteristics:

H2S = 1,400ppm
CO2 = 5ppm
Chloride = 30,000ppm
pH = not known, but should be around 6 to 7, as bicarbonate should buffer the pH considerably.

Although the O2 levels may be much lower when compared with full oxygenated system, I guess one cannot consider fully deaerated conditions as a basis for material selection on units handling produced water (hydrocarbon condensate separation unit and sour water stripper unit). As a minimum O2 ingress will happen through the upstream chemical injection systems (corrosion inhibitor injected the production site). My questions are:

1) Can the O2 ingress be considerable through (atmospheric) upstream chemical injection systems?
2) What other O2 ingress sources may exist in a unit with several pumps, drums and a surge tank (even if N2 blanketed)
3) Should the material selection consider the water to be deaerated?

Considering the water to be deaerated (with O2 in the ppb level - below 20ppb) seems a risky approach in my view for selecting appropriate materials, as oxygen contamination should never be excluded. Your thoughts would be well appreciated on this matter.

 

[EdStainless]

Once produced water hits the surface it will contain oxygen.
Likely not at saturation levels, but it will have some.
Anyplace that there is the slightest leak, even just a damp gasket you will find oxygen diffusing into the fluid stream.
Your examples 1 and 2 are all likely points of oxygen ingress also (especially pumps).
You have to approach material selection carefully.
I used to design downhole equipment and we could get away with using materials that stood zero chance of working in a surface system.
But there are a lot of materials in the middle ground.
They will work but degrade faster depending on the actual oxygen levels.
I have seen people make the mistake of testing systems and saying that 'O2 is <1ppm so I am safe', never mind that it is actually 600ppb which is still enough to cause plenty of issues.

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P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

rscosta - you haven't said how much oxygen is in the produced water?

I've seen this done but it comes at a risk that somewhere there would be oxygen ingress, even if it's only during maintenance.

As soon as you hit a tank then even if blanketed, your risk increases dramatically.

This looks like slightly sour salty water? Can't remember what O2 levels need to be to use say C stl, but from memory its down to something like 30 or 40 ppb?

I once installed a vacuum deaerator tower system to deaerate seawater so that we could use C Stl downstream the oxygen scavenger tank and that went ok with 3mm CA. Much cheaper than Duplex.... Even if you need to replace it after 10 years.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

The point that LI makes is critical, how long do you need it to last?
If CS only lasts 3 years then Duplex makes real sense.
In most systems they simply go to heavier CS and try to get a 10+ year life and live with that.
But everyone needs to know that everything in this system is a maintenance item and requires regular documented inspection.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[rscosta1976]

Thank you both for the valuable feedback. It is truly useful.

Now let me answer one of the questions and tell you the rest of the story.

I do not know the O2 level as the plant is still being designed. However, there is no O2 scavenger being injected nor any deaeration being performed, so we need to consider O2 contamination a real possibility.

The unit in question is a Sour Water Stripper (for the produced water). It has a HC coalescer,nSW filters and SW surge tank (N2 blanketed system) and pumps installed upstream the stripper feed/bottom heat exchanger and stripper column.

The engineers responsible for the design selected materials which I don't agree with. Just a few examples below:

i. bare CS for piping with temperature around 150F
ii. 316L for several internal components at 150F and valves' trim.
iii. 316L clad/WO for stripper feed piping at 200F
iv. bare CS for hot stripped sour water piping (bottom of stripper at 260F.
v. bare carbon steel for stripper section immediately below SW inlet nozzle and 316L for inlet nozzle elevation and above.

What are your thoughts on the above selection? Any chance it may work for 20 years design life?

I don't think so considering the high amount of chlorides, operating temperatures and O2 ingress possibility. A SDSS and 6Mo should be needed to avoid excessive degradation and achieve 20 years design life.

Thanks for the discussion