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Corrosion in the Oil Industry
Transcript of Corrosion in the Oil Industry
Ahmad El Nakah
Dr. Magdy Nasrallah
When two dissimilar metals in a conductive medium develop a potential difference between them.
One becomes anodic and the other becomes cathodic.
The problem is acute when the ratio of the cathode to anode area is large.
A localized form of corrosion.
Found exclusively in oxygen containing systems.
Most intense when chloride is present.
Another form of crevice corrosion.
Where a defect, usually a small scratch, or an impurity agitates the process.
A buildup of positive charges occurs in the small pit.
Chlorine ions migrate towards the pit.
Those coupled with the formation of the hydrogen ions act as a catalyst causing more metal dissolution.
Hydrogen Sulfide, Polysulfides, and Sulfur.
Mechanical and Mechanical/Corrosive Effects
This type of metal loss is due to high pressure shock waves, generated from the collapse of minute bubbles in high velocity fluids impinging on nearby metal surfaces.
Usually found on pump impellers developing a too low suction pressure.
When erosion removes the protective film of corrosion products, Corrosion might happen at a faster rate.
Results from subjecting a metal to alternating stresses in a corrosive environment.
At the point of greatest stress, the corrosion product film becomes damaged allowing localized corrosion to take place.
This eventually leads to crack initiation and crack growth.
Welded connections are subject to this type of corrosion,
Sulfide Stress Corrosion
Occurs when a susceptible metal is under tensile stress and exposed to water containing hydrogen sulfide, generally under anaerobic conditions.
Because its almost impossible to prevent
corrosion, corrosion engineers are therefore
increasingly involved in estimationg the cost
of replaced equipment and assessing the useful
life of the equipment through various Corrosion
detection Tools which can inspect corrosion rates
of all types of equipment whether located on surface
1- Visual Inspection:
is quite straightforward ans simple method used to inspect
In case of subsea structure such as offshore drilling rigs, it is
performed by divers or ROVs using video recording to check the
conditions of the legs and the risers.
Helps in estimating the rate of corrosion of sacrificial annodes
placed at the submerged part of the rig.
2. Coupon Rings:
Are steel rings which can be inserted in any pipeline to monitor corrosion rate.
Such rings are assumed to corrode with nearly the same rate as the rest of the
pipe linethen it can be retrieved later to determine the rate of corrosion by
measuring the amount of metal lost from them.
Coupon rings can give also information about the type of corrosion taking place.
3.Multifinger Imaging Tool :
Is a multifinger caliper tool that makes highly accurate radial measurements of the inside diameter of tubing and casing strings.
Obviously the bigger the diameter recorded the greater the metal loss.
The tool can be run in the presence of any wellbore fluid and in relatively small holes as long as the caliper passes through them.
Although the tool can determine quite accurately internal corrosion rates, sometomes it is not preferable as it may scratch the protective coatings and the chemical inhibitors.
4.Ultra sonic tools:
Sends ultra sonic waves that reflects off and reasonate the casing surface to be later recieved by the tool.
The transit time forst recieved echo gives the internal casing radius
The analysis of the reasonant waves enables the computation the casing thickness allowing internal and external calculation of metal loss.
Electromagnetic thickness tool uses induction methods to detect metal loss and changes in casing geometry, regardless of the fluid type inside the casing.
The Tool is typically used to detect large-scale corrosion or splits, and it can also be used to detect metal loss in the outer casing of multiple casing strings.
A coil centered in the borehole generates an alternating magnetic field that interacts with the casing; a second coil measures the phase shift. These electromagnetic measurements, made at multiple frequencies, are related to the casing wall thickness, inside diameter, and conductivity. each parameter is averaged around the pipe circumference.
6. Corrosion rate
The tool takes stationary measurments of potentials differences and casing resistance between its electrode pairs.
The casing current is calculated from these measurement at desired depths .
From the current difference between stations radial current density can be calculated enabling us to compute the corrosion rate.
The tool shows anodic regions where corrosion is more likely to exist.
"Corrosion in Oil Industry"
It means in Latin: falling into pieces, which is basically the breakdown or the failure of the material.
A chemical reaction occurs between the material and its surrounding causing this process.
It happens mainly with oxygen, oxidation.
It can occur not only to metals but also to other materials like ceramic.
Excess of oxygen and water may also increase corrosion rates.
They form weak acids when dissolved in water.
They give off hydrogen ions and are corrosive.
Effects are greater in deep wells, (pH is reduced by pressure).
Products are iron sulfides that form a scale that is protective at low temperatures.
However at high temperatures, its cathoidc wrt to the casing, and galvanic corrosion starts.
Must hydrate to carbonic acid before it becomes acidic.
Product is iron carbonate scale.
It could be protective under certain conditions.
Protective layer does not allow galvanic corrosion.
Can cause hydrogen embrittlment resulting in stress corrosion cracking.
Lack of turbulence.
Acids are used to stimulate wells by increasing formation permeability near wellbore.
Different concentrations are used.
Corrosion control is achieved by limitng exposure time, and by the loading of an inhibitor.
Used to balance formation pressure.
Maybe corrosive due to dissolved oxygen or entrained air.
Or by acidity generated by hydrolsis of metalic ions.
Prevented by use of brines lacking zinc.