Types of Corrosion
Corrosion on downhole equipment is nature's way of reducing
a man-made material of a higher energy state back to its basic
condition. Controlling this process by proper chemical inhibition
and application methods will help lengthen the economic life
of downhole equipment (refer to API Spec 11B and 11 BR).
Corrosion is one of the greatest problems encountered in the
use of downhole equipment. The following suggestions can be
used in the preliminary evaluation of the possibility of corrosion
when no other information is available. The following suggestions
may not be valid when specific test data or other test information
appears to be contradictory.
- Where downhole water percentages are between 24% and 100%,
and pH is less than 7.0, the possibility of corrosion depends
on the corrosivity of the water.
- For most wells, an oil soluable-water
disposable inhibitor should be used unless other types are indicated.
- A corrosion abating program should
be used, monitored, and documented for effectiveness, whether
continuous or batch treated on a weekly basis.
- A treating rate of 10 to 15 ppm
for mild corrosion, 15 to 25 ppm for moderate corrosion, and
25 ppm or more for serious corrosion, should be considered for
water rates of 0 to 300 barrels per day. A specialist in corrosion
control should be consulted for water rates above 300 barrels
per day.
Non-corrosive well fluid types are classified as follows:
- Well fluids that have less than 25% water cut and a pH of
7.0 and above.
- Well fluids that are effectively,
chemically inhibited, monitored and documented.
Consider installing Type 30, Type
54, Type 78, or Type 97 sucker rods where corrosion is not
a problem. The specific type sucker rod must be determined
by the design loading conditions.
All fluids that are either known or are tested to be corrosive
should be effectively, chemically inhibited with a film forming,
amine-polar-organic type inhibitor. The well fluids should be
monitored and documented. Changes in the inhibitor should be
made if corrosion is found on any article of downhole equipment.
Corrosive well fluid types are classified as follows:
Acid Gas Corrosion
- Hydrogen Sulfide (H2S), either as a gas or from bacterial
activity in brine, in any amount.
- Carbon Dioxide (CO2) as a gas between
a partial pressure of 7 psi (.048 Mpa) to 30 psi (.207 Mpa),
or between 600 ppm and 1200 ppm in brine.
Consider installing Type 40 or Type 90 sucker rods for service
in corrosive fluids. The specific type sucker rod must be determined
by the design loading conditions.
Oxygen Corrosion
- More than 50 parts per billion in brine water.
- For all rod types the oxygen source
should be stopped, an oxygen scavenger used, or a specially
designed trace-oxygen inhibitor used.
Bacteria Corrosion
- All sucker rod types can be corroded by bacteria.
- Most prevalent types of bacteria
are sulfate-reducing and acid-producing.
- A bottle test will quantify, and
a bactericide will control downhole bacteria.
Mechanical Corrosion
- Mechanical Corrosion occurs in the following ways, each of
which if left untreated, will destroy all types of sucker rods.
Fatigue Corrosion
- Resulting from cyclic motion of the rod string under load
Abrasive Corrosion
- Resulting from the rod string abrading and galling against the tubing
Flow Corrosion
- Induced
by the movement of fluid containing
either solids or gasses against
the rod string
Galvanic Corrosion- resulting form
one metal being in contact with another
metal in fluid
Electrolytic Corrosion
-
(Electrolysis), induced by flowing
electrical currents.
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