Page 193 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

OIL AND GAS EXPLORATION AND PRODUCTION 171

+
H S → H + SH −
2
−
Fe + SH → FeS
CO + H O → H CO
2 2 2 3
+
H CO → H + HCO −
2 3 3
The mechanisms of CO corrosion are generally well defined; however, the
2
reality inside a pipeline becomes complicated when CO acts in combination with
2
H S, deposited solids, and other environments. H S is highly corrosive, but can, in
2 2
some cases, form a protective sulfide scale that prevents corrosion.
Microorganisms can attach to pipe walls and cause corrosion damage. Solids such
as formation sand can both erode the pipeline internally and cause problems with
underdeposit corrosion, if stagnant.
Oxygenisnotfoundinoilreservoirsandmuchisdonetoensurethatnooxygenenters
the production environment; however, in many cases, a few parts per million (ppm)
of oxygen enters the production pipeline, greatly exacerbating corrosion problems.
External corrosion problems in oil and gas production normally are similar to those
found in the pipeline industry, but as the lines are shorter and smaller in diameter, their
economic impact on the total cost of production is limited. Atmospheric corrosion of
structures and vessels is a problem for offshore fields and those operating near marine
environments.
Improvements in the quality of protective coatings for offshore environments have
dramatically reduced the frequency of repainting platforms and tanks.
Corrosion in oil and gas production varies from location to location. Corrosion
can be classified into one of three general categories of internal corrosion caused by
the product fluids and gases, external corrosion caused by exposure to groundwater
or seawater, and atmospheric corrosion caused by salt spray and weathering offshore.
Of these, internal corrosion is the most costly as internal mitigation methods cannot
be easily maintained and inspected.
A typical oil and gas production flow diagram is shown in Figure 3.27. Oil, water,
and gas are produced in every oil field. Water is injected downhole to maintain
reservoir pressure and stability, and often water fording from seawater or freshwater
sources is used to drive oil out of the formation. As a field ages, the water cut
or the ratio of water to oil in the fluids produced, increases to levels of 95% or
higher depending on the economics of production. As the oil industry matures and
the number of old oil fields relative to new fields increases, the amount of water
produced increases and the internal corrosion increases.
Water injection from seawater or fresh water sources contributes to the “souring”
of oil fields with H S usually resulting in an increase in the corrosion rate, which
2
sometimes requires a complete change in corrosion strategy. These water sources may
necessitate biocide injection and will require deaeration to avoid introducing a new
corrosion mechanism into the existing system. Tertiary recovery techniques are often
based on miscible and immiscible gas floods. These gas floods invariably contain a