Page 55 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 43

oxygen. Oxygen depletion is normally not a problem for discharge in
deep water or in high-energy environments because of rapid dilution
of the produced water in the surrounding environment,

2.2.3 Production Chemicals

Produced water is responsible for a variety of problems in oilfield
operations. The most common problems are emulsions, corrosion,
scale, microbial growth, suspended particles, foams, and dirty equip-
ment. A variety of chemicals are often added to the water to avoid
those problems.

Emulsion Breakers

As previously discussed, produced water often consists of an oil-
in-water emulsion. Chemicals are commonly used to lower the electro-
static forces on the oil droplets to allow them to coalesce into larger
droplets. Common chemicals used for this purpose include surfactants,
alcohols, and fatty acids.

Corrosion Inhibitors

Produced water can be very corrosive to production equipment.
Corrosion is caused primarily by the presence of dissolved oxygen,
carbon dioxide, and/or hydrogen sulfide gases. A detailed discussion
of corrosion is given by Jones (1988).
Although produced water is initially oxygen depleted, oxygen can
enter the produced fluid stream as a result of agitation during pumping
or by atmospheric diffusion in holding tanks and surface impound-
ments. The oxygen content of water can be minimized by designing
the system to exclude oxygen contact with the water. Carbon dioxide
and hydrogen sulfide can occur naturally in the formation and be
produced with the water. Carbon dioxide forms carbonic acid, which
lowers the pH and increases the corrosivity of the water. Hydrogen
sulfide corrosion can occur as a result of bacterial action on sulfates
and is more often a surface or near surface phenomenon.
Complex inorganic salts like sodium chromate (Na 2CrO 4), sodium
phosphate (Na 3PO 4), and sodium nitrite (NaNO 3) are also effective in
slowing oxygen corrosion, particularly in high pH environments.