Page 56 - Environmental Control in Petroleum Engineering

44 Environmental Control in Petroleum Engineering

Sodium chromate, however, adds chromium to the produced water,
Sodium hexametaphosphate (Na 6P 6O l8) is used in cooling and boiling
water treatment. Zinc salts of organic phosphonic acids and sodium
molybdate (Na 2MoO 4) have also been used for corrosion control. Zinc-
based inhibitors are less toxic than chromates and should be used if
possible. Organic anionic inhibitors, such as sodium sulfonates and
sodium phosphonates, are also used in cooling waters and antifreeze.
Current regulations may limit the use of some corrosion inhibitors.
Hydrogen sulfide can be removed from produced fluids with a
zinc scavenger. Zinc carbonate (ZnCO 3-Zn[OH] 2) is widely used.
This chemical reacts with hydrogen sulfide, producing insoluble zinc
sulfide (ZnS).
For water injection systems, oxygen causes the largest problems
with corrosion. Oxygen can be removed from water by stripping it
with an inert gas, such as natural gas, steam, or flue gas, by vacuum
deaeration, or by chemical treatment. Oxygen scavengers include
sodium sulfite (NaSO 3), sodium bisulfite (NaHSO 3), ammonium bisulf-
ite (NH 4HSO 3), sulfur dioxide (SO 2), sodium hydrosulfite (Na 2S 2O 4),
and hydrazine (N 2H 2).
Cathodic protection can be used for external corrosion of casing and
pipes and for internal corrosion of tanks. Both internal and external
surfaces of surface equipment can sometimes be protected with liners
to prevent corrosion. These liners can be hydrocarbon, plastic, metal,
ceramic, or cement based.

Scale Inhibitors

The dissolved solids in produced water are normally in thermo-
dynamic chemical equilibrium with the downhole conditions. As water
is produced, however, its temperature and pressure are lowered,
altering the chemical equilibrium. One common result of this altered
chemical equilibrium is the precipitation of inorganic salts in produc-
tion equipment, i.e., scale. Scale can plug production equipment,
rendering it useless. Scale is commonly composed of calcium, stron–
tium, and barium sulfates, as well as calcium carbonate. A more
complete discussion of scaling is given by Jones (1988).
Scale can be inhibited by organic phosphate esters of amino-alcohols,
phosphonates, or acrylic acid type polymers (sodium polyacrylate poly–
mers). These chemicals adsorb onto the crystal nuclei when scale first