Page 198 - Handbook Of Multiphase Flow Assurance

Emulsions, foam, topsides separation, water treatment management 195

Hydrocarbon oil
Water content specification applies to sales oil. In the USA the crude oil should have a
water content under 5000 ppm or 0.5 wt%. Some contracts vary and the water content may be
lower at 2000 ppm or higher at 10,000 ppm or 1 wt%.
Methanol is soluble up to 1% in crude oil. Methanol content in USA oil is regulated by
contract with the receiving refinery. Some refineries cannot process oil with high methanol
content.
In Russia separation processes may vary, and oil may contain up to 1% water. The maxi-
mum salt content of product oil is set to 300 mg/L and is rated into three categories by a state
standard. Methanol content in oil is not regulated.

Hydrocarbon gas
Gas is expected to have water content below 6–7 pounds per MMScf.

Produced water

Water discharge requirements include hydrocarbon content and toxicity.
In the US Gulf of Mexico the allowed oil and grease hydrocarbon content limit in pro-
duced water averaged over 30 consecutive days is 29 mg/L. The temporary excursion limit
is 42 mg/L which allows for slightly-off-spec water discharge. This temporary excursion is
allowed only during any 24-h period. Once the limit is exceeded and the regulatory agency
notices that, usually in a form of a sheen, usually silvery in color and well seen from aerial
vehicle such as a helicopter.
The difficulty in obtaining the discharge water specification is that it includes both
water-insoluble and water-soluble organics. While the former may be separated by regular
means as free water knockout, flotation cell and electrostatic treater, the latter may not be-
cause organics are dissolved in water. This requires the injection of water clarifier chemicals.
The water may also contain hydrocarbons in form of a reverse emulsions. In a reverse emul-
sion a droplet of water is covered by a shell of hydrocarbon material, which is suspended
in bulk water. Reverse demulsifier chemicals (also called water polishers) also help remove
hydrocarbons from water. Reverse emulsions and water soluble organics usually manifest
themselves in form of a greyish color sheen on water near the discharge point.
Toxicity test is performed with shrimp in the water sample and survival rate is monitored
over several hours.

Emulsions, foam, topsides separation, water treatment management

Emulsions and foams both increase resistance to flow and should be avoided in wells and
flowlines.
Separation management is complementary to the topsides process design in controlling
emulsion or foam formation by physical or chemical means. Physical means include avoiding
flow shear or incompatible fluids comingling.