Page 57 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 45

precipitates and prevent further growth. Altering the design of the
production system may also minimize the probability of a solution
reaching a saturated state and forming scale in critical flow paths.
Because some oxygen scavengers can produce sulfates which
can react with calcium, barium, and strontium to produce scale, the
addition of oxygen scavengers where scaling may be a problem should
be minimized.
A problem related to scale formation is the precipitation of hydro-
carbon solids (paraffin) in production tubing and equipment. Paraffin
precipitation occurs when the temperature and pressure of the crude
oil no longer allow paraffin to remain dissolved in the oil. Various
organic additives are used to inhibit paraffin deposition.

Biocides

Microbial growth (bacteria) in produced water can produce hydro-
gen sulfide gas by the chemical reduction of sulfates. Dissolved
hydrogen sulfide gas makes produced gas highly corrosive. In addition
to causing corrosion, the presence of the bacteria themselves can
impact production operations. Bacterial fouling of equipment and
degradation of hydrocarbons can occur. Pads or mats of bacteria, iron
sulfide, and degraded oil can be formed at the oil/water interface in
tanks and separators, rendering them less effective.
To minimize these problems, biocides are often added to the pro-
duced water to inhibit microbial growth. Surfactants can also be added
to mobilize the microorganisims and make them more susceptible to
the biocide. Bacteria are rarely completely killed using biocides, so
long-term treatment is usually required once a system is contaminated.
Biocides used include aldehydes, quaternary ammonium salts, and
amine acetate salts. Chlorine compounds are used as biocides in
municipal drinking water systems.

Coagulants

Produced water often contains various amounts of produced
solids. While most of these solids are separated in surface settling
tanks, very small solids (clay particles) may remain suspended in the
water. Coagulants and flocculants can be added to the produced water
stream to agglomerate these fine particles and allow them to settle.