Page 152 - Handbook Of Multiphase Flow Assurance

148 5. Flow restrictions and blockages in operations

• periodic scraping
• periodic solvent wash
Following remediation technologies could also be considered.

• solvent soak
• coiled tubing to jet out the deposit, especially in a riser.

Bacterial growth

Topsides process equipment

Bacteria present in the production fluid may grow inside process vessels and restrict the
flow and the vessels' capacities. Periodic biocide treatment or mechanical removal may be
required for maintenance of the process vessels.

Water injection wells

Various bacteria may be present in the lower temperature reservoirs. However, their activ-
ity is subdued by the high reservoir temperature. As water is injected to maintain reservoir
pressure, it usually cools down to the ambient temperature of the water injection pipeline.
Injection of a colder water into the reservoir lowers the temperature near the wellbore and
activates the indigenous bacteria which may include sulphate reducing and nitrate reducing
bacteria.
In some cases the bacteria may grow close to the well perforations and cause resistance to
flow or injectivity damage.
If filtered seawater is injected, it contains sulphate SO 4 ions dissolved in sea water. Sulphate
reducing bacteria produce H 2 S which causes reservoir fluid souring and corrosion issues in
the producer wells. Sulphate reducing bacteria activity is mitigated by either water desulpha-
tion treatment or by injection of nitrate into the injection water.
If nitrate chemical is injected into the injection water, the nitrate reducing bacteria are acti-
vated more than the sulphate reducing ones and take over the favorable temperature space.
Their products are less harmful to the reservoir and producer wells.
One way to mitigate the low temperature impact of water injection is to preheat the water
downhole by drilling the injection well deeper than required and allowing water to flow
down the tubing and up the annulus, using reservoir heat to warm it up before entering the
reservoir through perforations. Injecting hotter water through perforations reduces the zone
of favorable temperature and prevents bacterial accumulation at perforations and injectivity
damage. The extra cost of a deeper well has to be weighed against lower cost of desulphation
equipment or chemical injection such as THPS biocide or calcium nitrate to control bacterial
growth downhole.

Bacterial growth management

Bacterial growth effects should be controlled within normal operating procedure.