Page 44 - Handbook Of Multiphase Flow Assurance

2. Initial diagnosis and solution of flow assurance production problems in operations 39

the reservoir. SRB bacteria flourish in reservoir zone with temperature between 10 and
50 °C and generate sour chemicals. Seawater treated with desulfation has less impact
on souring. Usually it takes several years for sour components to migrate through the
reservoir to producer wells. Faster souring may occur if there is streaming from injector
to producer wells.
Stuck scraper during pipeline commissioning or during flow line maintenance—can
occur when the energy of fluid propelling the scraper (a piston) is insufficient to
overcome the viscosity or Young's modulus of material being scraped. Can also happen
when a scraper gets tilted in a valve cavity, in a wye or on a deposit or obstruction and
loses seal against pipe wall allowing propelling fluid to freely bypass it.
Sulfur deposition in well tubing or in flow line—occurs when solubility of elemental
sulfur S 8 in produced fluid at reservoir conditions is higher than at well or flowline
conditions. Elemental sulfur is yellow, not transparent and in its solid form is not toxic.
Underdeposit corrosion generally occurs from neglect for regular maintenance cleaning
of the flowline during its operation, or from inability to clean the line if it was designed
without such ability. Deposits of sand, precipitated wax, asphaltene, scale and their
combinations occur where flow velocity is low, usually less than 1 m/s. Deposits prevent
corrosion inhibitor from reaching the pipe wall, or allow bacteria to grow depending on
conditions.
Viscous oil or viscous emulsion flow—usually occurs in wells where viscosity at reservoir
conditions exceeds 200 cP. Significant pressure drop reduces production from such wells
and may require artificial lift.
Wax deposition—can occur in subsea, deepwater and onshore fluids when flowing oil
or condensate cools below temperature of normal (straight chain) paraffin wax freezing
or crystallization. Wax appearance temperature depends on content of normal paraffins
and typically ranges from 10 to 50 °C. Wax is a soft pliable material. Typical wax melting
temperature ranges from room temperature (for a waxy gel) to over 80 °C (for a wax
deposit aged 2+ years).
Wax deposition from gas is less common than from oil but possible when reservoir
fluid and hydrocarbon condensate liquid contain normal paraffins heavier than C 18 .
Wax deposition requires both a heat loss and replenishment of waxy components to be
present, thus a wax deposit can only form during flow. Without flow, wax can precipitate
in the liquid but cannot be replenished. Without heat loss, wax cannot precipitate.
Water quality noncompliance due to organic content—occurs in topsides or surface
process equipment due to insufficient residence time or insufficient temperature to
resolve oil-in-water emulsion or due to high concentration of water-soluble organic
(negatively charged acid groups or aromatic) components which cannot be separated by
mechanical means and require water clarifier (reverse demulsifier) chemical and water
polisher (sorbent) filter. Offshore discharge of water with oil causes a sheen layer on
seawater with colors. Water with water-soluble organics causes a gray sheen. Either can
result in a noncompliance.
Each of the possible flow assurance issue causes needs to be evaluated to determine if the
production system entered the pressure-temperature-composition conditions for stability
of each cause. If samples are available for laboratory analysis, the sample identification
should be performed.