Page 157 - Handbook Of Multiphase Flow Assurance

Sand transport 153

correlation allows to estimate the minimum gas velocity to unload liquid in lateral sections of
deviated or near-horizontal wells. Belfroid presented the correlation in metric units.

.
. 05
A ( ρ
σ
/
V gas _min [ ms] = 31 ρ gas ( liquid − ρ gas g ) ) . 025 ×( sin. (17ω )) ) 038 /.
.
074
2
ω = deviation from horizontal (0° = horizontal), A = tube area [m ], surface tension σ
3
2
[N/m], density ρ [kg/m ], g = gravity acceleration [m/s ]. The correlation shows zero velocity
at ω = 0°.
Multiphase flow
Flow resistance of gas, oil and water
Hydraulic resistance to flow is composed of frictional pressure drop and hydrostatic pres-
sure drop. Most production systems operate in turbulent flow regime. As shown in Chapter 4,
turbulent pressure drop is proportional to diameter to the power of 5. Presence of liquids in
the line reduces the effective cross-section of pipe available for transport of the less viscous
fluid such as gas, which increases the pressure drop.

Vacuum condition and pressure surge during stock oil flow
In oil export pipelines going through mountainous terrain or in deepwater displacement
of the flowline live oil with stock oil there may be a vacuum condition at the highest point of
the flow system.
If pressure at the bottom of the riser or at a pumping station downstream of a mountain
is lower than hydrostatic head pressure for stock oil, vacuum may occur at the riser top or at
the crest of the mountain.
Vacuum condition has to be taken into account for design of flexible lines and flexible parts
and materials on topsides system.
Vacuum can occur at top of chemical injection lines causing flashing off of solvent and
deposition of active ingredient in the chemical tubing.
Deadheading or higher than normal flowing pressure may occur during start of stock oil
flow to move the stationary fluids in the pipeline or in the flowline. Pressure surge also occurs
during sudden valve closure.

Sand transport

Fine sand particles get produced with the fluid into the wellbore and production system
despite downhole sand screens and gravel pack well completions.
Maintenance scraping is one way to remove sand from production flowlines. High flow
velocity is another way to transport sand to the separator, from which it can be mechanically
removed during periodic maintenance.
Sand production from wells is one of the hindrances to subsea separation for oil fields.
Sand may also get produced from gas wells if rock consolidation is weak.