C3.2.3&4. Fluid Recovery _2020

1. Offshore Production Technology
Fluid Recovery – Gas Lift, Gas Injection,
Water Injection
Marine & Offshore Technology

2. Learning Outcomes
1. Understand the following recovery processes for a subsea oil
well:
a. gas lift process
b. gas injection process
c. water injection process
2. Seawater is treated before being used for high-pressure
water-injection. Explain the processes involved in the sea-water
treatment using a block diagram.
3. Understand the produced water system operating principle.

3. 1. Understand the following recovery
processes for a subsea oil well:
a. gas lift process

4. Video Links for the process
• Gas Lift by Weatherford (00:40:00 –
02:42:00)
– https://h5p.org/node/810586
• How does a gas lift oil well work? - Lab
demonstration by Dover Artificial Lift - Gas
Lift
– https://youtu.be/naI54wnAqFw

5. Gas Lift Principle

6. Gas Lift – Operating Principle
• An artificial-lift method in which gas is injected into the
production tubing to reduce the hydrostatic pressure of the
fluid column.
• The resulting reduction in bottomhole pressure allows the
reservoir liquids to enter the wellbore at a higher flow rate.
• The injection gas is typically conveyed down the tubing-casing
annulus and enters the production train through a series of
gas-lift valves.
• The gas-lift valve position, operating pressures and gas
injection rate are determined by specific well conditions.

7. Gas Lift Principle
• Gas-Lift is used when
large supply of natural gas
is available.
• Natural gas is introduced
into the well in the
annular space between
the tubing and casing.

8. Gas Lift Principle
• Gas lift valves close as the
gas enters the lowest
valve.
• As the gas enters the
production zone, it is
dispersed within the oil
phase, which makes it
lighter reducing static
head.

9. 1. Understand the following recovery
processes for a subsea oil well:
b. gas injection process

10. Gas Injection – Gas Flooding
• A reservoir maintenance or
secondary recovery method
that uses injected gas to
supplement the pressure in
an oil reservoir or field.
• In most cases, a field will
incorporate a planned
distribution of gas-injection
wells to maintain reservoir
pressure and effect an
efficient sweep of
recoverable liquids.

11. Gas Injection or Gas Flooding
• Gas is injected into the
gas cap above the oil
layer.
• It essentially builds up
pressure to push the oil
up once the natural
pressure decreases.

12. 1. Understand the following recovery
processes for a subsea oil well:
c. water injection process

13. Water Injection – Water Flooding
• A method of secondary recovery in
which water is injected into the
reservoir formation to displace
residual oil.
• The water from injection wells
physically sweeps the displaced oil
to adjacent production wells.

14. Water Injection – Water Flooding
• Potential problems associated with
waterflood techniques include
inefficient recovery due to variable
permeability, or similar conditions
affecting fluid transport within the
reservoir, and early water
breakthrough that may cause
production and surface processing
problems.

15. Tertiary Recovery - Water Injection

16. Water Injection –
Two Sources
What are the two
sources?
• Sea water (plentiful, all
around)
• Produced water (good
to inject them back into
reservoir, otherwise
they have to be
disposed off)

17. 2. Seawater is treated before being used for
high-pressure water-injection.
Explain the processes involved in the sea-
water treatment using a block diagram.

18. Specification of Water Injection
Flowrate : 80,000 BWPD
Temperature : 10 to 30C
Pressure : 205 bara at FPSO
Pressure 201 bara at well
Oxygen concentration : 0.10 mg/l (without O2 Scavenger)
O2 concentration : 0.01 mg/l (with O2 Scavenger)
Solids content : 95% removal of  5microns

19. Seawater Injection – Greater Plutonio

20. Sea-water treatment
process
Coarse &
Fine
Filtration
•Coarse and Fine Filtration
•Remove 95% of all solids and down to 5 microns
Chlorination
& Sulphate
Removal
•To kill living organisms
•To remove sulphates to reduce scaling
Vacuum
Deaeration
•Reduce oxygen to 0.01 mg/l or less
High-
Pressure
Injection
•2-stage pump to boost to 200
bara
Sterilisation •Downstream of injection
•UV or chlorinating
chemicals
Key Diagram to understand

21. Seawater Injection
Sea-water Lift Pumps
• A large amount of seawater is used for cooling various
processes and in providing seawater for injection.
• The seawater lift pumps may be located within the ship taking
suction from sea-chests or be large electric submersible
pumps located in caissons.
• These pumps are usually high-volume, low-head pumps.
• Seawater for water injection and cooling medium cooling is
normally supplied by three (3) units of 50% seawater lift
pumps located in seawater lift caissons or sea chests.

22. Seawater Lift Pumps

23. Water Injection Process
Filtration
• Filtration of the seawater will be carried out in two stages.
Coarse filters Package filters

24. Seawater Filters
Coarse filters

25. Water Injection Process
Filtration
• The first stage will be through a
coarse filter package. This will
remove particulate matter down to
300 microns. The coarse filter
package will be located on the
discharge of the seawater lift
pumps.
• All the seawater required for
process indirect cooling and for
injection will pass through the
course filter package.

26. Water Injection System
Coarse Filtration
• The coarse filters are usually
automatic back-washable
basket filters.
• During the bloom period, these
filters may require more
frequent backwashing to deal
with the high levels of plankton.

27. Water Injection System
Filtration
• The second stage of filtration after
the seawater has passed through
the cooling medium/seawater
plate cooler will be through a fine
filter package.
• This package normally comprises of
single or dual media back washable
filters which will remove 95% of all
solids down to and including 5
microns.

28. Water Injection System
Fine Filtration
• The package normally contains a number, approximately 6
filter pods, which provide the correct flux rate to be able to
remove virtually all particulate matter.
• The filters are backwashed using an air scour system to lift
and break up the bed in order to remove the solids contained
on the media.
• Fine filters take up quite a bit of space and are heavy. For
these reasons, careful analysis of the reservoir rock porosity is
necessary to ascertain what level of filtration is required.
(There is a big space and weight bonus if fine filters are not
required.)

29. Electro-Chlorination
• This occurs immediately after filtration has taken place
• This process will kill off marine organisms remaining in the
seawater so as to reduce marine fouling in piping or
equipment.

30. Sulphate Reduction
Intent : reduce the sulphate ion concentration in the sea water
Method: dose the low sulphate water with chemicals
Effect : prevent corrosion, scaling in the well and separation
equipment and reservoir souring when injected
Performance specification:
– Sulphate content 20 mg/litre
– Max particle size 10 micron

31. Sulphate Reduction Package

32. Water Injection Process
Deaeration
• Vacuum deaeration is the most common method used to
remove dissolved oxygen from the seawater.
• The warm filtered seawater will then be routed to the
deaerator where the oxygen content of the seawater will be
reduced to 0.1 mg/litre by vacuum stripping.
• Injection of an oxygen scavenger into the base of the
deaerator will reduce the oxygen content down to 0.01
mg/litre or less.

33. Vacuum Deaeration
Tower

34. Water Injection System
Deaeration – continued
• A vacuum package made up of a combination of vacuum
pumps and ejectors which creates a vacuum at each stage
within the tower.
• Each stage is separated from the next by a water seal. This
means that as the water travels down the tower it will pass
through two or three different levels of vacuum (2 or 3 stage
vacuum tower) depending on the number of stages of the
design.

35. Water Injection System
Deareator Tower

36. Water Injection Process
• The treated seawater from the deaerator will then be
pumped up to the required injection pressure by a two-stage
process:
– booster pump (to 10 bara)
– injection pump (to 200 bara)
Booster pump Injection pump

37. Water Injection System
Booster Pumps
• Two 50% booster pumps will take water from the deaerator
tower and boost the pressure up to around 10 bara.
• Booster pumps provided the necessary net positive suction
head (NPSH) for the high-pressure injection pumps.

38. Water Injection System
Injection Pumps
• The booster pumps will then feed straight into the suction
side of two 50% injection pumps which will further boost the
pressure up to its final injection pressure normally around
200 bara.

39. Water Injection System
Injection Pumps - continued
• The filtered and oxygen-free seawater is then finally pumped
up to the required injection pressure by multi-stage injection
pumps.
• These high-pressure pumps are normally one of the biggest
power users. Their power requirement is of the order of
several megawatts.
Water Injection manifold

40. Water Injection System
Sterilisation
• Sterilisation of the injection water will be carried out using a
UV sterilisation package or by use of chlorinating chemicals,
downstream of the injection pumps.

41. Water Injection System
Principal Items of Water Injection Equipment
Seawater Lift Pumps 3 × 50%
Seawater Coarse Filters 3 × 50%
Seawater Fine Filters 6 × 20%
Deaerator Tower 1 × 100%
Booster Pumps 2 × 50%
Injection Pumps 2 × 50%
Sterilisation Package 1 × 100%

42. 3. Understand the produced water system
operating principle.

43. Produced Water System
• The other type of water for injection is Produced water.
• Produced water from the HP Separator and LP Separator is
normally treated in a produced water treatment package.
• This package normally consists of a bank of hydrocyclones
and a degassing vessel.

44. Produced Water System
• Produced water from the electrostatic coalescer can either be
–pumped up to the pressure of the HP Separator,
commingled with the HP Separator water and prepared for
re-injection or
–sent to the ships slop oil system and treated as part of the
ships oily water system.

45. Produced Water System
Treatment for Re-Injection
•Hydrocyclones / Centrifuges
– The level of oil in the produced water is reduced by
hydrocyclones where the g-forces are increased by
hundreds
•Degassers
– The level of gas in the produced water may be reduced by
vacuum stripping in a degasser

46. Hyrdocyclones – Working Principle
Liquid/Liquid Hydrocyclones
• Based on the physics of enhanced gravity separation and free
vortex action
• a cylindrical inlet, a tapered tube with the liquids entering
tangentially via inlets
• These forces & differential pressures, set up across the
hydrocyclone, allow:
– the heavy phase to exit at the underflow
– the lighter phase reverses flow and exits the overflow at
the opposite end

47. Produced Water
Disposal of Unwanted Water
As oily water (water for disposal overboard):
• The treated produced water will contain a maximum 15 ppm
free oil and will have been degassed down to atmospheric
pressure before disposal to the sea via a produced water
disposal caisson or appropriate sea chest.
(ref: MARPOL Annex 1 Regulation 15)

48. Oil Content Criteria
Less than 15 ppm
− Will not produce an oil sheen on the
water
Between 15 to 100 ppm
– Produce a sheen (but not a slick) which
would be absorbed by the sea or
evaporate within 3 to 4 hours.
– 15 ppm can be smelled but not seen
– 100 ppm may be seen as a sheen with
slight discolouration
More than 100 ppm
– Produce an oil slick which would
eventually reach shore

49. Produced Water
Specification for Produced Water for discharge into sea:
Handling capacity : 70,000 BWPD
Discharge temperature range : 50 to 80C
Oil content : 15 ppm (free oil)

50. Key Question
Seawater is treated before being used for high-pressure water-
injection. Explain the processes involved in the sea-water
treatment using a block diagram.