Johan Castberg Production
Challenges
Brian Dahl Kristensen | Statoil
Outline
• General overview
– Drainage strategy
• Gas re-cycling and temperatures
– Wax & hydrates
– Network modelling
– Effect on design
• Producers
• Subsea
• Topside
• Gas injection
General Information
Licensees:
Statoil ASA
Eni Norge AS
Petoro AS
50 % (operator)
30 %
20 %
Location/Depth:
240 km from Hammerfest LNG
200 km from Bjørnøya
345-405 m water depth
Drainage Strategy
• Long horizontal producers
• Re-injection of gas
• Maintain voidage (and pressure) by water injection
Skrugard
Havis
Drivis
OWC
1372 m
OWC
1917 m
OWC
1570 m
Oil column
40 / 50 / 85 m
Oil column
130 m
Oil column
85 m
Pressure
148 bara
Pressure
209 bara
Pressure
166 bara
Temperature
47 oC
Temperature
o
38 C
Temperature
Oil properties
GOR 60
oAPI
o
59 C
Oil properties
Sm3/Sm3
GOR 110
oAPI
31
35
Sm3/Sm3
Oil properties
GOR 100 Sm3/Sm3
oAPI
35
Gas
Oil
Water
N
S
Subsea development
Wells
•
•
•
21 producers all with gas-lift
9 water injectors
5 gas injectors
Templates
•
•
•
•
9 production (4 combi with gas injection)
3 water injection
1 gas satelite
1 water satelite
Flowlines
• 5 production
• 3 gas lift / injection
• 2 water injection
Oil production
Gas injection
Water injection
Free slot
0
1
2
3
4
km
Gas cycling and temperatures
•
The drainage strategy with combined water and gas injection leads to large span in gas liquid ratio
(GLR) in the producers
•
Reservoir temperatures are low to start. Re-cycling of gas enlarges the temperature challenges in
the production network
– High GLR producers have low wellhead temperature and high pressure (WHT & WHP)
– High GLR producers will see a
significant temperature drop when
choked at wellhead to meet the
flowline pressure
– Temperatures down stream choke can
drop below wax appearance
temperature (WAT) and also into the
hydrate area
Wax content and precipitation
•
Small difference between reservoir temperature and WAT
•
Wax precipitation curves are tuned to experimental data
•
Limited precipitation above 10 oC at flowline conditions
Skrugard
Havis
Reservoir temperature [oC]
38
59
WAT [oC] Pipeline conditions
26
34
Wax content [weight%]
3.7
7.9
Wax deposition
•
Wax deposition depends on:
– The pipe surface temperature has to be below WAT
– The pipe surface temperature has to be colder than the fluid within the pipe
•
Deposition occurs by radial mass transfer of dissolved wax towards the cold pipe wall
•
The driving force is mainly molecular
diffusion given by a concentration
gradient (dC/dr) created by a
temperature gradient (dT/dr)
•
Deposition of solid wax particles is
considered negligible
Recommended
cut-off
Hydrate
• The operating margin for wax and hydrate coincide.
– The recommended cut-off for wax gives a small but sufficient margin on
hydrate
Recommended
wax cut-off
Network modeling
Network modeling
•
VBA code in Excel is used to run Prosper, GAP and PVTsim coupled using open source
– Prosper:
Calculate WHP and WHT
– GAP:
Calculate pressure and temperature at nodal points along the subsea network
– PVTsim:
Calculate temperature drop across chokes
•
PVTsim is used for thermal choke calculations to achieve required accuracy
•
Prosper and GAP are run in in black oil
mode
– Good consistency with Olga
•
Yearly profiles are run in apprx. 1 hour
Network results
•
Current reservoir models (Skrugard, Havis and Drivis) do not have any producers with temperature issues
•
The two producers with highest GLR are shown for Havis and Skrugard
– H03 produces slightly below WAT but this could be adjusted by increasing the flowline pressure
Network results cont.
•
Down stream temperatures have also been checked based on well rates from a reservoir
uncertainty study
•
Down stream temperatures are based on a flowline pressure of 55 bars
Cut-off
Cut-off
Effect on Design
Design - Producers
Completion
•
Producers are installed with autonomous inflow control devices (AICD)
– Good experience with AICD’s on Troll
– MLT’s are preferably installed with branch controll
Well placement
•
Optimized distance to GOC and OWC
•
Orientation is important
– S01, S03 and S04 at the DB template
are drilled in east – west direction
– These are the producers that see the
highest GLR according to the
uncertainty study
• Orientation is based on a trade
off with Drilling and Well
Design - Subsea
•
The subsea system will be insulated
– Pipelines, spools, manifolds etc..
•
Pigging
– Yearly frequency expected
– Frequency depends on insulation
•
Dual production flowlines
– Enables high / low pressure production
•
Hot oil circulation
Oil production
– Possible with dual flowlines
Gas injection
Water injection
Free slot
0
1
2
3
4
km
Design - Topside
•
The production stream has to be heated upstream the inlet separator
– Separation temperature is 45 oC
– Wax melting temperature is 55 oC
Design - Gas injection
•
Good flexibility on alternating rates between the
injectors
– Moving gas between the gas caps changes the
reservoir balance and hence the GLR in the
producers
•
Gas is preferably injected:
– Where there are dual production flowlines to
enable HP / LP production
– In Havis which has the highest reservoir
temperature
Oil production
Gas injection
Water injection
Free slot
0
1
2
3
4
km
Summary
•
Low temperatures result in issues with wax and hydrates
– Sets an upper limit on GLR in the producers
•
Uncertainty study suggest that the expected number of wells that have to be shut in due to low
temperatures is less than one
Mitigating actions
• Producers
– AICD’s
– Orientation and placement
• Subsea
– Insulation
– Dual pipelines
• Topside
– Heating to enable separation and melting of wax
• Gas injection
– Optimize injection with regard to GLR in the producers
Thank you!
Brian Dahl Kristensen | Statoil
bdkr@statoil.com