1. Johan Castberg Production Challenges
Brian Dahl Kristensen | Statoil

2. Outline General overview Drainage strategy
Gas re-cycling and temperatures
Wax & hydrates
Network modelling
Effect on design
Producers
Subsea
Topside
Gas injection

3. General Information Licensees: Location/Depth:
Statoil ASA 50 % (operator) Eni Norge AS 30 % Petoro AS 20 %
Location/Depth:
240 km from Hammerfest LNG
200 km from Bjørnøya
m water depth

4. Drainage Strategy Long horizontal producers Re-injection of gas
Maintain voidage (and pressure) by water injection
Skrugard
OWC m
Oil column 40 / 50 / 85 m
Pressure 148 bara
Temperature 38 oC
Oil properties
GOR 60 Sm3/Sm3
oAPI 31
Havis
OWC m
Oil column 130 m
Pressure 209 bara
Temperature 59 oC
Oil properties
GOR 110 Sm3/Sm3
oAPI 35
Drivis
OWC m
Oil column 85 m
Pressure 166 bara
Temperature 47 oC
Oil properties
GOR 100 Sm3/Sm3
oAPI 35
Gas
Oil
Water N S

5. Subsea development Wells Templates Flowlines 5 production
Oil production
Gas injection
Water injection
Free slot 1 2 3 4 km
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

6. 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

7. 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

8. 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

9. 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

10. Network modeling

11. 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

12. 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

13. 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

14. Effect on Design

15. 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

16. Design - Subsea The subsea system will be insulated
Oil production
Gas injection
Water injection
Free slot 1 2 3 4 km
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
Possible with dual flowlines

17. Design - Topside
The production stream has to be heated upstream the inlet separator
Separation temperature is 45 oC
Wax melting temperature is 55 oC

18. Design - Gas injection
Oil production
Gas injection
Water injection
Free slot 1 2 3 4 km
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

19. 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

20. Brian Dahl Kristensen | Statoil bdkr@statoil.com
Thank you!
Brian Dahl Kristensen | Statoil