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: Location/Depth: Statoil ASA 50 % (operator) Eni Norge AS 30 % Petoro AS 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 OWC 1372 m Oil column 40 / 50 / 85 m Pressure 148 bara Temperature 38 oC Oil properties GOR 60 Sm3/Sm3 oAPI 31 Havis OWC 1917 m Oil column 130 m Pressure 209 bara Temperature 59 oC Oil properties GOR 110 Sm3/Sm3 oAPI 35 Drivis OWC 1570 m Oil column 85 m Pressure 166 bara Temperature 47 oC Oil properties GOR 100 Sm3/Sm3 oAPI 35 Gas Oil Water N S

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

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 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

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 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

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

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