1. Total System Optimisation in Gas-Lifted Fields
ASME/API/ISO Fall 2003 Gas-Lift Workshop , Kuala Lumpur,
October 21-22, 2003
ZR Lemanczyk & CJN McKie
Edinburgh Petroleum Services 1

2. Optimisation in Gas-Lifted Fields
Optimisation: maximisation of ‘benefit’ subject to constraints imposed by external conditions and the performance of the producing system
Reservoir
Wells
Pressure drops in pipes
Performance of surface equipment
Delivery pressures 2

3. Single Well Optimisation
Gas Injection Rate
Oil Production or $/day
Increasing THP
Lift Gas -$
Oil
Water +$
Export Gas Q P 3

4. Single Well Optimisation
Assumptions
Fixed tubing head pressure for all gas lift rates
Lift gas is available to the well at the rate and pressure required
Considered in the optimisation
Combined reservoir inflow and tubing outflow performance
Not considered in the optimisation
Effect of other equipment on the well and vice-versa
How lift gas is supplied to the well
Whether lift gas injected into this well would give more benefit in another well 4

5. Multi-well Optimisation
Prod Manifold
Flowline
Prod Manifold
Prod Manifold
Export Gas
Lift Gas
Water
Oil -$ +$ +$
Flowlines
Flowlines
Flowlines
Lift Gas
Lift Gas
Lift Gas -$ -$ -$ 5

6. Multi-well Optimisation
Assumptions
Fixed separator pressure
Fixed total lift gas availability
Considered in the optimisation
Interactions between wells in production gathering network
Optimal allocation of limited supply of gas between wells
Not considered in the optimisation
How lift gas is supplied to the well
How changing operating conditions affect total amount of lift gas available 6

7. Total System Optimisation
Prod Manifold P
Prod Manifold
Prod Manifold
Water
Oil -$ +$
Lift Gas Manifold
Export Gas +$ P
Lift Gas P Q P P P P P
Fuel Gas Q
External Fuel Supply Q -$ 7

8. Compressor Performance
Speed
Power
100%
Decreasing Efficiency
Gas Turbine
Stonewall
Surge
Suction Flowrate
Discharge Pressure
Increasing Speed
Compression Stage
Fuel Gas 8

9. Relationship between CHP and Qgi
Gas Injection Rate
Increasing CHP
Ptub at operating valve
Gas Injection Rate
Valve
Tubing 9

10. Well E Performance 10

11. WPS for GL Optimisation
Well Completion Details
Tubing
Gas-lift Valves
Open interval(s)
Reservoir completion
Reservoir Pressures
kh, Skin, PI
Production Tests
FBHP, FBHT Observations
Flowing Gradient Surveys
Well Performance Surfaces
Production System Geometry/Dimensions
Well Active/Inactive Statuses
Production Choke Sizes
Current Flow Routing (Block Valve Statuses)
Surface Eqpt Active/Inactive Statuses
Current Pressures and Flowrates
Economic Parameters
Optimiser Recommendations 11

12. Solution Technique
A model is constructed containing all of the wells, the gathering & distribution networks and the surface equipment
The optimal solution to the model is found using Sequential Linear Programming (SLP)
Generic optimisation capability which can be applied to many different types of problems
Simultaneous simulation and optimisation
Proven ability to handle large, non hierarchical networks with loops and branches and hundreds of wells 12

13. Automation & Optimisation
Off-line
Data input manually into system model
Results from system model implemented manually
Open loop
Data input automatically into model from SCADA
Results from model implemented manually
Closed loop
Results from model implemented automatically via set point controllers
Operator review may be required to ensure that implemented results are “sensible” 13

14. Open Loop Optimisation
Closed Loop Optimisation
Open Loop Optimisation
Data Output
Automatic Data Input
Optimiser
Data Output
OPERATOR
Advice
Approval
Implementation
OPERATOR
Advice
Approval
Implementation
Field
Setpoint
Controllers 14

15. Offline Optimization Workflow
Current production equipment and network status
Corporate economic parameters
Production Report
Process takes weeks
Highly skilled resource required
performance curves
Enter and validate production test data, re-tune well models
Import updated well models into network models
Update network model
Run optimization Review and output optimiser results
Implement in field
Archive well and network models used for optimization
Update corporate information systems 15

16. Why Online? Large Number of Wells Complex optimisation problem
Reduce cycle time
Optimisation-to-implementation
Engineer’s time concentrated on value adding tasks
Goals
Automate Process
Automate Repetitive Tasks
Optimise 16

17. Sustainable Production Optimisation
Sustained Gains
Gains
Optimisation gains revert to norm as system changes: automation of process is key to sustain the gains
Increased Value Over ‘Do Nothing’
Time
Simple Manual Optimisation
Complex Manual Optimisation (Offline)
Complex Automated Optimisation (i-DO) 17

18. Network Model and Optimizer
Online Optimisation
SCADA Historian
Expired Data
Historical Data
Process takes minutes
Fully automated
Server
Real-time Data
Production Data
Management
Production Tests
Well Models
Targets
Conditions, Status
SCADA Server
Well Perf Curves
Network Model and Optimizer
Optimized Set-points
Corporate Database
Economic Parameters
Results
LAN/WAN/Internet/Intranet
Review, Approval
Engineer PCs
Process Data
Web Client 18

19. Conclusions
Optimisation considering the total system can deliver additional production gains and costs savings over and above considering the production gathering network alone
The capability to perform total system optimisation in gas-lifted fields exists today.
A number of online gas lift optimisation systems have been installed and are operational today. 19

20. Case Study: PDVSA - Venezuela
Lake Maracaibo, Venezuela
Large-scale implementation of gas lift
Pilot Area (Centro Lago)
Over 200 wells
4 separation plants
5 compressor trains
10 lift gas manifolds
3.5 million BOPD
1.5 million BOPD
MARACAIBO
CABIMAS 20

21. Case 1: PdVSA On-Line
SCADA data automatically loaded to give current block valve and compressor status and to constrain the optimisation to stay close to existing operating conditions
Price/cost and equipment constraint data loaded from Corporate databases
Gas injection well set-points sent directly to SCADA controllers (after production operator review as a block)
Recommended pressure control valve set-points and compressor operating conditions sent to production operators in open-loop advisory messages
Results stored in central database for access by other applications. 21

22. Optimal Separator Pressure
Separator pressure has to be high enough to transfer gas to compression plant
Total System Optimisation showed that it was possible to simultaneously reduce Psep and Qgi
3% increase in oil production, 14% decrease in lift gas requirement
From Wells
Significant Pressure Drop
Water
Oil
To Wells
Export Gas
Lift Gas 22

23. Field Implementation Results23

24. KOC POIS
In 2000 KOC awarded a contract for Production Optimization and Information Systems to a consortium including EPS
Objective - Deliver an integrated optimization and advisory system interfacing to the automation and SCADA systems covering four fields in North Kuwait
Scope
Four fields in North Kuwait including 411 well strings
Of these 33 are water injectors, 91 gas-lifted producers, and 30 wells producing with ESP’s.
Complex network configuration allows wells to be switched between high, medium and low pressure separators as well as between wet and dry trains. 24

25. KOC POIS Production Operation Information System (POIS)
EPS in partnership with Aspentech
On-Line Optimisation (inc. GL) implemented
253 NF + 91 GL + 34 ESP + 33 Injectors = ~400 wells
5 Fields; 8 Producing Layers; 13 Fluid Models
600,000 bopd
217 SCADA – RTU Systems
Engineering support contract awarded 2003 despite intense competitive pressure 25

26. KOC POIS 378 producers connected to 3 GCs through of 21 headers
Abdali:
16 Wells
to 2 GC
via 13 6in lines
378 producers connected to 3 GCs through of 21 headers
Ratqa:
20 Wells
to 2 GC
via 36in & 10in lines
Sabriyah:
142 Wells
to 1 GC thru’ 7 MF
Raudhatain:
195 Wells
to 3 GC
thru’ 7 MF each
Bahra:
5 Wells
to 1 GC thru’ 7 MF 26

27. KOC POIS Inside Raudhatain 5x7 Possible inputs from RQ & AD fields
Lift-Gas System
Lift gas to SA field
9 Sub-sheets holding wells each
7x3 MF interconnecting all 194 wells across the field
7x3 MF outlets to production terminals 27

28. Inside Raudhatain Subsheet RA-1
KOC POIS
Inside Raudhatain Subsheet RA-1
Multi-header choice for lift-gas
at present & for future
Multi-MF choice for production
at present and for future
NP & GLwells (FG) 28