total system optimisation in gas-lifted fields

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e-petroleumservices.com

Total System Optimisationin Gas-Lifted Fields

ASME/API/ISO Fall 2003 Gas-Lift Workshop , Kuala Lumpur,

October 21-22, 2003

ZR Lemanczyk & CJN McKieEdinburgh Petroleum Services

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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

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LiftGas-$

Oil

Water

+$

-$

Export Gas +$

Lift Gas

Single Well Optimisation

Gas Injection Rate

Oil

Pro

duct

ion o

r $

/day Increasing THP

PQ

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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

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ProdManifold

ProdManifold

ProdManifold OilWater

+$-$

ExportGas+$

LiftGas

-$

LiftGas

LiftGas

LiftGas

-$ -$

Multi-well Optimisation

Flowlines

Flowlines

Flowline

Flowlines

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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

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OilWater

Export Gas

Fuel Gas

Lift Gas

Lift GasManifold

ProdManifold

ProdManifold

ProdManifold

+$-$

+$

-$

ExternalFuel Supply

Total System Optimisation

P

P

PP

P

P

P P

Q

Q

Q

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Compressor Performance

Speed

Pow

er

100%

100%

DecreasingEfficiency

Gas Turbine

StonewallSurg

e

Suction Flowrate

Dis

charg

e P

ress

ure

IncreasingSpeed

Compression Stage

Fuel Gas

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ValveTubing

Relationship between CHP and Qgi

Ptu

b a

t opera

ting v

alv

e

Gas Injection Rate

CH

P

Gas Injection Rate

IncreasingCHP

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Well E Performance

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WPS for GL Optimisation

Well Completion Details• Tubing• Gas-lift Valves• Open interval(s)• Reservoir completion

Reservoir Pressureskh, Skin, PIProduction TestsFBHP, FBHT ObservationsFlowing Gradient Surveys

Production System Geometry/DimensionsWell Active/Inactive StatusesProduction Choke SizesCurrent Flow Routing (Block Valve Statuses)Surface Eqpt Active/Inactive StatusesCurrent Pressures and FlowratesEconomic Parameters

Well PerformanceSurfaces

OptimiserRecommendations

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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 andhundreds of wells

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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– Data input automatically into model from SCADA– Results from model implemented automatically via

set point controllers– Operator review may be required to ensure that

implemented results are “sensible”

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Open Loop Optimisation

Optimiser

FieldOPERATOR • Advice• Approval• Implementation

Automatic Data Input Data Output

OPERATOR •Advice•Approval•Implementation

Data O

utp

ut

SetpointControllers

Closed Loop Optimisation

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performance

curves

Offline Optimization Workflow

Production Report

Enter and validate production test data, re-tune well models

Import updated well models into network models

Update network model

Corporateeconomic parameters

Current production equipment and network status

Run optimizationReview and output optimiser results

Implement in field

Update corporate information systems

Archive well and network models used for optimization

• Process takes weeks

• Highly skilled resource required

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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

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Sustainable Production Optimisation

Time

Incr

ease

d V

alu

e O

ver

‘Do

No

thin

g’

Simple Manual Optimisation

Complex Manual Optimisation (Offline)

Complex Automated Optimisation (i-DO)

Gains

Sustained Gains

Optimisation gains revert to norm as system changes: automation of process is key to sustain the gains

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Data

SCADAServer

Server

Web Client

Online Optimisation

Data

SCADAHistorian

ExpiredData

Conditions,Status

OptimizedSet-points

Results

Targets

LAN/WAN/Internet/Intranet

Review,Approval

EngineerPCs

Well PerfCurves

Network Model and Optimizer

Historical Data

Real-timeData

Production DataManagement

ProductionTests

WellModels

Data

CorporateDatabase

EconomicParameters

• Process takes minutes

• Fully automated

Process Data

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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.

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3.5 million BOPD

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

1.5 million BOPD

MARACAIBOCABIMAS

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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.

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OilWater

Export Gas

Lift Gas

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

SignificantPressureDrop

From Wells

To Wells

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Field Implementation Results

20000

11/06/00 11/07/00

Production History of Flow Station(Group of Gas Lifted Wells)

TotalOil

Rate

(bbl/day)

Start of NewGL Set Points

Average Rate 23.9 Mbbl/d

Average Rate 23.1 Mbbl/d

TIME

24 hr pumped volume of oil

LakeMaracaibo

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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.

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KOC POIS

• Production Operation Information System (POIS)• EPS in partnership with Aspentech• On-Line Optimisation (inc. GL) implemented 2000-

2002.• 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

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Raudhatain-[RA]-GC15&GC25

Abdali-[AD]

Ratqa-[RQ]

Bahra-[BH]

Sabriyah-[SA]-GC23

C-GC15 C-GC25 C-GC23

Abdali:

16 Wells

to 2 GC

via 13 6in lines

Ratqa:

20 Wells

to 2 GC

via 36in & 10in lines

Raudhatain:

195 Wells

to 3 GC

thru’ 7 MF each

Sabriyah:

142 Wells

to 1 GC thru’ 7 MF

Bahra:

5 Wells

to 1 GC thru’ 7 MF

378 producers connected to 3 GCs through of 21 headers

KOC POIS

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Lift-Gas System5x7 Possible inputs from RQ & AD fields

9 Sub-sheets holding 20-25 wells each

7x3 MF interconnecting all 194 wells across the field

7x3 MF outlets to production terminals

Lift gas to SA field

Inside Raudhatain

KOC POIS

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NP & GLwells (FG)

Multi-header choice for lift-gas

at present & for future

Multi-MF choice for production

at present and for future

KOC POISInside Raudhatain Subsheet RA-1

total system optimisation in gas-lifted fields

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