Benefits of Detailed Benefits of Detailed Compressor Modeling in Compressor Modeling in Optimizing Production Optimizing Production from Gasfrom Gas--Lifted FieldsLifted Fields

Manickam S. NadarManickam S. NadarGreg StephensonGreg Stephenson

ContentsContents

Optimization of GasOptimization of Gas--Lifted FieldsLifted FieldsThe Need for Modelling of GasThe Need for Modelling of Gas--Lift Lift CompressorsCompressorsCompressor Models Compressor Models –– Simple vs. DetailedSimple vs. DetailedImplementation and Calibration of Implementation and Calibration of Compressor ModelsCompressor ModelsBenefits of Modelling Compressors Benefits of Modelling Compressors –– Case Case StudyStudyConclusionsConclusions

OilWater

Export Gas

Fuel Gas

Lift Gas

Lift GasManifold

ProdManifold

ProdManifold

ProdManifold

+$-$

+$

-$

ExternalFuel Supply

Optimization of GasOptimization of Gas--Lifted FieldsLifted Fields

P

P

PP

P

P

P P

Q

Q

Q

Total System OptimizationTotal System Optimization

Optimum separator pressure Optimum separator pressure Optimum gasOptimum gas--lift header pressure lift header pressure Interactions between wells in production gathering Interactions between wells in production gathering network and gas distribution systemnetwork and gas distribution systemOptimal allocation of available supply of gas among Optimal allocation of available supply of gas among wellswellsRevenues and costs of the operation considered:Revenues and costs of the operation considered:

Revenue (Revenue (+$+$): ): Oil production, Associated gas production Oil production, Associated gas production

Cost (Cost (--$$) : ) : Water disposal, Lift gas costs (compression / fuel gas)Water disposal, Lift gas costs (compression / fuel gas)

Role of GasRole of Gas--Lift CompressorsLift Compressors

Compressors Compressors -- source of high pressure gas for source of high pressure gas for continuous flow gascontinuous flow gas--lift operations lift operations Cost of gas compression is a significant factor in the Cost of gas compression is a significant factor in the optimizationoptimizationGas turbine (GT) driven compressors are dynamic Gas turbine (GT) driven compressors are dynamic units and they should be optimized as a part of the units and they should be optimized as a part of the networknetwork

Effects of Compressor VariablesEffects of Compressor Variables

Maximum gas throughput is not constant, but Maximum gas throughput is not constant, but depends on:depends on:

Suction pressure and therefore separator pressureSuction pressure and therefore separator pressureDischarge pressure and therefore gasDischarge pressure and therefore gas--lift header pressure lift header pressure (CHP, with (CHP, with ∆∆P in the injection line considered)P in the injection line considered)Power available and therefore ambient temperature and Power available and therefore ambient temperature and condition of the GG condition of the GG

All three variables (Psuct, Pdisc and Qg) impact the All three variables (Psuct, Pdisc and Qg) impact the compression power requirement compression power requirement -- should be should be considered together and not in isolationconsidered together and not in isolation

Types of Compressor ModellingTypes of Compressor Modelling

Can model in different levels of detailCan model in different levels of detailSimple compressor model:Simple compressor model:

−

−η

=η−

1PP.

1kk.T.Q.Z.0845.0Power

k1k

suct

discgasavg

Relates compression ratio, horsepower and flowRelates compression ratio, horsepower and flow--rate, but rate, but cannot model limits on cannot model limits on Qgas, Psuct and PdiscQgas, Psuct and PdiscSpeed not modelledSpeed not modelledGas Turbine not modelled Gas Turbine not modelled -- dynamics associated with the dynamics associated with the driver not considereddriver not considered

Detailed Compressor ModelDetailed Compressor Model

Models the performance of real machineModels the performance of real machineOperating range Operating range -- surge and stonewall, and recycle modelledsurge and stonewall, and recycle modelledEach compressor stage modelled separatelyEach compressor stage modelled separately

Splits the power from driver for multiSplits the power from driver for multi--stage compressionstage compression

Efficiency as a function of speedEfficiency as a function of speedGT modelled separately GT modelled separately

Can examine effects of degradationCan examine effects of degradationModels variation of power with ambient temperatureModels variation of power with ambient temperatureFuel gas rate calculated and automatically removed from process Fuel gas rate calculated and automatically removed from process streamstream

Accessories (gear box, interAccessories (gear box, inter--coolers, separators, recycle loop) coolers, separators, recycle loop) modelledmodelled

Compressor Stage Performance CurvesCompressor Stage Performance Curves

(Assuming constant suction conditions)

Speed

N2

N3

N4

N5

Surge region – Cannot operate

Stonewall

Surge line

Pd max

Pd

P Discharge

QQ minN1

Q Suction

Compressor Power CurvesCompressor Power Curves(Assuming constant suction conditions)

Surge line

N1

N2

N3

Speed

N5

N4

Power

Q Suction

Power Required by CompressorPower Required by Compressor

P Discharge

2

N3N4

N5

Pd4

Pd4 max

N1N

Q Suction

N5N4

N3N2

N1

P4P4 min

Power

Q4Q4 min

Q Suction

Compressor Solution Region Compressor Solution Region

Power

PT Speed

GT power vs. PT speed (design)

Compressor min power vs. speed(at surge limit)

Actual operating conditionsare above this line (for higher values of Q)

Min compressor speed for lowest desired discharge P

Typical operating

region

Model ImplementationModel Implementation

^Pipe_0_2

^Pipe_1_2

^Junction_0

In Gas Q = 1.784185 MMscf/day

In Pres = 107.97 psigIn Temp = 124.91 degFIn Gas Q = 68.391824 MMscf/day

Out Pres = 477.66 psigOut Temp = 333.85 degFOut Gas Q = 68.391824 MMscf/day

In Pres = 459.65 psigIn Temp = 134.00 degFIn Gas Q = 75.876465 MMscf/day

Out Pres = 1645.00 psigOut Temp = 195.00 degFOut Gas Q = 68.391824 MMscf/day

In Gas Q = 7.484625 MMscf/day

Gas Turbine

Gearbox

CompressorStage

Scrubber

Recycle loop

Drive shaft

Fuel Gas Cooler

Model Implementation (Contd.)Model Implementation (Contd.)

Construct GT performance surface table using the Construct GT performance surface table using the performance data of the GG in serviceperformance data of the GG in service

Speed 2500 3000 3500 4000 5750Power

10136 11566 10920 10602 10385 1038511403 10782 10057 9824 9712 971212670 10097 9424 9153 9023 908713937 9978 9153 8684 8481 848115204 -1 8959 8481 8208 812916471 -1 8866 8261 7951 778117738 -1 -1 8129 7757 757319005 -1 -1 9138 7687 744020272 -1 -1 -1 7573 724922553 -1 -1 -1 -1 7029

Model Implementation (Contd.)Model Implementation (Contd.)Model each compressor stage using performance curves using Model each compressor stage using performance curves using polynomial coefficients for head and efficiencypolynomial coefficients for head and efficiency

Hea

dC

oeffi

cien

tPo

lytr

opic

Ef

ficie

ncy

Q/N (ACFM/RPM)

Q/N (ACFM/RPM)

StonewallSurg

e

Suction FlowrateD

isch

arge

Pres

sure

IncreasingSpeed

Compression Stage

Calibration of Detailed Compressor Models Calibration of Detailed Compressor Models

Gas rates measured at different locations of the Gas rates measured at different locations of the network should be consistent network should be consistent Compressor models to be calibrated to the correct Compressor models to be calibrated to the correct SG of gas in the networkSG of gas in the networkGas flow in recycle loops should be modelled Gas flow in recycle loops should be modelled –– flow flow for surge protection or due to leaking valves?for surge protection or due to leaking valves?Head coefficient adjusted for pressure rise, and Head coefficient adjusted for pressure rise, and efficiency tuning factor adjusted for powerefficiency tuning factor adjusted for powerModel variation of power with ambient temperatureModel variation of power with ambient temperature

Example: power loss was 15% for increase in ambient Example: power loss was 15% for increase in ambient temperature from temperature from 70 to 10570 to 105°F °F

Case Study Case Study –– BenefitsBenefitsOptimization with detailed compressor models for a Optimization with detailed compressor models for a large, complex field with 200+ gas lifted wellslarge, complex field with 200+ gas lifted wellsDirect benefits: Direct benefits:

1 1 –– 3% increase in oil production3% increase in oil productionUp to 14% saving in lift gas; able to reactivate S/I wellsUp to 14% saving in lift gas; able to reactivate S/I wells

Compressor management: Compressor management: Reduced compressor operating costs by 3% Reduced compressor operating costs by 3%

Reduced separator pressure (by 5%) and gas lift Reduced separator pressure (by 5%) and gas lift header pressure (up to 3%)header pressure (up to 3%)

Benefits of Detailed Compressor ModellingBenefits of Detailed Compressor Modelling

Correctly handles tradeCorrectly handles trade--offs between Qgas, Psuct offs between Qgas, Psuct and Pdisc: resulted in lowered Psep and Pdiscand Pdisc: resulted in lowered Psep and PdiscTotal lift gas available Total lift gas available notnot assumed, but calculatedassumed, but calculatedCorrectly accounts for changes in power Correctly accounts for changes in power –– e.g. e.g. allowed shutting down a train in winterallowed shutting down a train in winterShows where the compressor is operating with Shows where the compressor is operating with respect to limitsrespect to limitsAllows shared understanding between petroleum Allows shared understanding between petroleum engineers and compressor O&M staffengineers and compressor O&M staff

ConclusionsConclusionsCompressors are an important component of a gasCompressors are an important component of a gas--lifted production systemlifted production systemThere are strong interactions between compressor There are strong interactions between compressor performance and conditions in other parts of the performance and conditions in other parts of the systemsystemTherefore, compressor performance needs to be Therefore, compressor performance needs to be considered and accurately represented when considered and accurately represented when optimising the gasoptimising the gas--lift systemlift systemWhen this is done, benefits are delivered in terms of When this is done, benefits are delivered in terms of increased production, cost savings and increased increased production, cost savings and increased understanding of total system performanceunderstanding of total system performance

End of PresentationEnd of Presentation