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IPA01-E-125

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Eighth Annual Convention & Exhibition, October 2001

DEHYDRATION AND BOOSTER COMPRESSION UPGRADE,

ARUN FIELD, NORTH SUMATRA

Syamsul Bahri* Tania Irani*

ABSTRACT The Arun Field has been producing since 1977 and, as of January 1, 2000, has produced over 90% of the total recoverable reserves. The average reservoir pressure is currently below 1000 psig, compared to the original pressure of 7,100 psig with an average reservoir temperature of 340o F. Because of the high reservoir temperature, a large amount of water is vaporized at reservoir conditions, which must then be removed at the surface. Upgrading the Arun Field dehydration facilities and booster compression will enhance gas deliverability. Currently, dehydration facilities are designed to operate down to a compressor suction pressure of 150 psig. At lower pressures, the dew point temperature of the gas in the facility and pipelines increases above 85o F. This condition has the potential for free water condensation that could lead to rapid corrosion in the gas pipeline to the LNG Plant. Upgrading of the existing facilities will allow operation down to 40 psig, while still meeting the dew point specification. Several processing alternatives were considered which included: glycol dehydration at the inter-stage of clusters, and re-commissioning of the temporarily abandoned High Pressure (HP) propane refrigeration dehydration equipment. Use of corrosion inhibitors in the 42” pipeline was considered but eliminated due to reliability concerns. ___________________________________________________________ * ExxonMobil Oil Indonesia, Inc.

Re-commissioning of HP refrigeration equipment is the most economical option due to lowest cost and the fact that most of the equipment is already installed at site, and it builds on the process knowledge of the operations and maintenance personnel. Upgraded compressors will result in lower community noise and lower fuel consumption, since fewer machines will be required on line. Redundant machines can potentially be used in other areas. INTRODUCTION The Arun Field was discovered in 1971 and has been producing since 1977. The current average reservoir pressure has now fallen to below 1000 psig, compared to the original pressure of 7,100 psig. The reservoir temperature is 340o F. The Arun and other fields have supplied the gas and unstabilized condensate since August 1978 to the P.T. Arun LNG Plant. Currently, besides the Arun fields, the P.T. Arun LNG Plant also receives gas and condensate from newer fields such as South Lhok Sukon (SLS), Pase and NSO (North Sumatera Offshore). The P.T. Arun LNG Plant is located about 10 kilometres west of Lhok Seumawe near the coast of the Straits of Malacca in the Aceh province of North Sumatera, Indonesia. Until recently, the Arun Field had excess deliverability to meet more than the P.T. Arun LNG Plant gas demand. Now, the Arun Field reservoir pressures have declined to low levels resulting in a significant decline in the gas production rate. One option that was evaluated to arrest the decline was to accelerate production from the Arun Field by a further reduction in wellhead pressures. This paper describes ExxonMobil's successful efforts in upgrading the surface facilities at Arun Field to overcome the impact of lower wellhead pressure and to

© IPA, 2006 - 28th Annual Convention Proceedings, 2002

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help meet future gas demands by maintaining overall production efficiency. METHODS Assumptions

The global assumptions that were made throughout this study are listed below: 1. PT. Arun LNG Plant minimum required gas arrival

pressure set at 860 psig 2. Water dew point target 85oF at Arun Field booster

compressor discharge 3. Ambient temperature 90o F 4. Process throughput set by compressor capacity Project Design Basis The design data used in this project were a combination of PEGASUS reservoir projections and current operating data. PEGASUS is an ExxonMobil proprietary reservoir modelling tool. PEGASUS projections were available for Arun through year 2005. For most of the other fields, the current operating data is assumed to be applicable in the future with changes only in water content. New fields were assumed to have similar compositions to operating fields. The design is based on making optimal use of the existing facilities and with a high degree of integration with the existing plant. Project Objective and Strategies The project objective is to ensure that dehydrated gas delivered to the P.T. Arun LNG Plant has a temperature at least 10o F higher than the water dew point temperature. The booster upgrade will maintain the overall efficiency of compressor trains by fully utilising all of the available horsepower. Completion of the Cluster IV booster compressors by 1st qtr, 2002, would yield another benefit by eliminating one of the turbine major overhauls that is scheduled for the year 2001, as well as increasing the fuel efficiency which will positively impact the LNG deliverability in years 2002 – 2004, and beyond.

This modification will also eliminate a major overhaul in Cluster-II. Both Clusters-II & -IV will require the Hot Gas Path Inspection (HGPI) for the turbines that will not undergo major overhaul. The Dehydration Upgrade project and Booster Compression Upgrade project have been covered under separate AFEs but managed by one project team. ARUN FIELD OVERVIEW The Arun Field is produced through four individual clusters (I, II, III and IV) and routed through a central facility known as Point "A" for metering and distribution. Each cluster has two parallel production trains consisting of producing wells, wellstream coolers, propane refrigeration for dew-point control, and liquid hydrocarbon and water separation. Wellhead tubing pressures vary among clusters ranging from 200 psig at Cluster I to 400 psig at Cluster IV. Separator gas flow rates vary from approximately 300 MSCFD to 350 MSCFD, respectively. The inlet gas stream is cooled with air and then chilled with propane to condense water and hydrocarbons resulting in a dehydrated gas stream. Unstabilized condensate and the dehydrated gas stream are transported to P.T. Arun. Each of the four clusters has its own propane refrigeration system supporting the two production trains. With the reduced pressures at Clusters I and II, chiller temperatures are operated at about 40o F to achieve dew-point requirements. The chillers at Clusters III and IV are operated at 45o F. In 1995, six GE Frame 5 turbine drivers operating two stages of compression each were installed. Three compressor trains were installed at Cluster II and three at Cluster IV. Dehydrated gas from Clusters I and II is commingled at the compressor suction at Cluster II, and dehydrated gas from Clusters III and IV is commingled at the compressor suction at Cluster IV. The dehydrated gas is compressed from 150 psig to 1000 psig at Cluster II and from 300 psig to 1070 psig at Cluster IV with the discharge headers linked at Point "A". From Point "A" gas is delivered to P.T. Arun at approximately 860 psig via a 42 in. pipeline.

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Current Operations Data Gas production from the Arun Field, SLS and Pase is gathered to supply gas flow to the LNG Plant. Peak production rate of separator gas is about 2.4 Bcfd, of which 2.2 Bcfd is delivered to P.T. Arun via a 42 in. pipeline. Approximately 160 MSCFD of separator gas is delivered to National Projects via a 16 in. pipeline. A second 16 in. pipeline transports about 75 MMSCFD of Pase and SLS commingled stream as fuel to PT. Arun. Since 1999, the NSO gas has been transported via a subsea pipeline at a rate of 370 MMSCFD directly to the onshore of P.T. Arun plant. Unstabilized condensate field volumes vary between 25 and 30 MBPD and are delivered to P.T. Arun through a 10 in. pipeline. Produced water is disposed of at each cluster through existing water disposal facilities. Projected Operations Data Beginning in late 2002 and extending through the middle of year 2005, field operations must be optimized to produce the gas supply volumes required to meet the P.T. Arun LNG sales demand. Projected Arun demand volumes require maximum production from the Arun Field combined with continued production from SLS, Pase and NSO. Also, some new fields have to be developed and are expected to be on line in 2002 to replace the decline from the Arun Field. Assessment of Existing Surface Facilities at Low Pressure Operation a. Dehydration Facilities Dew point control at the Arun gas production

clusters is necessary to prevent free water dropout in the carbon steel line from the discharge of the booster compressors to the LNG plant. Currently, the produced gas is dehydrated downstream of the wellhead coolers by dropping its temperature to approximately 40oF in "low pressure" (LP) propane chillers. The gas is reheated through gas-to-gas exchange. The gas is then compressed by the boosters, and sent to the LNG plant.

As the wellhead pressures decrease in the Arun

Field, the effectiveness of the existing dehydration system to control the dew point decreases. Figure 6 illustrates the change in water dew-point temperature as a function of wellhead

tubing pressure. As a basis for calculating dew-point temperature, the chiller temperature is set at 40o F and the compression discharge pressure is set at 1000 psig.

The chiller temperature is set at 39-40o F to

minimize the risk of producing hydrates. As operating pressures decrease, the chiller temperature becomes more critical. A swing in chiller temperature below 38o F increases the potential of hydrate formation. A swing in chiller temperature above 40o F results in an elevated water dew point temperature.

Similar to changes in chiller temperature,

fluctuations in compressor discharge pressure, at constant wellhead tubing pressure and chiller temperature, impact the water dew-point temperature. While a reduction in discharge pressure decreases the water dew point temperature, an increase in pressure elevates the water dew point.

b. Compression Facilities Booster compressors at the Arun Field are

operating with 2 stages, the Low Pressure (LP) side increases the pressure from 150 psig to 450 psig and the High Pressure (HP) side delivers the gas at approximately 1000 psig.

Without the compressor upgrade project, the six

existing Arun compressors would operate through 2004 at low efficiency. The existing booster compressor was designed for a minimum suction pressure of 85 psig. Operation at lower pressure will impact in the reduced efficiency of the compression.

Process Simulation Development Process simulations were performed to match the operation of the existing LP chilling systems. Appropriate parameters were then applied to the proposed HP chilling process. Cases were run for upgraded booster compression capability, which represents the greatest load on the LP and HP chilling systems. Pro-2 from Simsci was used to build-up the model and more rigorous heat transfer calculation was performed using Hextran 8.0.

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FEASIBLE ALTERNATIVES Dehydration Upgrade a. Interstage Glycol Dehydration Below 200 psig wellhead tubing pressure, the

ability of the existing dehydration system to achieve an 80-85o F water dew point becomes limited. This limitation occurs even if Low Pressure compression is not installed. To supplement the existing system, a Tri-Ethylene Glycol (TEG) dehydration system was considered.

b. Post Refrigeration Using HP Propane (HP

dehydration system) It was proposed to reinstate existing redundant

HP propane chilling dehydration equipment to lower the dew point after the HP booster discharge fin fan cooler, thereby eliminating the concern for water dropout downstream of the HP chilling system.

The redundant equipment includes a gas-to-gas

exchanger and propane chiller for each booster compressor set. Spare propane compression is available from the LP chilling systems. Dew point depression would be achieved in the HP system via a similar mechanism of cooling the gas (in this case to 80o F), condensing water vapor, and reheating the gas through gas-to-gas exchange, as is currently done in the LP chiller system (which would remain in operation).

c. Other options Other options considered include glycol injection,

corrosion inhibitor injected into the 42 in. pipeline and hot gas discharge into the 42 in. pipeline. Glycol dehydration has an advantage over the other options on the basis of process reliability and minimal impact to P.T. Arun gas.

Booster Compression Upgrade Several compressor design alternatives were evaluated by Mobil in their 1999 study. In an analysis of existing compression, the maximum capacity curve was constrained by ‘actual cubic feet per minute’ and maximum speed, not by available horsepower. At low

suction pressures, existing machines are utilizing only 60% of the available horsepower. The unused horsepower offers potential for lower wellhead pressures to be achieved by modifying compressors and not adding any more horsepower. Existing compressor manufacturer, Demag Delaval, was asked to propose compressor modifications to increase flow rate at low pressures up to the maximum constraint of installed horsepower. Demag proposed two options. The first one is to replace both LP and HP compressor internals (rotors and bundles). The second option is to replace the complete LP stage compressor with a new (larger) case and internals. During the 2000 re-evaluation, Demag Delaval developed a revised option of replacing the LP compressor using high efficiency brazed impellers. Initial work on the new LP case option assumed that all six Arun machines would be converted (the 6-0 option). The reservoir model indicated the resulting production uplift was not as high as expected. The field appeared to be reservoir constrained instead of compression constrained at lower pressures. The 4-2 upgrade option (4 compressors modified, 2 compressors not modified) became the base case due to reduced capital investment and more favorable economics. It was indicated that some of the acceleration in production would be delayed versus the 6-0 option, but overall reservoir recoveries and fuel savings would be similar. Subsequent evaluations then revealed that even with the 4-2 option, the field was reservoir constrained at lower pressures, and production uplift at the lower compression suction pressures would be marginal. Fuel and maintenance savings appeared to generate more benefits than the lower operating pressures yielded in increased production. The final project option of 3-2 was arrived at, comprising 3 modified machines, two unmodified machines for peak load and one machine decommissioned. With upgraded compressors, a higher capacity at lower pressure results in a significantly higher actual flow rate through process facilities, which in turn requires bigger diameter compressor scrubbers, increased interstage cooling capacity, and larger LP suction piping.

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Further detailed engineering showed the opportunity to reduce costs by switching around existing LP and the HP scrubbers to take advantage of the larger HP scrubber sizing on the compressor suction where it was needed. The existing LP scrubber has both the required size and rated pressure to switch to the HP service. A further benefit of upgrading the booster compressors is that initially one, and later two surplus machines may be relocated to SLS and Pase. This offers potential cost savings of $ 18 MM per machine to SLS and Pase compression projects against the purchase of new equipment. a. Process Simulation Result

Simulations indicate that the option of using HP dehydration is viable for protecting the pipeline between the HP chillers and the LNG plant. However, consideration must be given to the carbon steel equipment immediately upstream of the HP dehydration system (i.e., discharge cooler, HP booster recycle, and the 400 meter pipe to the HP gas-to-gas exchanger). Control of the HP discharge cooler temperature to maintain a buffer of at least 10 oF above the gas dew point is recommended to prevent water dropout in this part of the system.

b. Analytical Model To ensure that model results are valid, a quick

check with a simple analytical model was performed. At the wellhead, it is assumed that the gas is saturated with water vapor at 250 oF, regardless of pressure. Since the vapor pressure of water is approximately 30 psia at 250 oF, one can obtain a good estimate of water concentration in the vapor phase, based on total system pressure. For example, if the pressure at wellhead is 170 psig (185 psia), the saturated water concentration will be roughly 30/185 = 16.2 mol% = 16.2 vol%. Likewise, at a wellhead pressure of 95 psia (booster suction at 50 psig), the water vapor concentration will be 30/95 = 31.6 %, nearly double that of current conditions.

The vapor pressure of water at 40 oF is 0.127 psia.

Thus, at the LP chiller, the concentration of water

vapor at 83 psia (95 psia less 5 psi pressure drop each from the wellhead cooler and one side of the gas-to-gas exchanger, less 2 psi in the chiller) is 0.1217/83 = 0.146 vol%. Now, if this gas is compressed to 1000 psia, the total pressure of water becomes 0.1217 x (1000/83) = 1.46 psia. This corresponds to a dew point of approximately 116 oF. Thus, it is easy to see why dew point becomes a problem at low suction pressures. These simple calculations indicate that the model is functioning properly.

SELECTED OPTION Post Refrigeration Using HP Propane Re-commissioning of HP refrigeration equipment is the most economical option due to lowest cost and the fact that most of the equipment is already installed at site. A new 400 meter piping will be installed connecting the HP booster discharge cooler with the HP dehydration. A louver will be installed at HP discharge cooler to maintain the temperature in this part of process always 10 oF higher than gas dew point. The project cost is $ 4.5 million compared to $20.6 million for glycol dehydration. The project start-up is re-scheduled from early 2001 to end of year 2001. Replacement of LP Compressors Upgrading of Arun Field booster compression facilities in Clusters II and IV will allow operation at lower pressure. The modifications will allow operation down to 40 psig suction pressure with 3 compression trains. Replacement of low pressure compressor cases and rotors in three (3) out of six compression trains was investiged. Piping to the Compressor LP and HP scrubbers will be reconfigured, so that the current LP scrubber, strainer, and compressor suction piping will become that for the HP compressor service, and the new LP compressor service will have the current LP scrubber, strainer, and suction piping. The project cost is estimated at $ 9.5 million reduced significantly from the previous plan that

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would have cost $ 15 million. The project is planned to be completed by mid 2002. CONCLUSIONS This paper describes the use of an in-depth technical evaluation done by EMOI to justify the lowest cost option but achieving the target to upgrade the existing surface facilities. The dehydration upgrade will allow operation down to 40 psig, while still meeting the dew point specification and the LP booster upgrade will result in significant fuel savings with relatively minor deliverability impact. Modified machines have significantly higher capacity, so fewer machines can deliver the same volumes. Higher capacity reduces surface pressure and increases field deliverability.

ACKNOWLEDGEMENTS A special debt of gratitude is extended to Mike Pepper and other engineers in the Arun 2002-2004 Projects, who have directly contributed to the preparation of the text by their suggestion and criticism. A few select words are inadequate to describe the help gained from these truly professional engineers. Special thanks are extended to EMOI Management, D. Dale Pittman, Syahrizal Tamin, Suresh Batra and Jeffrey Tatarzyn who gave a full support to complete this paper. REFERENCES MTC-Upstream Facilities, 1994. Optimizing Surface Facilities, Deliverability Optimization Project (2002-2004), MEPTEC. Gas Processor Suppliers Association, 1987. Engineering Data Book, Volume II, Dehydration, GPA.

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FIGURE 1 – Aceh Production Operation – Flow Schematic .

CONDENSATE TO POINT A

CONDENSATE WATER SEPARATOR

WET GAS

PRODUCTION WELLHEAD

PRODUCED WATER TREATMENT

PLANT ( PWTP )

DEHYDRATION SYSTEM

BOOSTER COMPRESSOR

POINT “ A “

POINT “ B “ PT ARUN NGL

GAS CONDENSATE

NATIONALPROJECT

GAS LINE

LIQUID LINE

WATER LINE

CONDENSATE LINE

DRY GAS LINE

NOTES :

PRODUCTION SEPARATOR WELL STREAM

COOLER

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FIGURE 2 – Aceh Production Operation – Key Facilities.

• Currently dehydration facilities are designed to reduce the gas dew point using propane refrigeration down to 150 psig; dehydration upgrade is required to allow operation down to 50 psig.

• Current facilities are designed to cool the gas to 40+ F using propane refrigeration to avoid water dropout in the downstream carbon steel equipment (compression) and pipelines. Arun gas contains 16 %+ CO2 and presence of liquid water will lead to corrosion.

• For operation below 150 psig, the following modifications are planned to prevent water from condensing:

• Existing propane chiller temperature will be reduced to 40º F and compressor after cooler temperature will be controlled to prevent water dropout in the recycle loop.

• Redundant dehydration equipment (gas exchanger, propane chiller and separator) will be put in service to cool the gas to 85º F downstream of compression in Clusters II and IV to meet PT Arun pipeline specification (no water drop out).

COMPRESSOR REFRIG PROPANE

WELLSTREAM COOLER

WELLS

ACCUMULATOR

PRODUCTION

TO COMPRESSION TRAIN

GAS/GAS EXCHANGER

C3 CONDENSER

SEPARATOR SEPARATOR

C3

LP DEHYDRATION & SEPARATION LP DEHYDRATION & SEPARATION

CL-IV

CL-III

POINT-A

CL-II

CLUSTER PASE A

SLS-B3

S.LHOKSUKON-D

CLUSTER S.LHOKSUKON-AS.LHOKSUKON-D2

BUKIT HAGU

TRANSMIGRATION

MALACCA STRAIT

CL-I

LHOKSEUMAWE

NSO-A

0 10KILOMETERS

ACEH PRODUCTION OPERATION

Key fac i l i t i es

PT. KKA

LANDING

P.T. PIM

ROAD

PIPELINE

A-13

LHOK SUKON

PANTON LABU

SIMPANG ULIM

TO MEDAN

CUNDARAYEU

SLS 'B'

PASE 'B'

18” PIPELINE300 MSCFD CAPACITY

12” PIPELINE135 MSCFD CAPACITY

30” PIPELINE600 MSCFD CAPACITY

30” SUBSEA GASPIPELINE

100 KM

30” GASPIPELINE

PIPELINES 42” SEP. GAS 20” INACTIVE 10” NGL

PIPELINES 16” COND. 16” RESIDUE

ARUN LNGPLANT

HUMPUSAROMATIC

ASEANACEH FERTILIZER

EXXONMOBIL OIL INDONESIA

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FIGURE 3 – Dehydration System Upgrade Project.

3 BOOSTER COMPRESSION

400 METERS NEW PIPING TSC

FROM WELLSTREAM TRAIN 2

TRAIN 1

TRAINS

TRAIN 1

TO PIPELINE

TRAIN 2

TRAIN 1 HP DEHYDRATION TRAINS

(CURRENTLY ABANDONED)

TRAIN 2

EXISTING LP DEHYDRATION TRAINS

EXISTING PROPANE COMPRESSION TRAINS

CLUSTER PRESSURE

(PSIG)

LP CHILLER

TEMP/WATER CONTENT (°F/LB H2O/MMSCF)

BOOSTER AFTER COOLER TEMP *

(° F)

HP CHILLER **

TEMP/WATER CONTENT ( F/LB H2O/MMSCF)

DESIGN

PRODUCTION RATE (MMSCFD)

>150 40°/40 110° NA/37 675 + 150 46°/45 120° 85°/37 675 100 41°/60 123° 85°/37 675 50 38°/80 130° 85°/37 675

* DESIGN BASIS TO MAINTAIN OPERATING TEMPS A MINIMUM OF 10” ABOVE DEW POINT TEMPS ** 42” PIPELINE SPEC OF 37 LBS H2O/MMSCF

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FIGURE 4 – Booster Compressor Upgrade Project.

- NEW PIPING/EQUIPMENT - PIPING REMOVED/DISCONNECTED - EXISTING FACILITIES

KEY

PROJECT COST = $ 9.5 MM

HP DEHY TO

NEW REPLACEMENT 1ST STAGE COMPRESSOR

EXISTING 2ND STAGE COMPRESSOR

TURBINE

LP DEHY

FROM

INTERSTAGE &

AFTER COOLER SUCTION

SRUBBERS

INTER-STAGE

FUEL SAVINGS = 16 MMSCFD (shutdown 2 machines)

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FIGURE 5 – Existing vs Upgrade Compressor Capacity.

PASE

SLS

LP DEHY

CLUSTER-II HP DEHY

LP DEHY CLUSTER-II

BOOSTER COMPRESSOR

CLUSTER-I SEPARATION &

BOOSTER COMPRESSOR

CLUSTER-III

& LP DEHY

SEPARATION

EX.NGL

SLS BOOSTER

EX.NGL

7THBOOSTER

HP DEHY CLUSTER-IV

LP DEHY CLUSTER-IV

POINT A

X X X X X X X X X X X X

X X X X X X X X X X X X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

CLUSTER 2

CLUSTER 4

CLUSTER 3

PSIG

INTERNALS), CAPACITY,

150

55 90

194

0129

MMSCFD

260

160 210

MMSCFD

COMPRESSION

SUCTIONPRESSURE,

EXISTING

UPGRADED

(LP CASE &

CAPACITY

CAPACITY PER MACHINE

Upgraded compressor

Idle compressor

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FIGURE 6 – Dewpoint Temperature vs Wellhead Tubing Pressure.

DewPoint Temperature vs Wellhead Tubing Pressure

75

80

85

90

95

100

105

110

115

120

250 225 200 175 150 125 100 75

psig

Dew

Poi

nt T

empe

ratu

re