Nothing lasts forever. To many of us, forever isour life span, which can vary widely among indi-viduals. The permanence of inanimate objectsalso varies in absolute time and importance. Forexample, commercial communication satellitesare expensive to fabricate, difficult to deploy andgenerally inaccessible for repair, so it is impor-tant that they function properly for a long time.Replacement valves and pacemakers for humanhearts can be replaced or repaired, but not with-out considerable risk to the recipient. Equipmentsent to the remote research stations ofAntarctica is expected to stand up to harsh con-ditions. Buildings, bridges and monuments arealso built to endure, but they have finite life-times. Intelligent completions, which combineproduction monitoring and control, are becomingmore common, and require reliable downholegauges and flow-control valves.1

Downhole equipment in the oil field alsomust stand the test of time. The productive life

of an oil or gas well may be 10 or more years, sopermanent downhole equipment must last atleast that long to satisfy operators expectations.Because it is impractical to conduct equipmenttests of such long duration, reliability engineer-ing and failure testing have become mainstays ofthose people who develop permanent monitoringsystems. The result has been an impressive reliability track record for permanent monitoringinstallations worldwide.

In this article, we begin by examining thechallenges in permanent monitoring. Next, weconsider how engineers develop robust perma-nent gauges to provide a continuous stream ofdata for the life of a well. Finally, we presentexamples that demonstrate how the use of per-manent gauges adds value by helping to optimizeproduction and forewarning operators of prob-lems so that preventive or corrective action canbe taken.

FloWatcher, NODAL, PQG (Permanent Quartz Gauge),PressureWatch, PumpWatcher, Sapphire and WellWatcherare marks of Schlumberger.1. For more on flow-control aspects of intelligent

completions: Algeroy J, Morris AJ, Stracke M, Auzerais F, Bryant I, Raghuraman B, Rathnasingham R,Davies J, Gai H, Johannessen O, Malde O, Toekje J and Newberry P: Controlling Reservoirs from Afar,Oilfield Review 11, no. 3 (Autumn 1999): 18-29.

20 Oilfield Review

Downhole Monitoring: The Story So Far

Joseph EckHouston, Texas, USA

Ufuoma EwheridoJafar MohammedRotimi OgunlowoMobil Producing Nigeria UnlimitedLagos, Nigeria

John FordAmerada Hess CorporationHouston, Texas

Leigh FryShell Offshore, Inc.New Orleans, Louisiana, USA

Stphane HironLeo OsugoSam SimonianClamart, France

Tony OyewoleLagos, Nigeria

Tony VenerusoRosharon, Texas

For help in preparation of this article, thanks to FranoisAuzerais, Michel Brard, Jean-Pierre Delhomme, JosianeMagnoux, Jean-Claude Ostiz and Lorne Simmons, Clamart,France; Larry Bernard and David Lee, Sugar Land, Texas,USA; Richard Dolan and Brad Fowler, Amerada HessCorporation, Houston, Texas; David Rossi and Gerald Smith,Houston, Texas; John Gaskell, Aberdeen, Scotland; andYounes Jalali and Mike Johnson, Rosharon, Texas.We thank Philip Hall, Chief Executive of The Sir HenryRoyce Memorial Foundation, for information about SirHenry Royces bumping test machine.

Reservoir monitoring requires dependable downhole data-acquisition systems.

Products based on sound reliability engineering and failure testing, essential to

building durable permanent monitoring systems, are responsible for an impressive

track record for permanent gauge installations worldwide. Gauges supply data

useful for both short-term troubleshooting and for long-term development planning.

Winter 1999/2000 21

Challenges in Permanent MonitoringFrom the perspective of reliability, permanentdownhole gauges used in oil and gas wells aresimilar to commercial communication satellites,although other industries, such as the automotiveindustry, confront similar reliability challenges.Each system must endure a long life under harshenvironmental conditions. Once in place, thedevices are not routinely repaired, replaced orrecovered. Parts may never return to surface forlab analysis of what worked and what didnt; it isdifficult to determine what failed without retriev-ing and examining a malfunctioning device.

A typical approach to these challenges is toinclude redundant components in the hope that if one part fails, its backup will function. Whenused wisely, redundant designs can improve reli-ability significantly. However, in both downholegauges and satellites, redundant componentsoccupy valuable, limited space and consumeprecious power. Common failure modes must beavoided when specifying redundant components.For example, if a particular component is prone to failure in a particular environment, its backuppart should be made from different material sothat it too wont fail under the same conditions.The annals of aviation include numerous episodesof common-failure-mode disasters. CharlesLindbergh undertook a transatlantic flight in thesingle-engine Spirit of Saint Louis in 1927 onlyafter careful study convinced him that the lack ofbackup systems would not put him at risk.2

In addition to fabricating durable permanentdownhole equipment, engineers and designerswork together to address the complexity ofequipment installation and conditions at thewellsite. Competent field engineers and robustequipment are both essential for reliability. Forexample, it is difficult to maintain a high level ofmanual dexterity for hours at a time in an icydownpour or a fierce wind. It is important for thefield crew to install a monitoring system usingwell-designed installation tools that ensureinstallation consistency, especially in remotelocations. Simplifying the installation process asmuch as possible also improves success rates.Early failure of permanent monitoring systemsdecreases when a well-prepared crew performsthe installation with familiar tools.

Operators have used permanent downholepressure gauges since the 1960s.3 The vast bodyof experience is paying off in the latest genera-tion of gauges, for which statistically valid relia-bility data are now available. There are nowthousands of gauges deployed worldwide, over800 of which have been installed by Schlumbergersince 1973 (above and next page, top). A signifi-cant increase in installations occurred after anew generation of more reliable gauges wasdeveloped in the early 1990s.

22 Oilfield Review

Metal-to-metal sealedcable head

Hermetically sealedwelded housing

Cable driver andfault-tolerant regulator

Digital pressure,temperature and self-test11

010

Quartz crystal resonatorsto measure temperatureand pressure

Protection bellows

P/T

Pressure connection

Gland radialconnection

Autoclave axialconnection

or

1/4-in. encased cable

> Permanent downhole pressure guage. ThisPQG Permanent Quartz Gauge system measurespressure and temperature using quartz crystalresonators.

1973 First permanentdownhole gauge installationin West Africa, based onwireline logging cable andequipment

Depe

ndab

ility

1975 First pressure andtemperature transmitter ona single wireline cable

1978 First subseainstallations in North Seaand West Africa

1983 First subseainstallation with acousticdata transmission to surface

1986 Fully welded metaltubing-encased permanentdownhole cable

Winter 1999/2000 23

Dependability, the Sine Qua NonA basic permanent downhole gauge consists ofsensors to measure pressure and temperature,electronics and a housing (previous page, right).4

A mandrel on the production tubing holds thegauge in place. A cable, enclosed in a protectivemetal tube, is clamped onto the tubing. The cable connects the gauge to the wellhead and then to

surface equipment, such as a computer or controlsystem. Because acquiring and transmitting gooddata depend on proper functioning of each part,such systems are only as reliable as their weak-est component.

A complete monitoring and communicationsystem, such as the WellWatcher system, han-dles diverse sensors, including a FloWatcher

sensor to measure flow rate and fluid density, a PumpWatcher sensor to monitor an electricsubmersible pump and a PressureWatch gaugeto measure pressure and temperature (below).Surface sensors measure multiphase flow rateand pressure and detect sand production. Inaddition to surface controls for valves andchokes, there is a computer to gather data, which

Surface sensors and controls Multiphase flow rate Valve and choke control Pressure measurements Sand detection

Permanent downhole sensors FloWatcher sensor to monitor flow rate and density PumpWatcher sensor to monitor electric submersible pump PressureWatch gauges to measure pressure and temperature Host server and database

Data-retrieval andcommunications software

Integratedapplications

> A complete permanent monitoring system for measuring pressure, temperature, flow rate and fluid density downhole. Surface sensors measureflow rate and pressure. A data-retrieval and communications system facilitates data transfer to the office of the end user.

1986 Introduction of quartzcrystal permanent pressure

gauge in subsea well

1990 Fully supported copperconductor in permanent

downhole cable

1993 New generation ofquartz and sapphire crystal

permanent gauges

1994 PQG Permanent QuartzGauge performance substant-

iated by gauge accreditationprogram at BP. Start of long-

term lab testing

1994 FloWatcher installationfor mass flow-rate measurement

are stored at the wellsite or transmitted to theoffice (below).5

Permanent downhole systems must bedependable throughout their lifetimestheymust be reliable and stable. Dependability con-jures different meanings for different people, butis used in this article to refer to the combinationof reliability and stability. Reliability in the con-text of downhole gauges refers to proper instal-lation and ongoing delivery of data from thegauge. It can be defined as the probability thatthe gauge will perform as specified without fail-ure for a certain amount of time under therequired environmental conditions.

Stability refers to the actual measurement.Measurements from an unstable or excessivelydrifting gauge might prove more troublesome toan oilfield operator than outright failure of the

gauge. It is important to know whether gradualvariation in a measurement with time indicatesan actual change in the reservoir or reflects adrift problem with the measuring device.

To ensure a dependable product, it is essen-tial to maintain strict quality control throughoutthe entire engineering process. Quality is thedegree to which the product conforms to specifi-cations. To truly achieve world-class reliabilityand stability entails systematic product develop-ment and qualification testing, use of qualifiedcomponents and proven design methods, strictaudits and tracking of generic parts, failure analy-ses and consultation with industrial and academicpeers. Reliability and stability cannot be testedinto a product after it is built, but instead must beconsidered throughout the entire process, fromdesign and production to installation.

The Road to ReliabilityDuring the past 10 years, Schlumberger hasenhanced the dependability of its permanentmonitoring systems through improvements inengineering and testing processes, systemdesign, risk analysis, training and installationprocedures (next page, top).6 Like other tools andsystems developed by Schlumberger, permanentgauge development follows a logical sequence ofengineering phases. Dependability concerns areparamount during each phase.

The engineering phase begins with develop-ment of a mission profile, or a verbal descriptionof the technical concept that serves as an engi-neering framework. The mission profile definesthe role of each component and the environmen-tal conditions components will encounter during

24 Oilfield Review

WellWatcheracquisition unit

Sensors

Automaticdata-retrievalserver

Automatic data-retrieval client

Central storage

Central storageconfiguration

Archivingdatabase

ASCII files

Data browser

Data access library

Engineeringoffices

HELIKOPTER SERV

Wellsite Office

> Data flow. Measurements are transmitted from the downhole device through the cable to surface. The surface data-acquisition unit can send data bysatellite to engineering offices, where data are stored in a library for easy access.

Winter 1999/2000 25

their expected lifetime. All components of thesystem are screened and qualified to withstandthe expected conditions. Accelerated destructivetests subject components to conditions muchmore extreme than expected over their lifetime,such as greater mechanical shocks and vibrationsand higher-than-downhole temperatures andpressures. This type of testing helps determinefailure causes and failure modes. Long-term test-ing of the system enables engineers to validatereliability models and quantify measurementstability (below).

A drawback to accelerated testing is that failure can occur simply because of the stressfultest procedure, and the test might not be a goodpredictor of actual performance. It is impossibleto test everything, but it is important to test asmuch as possible to increase confidence that theproduct will perform as required in commercialoperations. Feedback from field engineers is a crit-ically important complement to laboratory testing.

Product engineering

Mission profile and requirementsPrototype product designRisk analysis and test plansComponents qualification testingReliability qualification testingTechnical reviews and auditsSustaining, product improvement

Training and personnel development

Training with development and field engineersWell completions installation trainingPerformance evaluation and growth planTechnique improvement

Project engineering

Reservoir engineering and production requirementsWell completions design and installation planningWell construction, installation and operationProject improvement

Reliability and data qualitymanagement

Collect field track records into databaseAnalyze results and feedback for improvementReview with operators, development and field engineers

>Permanent monitoring system development. From the initial mission profile to failure analysis, collaboration between engineers, field personnel and operators contributes to continual improvements in permanent monitoring systems.

Permanent gauge stability test. This plot of pressure versus time represents testing of a PQG Permanent Quartz Gauge system atelevated pressures and temperatures for morethan two years. The initial test conditions were140C [284F] and 7000 psi [48.2 Mpa]. Testingwas then accelerated, with the temperatureincreased to the maximum rated temperature of 150C [302F], and then to 160C [320F] and170C [338F], to make the gauge fail. Each time the temperature was increased, there was a brief period of measurement drift before the gauge reached stability. The gauge driftedless than 3 psi/yr [20 kPa/a]. During the test, the gauge performed as expected, but the testcell had to be repaired twice!

5. For a related article on data delivery in this issue: Brown T,Burke T, Kletzky A, Haarstad I, Hensley J, Murchie S,Purdy C and Ramasamy A: In-Time Data Delivery,Oilfield Review 11, no. 4 (Winter 1999): 34-55.

6. Veneruso AF, Sharma S, Vachon G, Hiron S, Bussear Tand Jennings S: Reliability in ICS* IntelligentCompletions Systems: A Systematic Approach fromDesign to Deployment, paper OTC 8841, presented atthe 1998 Offshore Technology Conference, Houston,Texas, USA, May 4-7, 1998.

010,000

10,005

10,010

10,015

10,020

10,025

10,030

100 200 300 400 500 600 700 800 900

PQGpressure reading

1 year 2 years

Test

cel

l rep

airs

Test

cel

l rep

airs

-3 psi/year drift

0 psi/year drift

Duration of testing, days

Pres

sure

, psi

150C 160C 170C

PQG Stability Test at 10,000 psi

>

Tests for susceptibility to mechanical shockand vibration, such as those expected duringtransport and installation, are also performed.7

These tests are similar in concept to thosedeveloped by Sir Henry Royce, the engineerbehind the success of the Rolls-Royce auto-mobile. By repeatedly bumping the car on anapparatus that simulated bumps in a road, Royce determined which parts of the chassiswere not strong enough and developed betterones (right).8 The changes included replacingrivets with bolts and using a few large boltsrather than many small ones.

In the system-design phase, engineers ensureproper interfacing between the completion components. Communication with completionengineers and third-party vendors has resulted incontinual improvement in downhole cable con-nections and protection of the system.

Both experts and end users provide input dur-ing the development phase, as engineers performsimulations and build mock-ups. Conducted fre-quently, design reviews include field personnel.Design rules have been prepared to address theneed for low stress on components, minimalexternal connections and other concerns.

Once the system is built and is ready forinstallation, a specially trained crew reviewsdetailed installation procedures and projectplans with operations personnel and third-partyvendors. Performance of the field installationcrew plays an important role in system reliability,so formal training programs for both systemdesign engineers and field installation techni-cians are conducted. Whenever possible, systemdesign engineers attempt to simplify installationrequirements because factors such as frigid temperatures, gusty winds and long hours maypresent additional challenges to the crew. Adesign that allows fast, easy installation relievessome of the burden on the field crew and minimizes risk and rig time.

26 Oilfield Review

>Torturing tools. By exposing an automobile chassis to repeated mechanical shocks (top), Sir HenryRoyce observed which parts were prone to failure and built better ones for Roll-Royce, beginningaround the turn of the last century. Today, highly specialized testing machines and accelerated testtechniques developed by Schlumberger verify the endurance of downhole equipment againstmechanical shocks (bottom).

7. Veneruso A, Hiron S, Bhavsar R and Bernard L:Reliability Qualification Testing for PermanentlyInstalled Wellbore Equipment, abstract submitted to the2000 SPE Annual Technical Conference and Exhibition, to be held in Dallas, Texas, USA, October 1-4, 2000.

8. We thank Philip Hall for information about the bumpingtest machine. Mr. Hall retired from Schlumberger after22 years of service, both in the oilfield and in electronics.He is Chief Executive of The Sir Henry Royce MemorialFoundation, The Hunt House, Paulerspury,Northamptonshire, NN12 7NA, England.

Winter 1999/2000 27

Learning from ExperienceIf a permanent downhole gauge fails, engineersanalyze the circumstances and sometimesattempt to reproduce the failure modes in theengineering center or other testing facility. Failuremechanisms are not random; in most cases thereare underlying causes at work that must beuncovered, such as design problems, faulty mate-rials or improper installation. Schlumberger hasestablished an on-line database to capture dataabout system installations, including detailsabout environmental conditions, to identify anypatterns in failures (right). The database allowsstatistical analysis of the data by region, operator,environmental conditions and other operationalparameters. Careful analysis of the worldwidedatabase increases confidence that the appropri-ate lessons are learned from field experiencesand helps focus efforts on possible areas ofimprovement.

From August 1, 1987, to the present, the per-formance of 712 permanent gauge installationshas been tracked. The oldest system is more than16 years old, having been installed a few yearsbefore the database was established. Analysis of572 new-generation digital technology installa-tions made since their introduction in March1994 indicates that over 90% of thesePressureWatch Quartz and Sapphire systemswere still operating after 2.5 years (below). Theanalysis, based on methods introduced by

> Permanent downhole gauge database. Careful tracking of each system enables analysis ofgauge performance. Comparison of environmental conditions helps teams prepare to installgauges in new locations by learning from past experience in similar areas.

00.0 0.5 2.01.51.0 2.5 3.0 4.0 4.53.5 5.0

10

20

30

40

50

60

70

80

90

100

Operational life, years

Surv

ival

pro

babi

lity,

%

Permanent gauge operating life. Since record-keeping began in 1987, Schlumberger has installedmore than 700 permanent gauges worldwide.Analysis of 572 new-generation digital technologyinstallations made since March 1994, shown by the purple line, indicates that over 88% of thesePressureWatch Quartz and Sapphire systemswere still operating after 4 years. The lavendertrend line begins at 97% and decreases by 3% per year, a higher failure rate than that of theactual data. The photograph shows the productionfacilities of the Baldpate field, operated by Amerada Hess.

>

Mltoft, helps reveal the key factors influencingthe reliability of permanent monitoring systems(above right).9 The Mltoft method addresses asystems actual operational time rather than itscalendar time, a key advantage when studyingfield installations over a long time period. Themethod helps pinpoint areas for improvement insystem design and deployment.

Operating companies have independentlystudied the reliability of permanent gauges.10

Different manufacturers and operators measureperformance according to their own standards.Schlumberger has chosen to focus on the wholesystem rather than a single component becauseit is vital that the entire system operate properlyand provide usable data.

Downhole to Desktop: Using the DataAfter the equipment has survived the ordeal oftesting and installation, the real challenge beginsonce a permanent monitoring system is placedsecurely in a well. A system that takes a mea-surement every second of the day produces over31 million data points per year. Coping with thevolume of data from permanent monitoring systems is an issue that operators and servicecompanies continue to address.11 Some operatorshave chosen to sample their data at specifictimes or when the change in a measurementexceeds a predetermined threshold. Others sam-ple their data at greater time intervals, such as30 seconds, to reduce data volume.

Once reaching the end user, the data are appliedto two general production issues: reservoirdrainage and well delivery (right). Reservoir-drainage aspects include pressure monitoring,pressure maintenance, material-balance modelsand simulation models. Well-delivery issues,such as skin and permeability, affect productionengineering.

When a well is shut in for maintenance, apressure gauge offers the small-scale equivalentof a pressure buildup test. Subsequent well shut-ins allow engineers to analyze the repeatability

28 Oilfield Review

Reservoir drainage

Application Description

Well delivery

Application Description

Pressure monitoring Static bottomline pressure survey

Pressure maintenance Future development plans (reservoirrepressurization: install injection facilities?)

Real-time fracturing and stimulationoperation monitoring

Appraisal of injection and production profile along the well

Material balance model updating Input data for continuous update andrefinement of material balance model

Well test interpretation and analysis(buildup, drawdown, multirate andinterference well testing)

Reservoir boundaries, well spacingrequirements, interwell pressurecommunication

Water and gas injection monitoring Evaluate degree of pressure support from injector wells

Appraise performance of injection program

Reservoir simulation model refinement and validation

Historical database for pressure history matching

Calibration tool for simulation model

Well test interpretation and analysis(buildup, drawdown, multirate andinterference well testing)

Skin, permeability and average reservoir pressure

Production engineering Input for NODAL analysisProductivity Index (PI) and long-term

variation in PI measurement;generation of water, gas and sandproduction rate correlation as afunction of pressure

Flowing bottomhole pressure survey to determine maximum offtake _

Flow well at optimal pressure abovebubblepoint pressure to avoidliberation of free gas

Complement or corroborate other reservoir monitoring measurements

Corroboration of information provided by innovations such as 4D seismicsurveys, time-lapse well logging

>Typical applications of permanent downhole gauge data. Data from downholegauges can be used to improve both reservoir drainage and well delivery.

Operational time

Accu

mul

ated

failu

res,

%

Flaws(manufacturing and installation related)

Random overload(design related)

Predictable wear-out(design and environment related)

Characterizing performance over time. Even the most reliable permanent gauge canfail and the root cause often is a matter ofspeculation. Production-related or installationflaws account for many early failures. Atintermediate stages, failures occur at a low,relatively steady rate, apparently because ofrandom overloads. After many years of service,failures may occur as components age.

>

Winter 1999/2000 29

of the tests and improve confidence in selectinga reservoir model. If all the wells in a field areshut in, downhole gauges can measure the aver-age reservoir pressure. The average reservoirpressure measured this way is a key componentof decline rate and reserve estimations and aparameter for reservoir simulations.12

In fluid-injection projects, permanent downholepressure gauges can be used to better maintainpressure, displace oil, arrest subsidence and dis-pose of fluids. By monitoring a continuous streamof pressure data, operators can control reservoirperformance by injecting fluids to keep reservoirpressure above bubblepoint pressure to ensureproduction of oil rather than gas. Permanentgauges can also help determine the optimal pro-duction rate when there are concerns about sandproduction or water coning at high flow rates.

Downhole pressure gauges allow engineersto allocate production to specific wells. Knowingthe downhole pressure, the wellhead pressureand the general properties of the produced fluidsallows calculation of the flow rate for a well andcalibration of flow rates with test data. Offshoresatellite fields tied back to platforms and fieldsowned by multiple partners are good candidatesfor this particular application of downhole pres-sure gauges.

In artificial-lift applications, downhole pres-sure gauges help engineers determine how wellthe artificial-lift system is performing. For exam-ple, a prolific, highly permeable, unconsolidatedoil reservoir might have high deliverability, butthe bottomhole pressure of the well might beinadequate to produce the fluid to surface. If anelectric submersible pump or gas-lift system isinstalled in the well, the operator can add adownhole gauge to assess the performance ofthe lift system.

Gauges in ActionThe permanent monitoring applications that fol-low come from widely separated regions withdifferent operational challenges and operatorpriorities. In each case, the operator might mea-sure the value of permanent monitoring systemsin a variety of ways, such as additional barrels ofoil recovered through more efficient reservoirdrainage or delivery from individual wells, or incost savings through decreased well interven-tions. Appraisal of a deep, sour, high-pressure,high-temperature (HPHT) discovery in the MiddleEast presented numerous operational and inter-pretation challenges. Unlike the prolific shallowoil fields nearby, the discovery well producedanomalously high API gravity oil for the regionfrom a fractured carbonate reservoir with limitedmicroporosity. A thick salt layer above the reser-voir complicated interpretation and operations.Nevertheless, the accumulation presented fasci-nating opportunities to evaluate fracture fairwaysbelow structural spillpoints and hydrocarbon self-sourcing in a kerogen-rich reservoir rock.

Data from the initial discovery well were inad-equate to calibrate reservoir simulations or toplan development. A deep appraisal well, drilledover the course of a year with mud weightsexceeding 20 pounds per gallon [2.4 g/cm3], pro-vided core, mud log and wireline log data. Anextended well test generated enough data forengineers to decide how to proceed.

The extremely high formation pressures anduse of kill-weight mud in wellbores meant thatwireline-conveyed pressure measurements werenot possible. Instead, the operator selected aFloWatcher system to measure pressure, temper-ature and flow rate continuously. This installation

was the first use of the FloWatcher system at apressure of 15,000 psi [103.4 Mpa], so advancepreparations were necessary. The wellhead,which had already been procured, was modifiedto allow an exit for the cable. A shed was built toaccommodate surface monitoring equipment.

The permanent monitoring system wassafely installed and an extended well test wasconducted for four months, with oil flowingthrough a 70-km [43.5-mile] flowline. TheFloWatcher system was selected in partbecause pressure measurements at the Venturiinlet and throat allowed determination of theabsolute pressure, the pressure change acrossthe Venturi and the flow rate. Despite arepairable seal failure in the Venturi, it was stillpossible to obtain pressure measurements fromthe pressure gauge, which functioned asexpected throughout the test. Also, the mandreldesign for the system was relatively inexpensive.

The permanent monitoring system enabledengineers to produce at the maximum rate whilemaintaining pressure above the bubblepoint, andto gather the data they needed to formulatedevelopment plans. Given the operational chal-lenges of this particular well and area, theremote location and the importance of gaininguseful data, an extended well test with a perma-nent downhole monitoring system proved to bethe optimal approach.

Permanent downhole monitoring systemshave been used in the Gulf of Mexico for severalyears. Shell Offshore, Inc., has installed perma-nent gauges in each of the 10 wells it operates inthe Enchilada area in the continental Gulf ofMexico (above). The Enchilada area comprisesthin-bedded turbidite reservoir sands located both

>Enchilada field. The Enchilada area includes several blocks in the Garden Banks area offshoreLouisiana, USA. The blocks are 3 miles [4.8 km] long and 3 miles wide.

9. Mltoft J: Reliability Engineering Based on FieldInformationthe Way Ahead, Quality and ReliabilityInternational 10, no. 5 (May 1994): 399-409.Mltoft J: New Methods for the Specification andDetermination of Component Reliability Characteristics,Quality and Reliability International 7, no. 7 (July 1991):99-105.

10. van Gisbergen SJCHM and Vandeweijer AAH:Reliability Analysis of Permanent Downhole MonitoringSystems, paper OTC 10945, presented at the 1999Offshore Technology Conference, Houston, Texas, USA,May 3-6, 1999.

11. A complete discussion of processing and reducing datafrom permanent downhole gauges is beyond the scopeof this article. For one example of how to process data:Athichanagorn S, Horne R and Kikani J: Processing andInterpretation of Long-Term Data from PermanentDownhole Pressure Gauges, paper SPE 56419, pre-sented at the SPE Annual Technical Conference andExhibition, Houston, Texas, USA, October 3-6, 1999.

12. Baustad T, Courtin G, Davies T, Kenison R, Turnbull J,Gray B, Jalali Y, Remondet J-C, Hjelmsmark L, Oldfield T,Romano C, Saier R and Rannestad G: Cutting Risk,Boosting Cash Flow and Developing Marginal Fields,Oilfield Review 8, no. 4 (Winter 1996): 18-31.

TEXAS

LOUISIANA

Garden Banks

Baldpate

BaldpateNorth

Enchilada

0

0 160 km

100 miles

above and below salt. The first gauge wasinstalled in September 1997, and to date all ofthe gauges continue to operate without failure.

Permanent downhole pressure gauges fulfilltwo major requirements for Shell Offshore: dailyoperations improvements and better long-termreservoir management. In both cases, pressuredata must be accessible to reservoir specialistsin a format they can use efficiently. The systeminstalled by Schlumberger stores the data forsubsequent pressure transient analysis. ShellOffshore retrieves the data from the system anduses its own computer-assisted operations (CAO)system to manage the data stream on a long-term basis.

Shells CAO acquisition unit captures surfaceand downhole pressure measurements atapproximately 30-second intervals for trend analy-sis and long-term archiving of pressure data. Inthe past, most decisions about daily operationswere made on the basis of surface pressure ortubing pressure measurements with infrequentdownhole wireline pressure measurements. Adecline in surface pressure could indicate reser-voir depletion or a downhole obstruction, but thisambiguity could not be resolved with surfacedata alone. Now, with both surface and down-hole pressure measurements, it is possible toquickly diagnose production problems. For exam-ple, if both surface and bottomhole pressurecurves track each other on a declining trend, thenthe probable cause is reservoir depletion. On theother hand, if the surface pressure is droppingbut the downhole pressure remains constant orincreases, then the engineer might suspect thatsalt, scale or paraffin is plugging the tubing(right).13 Therefore, engineers for the Enchiladaarea use surface and downhole measurements todiagnose production problems and optimizeremediation treatments.

Permanent downhole pressure gauges areespecially important for effective reservoir man-agement in the Enchilada area and areas like it.Thin-bedded reservoirs, such as turbidite sands,can be difficult to evaluate by wireline methods.Producers want to determine if the reservoir iscontinuous. During the initial development, fewappraisal wells had been drilled and the subsaltlocation of several prospects made it difficult todefine the reservoir geometry and extent.Gathering early reservoir pressure data fromeach well aided development planning. In addi-tion, the long-reach, S-shaped wells in theEnchilada area are expensive to drill and noteasily accessed by wireline methods.Furthermore, the mechanical risk of runningwireline pressure devices into these high-ratewells is unacceptable. Therefore, the perma-nent gauge system allows frequent reservoir

pressure monitoring without mechanical riskand with minimum deferred production.Frequent pressure measurements help optimizeproduction rates, and enhance understanding ofultimate reserve potential.

The Enchilada area example affirms that datafrom permanent gauges are valuable throughoutthe life of the well. Run time is a major concern forShell Offshore because the Enchilada wells areexpected to produce for at least 10 years. The reli-ability and durability of these permanent gaugeshave a direct impact on the assets value. The suc-cessful application of permanent monitoring tech-nology convinced Shell to install gauges in twowells on their deepwater Ram-Powell platform,offshore Gulf of Mexico. The second of theseinstallations, a PQG Permanent Quartz Gauge sys-tem set at a depth of 23,723 feet [7230 m], is thedeepest installation by Schlumberger to date.

30 Oilfield Review

Pres

sure

Time

Psurface

Pbhp

Psurface

Pbhp

Pres

sure

Time

Diagnosing production problems. Plots of bothbottomhole, Pbhp, and surface pressure, Psurface,versus time help engineers diagnose productionproblems. In the top example, surface and bottomhole pressures are declining, but thecurves track each other, suggesting reservoirdepletion. In the bottom plot, the surface pressure diverges and drops at a faster ratethan the bottomhole pressure. One possible conclusion is that scale is plugging the production tubing.

>

Winter 1999/2000 31

Complicated deepwater developments, suchas the Baldpate field in Block 260 of the GardenBanks area of the Gulf of Mexico, challenge oper-ating companies (above). The first downholegauge in the Baldpate field was installed inAugust 1998. Seven of eight wells have down-hole gauges. The field is expected to produce for6 to 10 years.

Baldpate field comprises two major Pliocenereservoirs at depths of 15,500 to 17,500 feet[4724 to 5334 m]. Original reservoir pressuresexceeded 13,000 psi [89.63 MPa]. Productionfrom the sands in the Baldpate North area iscommingled in a seventh well. The field reachedpeak production of 58,000 BOPD [9216 m3/d] and230 MMscfg/D [6.5 MMm3/d] by June 1999.

Installation of permanent downhole gauges isparticularly demanding at the well depths andpressures of Baldpate field. Success depends ona thoroughly trained, competent wellsite crew.For example, the crew must avoid potential pit-falls such as damaging the cable and making badsplices. Extensive prejob planning allows theentire team to anticipate problems and work outsolutions before installation. Having many of thesame crew work on every installation buildsexperience and carries lessons learned from onejob to the next.

Amerada Hess Corporation, operator ofBaldpate field, elected to install permanentdownhole pressure gauges for both mechanicaland reservoir management purposes. Expensivegravel-pack completions and tubing in high-ratewells are prone to damage if there is excessivedrawdown or if the erosional velocity is toohigh.14 As flow rates were ramped up during theinitial stages of production, pressure data helpedavoid damage by ensuring that predeterminedlimits on drawdown and erosional velocity wouldnot be exceeded. By measuring the pressure dropacross the completion, engineers calculated the mechanical efficiency, or mechanical skin, of the completion.15

Acquiring a constant stream of pressure dataenables reservoir engineers to fine-tune compo-sitional models for reservoir simulation, performhistory matching of pressure depletion of thereservoirs over time, test secondary recovery scenarios and predict ultimate recovery. Thepressure data are also used for frequent pres-sure-transient analysis. This analysis providescalculations of effective permeability, mechanicalskin, non-darcy flow effects, average reservoirpressure and approximate distance to variousreservoir boundaries.

Interference tests can be performed becausethere are permanent downhole pressure gaugesin all the wells. Each well responds to rate adjust-ments in offset wells within hours. The pressureresponses can be used to assess reservoir conti-nuity. Data from pressure gauges confirmed thegeologic model of laterally continuous basin floorfan sands.

Of seven gauges installed in the Baldpatefield, six are working. The lone failurethe onlyfailed gauge out of 43 gauges installed bySchlumberger in North Americaappears tohave resulted from a problem within the gaugeitself, although it has not been recovered forpostmortem analysis. The installation of gaugesin all the wells meant that the loss of one gaugewas an inconvenience rather than a major diffi-culty. It was not worth retrieving or repairing thefailed gauge because of the cost and mechanicalrisks of pulling tubing. Data from the gauges inthe other wells are sufficient for ongoing reser-voir management.

Amerada Hess carefully manages the highvolume of data from permanent downhole pres-sure gauges. The data are stored in the hard driveof a personal computer on the production tower.From the office, an engineer can control samplingrate and electronically retrieve data from theremote production tower and move them to theoffice. Eventually, however, Amerada Hessexpects to move and store the complete data vol-ume elsewhere. Data can be downloaded into apressure-transient software package and ana-lyzed within minutes.

13. For more on scale: Crabtree M, Eslinger D, Fletcher P,Miller M, Johnson A and King G: Fighting ScaleRemoval and Prevention, Oilfield Review 11, no. 3(Autumn 1999): 30-45.

14. Erosional velocity is the velocity at which an impingingfluid degrades a metal at the molecular level. In thiscase, the operator was concerned about the possibilityof high-flow rate wells producing sand from the uncon-solidated reservoir and damaging the production tubing.

15. Pahmiyer RC, Fitzpatrick HJ, Jr. and Dugan J:Completion Efficiency Measures for High-Permeability,Unconsolidated Sand Environments, presented at the1999 SPE European Formation Damage Conference, The Hague, The Netherlands, May 31-June 1, 1999.

>Baldpate field location. Baldpate field is located offshore Louisiana in Block 260 of the GardenBanks area.

TEXAS

LOUISIANA

Garden Banks

Baldpate

BaldpateNorth

Enchilada

0

0 160 km

100 miles

An example from Africa demonstrates otherapplications of downhole gauges. Since 1992,Mobil Producing Nigeria Unlimited has installedpermanent downhole pressure gauges in 12 of itsfields offshore Nigeria: Usari, Oso, Mfem, Ubit,Iyak, Enang, Asasa, Ekpe, Asabo, Unam, Edopand Etim (above).16

Mobil has used continuous pressure mea-surements from downhole gauges in many ways.The most basic applications include determiningthe reservoir drive mechanism, assessing deple-tion patterns and reservoir discontinuities, andplanning pressure maintenance programs.Permanent downhole gauges measure downhole

pressure in wells whose high wellhead pressureprecludes use of wireline pressure measurementtechniques. Mobil can avoid the costs of shuttingin wells with high flow rates solely for gatheringdata. In fields with many wells, data from strate-gically placed pressure gauges allow reservoirengineers to calibrate pressure measurementsgathered by wireline methods with those frompermanent gauges.

In the Edop field, 7 of approximately 40 wellshave downhole pressure gauges. Mobil expectedto inject gas to maintain reservoir pressure, sothe initial plan was to place a downhole pressuregauge in a well in each of four fault blocks in theEdop field and assess the connectivity of the

reservoir across fault blocks. Results from thegauges showed no communication across thefault blocks, and that separate injectors would berequired for each fault block. The downhole pres-sure gauges also indicated that the plannedinjection patterns needed to be changed, so thedownhole pressure gauge data were then inte-grated with the 3D geological model to modifyand optimize producer and injector locations.

32 Oilfield Review

16. Ogunlowo RF, Ewherido UJ and Oyewole AA: Use ofDown-hole Permanent Gauges in Reservoir Descriptionand Management of a Gas Injection Project in EdopField, Offshore, Nigeria, prepared for the 23rd AnnualInternational Conference and Exhibition, Abuja, Nigeria,August 4-6, 1999.

17. Algeroy et al, reference 1.Huck R: The Future Role of Downhole Process Control,Invited Speech, Offshore Technology Conference,Houston, Texas, USA, May 3, 1999.

18. Christie A, Kishino A, Cromb J, Hensley R, Kent E,McBeath B, Stewart H, Vidal A and Koot L: SubseaSolutions, Oilfield Review 11, no. 4 (Winter 1999): 219.

Niger Delta

Qua Iboeterminal

Oil fields with downhole gauges

0 15 miles

0 24 km

AFRICA

Asabo

Enang

Edop

Asasa

Etim

UnamUbit

Iyak

Mfem

Oso

Usari

Ekpe

>Offshore Nigeria. Since 1992, Mobil Producing Nigeria Unlimited has installed permanent downholegauges in the 12 offshore fields shown in red-rimmed green. Approximately 95% of the gauges are stilloperating today.

Winter 1999/2000 33

Pressure data provided by downhole gaugeswere critical in determining communication effi-ciency around shale baffles that had escapeddetection by seismic and well logging methods.Also, the continuous data provided by the gaugesled to better reservoir simulation results than sin-gle data points from wireline measurementmethods. As the injection project proceeded,instantaneous pressure responses within thecontinuous stream of data enabled engineers todetermine how much compressor downtime theirinjection project could accommodate (right).

In other fields operated by Mobil offshoreNigeria, 20 to 25% of the wells have downholepressure gauges. Approximately 95% of thegauges provided by Schlumberger are still oper-ating. The rare instances of failure have beenattributed to problems in control lines, badcable splices, failure at the wet connector orproblems at the Christmas tree rather than prob-lems with the gauges themselves. However,these are still considered failures of the system.Improvement beyond the current 95% successrate is expected.

Outlook for Reservoir MonitoringPermanent reservoir monitoring is vital to intelli-gent completions, a modern approach to improvingreserve recovery.17 Efficient, beneficial operationof downhole flow-control valves depends onunderstanding reservoir dynamics, so the combi-nation of acquiring downhole data and usingflow-control valves is essential. At present,knowledge of the reservoir comes from analyzingpressure and production data and, in some cases,data from downhole flowmeters. Ongoingresearch and development of flowmeters areexpected to provide accurate measurement offlow rates as well as multiphase fluid properties.In addition, researchers are addressing the chal-lenges of accurately measuring flow rates indirectional and horizontal wells.

Improved links between data acquisitionsystems and operators will facilitate real-timedata transmission and display. Permanent mon-itoring allows engineers to get a sense of thereservoir, but to see the reservoir requiresthat the data be transformed into a usable for-mat. If data access or display is too cumbersome,downhole pressure gauge data are in danger ofbeing ignored.

The costs and economic benefits of perma-nent monitoring must be considered together.Success stories from around the world, such asthose presented in this article, should serve tobolster confidence in permanent downhole pres-sure gauges. As confidence in the dependabilityof permanent gauges and other systems contin-ues to grow, the value of the data will overcomeshort-term concerns about cost in many cases.

Today, operators are venturing into remoteareas and water depths approaching 10,000 ft[3048 m] and are completing wells subsea withthe expectation of limited or no interventions.18

Optimal production in these arenas will necessi-tate permanent monitoring systems that are compatible with other completion equipment. As with permanent downhole pressure gaugesand flow-control valves, dependability of down-hole flowmeters and other permanent equipmentin wells will remain the key criterion for choosingto deploy these devices in expensive, inaccessi-ble wells.

The successful application of rigorous prod-uct development and testing processes withconcurrent reliability engineering and field ser-vice quality control has set the standard fordependable permanent monitoring systems. Thisreflects a long-term commitment of people andresources. Employing these engineering pro-cesses enhances future permanent monitoringsystems. For operators, these enhancementstranslate into early diagnosis of problems, fewerwell interventions, reduced risk and greaterreserve recovery. GMG

2150

2100

2050

2000

1950

1900

1850

1800

1750

1700

1650

tmin = 4/00Pmax = 2100 psia tmax = 7/00

Pres

sure

, psi

a

12/98 2/99 4/99 6/99 8/99 10/99 12/99 2/00 4/00 6/00 8/00

>Pressure response in Edop field. In the central fault block, gas injection is increasingreservoir pressure, as shown in this plot of pressure measured in four different wells versus time in the Intra Qua Iboe 3 reservoir. Predicted pressures, shown in dashes, werecalculated on the basis of well placement, drainage radius, production rates and expectedgas injection rates. tmin, or April 2000, represents the earliest predicted date when thereservoir pressure will attain the target pressure (Pmax), while tmax represents the latestprojected date to reach the desired pressure and occurs in July 2000.