Optical Fiber Sensors for Permanent Downwell Monitoring ... · to downhole production monitoring, but also for seis-micsensing,down-streamprocessmonitoring,platform structural and

400IEICE TRANS. ELECTRON., VOL.E83–C, NO.3 MARCH 2000

INVITED PAPER Special Issue on Optical Fiber Sensors

Optical Fiber Sensors for Permanent Downwell

Monitoring Applications in the Oil and Gas Industry

Alan D. KERSEY†a), Nonmember

SUMMARY This paper reviews the use of fiber optic sen-sors for downhole monitoring in the oil and gas industry. Due totheir multiplexing capabilities and versatility, the use of Bragggrating sensors appears to be particularly suited for this appli-cation. Several types of transducer have been developed, each ofwhich can be addressed along a single (common) optical fiber inthe well and read-out using a common surface instrumentationsystem.key words: Bragg gratings, �ber optic sensors, pressure sensors,

ow sensors

1. Introduction

Fiber optic sensor technology has been under develop-ment for the past 20 years [1], [2] and has resulted inseveral successful new products; fiber optic gyroscopes,temperature sensors, acoustic sensors, accelerometersand chemical probes (particularly biochemical) are ex-amples. Applications of the technology include civilstructural monitoring (e.g., smart structures), militarysystems (e.g., underwater acoustic arrays), industrialapplications (e.g., process control sensor networks),chemical sensing (distributed spectroscopy), and secu-rity monitoring (intrusion detection) to name a few.

This technology is now opening up new capabil-ities for sensing a wide range of parameters, such aspressure, temperature, vibration, flow, acoustic fields,in downhole oil and gas reservoir monitoring applica-tions. Significant interest is being directed towards thisarea, and several types of fiber optic sensors have beendemonstrated for such downhole use. These includea Distributed Temperature Sensor (DTS) for tempera-ture profiling [3], an optically excited micro-machinedresonant pressure sensor [4], an interferometric pointsensor [5] for pressure monitoring and Bragg gratingbased sensors for a range of parameters [6].

Sensors designed for downhole monitoring applica-tions in the oil and gas industry are subjected someof the most extremely hostile environments on earth.Timely, accurate, reliable information about how thereservoir is performing is an important component tooptimizing oil yield and production rates. To date,the industry has relied largely on “wireline” retrievablesensors that are lowered into the well to make mea-surements of key parameters. Such wireline monitor-

Manuscript received November 8, 1999.†The author is with CiDRA Corporation, 50 Barnes

Park North, Wallingford, Ct 06492.a) E-mail: [email protected]

ing provides a “snapshot” of the well/reservoir, and areusually repeated months or years apart. Permanentlyinstalled monitoring systems provide real time data ina continuous manner without interrupting productionor requiring a well intervention. Consequently, relia-bility in the transducer technology is one of the mainchallenges to realizing the full benefits associated withthe permanent deployment of downhole monitoring sen-sor systems. Conventional electronic gauge technologyhas been successfully deployed in a range of downholemonitoring applications, predominantly for use in wire-line retrievable systems, but also more recently for per-manent reservoir monitoring. Unfortunately, electronicsystems have inherent limitations that render down-hole applications particularly challenging. One fun-damental issue with electronic systems is a dramaticdecrease in reliability at elevated temperatures. Fur-thermore, the large number of differing electronic tech-nologies used for downhole measurements (e.g., quartzresonator, thermocouples, strain gauges, gamma detec-tion, resistivity etc.) complicates the telemetry aspectsof the overall sensor system.

Over the past decade, fiber optic sensing technol-ogy has been developed and applied to a wide rangeof industrial and commercial applications. In efforts toextract the full benefits of fiber sensing systems, theseapplications have tended to exploit the ability of fiberoptics to accurately measure a large number of param-eters in harsh conditions on a widely distributed ba-sis, with the fiber serving as both the measurementmeans and the telemetry channel. These, and otherattributes, make them particularly well suited for ap-plications within the oil and gas industry.

Since the advent of the ‘side-written’ grating fab-rication approach [7] there has been intense interestin grating based sensors. We have developed trans-ducers based on grating technology for a wide rangeof applications in the oil and gas industry not limitedto downhole production monitoring, but also for seis-mic sensing, down-stream process monitoring, platformstructural and pipeline monitoring, and other sensingrequirements covered under the generic fields of explo-ration, production and transportation.

2. Permanent Downhole Monitoring

Compared to simple vertical wells drilled for the ear-

KERSEY: OPTICAL FIBER SENSORS FOR PERMANENT DOWNWELL MONITORING APPLICATIONS401

liest oil production in the industry, modern wells usedfor the extraction of hydrocarbon reserves (oil/gas) in“high production rate” fields can be typically complexconfigurations involving vertical, deviated and multiplehorizontal/lateral branches, or components. Further-more, the reservoir can be split between multiple levels,each producing at differing rates, gas/oil/water ratios,pressures and temperatures. The vision of the reservoirengineer is to be able to extract the most of the hydro-carbon reserves prior to the reservoir being reduced to awater producer, and being abandoned. Unfortunately,the current ‘average’ recovery efficiency is ∼35%: i.e.,65% of the oil/gas being left in the reservoir. To opti-mize the recovery efficiency of such wells, the industryis turning towards the implementation of ‘smart-wells’or ‘intelligent’ completions. Inherent to this trend isthe need to add additional sensory capability to thewell bore environment on a permanent basis.Bragg Grating Sensors: Fiber Bragg gratings(FBG) are intrinsic sensor elements that can be ‘writ-ten’ into optical fibers via a UV photo-inscription pro-cess. The photo-inscription process produces a periodicmodulation in the index of the glass in the fiber, whichhas been show to be a stable structure even at elevatedtemperatures experienced downhole. The advantagesof Bragg grating sensors are well known in the fibersensor community and include:

• Simple UV photo-inscription fabrication process• Intrinsic ‘intra-core’ sensor element• Wavelength-encoded operation• Electrically passive• No downhole electronics• Intrinsically safe• Immune to EMI

• Capable of high temperature operation• Distributed sensing capabilities• Low-profile sensor• Minimally invasive in the wellbore

• Compatible with components developed for thefiber telecommunications market

Of these advantages, the multiplexed or distributedsensing capabilities of fiber optic sensors is of partic-ular pertinence for downhole applications, were thereis the need to monitor a parameter, or parameters,at many spatial locations through the wellbore, orhorizontal/multi-lateral components of the well is of in-terest.

We have developed a range of transducers that uti-lize gratings as the core building block for a suite ofwavelength-encoded sensors. This allows us to base ourdevelopment efforts on a ‘common technology platform’approach, in which all types of transducers are compat-ible with a single form of surface instrumentation hard-ware. Transducers for pressure, temperature and vibra-tion have been designed, built and tested, and conceptsfor flow, differential pressure, acoustics, corrosion and

Fig. 1 Downhole reservoir monitoring using fiber optic Bragggrating sensors.

resitivity are under development. These sensors can beapplied to a number of applications for retrievable orpermanently installed reservoir monitoring systems.

3. Applications

Reservoir Pressure and Temperature Monitor-ing: Pressure and temperature are fundamental reser-voir engineering parameters and permanent monitoringof downhole pressure and temperature is widely uti-lized. While conventional pressure and temperaturesensors are proving to be important reservoir manage-ment tools, the current technology has some signifi-cant limitations. Due primarily to the required down-hole electronics, conventional pressure and tempera-ture sensing technology becomes unreliable at elevatedtemperatures. Also, multiplexing limitations restrictthe spatial resolution provided by conventional sensors.The high operating temperatures and multiplexing ca-pability of fiber optic sensors have the potential to in-crease both the reliability and resolution of downholepressure and temperature measurements. The trans-ducer design challenge in producing a precise and re-peatable response to pressure and temperature, whileprotecting the fiber optics from the harsh well environ-ment.

Figure 2 shows an example of the calibration char-acteristics of a FBG-based pressure sensor developedfor production monitoring that uses a mechanical trans-lation of pressure into strain in the fiber. The packagedCiDRA pressure transducer is shown in Fig. 3. The sen-sor exhibits a highly linear response in wavelength shiftwith pressure. Current transducers have been testedto ∼175◦C, but devices with operating temperature to200◦C and above are currently under development, withsensors capable of operation to 250◦C as a design goal.

Figure 4 illustrates the accuracy that can beachieved with a 5,000 psi range FBG based sensor.Here, the residual error of the transducer over its pres-sure and temperature operating range is presented. Theerror from calibration is within approximately +/−1

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Fig. 2 Linearity of a Bragg grating based pressure transducer.

Fig. 3 Grating based pressure transducer.

Fig. 4 Accuracy of transducer.

psi over the full operating range. This level of accuracyis comparable to the performance found with the bestelectrical gauges used in the industry.

In addition to the pressure and temperature trans-ducers, we have developed surface instrumentation, ca-bles, connectors and fiber feed-throughs to field com-plete fiber optic pressure and temperature measuringsystems. The surface instrumentation, which interro-gates the fiber optic sensor and converts fiber optic out-put into engineering units, is shown in Fig. 5. The fiberoptic cable is deployed in a manner similar to the elec-trical cables commonly deployed in conventional reser-voir monitoring systems.

Fig. 5 Suface instrumentation unit.

Fiber optic fiber Bragg gratings have the ability tomeasure multiple parameters in a distributed mannerall along a single, common, deployed fiber optic cablethat is mechanically identical to a 1/4 in. control linecommonly used in the oil and gas industry. Fiber opticsensing systems thus enable a significant increase in thespatial resolution of pressure, temperature, and multi-phase flow data provided in a permanent monitoringsystem. The advantages of distributed measurementswithin wells are numerous, particularly in wells usingadvanced completion technologies.Flow Monitoring: Multiphase flow meters measurethe flow rates of individual components within co-flowing mixtures of oil, gas, and water without requir-ing separation of the components. Over the past 10–15years, the industry has predominately focused on top-side and subsea multiphase flow meters, advancing thetechnology such that today these meters are commer-cially available.

The next step in multiphase flow metering is tomove the flowmeters downhole [8], [9]. While maintain-ing the ability to monitor the entire well stream pro-duction, downhole flow meters provide the additionalcapability of determining the flow rates on a spatialdistributed basis from within the well. Unfortunately,the challenges associated with developing accurate, re-liable, downhole multiphase flow meters are numerous.The combination of harsh operating conditions, var-ied multiphase flow regimes, restrictive packaging con-straints, data transmission challenges, environmentalconstraints, and non-intrusive (full bore) requirementshave proved to be difficult barriers to overcome.

Fiber optic technology enables these measurementsto be performed downhole. Exploiting the inherent ca-pabilities of fiber optic sensors to accommodate theharsh temperatures, packaging, environmental, anddata transmission requirements, we have developed adownhole, fiber optic multiphase flow meter. The flowmeter is non-intrusive, intrinsically safe, and contains

KERSEY: OPTICAL FIBER SENSORS FOR PERMANENT DOWNWELL MONITORING APPLICATIONS403

no downhole electronics. The long-term goal is to de-velop multiphase flow meters capable of accurate mea-surement over an expanded range of multiphase flowregimes. The initial system, however, has been de-signed for quasi-homogenous flow regimes:

• oil, water cut binary liquid mixtures• oil, water, and gas cut in low gas volume fraction

mixtures (<∼20%)

To evaluate the performance of this novel fiber optic-based multiphase flow meter, we recently completedtechnology demonstrator test of a prototype downholemeter at an independent, industry-recognized, test well.The flow meter is designed into a standard piece of3 12 in. production tube, approximately 12 ft long. The

test facility was capable of producing specified mixturesof oil/water/ and natural gas through the productiontubing. The oil, water, and gas components consistedof a 32◦ API crude, a 7% salinity brine solution andmethane, respectively. These fluids were selected asrepresentative of typical production conditions. Thetest was operated at low temperature (100◦F) and lowpressure (< 400 psi) conditions.

The fiber optic multiphase flow meter was inte-grated into the production tubing of a 300 ft test well.The flow meter was oriented vertically within the welland performed the structural role of a section of stan-dard production tubing. The flow meter communicatedwith the surface-based optoelectronics solely via a sin-gle armored fiber optic cable assembly. The test facil-ity combined the oil, water, and gas prior to the testsection and separated the three components on the sur-face prior to recirculation through the test section. Bymonitoring the flow rates of each component, the facil-ity could produce arbitrary multiphase flow mixtureswithin the desired accuracy. Figure 6 shows the de-ployment of the flow meter into the test well facility.

A primary objective of the test was to assess theability of the flow meter to determine water cut in crudeoil and brine mixtures. Figure 7 shows the measuredvolumetric phase fraction versus the reference measure-ment for water cut ranging from 0–100%. The referencemeasurement was determined from the flow rates ofthe individual liquids prior to being mixed and passedthrough the test section. For the objectives of this test,the meter was calibrated by measuring 100% water and100% oil mixtures prior to calculating the phase frac-tion of intermediate mixtures. For production monitor-ing applications, industry required measurement accu-racy is approximately 10% relative uncertainty in gasliquid rates and 5% uncertainty in water in liquid ra-tio. As shown, with the exception of a few outlyingdata points, the flow meter was able to determine wa-ter cut in crude and brine mixture to within +/−5%over the full range of water cuts.

Multiphase flow metering requires flow rate mea-surement in addition phase fraction. The ability of

Fig. 6 Flow meter deployment into a test well.

Fig. 7 Phase fraction measurement (water/oil ratio).

the flow meter to measure velocity was evaluated foroil/water mixtures from 0–100% ranging from ∼1 ft/sec(22 gpm) to 25 ft/sec. Figure 8 shows the measured ver-sus reference flow rate for oil/water mixtures over theoperating range of the facility. As shown, the flow ratemeasured by the CiDRA flow meter agrees with the ref-erence flow rate to within approximately 5% over thetested flow range. Although not presented herein, theflow meter performed similarly for low gas volume frac-tions mixtures of oil, gas, and water.Seismic Applications: Fiber Bragg grating sensorsalso have great potential for providing distributed sens-ing of acoustic pressures in downwell environments forthe purpose of seismic monitoring. The trend in theindustry is currently to utilize the sub-surface imagingcapabilities of seismic monitoring not just for oil andgas exploration, but for monitoring reservoir depletion.This ‘time-lapse’ for 4D seismic monitoring is a power-ful new approach to increasing yields in the oil and gas

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Fig. 8 Flow rate measurement using the fiber optic flowmeter.

industry. Permanently installed sensors located deepin wells can provide such imaging, but are required tosurvive in-well for > 10 years. Here again, the passivenature of fiber optic sensors allows for high reliabilityin harsh environments over extended periods.

4. Conclusion

We have developed and demonstrated high tempera-ture, fiber optic reservoir monitoring system based onfiber Bragg gratings, including pressure, temperatureand multiphase flow. This full suite of sensors sharea common fiber optic operating principle and can bemultiplexed on a single fiber optic cable, and provideenhanced capability to monitor oil and gas reservoirproduction on a real time, spatially distributed basis.

Acknowledgements

The author wishes to acknowledge the significant con-tributions of coworkers at CiDRA Corporation and Op-toPlan A.S. to the work presented in this paper.

References

[1] E. Udd, Fiber Optic Sensors: An Introduction for Engineersand Scientists, Wiley, New York, 1991.

[2] A.D. Kersey, “A review of recent developments in fiber op-tic sensor technology,” Optical Fiber Technol., vol.2, p.291,1996.

[3] A.H. Hartog, et al., “Distributed temperature sensing in solidcore fibers,” Electron. Lett., vol.21, p.1061, 1985.

[4] K.A. Murphy, “Extrinsic Fabry-Perot optical fiber sensor,”Proc. Optical Fiber Sensor Conference (OFS’92), p.193,1992.

[5] OptoPlan A.S., Norway, unpublished work.[6] A. Kersey, et al., “Fiber grating sensors,” J. Lightwave Tech-

nol., vol.15, no.8, 1997.[7] G. Meltz, W.W. Morey, and W.H. Glenn, “Formation of

bragg gratings in optical fibers by a transverse holographicmethod,” Optics Lett., vol.14, p.823, 1989.

[8] A. Aspelund, et al., “Challenges in downhole mulitphase flow

measurments,” SPE 35559, Presented at the European Pro-duction Operations Conference and Expositon, Stavanger,Norway, 1996.

[9] J.P. Brill, “Multiphase flow in wells,” J. Petroleum Technol-ogy, Jan. 1987.

Alan D. Kersey is vice president and chief technology officerfor CiDRA Corporation. Alan possesses over twenty year’s expe-rience in the field of fiber optic sensing, particularly in the fiberinterferometry and bragg grating sensor systems. He receivedhis B.S. in physics and electronics from the University of War-wick, and his Ph.D. from the University of Leeds, United King-dom. Alan held a Research Fellow position in the Applied OpticsGroup at the University of Kent, Canterbury, United Kingdom,from September 1981 through November 1984. Dr. Kersey’s workin the area of applied optics has led to over 100 journals and morethan 150 conference publications, 25 US patents and over 20 ap-plications pending, and contributions to two books on fiber opticsensing. In 1993 he was elected Fellow of the Optical Society ofAmerica.