Monitoring - Fiber Optics

Using Fibre Optics

Contacts

Ahmed Abou-Sayed ahmed@advantekinternational.com Advantek International

Abstract

Fiber optics involves the transmission of light through fibers or thin rods of glass or some other transparent material with a high refractive index. Light that enters one end of a fiber can travel through the fiber with very low loss, even if the fiber is curved. Images can be transmitted, as can data. The fiber optics itself can be used as a sensor for variations in environmental parameters such as temperature, pressure, rate and sand concentration in a flow stream.

Key Issues

Background Precautions Complications References

Key Issue 1: Background

Concept 1.1:

Total Internal Reflection

Light traveling inside the fiber center (core) strikes the outside surface at an angle of incidence that is greater than the critical angle.
This light is reflected toward the inside of the fiber without loss.
Light can therefore be transmitted over long distances by being reflected inward a multitude of times.
In order to avoid losses through the scattering of light by impurities on the surface of the fiber, the optical fiber core is clad with a glass layer of much lower refractive index and the reflections occur at the interface of the glass fiber and the cladding. This avoids losses caused by scattering of light by impurities on the fiber surface.
It is possible to construct networks of Bragg gratings to measure temperature, pressure, strain, vibration, and acoustic signals over a single fiber. The change in reflected wavelength, indicating the change in an environmental parameter, can be detected by coupling a "white" light source onto the fiber. The reflected signals are for any wavelength shift from a calibrated value.

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Concept 1.2:

There are diverse applications:

Instrumentation:Petroleum applications include sensors.
Communication:In addition to sensing environmental variations, fiber optics can be used to transmit the measurements to the surface. Fiber-optic laser systems are being used in communications networks. Long distances (100 km) can be achieved before signal repeaters are needed to regenerate signals. For example, the Dunbar Platform is operated from the North Alwyn Platform, 22 km away.
Extreme Environments:Applications supporting technology that cannot function with "standard" electrical communication (to provide communication where high levels of electrical noise prevent the use of copper-based communication), for direct access to optical sensors (subsea and downhole) and for communication with sensor systems providing either continuous real-time data, or information at data rates higher than can be supported by existing electrical communication. In comparison to electrical equipment, survivability in the subsea and downhole environments, with contact with water is allegedly superior. Hydrogen darkening can be addressed.
Subsea Machinery:The electrical noise environment around new subsea production techniques (e.g. subsea separation, multiphase pumping), and the power umbilicals that supply them, are problematic for conventional electrical communication and have driven the move to inherently noise immune, fiber optic communication configured for the modular offshore installation.
Well Diagnostics:For example, downhole, passive optical sensors (e.g., North ETAP).
UmbilicalsSome recent deepwater development programs have realized significant financial advantages using optical fiber in place of copper. There can be reduced installation costs and weight.
Fault Tolerant Systems:These can/could include modular control systems, ROV installable valves and chokes, position sensors, metering and multi-phase flow measurements.
Subsea Production:Multiphase pumps, and/or subsea separation systems (e.g., Troll Pilot subsea separator system, Petrobrass deepwater (1,000 meters water depth) compatible multiphase pump, TFE's Nautilus multiphase pump program, and the CoSWaSS (configurable subsea water separation system) joint industry project).

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Concept 1.3:

You might be surprised at some of the potential/current oilfield applications?

Position and movement with fiber gyroscopes
Acoustics with fiber hydrophones
Strain in "smart structures" ("smart composite" material technology is being investigated for drilling and production risers as well as composite tension legs and tethers)
Chemicals and reactions
Electrical supply characteristics
Subsea and downhole communication
Temperature, pressure, sand, vibration, flow
Pipeline temperature and early warning of waxing or hydrate formation

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Key Issue 2: Sensing

Precautions 2.1:

Precautions

Avoid excessive build up of either intrinsic or extrinsic energy losses in the optical fiber.
Hydrogen attenuation
Water ingress will damage fibers. Hermetic coatings are used.
Fragility: Micro-bending (small-scale bends in the core-cladding interface), sometimes caused by extreme thermal cycling, is usually not an issue although care before it is placed in the control line.
Cost:
Is there a response time to sudden temperature events?
Ensure adequacy of wellhead barriers and abide by legislative requirements
Plan for wellhead and tubing hanger modifications that may be necessary for passage of the control line through the wellhead.
Plan for clamps and straps to protect the control line.
Specific running equipment (e.g., on-off disconnects) may be required.
Plan for packer penetrators.

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Key Issue 3: Sensing

Sensing 3.1:

Temperature Measurements?

Temperature Measurement: Distributed temperature sensing can be used to provide a real-time thermal profile along the entire length of the well.

Measured at every point along the line.
Laser light pulses are launched into the optical fibre sensor. As the light pulse encounters temperature features along the fibre the pattern of back-scattered light returning changes.
The fibre optic system serves as both a sensor and the means of transmission.
Temperature is calculated for every meter along the fibre to an accuracy of 0.1°C and a resolution of 0.1°C.
Changes in temperature can be used to identify and monitor:

Flow profiling
Injection monitoring
Flow behind casing
Gas lift efficiency
Fracture zones
Casing leaks and channels
Lost circulation zones
Cement top identification
Cement squeeze validation
Steam breakthrough
Steam channeling

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Sensing 3.2:

Pressure?

Pressure Measurement: Pressure is measured using a self-referencing optical technique. T

The system detects small changes in distance that occur between two points in a ceramic pressure head when the pressure is varied.
The micron scale gap in the sensor head is monitored using optical interference patterns to provide accurate pressure readings, with calibration..
Sensor temperature comensation is provided from concurrent temperature measurements.
Necessary equipment is at the surface.
One vendors equipment allows up to 11 pressure sensors and a distributed temperature measurement to be installed in a single well with one cable, if required.
Applications include:

Evaluating fractured systems
Measurement of drawdown
Skin determinations
Layered formation pressure transient analysis
Permeability
Reservoir heterogeneity
Completion efficicency
Infer radii
Performance curves
...

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Sensing 3.3:

Rate?

Rate Measurement:

Using a fiber optic distributed temperature measurement system, changes in temperature with time can be correlated with fluid flow.
One vendor's system uses a pre-installed distributed temperature system and heating coils to track the velocity of single-phase fluid at low flow rates at specified points downhole.
A transient temperature anomaly is caused in the produced fluid with a heating coil and this anomaly is tracked as it moves up the well.
Records every meter along the fiber at sampling frequencies down to 1 Hz.
Can be installed to measure low flow rates near the toe of horizontal wells
Can be installed on either side of laterals in multilateral completions to define lateral contribution
Can be installed and used with viscous fluids
Will measure fluid velocity in either direction
Can be actuated at any time from the surface.

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Sensing 3.4:

Acoustic Measurements?

Acoustic Measurementa:

Applications could include measuring flow rate, discerning flow type/regime, monitoring pump condition, leak detection, and sand production monitoring.
Optical acoustic monitoring of near-field acoustic noise and vibration.
The acoustic flow sensor is a permanent, fiber optic sensing system that can be installed at several points.
Measurement uses fibre optic time domain reflectrometric interferometry technology.
Sensor consists of a wound length of fibre , encapsulated in a pressure housing acoustically mounted at defined locations on the completion.
The sensors are interrogated with pulses of laser light.

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References

Wright, P.J., "Optical fiber's gigabit bandwidth, 200 km range attractive for subsea work." Offshore Magazine (May 2000).
Lemaire, P.: "Reliability of Optical Fibers Exposed to Hydrogen: Prediction of Long-Term Loss Increases," Optical Engineering 30(6) (June 1991) 780-789.
Osato, K., Takasugi, S., Osawa, S., Hashiba, K., and Perales, K.: "Temperature Profiling/Bottom Pressure Monitoring System Using Optical Fiber and Capillary Tube - Field Test in a Geothermal Well," 1995 Annual Meeting Geothermal Research Society, Japan.
Orrell, P., and Harjes, B.: "Borehole Temperature Measurements using Distributed Fibre Optic Sensing," International Institute for Geothermal Research (1993).
Karakan, S., Kutlik, R., and Kluth, E.: "Field Trial to Test Fiber Optic Sensors for Downhole Temperature and Pressure Measurements West Coalinga Field, California, SPE 35685 (1996).
Van der Spek, A (1998) "Neural Net Identification Of Flow Regime Using Band Spectra Of Flow Generated Sound," SPE 50640,SPE, The Hague, The Netherlands.
Mckinley, R.M., Bower, F.M. & Rumble, R.C.: "The Structure And Interpretation Of Noise From Flow Behind Cemented Casing," SPE 6784, SPE-AIME San Antonio, Texas (1973).
Tague, J.R., Hollman, G.F.: "Downhole Video: A Cost/Benefit Analysis," SPE 62522, 2000 SPE/AAPG Western Regional Meeting, Long Beach, California (19-23 June 2000).
McKay, G., Bixenman, P.W., Watson, G.: "Advanced Sand Control Completion With Permanent Monitoring," SPE 62954, 2000 SPE Annual Technical Conference and Exhibition, Dallas, Texas (1-4 October 2000).
Buchwalter, J.L., Calvert, R.E., McKay, C.S., Thompson, S.J.: "Maximizing Profitability in Reservoirs Using New Technologies For Continuous Downhole Pressure Systems," 2000 SPE Annual Technical Conference and Exhibition, Dallas, Texas (1-4 October 2000).
Carnahan, B.D., Clayton, R.W., Koehler, K.D., Harkins, G.O. and Williams, G.R.: "Fiber Optic Temperature Monitoring Technology," SPE 54599, paper presented at 1999 SPE Western Regional Meeting, Anchorage, AK (May 26-28).

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