Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2000-08-14
2002-08-27
Ngo, Hung N. (Department: 2874)
Optical waveguides
Optical waveguide sensor
C385S125000
Reexamination Certificate
active
06442304
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to apparatus and method for protecting devices in hostile environments. It has particular relevance for protecting optical fibre cables, transducers and components in oil, gas and geothermal wells.
In the normal production of oil and gas it is recognised that accurate and detailed information of the pressure and temperature in the oil and gas wells is important in order that adjustments can be made to flow rates and in order that preventive action can be taken to remedy damaging or potentially damaging conditions in the well. Similarly it is common practice for operators to stop the well from producing periodically, in order to observe the rate at which the downhole pressure changes after flow has been stopped. Accurate recording of the pressure profile provides the operator with valuable information regarding the condition of the well assembly and the condition of the region in the reservoir near the producing section of the well. The variation of the pressure during the period following cessation of production also helps to establish the physical extent of the region in the reservoir which is in pressure communication with the well. Furthermore, when electrical pumps are installed in oil wells in order to assist and speed up the rate of production, the knowledge of pressure and temperature along the pump is useful in adjusting the pump operating conditions such that undesirable conditions are avoided which can lead to damage to the pump assembly because repair and replacement can be extremely expensive.
Pressure sensors and temperature sensors are commercially available which are capable of being installed in the difficult conditions encountered in many oil and gas wells. Commonly used sensors are ones based on quartz transducers or silicon strain transducers. Such sensors generally have active electronic modules associated with them that must be located very near the transducers. Electrical cables then link the sensing assembly to the surface, providing electrical power for the sensor assembly and transmission of the sensor signal. It is well known that as oil is produced from deeper reservoirs, the downhole temperature and pressure increases and the sensor and sensor electronics have to survive under increasingly difficult conditions. The conditions are made all the more difficult as the surrounding environment contains water and many other reactive chemicals which react with the sensor assembly. Temperatures are often higher than 100 degrees Centigrade and can reach 200 degrees C or higher. Pressures encountered are often in excess of 10,000 psi and can exceed 20,000 psi.
Oil companies frequently have experienced failures of sensors under such conditions, sometimes within very short periods after installation and often within one or two years. Replacement of failed sensors or the associated cables and connectors is often economically impractical since it involves the shut-down of the well and requires expensive procedures to extract the sensor system from the well and to replace it.
Optical fibre sensors have been developed in order to overcome the short-comings of electronic sensors such as silicon based or quartz based gauges. Optical fibre gauges are passive devices that do not require active electronic assemblies near the measurement point. Generally optical fibres used for such purposes are made of silica which has a melting point near 2000 degrees Centigrade and which has many excellent engineering qualities. It is a very elastic material, with a very low coefficient of thermal expansion and remains elastic at pressures as high as 20,000 psi or greater. During the manufacturing process, optical fibres are coated with a protective material to prevent chemical attack of the silica which results in weakening of the fibre.
SensorDynamics has developed a fibre optic pressure sensor assembly consisting of a polariser, a pressure sensitive sidehole fibre and a mirror, all fusion spliced to an optical fibre lead. Such pressure sensors have been shown to have excellent performance features such as linearity, high resolution and survival at temperatures above 300 degrees C and at pressures in excess of 15,000 psi.
These sensors have a further and very important advantage derived from the fact that they are typically very thin and flexible. Optical fibres are typically between 100 microns and 500 microns in diameter, hence can be simply joined to optical fibre cables of similar diameter and can be deployed over many kilometres through hydraulic control lines using fluid drag. Hydraulic control lines typically have outside dimensions of ¼ inch and are frequently a feature of oil and gas wells and are used to control valves and chokes and also to inject chemicals or gas to assist the efficient production of reservoir fluids. More recently hydraulic control lines have been included in the construction of oil wells in order to provide a conduit through which optical fibre cables and sensors can be transported to the remote regions of the oil or gas well in order to acquire pressure, temperature and potentially other information. The ability to deploy sensors over long distances is important for many reasons. It removes the need for complicated electrical connectors or optical connectors in difficult locations along the well construction, allows different types of sensors to share the same control line conduit, allows other sensors to be added to the same network without interrupting the normal operation of the oil or gas well and, in the event of a sensor failure, makes replacement of sensor and cable practical and economically acceptable and recalibration possible and simple.
Whereas the optical fibre pressure sensors have displayed these excellent characteristics described above, they have all exhibited a rapidly changing zero point when exposed to a high temperature, high pressure environment which contains water in liquid phase, forming either the major component of the liquid material, or as a dissolved component in another liquid material. This zero point instability has been investigated extensively and it has been established that the rate of drift of the zero point is greater at higher temperatures and is faster when the pressure sensors are surrounded by water than if immersed in another fluid such as silicone and polysiloxane oils (such as Syltherm 800 Heat Transfer Liquid supplied by Univar plc) have shown that the drift of the zero point is caused by ingress of water or OH radicals into the silica body of the optical fibre, resulting in a highly stressed layer which starts to form at the surface of the optical fibre sensor and gradually extends inward. This stress layer has been shown to form in coated fibres as well as in uncoated silica fibres. Carbon coatings have been shown to slow down the formation of the stress layer. Stress layers alter the response of pressure sensors significantly and mask the true variation of pressure in the well. Where an in-fibre Bragg grating has been used as a pressure transducer, this drift has been shown to be as high as 30,000 psi, after a period of a few weeks or months, when exposed to water at 250° C. and 5000 psi pressure. In the case of a polarimetric sensor the change has been significantly lower but still resulted in a zero drift of 6000 psi under similar environmental conditions. Further measurements were carried out on commercially available, optical fibres and have established that the ingress of OH radicals causes the significant increase in the physical length and in the optical length of the fibres. Clowes et al. (see Electronics Letters, May 27
th
1999, pages 926 to 927) reported changes greater than 0.1% in the physical length. Again it was found that carbon coatings that were developed to provide hermetic protection, to prevent the ingress of hydrogen into optical fibres employed in subsea communications cables, also reduce the rate at which the effect occurs. It has also been shown that no polymer coatings have been able to prevent the ingress of OH at 250°
Clowes John Redvers
Crawley Charles M.
Kluth Erhard Luther Edgar
Rutt Harvey N.
Varnham Malcom P.
Chevron U.S.A. Inc.
Ngo Hung N.
Schulte Richard J.
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