Distributed optical pressure and temperature sensors

Detailed Description

As illustrated in the accompanying drawings and discussed in detail below, the present invention is directed to optical sensors distributed along an optical fiber. According to the present invention, a plurality of temperature / pressure sensors are formed on an optical fiber. Although it is possible to use any type of sensor, such as intrinsic or extrinsic Bragg (FBGs) or Fabry-Perot reticles, FBGs are preferred because they can be recorded immediately on the optical fiber. The optical fiber with optical sensors distributed over it is preferably loaded on the side wall of a capillary tube. The optical sensor and capillary tube can extend over long distances, for example, several kilometers or miles, and can cover the entire depth of an oil and gas well. In a preferred embodiment, the capillary tube is a thick-walled metal capillary tube that is typically used to carry discrete optical pressure and temperature sensors, such as an intrinsic Fabry-Perot sensor or an extrinsic Fabry-Perot sensor.

Referring to figure 1, a thick-walled metal capillary tube 10 is illustrated. Capillary tube 10 can be of any length and, in one example, tube 10 has an outer diameter of about 6.35 mm (0.250 inches) and an internal diameter of about 4.69 mm (0.185 inches). Capillary tubes of any thickness can be used, as long as the capillary tube is thick enough to support the optical fiber and optical sensors. Tube 10 has a longitudinal slit 12 formed along its entire length. Slit 12 should be wide enough to carry a core-covered optical fiber in it and be small enough not to have a significant impact on the structural integrity of capillary tube 12. Typically, slit 12 can be machined or cut to size. from a conventional capillary tube, as shown in figure 2. Along the surface of the tube 10 at predetermined locations, areas 14 are modeled. As best shown in figures 3A and 3B, a portion of the side walls of the tube 10 is machined to form a thin wall section 16, which acts as a diaphragm sensitive to the pressure differential across it. The thin-walled section 16 may have a flat surface, as shown in figures 1 and 3A, or the slot 12 may remain on the surface of the thin-walled section 16, as shown in figure 3B. Although only two modeled areas 14 are shown, any number of modeled areas 14 can be formed in tube 10. The spacing between adjacent modeled areas 14 can be selected, wherever pressure and temperature measurements are desired. In one example, the spacing can be a few centimeters and more.

Alternatively, the slit 12 can be omitted and the optical fiber 20 can be attached to the capillary tube 10 in a serpentine shape to absorb the thermal expansion / contraction of the tube 10. The fixation can be continuous or at discrete points.

Within each modeled area 14, at least one optical sensor, for example, FBG 18, is formed in optical fiber 20, as best shown in figure 4. FBG 18 is fixed to the thin-wall section 16, by any method known as laser or epoxy welding or adhesive, such that, according to section 18 of thin wall flex or bend, FBG 18 also flex or bend. FBG 18 can also be metallized, by metal vapor deposition on the sensor or by other known techniques. The internal space 22 is preferably sealed to retain a reference pressure (P_ref) substantially constant within it. As the pressure to be measured outside the capillary tube 10 changes, the pressure difference arches the thin-walled section 16, which acts as a diaphragm. FBG 18 also curves along the thin-walled section 16 and changes the frequency of the optical signal reflected by the FBG. A surface instrumentation unit (SIU) (not shown) receives the altered frequency and reads the pressure in the modeled area 14.

In an alternative embodiment, capillary tube 10 and inner space 22 are segmented into a plurality of sections sealed, for example, by walls or membranes orthogonal to the longitudinal axis of the capillary tube, similar to that of a bamboo stem. One or more optical sensors can be located on each segment. An advantage of segmenting the inner space 22 into sealed sections is that if the inner space 22 is ruptured, that is, exposed to pressure from the well, only the ruptured section is affected and the rest of the capillary tube remains sealed for the remaining optical sensors to function .

Between adjacent profiled areas 14, the optical fiber 20 is preferably freely filled or placed inside the slot 12, as best shown in figure 4. The freedom of the optical fiber 20 between profiled areas 14 allows the loose part to absorb the expansion and the thermal contraction of the metal capillary tube 10 and allows the necessary clearance to wind the capillary tube 10 on spools. The amount of loose part can be determined from the thermal expansion coefficient of the capillary tube material 10 and / or the reel radius. Optionally, a second FBG 24 is provided next to FBG 18 to measure changes in temperature. In other words, FBG 18 arches with the thin wall section 16 to measure stress / deformation and FBG 24 measures temperature changes and compensates for the temperature effect on FBG 18.

As the optical fiber 20 can extend over long distances, it is expected that a large number of optical fibers will be recorded or located on the optical fiber. As such, it is preferred that advanced signal processing techniques are employed to distinguish the signals reflected by the multiple optical sensors. Such advanced techniques are described in the commonly owned US patent application, serial number 11 / 222,357, whose title is System and Method for Monitoring a Well and filed on September 8, 2005. The '357 patent application is incorporated into the context as a reference in its entirety, among other things, the' 357 application describes a physical interleaving technique, where the pluralities of sensors are arranged along the length of an optical fiber in each side of a reference reflector. In this technique, the corresponding sensors are placed at similar distances from the reflector to increase the sensing length. In addition, the physical interleaving technique can be expanded to combine multiple sensing lengths within an optical fiber to increase an overall sensing length. The '357 order also discusses the combination of the physical interleaving technique of multiple sensing lengths with wavelength division multiplexing (WDM), where each individual sensing length is designed to respond only to a slightly different wavelength of the next wavelength. This can further increase the sensing length by using a function of the number of wavelength divisions that are present. Additionally, it is possible to generate additional sensing lengths using a polarization technique, more specifically, using narrowband FBGs placed outside the Nyquist sampling distance. Additional signal processing techniques are discussed or cited in the '357 patent application.

While it is apparent that the illustrative modalities of the invention described here fulfill the objectives of the present invention, it is appreciated that numerous modifications and other modalities can be viewed by those skilled in the art. For example, capillary tube 10 can be replaced by a carrier of another shape, such as spherical or cylindrical pressure vessels that have been profiled to form thin-walled sections therein. In addition, characteristic (s) and / or element (s) of any modality may be used alone or in combination with characteristics and / or elements of other modality (s). Therefore, it will be understood that the appended claims are intended to cover all such modifications and modalities that are within the spirit and scope of the present invention.