WO2008031181A1

WO2008031181A1 - Temperature and pressure optical sensor and use thereof

Info

Publication number: WO2008031181A1
Application number: PCT/BR2007/000235
Authority: WO
Inventors: Roberto Waldo Angulo Llerena; Luiz Carlos Blanco Linares; Arthur Martins Barbosa Braga; Carlos Guedes Valente
Original assignee: Faculdades Católicas Sponsor Of Puc-Rio
Priority date: 2006-09-14
Filing date: 2007-09-13
Publication date: 2008-03-20
Also published as: BRPI0606051B1; BRPI0606051B8; BRPI0606051A

Abstract

An optical sensor (100) for attachment to a carrier structure for the measurement of temperature and pressure comprises an optical connector (51) attached to an opening (21) machined in a pin (11d) formed on a cylindrical body (11c) provided with threads (20). Part of the body (11a) is of general rectangular shape, having rounded edges (12) and surfaces (13, 18 and 22), said body (11a) being integral to said cylindrical body (11c) and being of geometry such that the larger diagonal of body (11 a) is equal or smaller than the diameter of cylindrical body (11c) so that there is always a distance (11b) between the surface of body (11a) and the axial slope parallel to said surface in cylindrical body (11 c) so that body (11a) is always in balance relative to body (11c). Further, body (11a) has a first recess (13a) in surface (13) for forming one of the surfaces of membrane (17), and where said recess (13a) houses a plug (31) provided with a channel (32) and an elastic locking ring (41) and a second recess (18a) in surface (18) opposite to said surface (13), said recess defining the thickness of said membrane (17); and an optical fiber (15) containing at least two Bragg gratings (16a, 16b) attached to said membrane (17), said fiber (15) being integrally contained in the interior of said sensor between the optical connectors (51) throughout axial channel (15a) and a flexible connection device (23), whereby the absence of the offset pressure and temperature. effect for the performed measurement is assured.

Description

TEMPERATURE AND PRESSURE OPTICAL SENSOR AND USE THEREOF

FIELD OF THE INVENTION

The present invention relates to the field of temperature and pressure optical sensors, more specifically to a pressure and temperature optical sensor free from the offset phenomenon in pressure and temperature measurements, said sensor being applicable to oil and gas wells, engineering and everyday life.

BACKGROUND

In petroleum production, production wells should be monitored as for pressure and temperature. Up to 1 ,000 bar hydrostatic head can be found in the well, with temperatures over 200⁰C. Electrical sensors such as for example piezoelectric resistors, piezoelectric elements, capacitive probes or vibrating crystals are frequently used in the measurement of pressure up to nearly 170⁰C. It is also known to use optical pressure sensors which are distinguished by their good temperature ability, corrosion resistance and insensitivity to electromagnetic interference. A pressure and temperature sensor is installed in a carrier structure and a source of light in an optical module is used for feeding optical signals to the pressure sensor through the optical fiber. The pressure indicating signal at the measurement location provided by the pressure sensor is directed back to the optical module for processing.

For the measurement of pressure in multiple locations within the well, multiple pressure sensors can be multiplexed in series for the measurement of distributed pressure using wavelength division multiplexing (WDM) and/or time division multiplexing techniques (TDM).

The concept underlying optical fiber sensors is that environment effects can influence the amplitude, phase, frequency, spectral content or polarization of the light propagated through an optical fiber. Advantageously, optical fiber sensors are low-weight, small-size, passive, energetically efficient and electromagnetic interference immune devices. Besides, optical fiber sensors are potentially highly sensible, of wide dynamic range and wide bandwidth.

The largest potential use tool in multifunctional and near distributed sensor systems in applications for measurements in the permanent monitoring of wells and reservoirs is fiber Bragg gratings (FBG).

The concept underlying FBG is that on building/designing a network located at a certain area of the fiber, typically of 1 to 10 mm length, according to a preset wavelength, a certain wavelength of the incident light is affected (FBG filter) As a result from the medium perturbations there is variation in the wavelength absolute value, this having a straight relationship with the physical magnitudes which have caused said variation

The main feature of an optical sensor is that it is completely passive, that is, it does not need any on-board electronics for transducing the measured magnitude, the only requirement being a mechanical element coupled to the FBG element which is the fiber itself. In a fiber, the core (the core being the central portion thereof) diameter determines if the optical fiber is single-mode or multimode. The single-mode and multimode expressions refer to the dimensional orientation of the rays that propagate throughout the fiber. The core diameter of single-mode fibers is relatively small (2-12 microns) and support one single propagation mode, the axial one. The core diameter of multimode fibers is relatively large (25-75 microns) and enables non-axial propagation rays or modes throughout the fiber core. Actually, the so-called single-mode fibers are actually two-mode fibers in the sense that there are two different optical polarization states which can propagate throughout the nucleus. In an ideal, straight fiber, which is free from deficiency and of perfectly circular symmetry, the light propagation rate is independent from the direction of polarization.

Sensors for the measurement of several physical parameters such as pressure and temperature are often based on the transmission of deformation from an elastic structure such as a diaphragm or a bellows up to a sensor element. In a pressure sensor, the sensor element can be fixed to the elastic structure by means of a suitable adhesive. It is well-known that the fixation of the sensor element to the elastic structure can be the source of many errors if the fixation is not highly stable. In the case of sensors, which measure parameters which are static or that change very slowly, the long-term stability of the fixation to the structure is of paramount importance. One main source of such instability of the long-term sensor is the phenomenon of creep, that is, variation in sensor element deformation without any variation in the load applied on the elastic structure, which results in DC shift or error by deviation in the sensor signal.

An optical sensor is described in US patent N° 6,016,702 for the measurement of pressure with temperature compensation for a point where an optical fiber is mounted to a pressure responsive bellows structure in a location along the fiber and to a rigid structure in a second location along the fiber, with a Bragg grating printed in the fiber between said two mounting locations of the fiber, the grating being under strain. As the bellows is pressed as a result of a variation in the outer pressure, tension on the fiber grating is reduced, this leading to a variation in the wavelength of the light reflected by the grating. However, temperature compensation can be performed by isolating the grating for temperature in an isolated pressure chamber, which adds costs to the equipment. The proposed sensor can be used in the single-point or multi-point version, in series along one single optical fiber. In the mounting of the sensors in series, the optical fiber is conveyed through a passage at the end of a bellows structure for interconnection to the following pressure sensor. The several pressure and temperature signals of the different sensors can be distinguished with the aid of Wavelength Division Multiplexing (WDM) techniques. Thus, each Bragg grating works at a central wavelength λ in the interior of a wave amplitude ω which is not superimposed to the amplitude of the other Bragg grating sensors. Therefore, based on the received wavelength the temperature and pressure signals of each of the sensors in series can be easily distinguished. Also, Time Division Multiplexing (TDM) techniques can be used for distinguishing among signals of different Bragg gratings sensors. However, the sensor of said US patent does not describe nor suggest the optical pressure and temperature sensor making the object of the present invention, the inventive sensor making use of at least two Bragg gratings in one single optical fiber.

Brazilian Application PIBR 0403240-3 (and its corresponding US published application 20060034559) relates to an optical transducer for the simultaneous measurement of pressure and temperature in oil and gas wells, said transducer being inserted within a system which comprises a light emitter propagating along the core of an optical fiber containing Bragg gratings printed on it, until said emitter meets a Bragg grating which reflects a portion of said light, said transducer comprising a predominantly cylindrical body including an elastic membrane for attaching at least one Bragg grating, and at least one more Bragg grating printed on the same optical fiber, and wherein said body is: a) bored along a main axis coaxial with or parallel to a longitudinal axis thereof thereby to allow the passage of said optical fiber therethrough; b) provided with two symmetry planes in order to facilitate installation and serial connection with other optical fiber transducers and sensors; c) provided with a central access and respective stop, perpendicular to the longitudinal axis of said body, on the opposite side of said elastic membrane with respect to said optical fiber, to allow the attachment of said at least one Bragg grating to said elastic membrane; and d) provided with sealing means against the outer pressure, at the inlet and at the outlet of the optical fiber and in the bore, whereby the elastic membrane conveys a strain that is proportional to the pressure and temperature conditions to which the transducer is submitted, the portion of non-reflected light by the Bragg gratings continuing along the remaining length of the optical fiber, where said fiber can be utilized to interrogate other sensors and transducers connected along the same optical fiber or in one or more optical fibers coupled to said one.

A drawback of said optical transducer is that when it is attached to a mandrel or any other structure meant to carry said sensor, it becomes integral to said carrier structure and consequently, the P and T measurement performed by said sensor will be affected by any deformation of for example the carrier structure and production string set, causing an offset. Offset is defined as a deviation in the measurement of absolute pressure and temperature.

Besides, when the mounting of the sensor is completed the length of the device becomes rather extended, this contributing for the offset phenomenon in case there is any bending of the string-mandrel set.

Contrary to said Brazilian Application PIBR 0403240-3, the mode of attachment of the sensor which is the object of the present application on the carrier structure makes said sensor free from the offset phenomenon, since the geometry of said sensor is such that, upon attachment to the carrier structure, the body of the sensor and the carrier structure do not involve any physical contact since the sensor itself is in balance. Advantageously, the distance between sensor and carrier structure avoids any deviation in the P and T measurements.

Further differences relate to the way of mounting the sensor, which in case of the sensor of said Brazilian application PIBR 0403240-3 requires an optical joint and the use of two metal parts to be complete. While in the present application, the optical joint as well as the metal parts are dispensed with. US patent N° 6,726,215B1 describes a sensor for measuring deformation, the sensor being free from any membrane, where the optical fiber is attached to a housing and where the fiber Bragg gratings are free in the structure. The sensor is not directed to the measurement of pressure, instead it aims at the measurement of deformation of structures as bridges, platforms, pipes, flexible tubing, dams and the like. In order to measure deformation, the sensor undergoes deformation together with the deformation of the structure being measured.

According to the concept of the present invention, the body of the sensor is unaltered by pressure variation, the membrane only undergoing deformation consequent to variation in pressure. The above US patent measures deformation by the deformation of structure while the present application measures deformation in a membrane consequent to the working of pressure. In US patent 6,726,215B1 Bragg grating (121) which measures deformation is free, that is, it is not attached to any support or structure while in the present application the Bragg gratings are attached to the surface of the sensor membrane. Were they not attached the Bragg gratings would not be able to perform the desired pressure measurements.

US patent 6,898,339B2 relates to a multiple mode pre-loadable fiber optic pressure and temperature sensor. The sensor includes a generally cylindrical structure having two compression elements, a fiber optic having a Bragg grating and a cover. A diaphragm or membrane is integral to said cover so as to define a pressure chamber. The Bragg grating-containing optical fiber is positioned between the compression elements - there is no clue if the grating is attached or supported on the compression elements. The proper working of said sensor requires at least one compression element adjacent to the optical fiber and the membrane which is positioned on the sensor cover. The measurement itself occurs when the membrane presses the compression element, therefore, it is expected that the fiber is also pressed by undergoing a lateral force. The sensor becomes a true sensor when the three portions - the cover containing the diaphragm or membrane, at least one compression element, and the generally circular housing - are duly joined by screws. In the absence of said three parts the sensor does not work. Thus, in the presence of only the diaphragm-containing cover, the fiber optic and the housing it is not possible to perform the measurement since at least one compression element is absent. In this way, it can be observed that according to the concept of the sensor configuration described in this US document, only a diaphragm or membrane forming a pressure chamber and an optical fiber do not constitute sufficient elements to perform pressure measurements. On the other hand, the present application relates to a sensor made up of a single part, the membrane being a portion of this single part. There is no need of any compression element since the Bragg gratings are directly attached to one of the membrane surfaces. Thus, in spite of the technical developments, there is a need of a pressure and temperature optical sensor, the sensor having an axial bore for the passage of an optical fiber on which are printed at least two Bragg gratings, the optical fiber starting at the tip of the inlet optical connector and ending at the tip of the outlet optical connector, the body of the sensor being further provided with two recesses forming a membrane, said Bragg gratings being attached to one of the surfaces of said membrane, said sensor being described and claimed in the present application. One or more sensors connected in series can be used.

SUMMARY

Broadly, the optical sensor of the invention for the measurement of pressure and temperature, said sensor being attached to a carrier structure and inserted in a system including i) a light emitter propagating along the core of an optical fiber containing Bragg gratings printed thereon, until it meets a Bragg grating which reflects a portion of said light, and /V) a body having a membrane for attaching at least two Bragg gratings, and where said sensor comprises: a) optical connectors 51, 51 ^' attached to housings 21, 21 ^' machined in pins 11d, 11d^' integral with cylindrical bodies 11c, 11c^' and provided with threads 20, 20^'; b) body 11a of general rectangular shape, having rounded edges 12 and surfaces 13, 18 and 22, said body 11a being integral to said cylindrical body 11c and has a geometry such that the larger diagonal of body 11a is equal or smaller than the diameter of cylindrical body 11c so that at all times there is a distance 11b between the surface of body 11a and the axial slope parallel to said surface in cylindrical body 11c so that body 11a is always in balance relative to body 11c, and where said body 11a has: b1) a first recess 13a in surface 13 for forming one of the surfaces of membrane 17, and where said recess 13a houses: a plug 31 provided with a channel 32 and an elastic locking ring 41; and b2) a second recess 18a in surface 18 opposite to said surface 13, said recess defining the thickness of said membrane 17; and c) an optical fiber 15 containing at least two Bragg gratings 16a, 16b attached to said membrane 17, said fiber 15 being integrally contained in the interior of said sensor between optical connectors 51, 51' throughout axial channels 15a, 15a' and a flexible connection device 23, whereby is assured the absence of the offset pressure and temperature effect for the performed measurement.

Thus, the invention provides an optical sensor for the measurement of pressure and temperature, said sensor being free from deviations in the measurement of pressure and temperature values caused by the offset effect.

The invention provides also an optical sensor for the measurement of pressure and temperature having a compact geometry, this including the optical connector. The invention provides further an optical sensor for the measurement of pressure and temperature where the optical fiber containing the Bragg gratings is completely contained in the boundaries of the sensor which makes it a self- contained kind of sensor. The invention provides also an optical sensor for the measurement of pressure and temperature where the optical fiber containing the Bragg gratings is completely contained in the boundaries of said sensor, so as to assure the integrity of the optical fiber and consequently of the Bragg gratings against any reactive or degrading agent present in the harsh oil and gas environment. The invention also provides an optical sensor for the measurement of pressure and temperature applicable in injection or production oil and gas wells, besides other engineering applications, and in everyday life.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIGURE 1 attached is an exploded three dimensional view of one mode of the sensor of the invention in the multiplexed configuration.

FIGURE 2A attached is a cut of the same configuration of the sensor of Figure 1. FIGURE 2B e is a cut of the plug.

FIGURE 3 attached is an exploded three dimensional view of another mode of the sensor of the invention in the configuration of one single sensor or a terminal sensor of a series of sensors.

FIGURE 4 attached is a cut of the same configuration of the sensor of Figure 3.

FIGURE 5A attached is a graph showing the response of one mode of the sensor of Figure 1 , containing two Bragg gratings when submitted to temperature variations at constant pressure. FIGURE 5B attached is a graph showing the response of one mode of the sensor of Figure 1 , containing two Bragg gratings when submitted to pressure variations at different temperatures (Ti, T₂, T₃, T₄).

DETAILED DESCRIPTION

The optical sensor of the invention comprises therefore, a body having an axial bore for the passage of an optical fiber on which are printed at least two

Bragg gratings, the optical fiber starting at the tip of the inlet optical connector and ending at the tip of the outlet optical connector, the body of the sensor being still provided with two recesses forming a membrane where the gratings will be attached. One single sensor can be used. Alternatively, several sensors connected in series can be used.

A first mode of the inventive sensor relates to a multiplexed sensor where a cylindrical body is designed to be a support to be attached to the carrier structure and, integral to said cylindrical body, the body of the sensor itself, which is in a state of balance. This mode is illustrated in Figure 1 of the present specification. A second mode of the inventive sensor relates to the same multiplexed sensor, with the difference that said cylindrical body is a separate part of the body of the sensor itself. A third mode of the invention is a sensor to be used by itself or as terminal in a set of sensors arranged in series, where the said cylindrical body is meant to be a support to be attached to the carrier structure and, integral to said cylindrical body, the body of the sensor in itself, the sensor being in a state of balance. This mode is illustrated in Figure 3 of the present specification. A fourth mode is a sensor to be used by itself or as terminal in a set of similar sensors, where the cylindrical body is a separate part of the body of the sensor in itself.

According to the optical fiber sensors utilizing Bragg grating technology, the sensor of the invention is inserted in a system which comprises a light emitter propagating along the core of said optical fiber containing Bragg gratings printed in said optical fiber, until it meets a Bragg grating which reflects a portion of this light, and includes a body containing a membrane for attaching at least two Bragg gratings.

The body itself of the sensor of the invention is of generally rectangular shape and has two recesses in opposite surfaces, said recesses defining the said membrane where the Bragg gratings will be attached to. The section of said membrane is constant. Alternatively, the section of the membrane is variable.

The recesses are not symmetrical, any of the recesses being deeper than the other one. One of the recesses together with one plug defines a low-pressure chamber while the other recess is under the direct action of the pressure to be measured.

It should be pointed out that although the following Figures show the recesses in the largest dimension of the section of the sensor body, the invention comprises a non-represented mode where the recesses are positioned in the smallest dimension of the section of said body. For the measurement of pressure and temperature the modes of the present sensor require at least two Bragg gratings. If a still more precise result is desired, a further Bragg grating can be included, it being attached to membrane 17 or along any portion of the interior of surface 18.

The optical fiber can be single- or multi-mode. According to the concept of the invention, the present sensor is free from the offset effect during pressure and temperature measurements. Advantageously, this results from the fact that the body of the sensor itself is in balance, any physical contact occurring with the structure of the carrier structure. In the case of oil and gas wells, carrier structures mean a mandrel, clamps or the well string itself. In other applications the carrier structure will be the structure corresponding for that application. In order to place the body of the sensor in balance, the diameter of the said cylindrical body is the same or larger than the largest diagonal of the sensor body.

When the sensor is to be applied in oil and gas wells, the mounting of the sensor in the multiplexed mode in the carrier structure is performed so that the cylindrical body of the sensor is necessarily parallel to the axial axis of the carrier structure. In other applications the mounting is performed in the position which is deemed more suitable to the desired end.

Further, as relates to the mounting of the sensor in the multiplexed mode, there is no limitation whatsoever as to which extreme is to be the beginning or the end of the sensor, that is, any extreme can be the beginning or the end.

Still, the concept of the invention relative to the multiplexed sensor which utilizes a flexible pipe to stand the outer pressure, convey the optical fiber and connect two cylindrical bodies forming the ends of the sensor, enables in view of its elastic feature resulting from the spiral, senoidal or similar geometry, that any effect of bending or axial deformation of the carrier structure is not transmitted to the sensor element (membrane and Bragg grating), this assuring the absence of offset effect for pressure and temperature in this mode.

The geometry of the said flexible pipe is such that the plies or leveling off which make it up do not cause any attenuation nor any polarization effect in the optical fiber which traverses said pipe.

As a result from the building features of the present sensor, the optical fiber containing Bragg gratings is entirely restricted to the interior of said sensor, this assuring the integrity of said optical fiber and consequently the integrity of the Bragg gratings against reactive or degrading agents present in the harsh oil and gas environment.

The confinement of the optical fiber within the said sensor configures a low- pressure chamber assuring pressure measurement since any fluid invading the environment eliminates the possibility of measuring the desired pressure. Such condition represents an important, patentably distinguishing feature of the present sensor relative to similar state-of-the-art devices, for example, in the sensors which make the object of the above cited references US 6,898,339B2 and US 6,276,215B1 as well as Brazilian Pl 0403240-3 the respective optical fibers leave the sensor and meet the outer medium. In order to prevent this to occur, one or more further devices are needed, the mounting of which requires several steps which are time-consuming manipulations by specialized, expensive manpower. The sensor of the invention permits the measurement of pressure and temperature values in various applications, one of the most important ones being such measurements in oil and gas wells.

Besides, in view of its versatility derived from the possibility of variation in membrane thickness, the sensor of the present application can be applied to various engineering areas at high or low pressure, together with temperature measurement, such as for example, as a piezometer.

The present invention will now be described as per the attached Figures, which represent only certain configurations thereof, so the experts should consider that various variations and modifications can be made in it without departing from the spirit thereof.

Figure 1 is an exploded view of one mode of the present invention with the sensor is in the multiplexed configuration and where a cylindrical body integral to the body of the sensor is used as support for attachment of the sensor to the carrier structure.

According to Figure 1 , one mode of the optical sensor of the invention, generally designed by numeral 100, comprises: a) an optical connector 51 attached to a housing 21 (non represented) machined into a pin 11d provided with threads 20, said pin 11d being integral with a cylindrical body 11c; b) body 11a, of general rectangular shape, having round edges 12, with surfaces 13, 18 and 22, said body 11a being integral with said cylindrical body 11c, and having a geometry such that the largest diagonal is the same or smaller than the diameter of cylindrical body 11c so as to present at all times a distance 11 b between the surface of body 11a and the axial slope parallel to said surface on cylindrical body 11c so that body 11a will at all times be in balance relative to body 11c, body 11a having a first recess 13a on surface 13 for forming one of the surfaces of membrane 17 (non represented), in said recess 13a being housed a plug 31 provided with a channel 32 and an elastic blocking ring 41 ; c) Thread 22a at surface 22 for insertion of a flexible connection device 23 formed by two male connections 23a, 23a' and between said two male connections, a flexible pipe 23b molded in two or three dimensions (spiral, senoidal or similar); d) Thread 22a' (non represented) at surface 22' for insertion of connection 23a' into a body 11c' provided with a pin 11d' with thread 20' integral to said body 11c', channel 19' for insertion of a sealing device and connector 51' attached to cavity 21' (non represented); and e) An optical fiber 15 containing at least two Bragg gratings 16a, 16b (non represented), said fiber 15 being integrally contained within sensor 100 between optical connector 51 and a counterpart optical connector 51' throughout axial channels 15a, 15a' (non represented) and said flexible connection device 23.

Parts 11a, 11c and 11d make up the whole sensor body 11. Notations 11a', 11c' and 11d' refer to the corresponding part of a multiplexed sensor mounted in series. Alternatively body 11a of sensor and cylindrical body 11c are separate parts joined together during the mounting of the optical sensor.

Sealing device to be inserted in channel 19 or 19' is a state-of the-art device, like an O-ring or the like.

Figure 2A is an illustration of a cut view of the multiplexed mode of the sensor object of the invention. In Figure 2A it is possible to observe: a) axial channels 15a, 15a' for passage of optical fiber 15 containing at least two Bragg gratings 16a, 16b; b) surface 18 opposite to surface 13 of body 11a. Surface 18 has a recess 18a defining the thickness of membrane 17, the section of which can be constant or variable; c) channel 19 for insertion of a sealing device; d) housings 21 and 21' of optical connectors 51 and 51'; and e) surface 22' and thread 22a'.

Said recesses 13a, 18a are located at any lateral surface of said body 11a, provided that they are opposite surfaces. Notations 15a', 21 ', 22', 22a', 51' and the like mean the corresponding part of a multiplexed sensor mounted in series with a sensor having parts 15a, 21, 22, 22a, 51 and the like.

Figure 2B illustrates a cut of plug 31 with channel 32 for adapting a sealing device.

Figure 3 is an exploded view of one further mode of the invention, such mode being utilized by itself or as a terminal sensor of a set of sensors arranged in series.

The sensor of Figure 3, generally designed by numeral 200 comprises: a) an optical connector 51 attached to a housing 21 (non represented) machined into a pin 11d provided with a thread 20, said pin 11d being integral to a cylindrical body 11c; b) body 11a, of general rectangular shape, having round edges 12, with surfaces 13, 18 and 22, said body 11a being integral with said cylindrical body 11c, and having a geometry such that the largest diagonal is the same or smaller than the diameter of cylindrical body 11c so as to present at all times a distance 11 b between the surface of body 11a and the axial slope parallel to said surface on cylindrical body 11c so that body 11a will at all times be in balance relative to body 11c, body 11a having a first recess 13a on surface 13 for forming one of the surfaces of membrane 17 (non represented), in said recess 13a being housed a plug 31 provided with a channel 32 and an elastic blocking ring 41 ; The view of Figure 3 does not show optical fiber 15 containing at least two Bragg gratings 16a, 16b. Figure 4 is a cut view of the same sensor 200 where it is possible to see: a) channel 15a for passage of optical fiber 15 containing at least two Bragg gratings 16a, 16b; b) surface 18 opposite to surface 13 of body 11a. Surface 18 has a recess 18a defining the thickness of membrane 17, the section of said membrane being constant or variable; c) channel 19 for insertion of sealing device; and d) housing 21 of optical connector 51. It should be obvious for the experts that, although not represented, the invention mode of Figures 1 and 3 comprises a variation which is within the scope of the present invention.

According to this variation, cylindrical body 11c is separately mounted in body 11a during the manufacturing of sensors 100 and 200.

Also, for more accurate pressure and temperature measurements, the invention comprises a third Bragg grating attached to said membrane 17 or along any inner portion of surface 18.

Figures 5A and 5B are graphs that illustrate the typical behavior of the sensor of the invention mounted with two Bragg gratings, when the sensor is submitted to the effects of pressure and temperature.

Figure 5A is a graph showing the behavior of one mode of the sensor of the invention for zero pressure or for a condition of constant pressure with temperature variation. From Figure 5A a linear variation of wavelength with temperature can be observed.

Figure 5B is a graph showing the behavior of one mode of the sensor of the invention for temperatures T₁, T₂, T₃, T₄, respectively 40, 50 and 65 ⁰C. From this Figure it can be observed that for each temperature wavelength varies linearly with pressure. The behavior of the sensor in response to pressure and temperature can be described, as an example, by equations (1) and (2) below:

P(A_x, A₇) = a_p + b_p A_x + C_pA₇ + d _PA_x A₂ Eq. 1

T(Z₁, A₇) = a_τ + b_τ X_x + c_τ A₂ + d_τ X_x A₇ Eq. 2

where P and T represent, respectively, pressure and temperature, while X_x and

X₁ , in the sensor mode of Figure 1 utilizing two Bragg gratings, are the values for the central wavelengths of said gratings. In Equations (1) and (2) above, a_P, b_P, Cp, dp, a_τ, b_τ, c_τ, and d_τ are calibration constants.

Claims

1. An optical sensor for the measurement of pressure and temperature, said sensor being attached to a carrier structure and inserted in a system which comprises i) a light emitter propagating along the core of an optical fiber containing Bragg gratings printed thereon, until it meets a Bragg grating which reflects a portion of said light, and /7) a body having a membrane for attaching at least two Bragg gratings, and wherein said sensor comprises: a. optical connectors 51, 51 ^' attached to housings 21, 21 ^' machined in pins 11d, 11d^' integral with cylindrical bodies 11c, 11c^' and provided with threads 20, 20^'; b. body 11a of general rectangular shape, having rounded edges 12 and surfaces 13, 18 and 22, said body 11a being integral to said cylindrical body 11c and being of geometry such that the larger diagonal of body 11a is equal or smaller than the diameter of cylindrical body 11c so that at all times there is a distance 11b between the surface of body 11a and the axial slope parallel to said surface in cylindrical body 11c so that body 11a is always in balance relative to body 11c, and where said body 11a has: b1) a first recess 13a in surface 13 for forming one of the surfaces of membrane 17, and where said recess 13a houses: a plug 31 provided with a channel 32 and an elastic locking ring 41 ; and b2) a second recess 18a in surface 18 opposite to said surface 13, said recess defining the thickness of said membrane 17; and c. an optical fiber 15 containing at least two Bragg gratings 16a, 16b attached to said membrane 17, said fiber 15 being integrally contained in the interior of said sensor between optical connectors 51, 51' throughout axial channels 15a, 15a' and a flexible connection device 23, whereby the absence of the offset pressure and temperature effect for the performed measurement is assured.

2. An optical sensor according to claim 1 , wherein alternatively body 11a of sensor and cylindrical body 11c are separate parts joined together during the mounting of said sensor.

3. An optical sensor according to claim 1 , wherein membrane 17 is of uniform section.

4. An optical sensor according to claim 1 , wherein membrane 17 is of variable section.

5. An optical sensor according to claim 1 , wherein recesses 13a, 18a lack symmetry, any of said recesses being deeper than the other one.

6. An optical sensor according to claim 1 , wherein one of said recesses

13a/18a together with plug 31 defines a low-pressure chamber while the other recess 18a/13a is directly under the influence of the pressure to be measured.

7. An optical sensor according to claim 1 , wherein said recesses 13a, 18a are located at any lateral surface of said body 11a, provided that they are opposite surfaces.

8. An optical sensor according to claim 1 , wherein for more accurate pressure and temperature measurements, a third Bragg grating is attached to said membrane 17 or along any inner portion of surface 18.

9. An optical sensor according to claim 1 , wherein flexible connection device

23 is inserted in a thread 22a of surface 22 of body 11a with the aid of male connection 23a and in a thread 22a' of surface 22' of body 11c' with the aid of male connection 23a', with a flexible pipe 23b molded in two or three dimensions (spiral, senoidal or similar) between said two male connections 23a, 23a'.

10. An optical sensor for the measurement of pressure and temperature, said sensor being attached to a carrier structure and inserted in a system which comprises i) a light emitter propagating along the core of an optical fiber containing Bragg gratings printed thereon, until it meets a Bragg grating which reflects a portion of said light, and O) a body having a membrane for attaching at least two Bragg gratings, and wherein said sensor comprises: a) an optical connector 51 attached to a housing 21 machined into a pin 11d provided with a thread 20, said pin 11d being integral to a cylindrical body 11c; b) body 11a, of general rectangular shape, having round edges 12, with surfaces 13, 18 and 22, said body 11a being integral with said cylindrical body 11c, and having a geometry such that the largest diagonal is the same or smaller than the diameter of cylindrical body 11c so as to present at all times a distance 11b between the surface of body 11a and the axial slope parallel to said surface on cylindrical body 11c so that body 11a will at all times be in balance relative to body 11c, where said body 11a has: b1) a first recess 13a on surface 13 for forming one of the surfaces of membrane 17, in said recess 13a being housed a plug 31 provided with a channel 32 and an elastic blocking ring 41 ; and b2) a second recess 18a in surface 18 opposite to said surface 13, said second recess defining the thickness of said membrane 17; and c) an optical fiber 15 containing at least two Bragg gratings 16a, 16b attached to said membrane 17, said fiber 15 being integrally contained in the interior of said sensor between optical connectors 51 , 51' throughout axial channels 15a, 15a' and a flexible connection device 23, whereby the absence of the offset pressure and temperature effect for the performed measurement is assured.

11. An optical sensor according to claim 10, wherein alternatively body 11a of sensor and cylindrical body 11c are separate parts joined together during the mounting of said sensor.

12.An optical sensor according to claim 10, wherein membrane 17 is of uniform section.

13.An optical sensor according to claim 10, wherein membrane 17 is of variable section.

14.An optical sensor according to claim 10, wherein recesses 13a, 18a lack symmetry, any of said recesses being deeper than the other one.

15.An optical sensor according to claim 10, wherein one of said recesses 13a/18a together with plug 31 defines a low-pressure chamber while the other recess 18a/13a is directly under the influence of the pressure to be measured.

16.An optical sensor according to claim 10, wherein said recesses 13a, 18a are located at any lateral surface of said body 11a, provided that they are opposite surfaces.

17.An optical sensor according to claim 10, wherein for more accurate pressure and temperature measurements, a third Bragg grating is attached to said membrane 17 or along any inner portion of surface 18.

18. Use of the optical sensor for the measurement of pressure and temperature according to claims 1 and 10 in oil and gas wells.

19. Use of the optical sensor according to claim 18 in engineering and in everyday life.