The invention relates to a pressure sensor and in particular, a sensor which uses the photo-elastic effect to detect changes in pressure.

When light propagates through an anisotropic material, it does so as two orthogonally polarised waves with different phase velocities. When these two orthogonal polarisation states correspond to linearly polarised states, the phenomenon is referred to as optical birefringence. As used herein, the terms "photo- elastic" or "elasto-optic" effect refer to a change in the birefringence of a material by the application of mechanical stress to the material.

The reserves contained in a sub-surface oil or gas reservoirs can be estimated by determining the pressure/time relationships following abrupt changes in the flow states of the reservoir. Most pressure monitors used in downhole environments are of the non- real time variety. These types of sensors do not communicate directly with the surface, but are examined for pressure readings after being retrieved from the bottom of a well or borehole.

The practice of stopping production to make spot readings decreases the productivity of the well.

There are direct reading pressure monitors available and the primary type of sensor that is currently used is based on the phenomenon of the piezo-electric effect in quartz. However, this type of sensor has the disadvantage that it requires electronics to be present in a downhole location. As well pressures can reach over 1,000 bar and temperatures can exceed 200°C, conditions may exist downhole which impair the operation of the electronics. Hence, as drilling technology improves and the depth of wells increases, the difficulty in obtaining reliable operation of electronic based sensors increases.

It has been known to measure pressure using the photo- elastic effect exhibited by birefringent optical fibres. However, these known pressure sensors have the disadvantage that the fibres are amorphous and are therefore likely to suffer from creep. Hence, such pressure sensors are not thought to be suitable in a downhole environment or for other applications where temperature differentials may be relatively high and/or it is desired that the pressure sensor be maintenance free for a relatively long period of time, such as a number of years.

In accordance with the present invention, a pressure sensor comprises a crystalline material, a pressure transmission device which transmits the ambient pressure at the location of the pressure sensor to the crystalline material, an input coupling device to couple a source of electromagnetic radiation in to the crystalline material, the input coupling device comprising a polariser, an output coupling device to

couple the electromagnetic radiation out of the crystalline material to a detector, the output coupling device comprising an analyser, and the polariser having an electric vector orientated at an angle to the x-y co-ordinates of the crystalline material.

An advantage of the invention is that it provides a pressure sensor which permits pressure to be monitored at a location without requiring electronics to be present at the location.

Preferably, the electromagnetic radiation is optical radiation.

The terms "optical radiation" and "light" as used in this specification cover infrared, visible and ultraviolet wavelengths of electromagnetic radiation.

Preferably, the electric vector (E vector) of the polariser is at an angle of substantially 45" to the x- y co-ordinates of the crystalline material.

Preferably, the E vector of the analyser is substantially orthogonal or parallel to the E vector of the polariser.

Preferably, the crystalline material is a single crystal. However, it is possible that the crystalline material may be a crystalline optical fibre.

Where the crystalline material is a crystal, the crystal may be a cubic crystal or a non-cubic crystal.

Preferably, the crystal is such that birefringence of the crystal has a low dependence on temperature. However, preferably, the crystal exhibits strong

birefringence when subjected to uni-axial loading.

Typically, a temperature sensor is also included to permit compensation of the effects of temperature on the pressure sensor.

Preferably, the crystal is also robust and has high stability. A suitable cubic crystal may be gallium phosphide (GaP) . An example of a non-cubic crystal which may be suitable is potassium titanyl phosphate (KTP).

Preferably, where the crystal is a cubic crystal, the pressure transmission device is a uni-axial pressure transmission device so that the pressure in the vicinity of the pressure sensor is applied to the crystal in a uni-axial direction.

An advantage of using a non-cubic crystal is that it is not essential that the pressure transmission means applies pressure uni-axially to the crystal in a uni- axial direction. Hence, it is possible to adopt hydrostatic loading of the crystal.

In accordance with a second aspect of the present invention, a method of measuring the birefringence of a material comprises passing broad band radiation through a polariser to polarise the broad band radiation in orthogonal directions, passing the polarised broad band radiation through the material, passing the polarised broad band radiation through an analyser after the polarised broad band radiation has passed through the material, and detecting the variation of intensity of the output radiation from the analyser as a function of wavelength.

Preferably, the step of detecting the variation of intensity with wavelength comprises passing the output radiation through a diffraction device, such as a grating, or a refraction device. Typically, the refracted or diffracted radiation is detected by a detector array.

Typically, the output signals from the detector array are used to calculate the fringe spacing which gives an indication of the pressure.

Preferably, the broad band radiation has a band width as broad as possible. Typically, this would be approximately 90 nm or more.

Preferably, the detection method in the second aspect of the invention is used as the method of detection for the detector to which the output radiation from the pressure sensor in the first aspect of the invention is coupled.

Alternatively, the detector may comprise two radiation sources of two different wavelengths. Typically, both wavelengths of radiation are combined and passed through the crystal, polariser and analyser simultaneously and the detection device detects the output light intensity amplitude of each wavelength and calculates the difference in amplitude between the wavelengths.

Preferably, the difference between the two different wavelengths is such that there is no ambiguity in the pressure measurement range.

An example of a pressure sensor in accordance with the invention will now be described with reference to the

accompanying drawings, in which:-

Fig. 1 is a schematic block diagram showing a first example of a pressure sensor coupled to a control unit; Fig. 2 is a schematic block diagram of a second example of a pressure sensor coupled to a control unit; Figs. 3a to 3d show typical output graphs from the system of Fig. 2; Fig. 4 is a schematic diagram of a detector optical arrangement for the control unit of Fig. 2; Fig. 5 is a plan view showing an example of a pressure transmission arrangement for use with the sensor of Fig. 2 and incorporating a diaphragm; and, Fig. 6 is a cross-sectional view through an example of a pressure transmission arrangement for use with the sensor of Fig. 1 and incorporating a piston.

A downhole pressure sensor 1 is intended to be used downhole in an oil or gas well. The sensor 1 comprises a crystal 2 mounted in a housing 3. Also mounted in the housing 3 are a polariser 50 and input coupling optics 4 and an analyser 51 and output coupling optics 5. The crystal 2 is located between the optics 4 and the optics 5. The housing 3 includes means for permitting the ambient pressure 6 surrounding the housing 3 to be transmitted to the crystal 2 in a uni- axial direction.

The crystal 2 typically has a diameter of 2.5cm and is about 5mm thick.

The polariser would have the E vector set at 45° to the crystal XY co-ordinates to ensure that equal intensity of polarisation components enters the crystal. The E vector of the analyser is orthogonal or parallel to the E vector of the polariser.

In order to couple the light from fibre optic 8 into the crystal 2, the input optics 4 include a GRIN lens of approximately 1mm diameter and a length of approximately 3.3mm. Such lenses are commercially available. The fibre optic links 8, 9, 10 are in this example multi-mode fibres. A similar GRIN lens is included in the output optics 5.

In this example, the crystal 2 is a crystal of gallium phosphide (GaP) which is a cubic crystal. GaP exhibits zero birefringence at atmospheric pressure but becomes strongly birefringent when pressures of the order of 1,000 bar are applied to it in a uni-axial direction.

A control unit 7 is coupled to the sensor 1 by means of an input fibre optic link 8 and two output fibre optic links 9, 10. Located in the control unit 7 are dual wavelength light sources 11, which provide a source of optical radiation into the input fibre optic link 8. A micro-processor 12 is also located in the control unit 7, and wavelength splitters 13, 14 are coupled to the output fibre optic links 9, 10 respectively. The outputs from the wavelength splitter 13 are coupled to detectors 15, 16, and the outputs from wavelength splitter 14 are coupled to the detectors 17, 18. Each of the respective detectors 15-18 detects a specific wavelength and polarisation state. The detector 15 detects radiation of polarisation state 1 with wavelength λ ₂ ; the detector 16 detects radiation of polarisation state 1 with wavelength λj; the detector 17

detects radiation of polarisation state 2 and wavelength λ ₂ ; and the detector 18 detects radiation of polarisation state 2 with wavelength λ ₁ .

In use, the sensor unit 1 is positioned downhole in a well and two wavelengths of radiation λ _{l f} λ ₂ are emitted by the light sources 11 and coupled into the input fibre optic link 8 which passes the dual wavelength radiation to the polariser 50 and coupling optics 4. The polariser 50 polarises the dual wavelength light into orthogonal polarisation states of polarisation 1 and polarisation 2. The polarised dual wavelength radiation is then coupled into the crystal 2 and passes through the crystal 2 and exits into the analyser 51 and coupling optics 5. After passing through the analyser 51 and coupling optics 5, the radiation is split into the two polarisation states, polarisation 1 and polarisation 2 and coupled into the respective output fibre optic link 9, 10 for transmission to the surface. Light of polarisation state 1 enters the wavelength splitter 13 where the radiation is split into radiation of wavelength λ, and radiation of wavelength λ ₂ . Radiation of wavelength λ, and polarisation state 1 is detected by detector 16 and radiation of wavelength λ ₂ and polarisation state 1 is detected by detector 15. Output fibre optic link 10 is coupled to wavelength splitter 14 which splits the light of polarisation 2 into the radiation wavelengths λi and λ ₂ and passes the split wavelengths to the detectors 18, 17 respectively. The detectors 15-18 detect the intensity of each radiation wavelength in each polarisation state.

The outputs from the detectors are passed to the micro- processor 12 which analyses the information from the detectors 15-18 by calculating the amplitude

differential of the light intensities of λj and λ ₂ , and displays the information on a display device (not shown) . Because the difference in wavelengths between λj and λ ₂ is only about 15nm, the differences in the intensities of radiation detected by the detectors 15- 18 is a unique value and corresponds to the uni-axial pressure 6 exerted on the crystal 2.

As an alternative to the control unit 7, it would be possible to replace the dual wavelength radiation sources 11 with a single broad band radiation source 57 and such an arrangement is shown in Fig. 2. In Fig. 2 the sensor 55 is similar to the sensor 1. However, the important differences are that an end face of the crystal is mirrored so that radiation entering the crystal from the opposite end reflects off the mirrored end and exits the crystal also through the opposite end (see Fig. 5). This permits one single mode optic fibre 56 to be used to transmit radiation to and from the sensor 55. In this example, a polariser/analyser combination would be located between the end of the fibre 56 and the crystal 2. The broad band radiation source 57 is typically a light emitting diode (LED), such as a zinc oxide (ZnO) doped GaP LED. Typically, the broad band radiation source would have a wavelength spread of approximately 90nm. The radiation from the source 57 is coupled into the fibre 56 by a beam splitter 58. Output radiation returning from the sensor 55 through the fibre 56 passes straight through the splitter 58 as a beam 59. The beam 59 is diffracted by a diffraction grating 22 and the diffracted light 60 is detected by a detector array 25.

As shown in Fig. 4, the detector optical arrangement includes a Littrow mounted diffraction grating 22, a focusing lens 61 and a 1024 pixel silicon detector

array 25. The Littrow mounted grating 22 diffracts the broad band radiation 59 from the output fibre optic link 56 into its constituent components which are then focused by the lens 51 and detected by the detector array 25. Output signals 20 from the array 25 are passed to a computer and display unit 62 and displayed on an appropriate display device. Typical outputs for the system shown in Fig. 2 are shown in Figs. 3a to 3d for, respectively, no applied pressure, a pressure of 100 atmospheres, a pressure of 400 atmospheres, and a pressure of 600 atmospheres. Hence, Figs. 3a to 3d show that the number of maxima and therefore, the fringe spacing varies with pressure.

Fig. 5 shows an arrangement in which ambient pressure 6 may be transmitted to the crystal 2 by means of a diaphragm. The crystal is mounted in a housing 35. In Fig. 5 one section of the crystal is mirrored and so the arrangement shown can be used with the system shown in Fig. 2.

The ambient pressure 6 is transmitted to the crystal 2 by a stainless steel diaphragm 30. The diaphragm 30 is in intimate contact with the crystal 2.

When pressure 6 is applied to the diaphragm 30, the diaphragm 30 moves and exerts a uni-axial pressure on the crystal 2.

Fig. 6 is a cross-sectional view through an example of an arrangement for transmitting ambient pressure 6 to the crystal 2 and which may be used with the sensor of Fig. 1.

A piston 35 is mounted in the housing 3 by means of welded bellows 32. Ambient pressure 6 forces the

piston against the crystal 2 and exerts a uni-axial force on the crystal 2. As the ambient pressure 6 changes, the magnitude of the uni-axial force on the crystal 2 will change correspondingly.

Advantages of the sensors 1, 55 are that they do not include electronics and are based on optics and optical components. This makes the sensors less susceptible to failure due to high temperatures and/or pressures.

It is expected that continuous monitoring of the reservoir pressures by means of the invention may increase the total reserves recoverable from many fields by perhaps 10%-30%.

Modifications and improvements may be incorporated without departing from the scope of the invention.