Fiber optic sensor

An apparatus, system and method are provided for measuring state properties without requiring an internal or proximate power supply. The apparatus provides a sensing device that is responsive to fluctuations in a state property that are encountered within the test zone. Due to the fluctuations experienced by the sensing device, a sensing signal is generated and transmitted to an optical device, such as an interferometer. The sensing signal's affect on the optical device produces an interference pattern that may be detected remotely and correlated to the fluctuation experienced by the sensing device.

BACKGROUND

The present disclosure relates to the field of borehole drilling for the production of hydrocarbons from subsurface formations. In particular, the present disclosure relates to sensors placed downhole to measure conditions within the subsurface formations.

Downhole drilling equipment works in harsh environments. Devices that are placed downhole, including sensors, are typically required to operate under high pressure and at high temperature. Because of the harsh environment, sensors often require certain supporting technologies, such as power and cooling, which increase the cost and decrease the reliability of the overall system. The inclusion of a sensor in downhole equipment, therefore, imposes reliability constraints on the entire system, increases the cost of the system itself, and increases the cost of operation thereof. There is, therefore, a need in the art for a robust reliable sensor system that does not require ancillary support technology, such as power, at the downhole location.

DETAILED DESCRIPTION

The sensor and system disclosed herein operates on the principle of having one state-measuring device measure a fluctuation in a state property, such as pressure or temperature, and having the output of that state-measurement device affect an optical device, such as an interferometer. The optical device can be used to affect the optical characteristics of an optical signal that is transmitted down to the sensor in the test zone and reflected back to a detector. Because the optical signal can be generated remotely, and because the reflected signal can be received remotely from the sensor, the sensor itself need not have a power source. In one embodiment, the sensing device can be a piezoelectric device. However, other embodiments can use other sensing devices to measure pressure, temperature, and/or other state properties of the environment in the test zone. The sensing devices are used to generate a sensing signal, typically in response to a fluctuation in a state property. The sensing signal is used to prompt the optical device to modify the light signal that is sent to the detector. Other equipment, such as a computer, can analyze the interference signal and associate the modified light signal to the fluctuations in state properties experienced by the sensing device. With proper calibration, the modified light signal can be used to monitor the fluctuations in the state properties experienced by the sensing device. In other embodiments, multiple sensors can be used in series and/or in parallel to provide indications of state-property fluctuations at multiple points within the test zone.

The optical device disclosed herein is typically an interferometer, although the use of other optical devices is possible. The interferometer measures modifications of light signals (from a light source) responding to the sensing signals generated by the sensing device. The optical devices can be interferometers, such as Fabry-Perot, Michelson, Mach-Zender interferometers, or other types of optical devices that can change its optical reflection and/or reflection spectrum produced in response to an electrical and/or mechanical stimulus or signal from the sensing device. A suitable stimulus can include, for example, a movement of the optical filament, a movement of a portion of an optical device within an optical cavity, and/or a change in the electromagnetic field in proximity to the test zone.

It is useful to begin with an illustration of the basic equipment used in downhole operations. As shown inFIG. 1, oil well drilling equipment100(simplified for ease of understanding) includes a derrick105, derrick floor110, draw works115(schematically represented by the drilling line and the traveling block), hook120, swivel125, kelly joint130, rotary table135, drill string140, drill collar145, LWD tool or tools (also known as node or nodes)150, and drill bit155. Mud is injected into the swivel125by a mud supply line (not shown). The mud travels through the kelly joint130, drillstring140, nodes150, drill collar145, and exits through jets or nozzles in the drill bit155. The mud then flows up the annulus between the drill string140and the wall of a borehole160. A mud return line165returns mud from the borehole160and circulates it to a mud pit (not shown) and back to the mud supply line (not shown). The combination of the drill collar145, node(s)150, and drill bit155is known as the bottomhole assembly (or “BHA”). In rotary drilling, the rotary table135may provide rotation to the drill string140or, alternatively, the drill string140may be rotated via a top drive assembly (not shown). The term “couple” or “couples” used herein is intended to mean either an indirect or a direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect electrical connection via other devices and connections. Other equipment on the surface101may include a real-time processor185that can receive and process signals emanating from equipment downhole. Additional equipment, such as a computer190can be coupled to the surface real-time processor185in order to compute and/or display results of the information that is gathered downhole.

It will be understood that the term “oil well drilling equipment” or “oil well drilling system” is not intended to limit the use of the equipment and processes described with those terms to drilling an oil well. The terms also encompass drilling natural gas wells or hydrocarbon wells in general. Further, such wells can be used for production, monitoring, or injection in relation to the recovery of hydrocarbons or other materials from the subsurface.

FIG. 2illustrates a completed well. As a well is drilled, several casing strings are inserted into the bore hole. Each casing string is made up of casing joints that are connected using threaded couplings. Each string of casing is connected into the borehole as it is drilled. Therefore, the casing is rarely recovered. During drilling and completion, the drilling crew runs several strings of casing into the hole. Each casing string fits inside the last, so each string is smaller in diameter than the casing string set before it. Referring toFIG. 2, the first string is a conductor casing220, which is a relatively short string of between 20 to 100 feet in length. The conductor casing220is a large diameter pipe that keeps the top part of the hole from caving in during drilling. The conductor casing220is drilled to just past the depth of the deepest fresh water in the formation in order to prevent drilling mud and hydrocarbons from contaminating fresh water (for drinking and/or irrigation purposes), and to keep loose sand or gravel from falling into the hole. The surface casings222is run inside the conductor casing220and extends from the bottom of the hole223(the surface hole) to the surface208.

The crew then continues drilling down to the oil reservoir. When the reservoir is very deep, the driller will often encounter troublesome formations, for example, one with high-pressure fluids in it. A high-pressure formation can cause oil and gas to blow out of the hole into the air, which is both dangerous and wasteful. By adjusting the properties of the drilling mud, a crew can successfully drill such formations. Later, however, as the hole passes through deeper formations, the mud they used to drill the high-pressure formation may no longer be suitable. So, to make it possible to drill deeper, the drill crew inserts another string of casing, called the intermediate casing string224, and cements it with cement250. This intermediate string224seals off the high-pressure zone or other troublesome formations. The intermediate casing224fits inside the surface casing222and runs from the bottom of the hole thus far to the surface208. As drilling progresses into the production zone, the drilling crew may set a second intermediate string of casing if they encounter more troublesome formations above the production zone.

When and if testing confirms the presence of hydrocarbons, the drilling crew may run the last string of casing, namely the production casing226(also called the oil string or long string). The production casing226usually runs from the bottom of the hole, or near the bottom, to the surface208. At the bottom end of the production casing226is a casing shoe228(also called a guide shoe), at the end of the last joint. The casing shoe228is a short, heavy, cylindrical section of steel filled with concrete and rounded on the bottom. It prevents the production casing226from snagging on irregularities in the borehole as it is lowered.

A driller may pump salt water into the hole to contain pressure in the reservoir and formation until the well is completed and ready to produce. The example illustrated inFIG. 2is a cased-hole completion. To make a cased-hole completion, one or more perforations270are made in the production casing226and the surrounding cement250. The perforations

The crew then continues drilling down to the oil reservoir. When the reservoir is very deep, the driller will often encounter troublesome formations, for example, one with high-pressure fluids in it. A high-pressure formation can cause oil and gas to blow out of the hole into the air, which is both dangerous and wasteful. By adjusting the properties of the drilling mud, a crew can successfully drill such formations. Later, however, as the hole passes through deeper formations, the mud they used to drill the high-pressure formation may no longer be suitable. So, to make it possible to drill deeper, the drill crew inserts another string of casing, called the intermediate casing string224, and cements it with cement250. This intermediate string224seals off the high-pressure zone or other troublesome formations. The intermediate casing224fits inside the surface casing222and runs from the bottom of the hole thus far to the surface208. As drilling progresses into the production zone, the drilling crew may set a second intermediate string of casing if they encounter more troublesome formations above the production zone.

When and if testing confirms the presence of hydrocarbons, the drilling crew may run the last string of casing, namely the production casing226(also called the oil string or long string). The production casing226usually runs from the bottom of the hole, or near the bottom, to the surface208. At the bottom end of the production casing226is a casing shoe228(also called a guide shoe), at the end of the last joint. The casing shoe228is a short, heavy, cylindrical section of steel filled with concrete and rounded on the bottom. It prevents the production casing226from snagging on irregularities in the borehole as it is lowered.

A driller may pump salt water into the hole to contain pressure in the reservoir and formation until the well is completed and ready to produce. The example illustrated inFIG. 2is a cased-hole completion. To make a cased-hole completion, one or more perforations270are made in the production casing226and the surrounding cement250. The perforations allow the hydrocarbons to flow from the reservoir into the production casing and eventually up to the surface208.

In most cases, a production tubing string (not shown) is inserted within the production casing to improve the production of fluids from the production zone. The production tubing is perforated in the same place as the production casing (or is terminated with an open bottom at that depth), and the annulus between the production tubing and the production casing is sealed by a packer so that production will occur through the production tubing. Unlike the production casing, which is cemented in place, the production tubing can be removed with relative ease.

Once the well is producing, the production flow goes upward as illustrated inFIG. 2. The movement of the fluid and other underground effects produce pressure fluctuations and acoustic signals that may be detected by one or more sensors that are placed within the gap between casings.FIG. 2illustrates two such sensors210and212that are between two casings and are well positioned to monitor the production flow. It will be understood that the sensors210and212may be coupled to the production tubing and may be located to monitor conditions inside the production casing or outside the production tubing. Similarly, the sensors210and212may be coupled to any of the casing strings and located to monitor conditions inside or outside the string to which it is mounted. Further, the sensors210and212could be mounted in the cement or in the borehole wall or further into the formation surrounding the borehole. For example, the sensors210and212may be positioned so that they may detect a variety of phenomena, such as acoustic signals (to aid in seismic analysis) as well as monitoring production flow in one or neighboring completed wells. One or more filaments211(composed in whole or in party by, for example, fiber optic cable) can be used to route light signals from the light source206through one or more sensors210and212and the analyzer/detector207. The analyzer or detector can be an optical sensing analyzer, or a photo detector, or other equivalent device. Although a single filament211is shown inFIG. 2, multiple filaments211, each servicing one or more sensors210, can be used.

In those instances where power at downhole locations is either unavailable or insufficient, but measurement of physical conditions is essential, an alternate or remote power source is necessary. Techniques employing fiber optics can be used to send signals (using, e.g., surface powered equipment) to remote devices in the test zone that return signals that are altered based upon the conditions at the test zone. This disclosure will present examples of two sensor types: fiber grating sensors and interferometric sensors. It will be noted, however, that other types of fiber sensors, including other types of intrinsic, extrinsic, evanescent and intensity-based sensors can be used with equivalent effect.

Fiber grating sensors using, for example, Bragg grating and long-period grating use changes in a wavelength of reflected or transmitted light that can be detected at, for example, by a surface-located analyzer or detector207. Bragg gratings are responsive to changes in the strain on the filament. Long-period gratings are responsive to changes in the curvature of the grating, and are also sensitive to axial strain in the grating as well as to changes in the refractive index of the material or materials surrounding the grating. Each grating can be designed to provide a single reflected or absorbed peak. This is useful for situations when multiple sensors are used because each sensor can have a single reflected peak that is distinguished from the other sensors.

Other sensors that can be used for the purposes disclosed herein include interferometers. There are many types of interferometers that can be used for displacement or state-property measurement, including, but not limited to, the Mach-Zender, the Michelson, and the Fabry-Perot. Unlike the grating sensors, the Fabry-Perot sensors can provide one or more reflected peaks shifted in response to the sensor stimuli. The principle of an interferometer is that the light is split into two beams, one beam is exposed to the sensing environment and undergoes a phase shift and the other beam is isolated from the sensing environment and is used for comparison. When the beams are re-combined (such as in a fiber optic cable), they interfere with each other and the level of interference can be detected and measured. A Mach-Zender interferometer is shown inFIG. 3a. A Michelson interferometer is illustrated inFIG. 3b. In both cases, there can be a light source302, such as a light emitting diode (“LED”) that emits light into a fiber optic filament to a first coupler304(such as the 3 dB coupler shown inFIGS. 3aand3b).

In the Mach-Zender interferometer ofFIG. 3a, a second fiber optic filament307can be introduced at the (optical combiner) coupler304that combines the light signal from the light source302and the first filament303with the second filament307. The two filaments303and307then travel along separate paths, with only one filament307going through the transducer306at the measurement location and both filaments being coupled again at the (optical combiner) second coupler304′ where the two light signals (one affected by the transducer306and one not) recombine and interfere with each other resulting in an interference pattern that can be then detected by the analyzer or detector308, as illustrated inFIG. 3a. The analyzer or detector308can be an optical spectrum analyzer or a photo detector or an equivalent device. A suitable optical sensing analyzer is the si720, which is manufactured by Micron Optics of Atlanta, Ga.

In contrast, the Michelson interferometer uses the light source302for the first filament303with the detector308that is coupled (operative) with the second filament307. The (optical combining) coupler304can be used to combine the (downward) light signal for both filaments. Thereafter, the light in one of the filaments can be affected by the transducer306before being reflected off of mirror354(and traveling back through the transducer306). However, the light on the second filament is not affected and is merely reflected off of mirror352and returned. The two returning signals are coupled at (optical combining) coupler304and interfere with each other, resulting in an interference pattern that is detected at detector308via the second filament307. The significant difference between the Mach-Zender and Michelson interferometers is that light passes through the sensing region (transducer306) twice in the Michelson interferometer, making it potentially more sensitive.

Fabry-Perot sensors allow multiple sensors to use the same fiber optic filament, and thus may be preferable for multiplexing. However, Bragg gratings may be preferable for a single fiber in some instances. The Fabry-Perot sensor can be either intrinsic or extrinsic.FIG. 4shows an extrinsic Fabry-Perot interferometer. Referring toFIG. 4, light from a light source302is transmitted through a filament303to the interferometer500. The interferometer500is discussed in more detail below in conjunction withFIG. 5. In general, however, the interferometer generates an interference pattern based upon the conditions within the interferometer, namely the length of an air gap within the interferometer, which can be measured with the detector308and the interference pattern correlated to the measured conditions. Changes in the length of the air gap affects the measured interference pattern, and thus any fluctuations in conditions with the test zone (within which the interferometer500is placed) can be measured.

FIG. 5is a close up view illustrating a classic extrinsic Fabry-Perot cavity. In the case of the sensor500ofFIG. 5, a glass tube501can be used to connect two fibers, namely a lead fiber502and a reflecting fiber504, the latter of which may be a glass plug. The two fibers are separated by an air gap506of length “d,” as shown inFIG. 5. A typical air gap “d” is on the order of 150 microns. Changes in the length of “d” by a few Ångstroms will make a detectable change in the reflected signal. In operation, a light signal from a broadband source is sent through the lead fiber502. Upon encountering the beginning of the air gap506at the interface534, a first portion524of the source light signal is reflected at the fiber/air interface534. A second portion535of the incoming light signal is transmitted into the air gap534and a portion535aof that second portion535is reflected at the air/fiber interface536, as illustrated inFIG. 5and reenters the lead fiber502. The second reflection535atravels an extra distance to the detector308that is equivalent to twice the length “d” of the air gap506. Since the light is composed of many wavelengths, and the length “d” of the air gap506is large compared to those wavelengths, there will be more than one wavelength that undergoes interference. Thus, interference will take place and the reflected light will have an interference pattern where some wavelengths interfere constructively and some destructively. Through known methods, very small changes in the length “d” of the air gap506are detectable. In general, a wide gap “d” results in more interference fringes and a narrow gap “d” results in fewer interference fringes. In other words, the longer the gap “d,” the greater the number of interference fringes.

FIG. 6shows one embodiment of an electromechanical device, specifically a micro-electromechanical system (“MEMS”) charge-coupled mirror platform600, although other electromechanical devices (besides MEMS) may be used with equivalent effect. A front view of the mirror600is illustrated inFIG. 6a. A cut-away side view is shown inFIG. 6b. The mirror600has a moveable plate602that may act as a mirror to reflect light signals. The mirror600itself can be a moveable plate602that can be attached to a base606by two or more springs604. The springs604enable the moveable plate602to move along a longitudinal axis610in response to a suitable stimulus. While the present embodiment shows the moveable plate602moving along a longitudinal axis610, other embodiments may have the moveable plate602move in other ways, such as radially or laterally or some combination thereof.

In one embodiment, for example, the moveable plate602may be placed in proximity to a charged plate. Charging the (static) plate would impart an (capacitive) electromagnetic force onto the moveable plate602and compel the moveable plate602to move to a new position laterally with respect to the base606. Once the force is removed, the springs604ensure that the moveable plate602returns to its original position. The moveable plate602can be polished or otherwise fitted with a mirror face603that can reflect optical signals, such as light. The mirror600can be used as part of an interferometer within the sensors described herein.

The physical arrangement of the interferometers, and how their interference patterns are changed by the signals generated from the sensing device (such as a piezoelectric device) is capable of wide variation.FIGS. 7,9,10,11and12illustrate some of the many embodiments having interferometric arrangements that may be employed to measure fluctuations in state properties using, generally, the method depicted inFIG. 8.

FIG. 7illustrates a downhole sensor700that employs the mirror600. The sensor700contains the essential elements for an optically integrated sensor. In the embodiment illustrated inFIG. 7, the sensor700has two main sections, a high-pressure piezoelectric section738and a sensor section726. Within the piezoelectric section738, there may be a piezoelectric element706that may be surrounded by a pressure-transfer medium704that can be contained within the section738by a membrane702. In this embodiment, the membrane702can be a highly compliant membrane. However, use of other membranes702are possible if they admit pressure or other state-property changes. The pressure-transfer medium704can be a fluid or other material. The piezoelectric element706can be electrically connected to the sensor section726through a high-pressure feed-though section734that contains one or more electrical feed-through conductors732. The high-pressure feed-through section734separates the piezoelectric section738from the sensor section726. The sensor section726can be used to hold the mirror600and the end740of a filament722in such a way as to create a Fabry-Perot cavity between the mirror face603(shown inFIG. 6a) and the end740of the fiber optic filament722. The filaments identified in the various embodiments disclosed herein can be, for example, fiber optic filaments, or other types of filaments equivalent to fiber optic filaments.

Referring again toFIG. 7, in operation, pressure fluctuations experienced by the membrane702will cause the piezoelectric element706to generate, for example, a current that can be transferred to a static plate714via conducting elements732. The resulting electric charge on the static plate714, which can be capacitively coupled to the moveable plate602of the mirror600, causes the moveable plate602(seeFIG. 6) to change position, and thus increase or decrease the distance between the mirror face603and the end740of the fiber optic filament722. The increased distance, between the mirror face603and the end740of the fiber optic filament722results in a change in the interference pattern of the light signal that can be sent down the fiber optic filament722. The change in the reflected interference pattern can be detected by the detector207(seeFIG. 2, for example) and calibrated to the pressure fluctuation experienced by the membrane702. The light source206can be used to generate the reference signal used in conjunction with the Fabry-Perot interferometer. The light source206and/or the detector207can be stationed remotely from the sensor700and remain operative via the filament211/722.

The mirror600within the sensor section726can be, for example, the charge-coupled mirror platform600as illustrated inFIG. 6, or any other type of mirror system that can be capable of reflecting a light signal emanating from the fiber optic filament722. In the embodiment illustrated inFIG. 7, the mirror600can be mounted adjacent to the static plate714via, for example, electrically insulative pads715. The moveable plate602of the mirror600can be capacitively coupled to the static plate714such that the voltage generated by the piezoelectric element706results in a movement of the moveable plate602along the longitudinal axis610. As mentioned previously, the mirror plate602has the reflective surface603that can be constructed and arranged to reflect the light that emanates from the end740of the fiber optic filament722, thereby forming a Fabry-Perot cavity. Because the mirror602can be deflectably mounted onto the base606, the distance between the end740of the fiber optic filament722and the mirror face603can vary, depending upon the amount of charge experienced in the static plate114(which can be a direct result of the charge supplied by the piezoelectric element706).

FIG. 8illustrates the operation of the system200that uses the sensor700that begins generally at block802. The sensor700can be provided and coupled to an interferometer at block804, and then be positioned downhole at block806. The fiber optic filament722connects the sensor700to the light source206and/or a detector207that can be located at the surface208as illustrated inFIG. 2. The light device206can be the device that sends light signals (either in a continuous or pulse fashion) down the fiber optic filament722to the sensor700at block808. The light signals may be reflected off the mirror face603and are accepted by the fiber optic filament722that transmits the reflected light back to the detector207at block810. At block812, the resulting interference pattern that is received by the detector207can be analyzed and correlated to the state-property fluctuations experienced by the sensor700. At block814, any output data can be forwarded to an appropriate device, such as a monitor, printers, storage devices, etc.

As illustrated inFIGS. 6 and 7, the reflective surface603of the mirror600faces a cleaved (and possibly polished) end740of a fiber optical filament722forming, for example, a Fabry-Perot cavity. It will be understood by those skilled in the art having the benefit of the present disclosure that other arrangements of mirrors603are possible with the sensor700disclosed herein.

Other embodiments of the sensor and the supporting equipment are possible. For example, any sensing element that produces a change in voltage (e.g., voice coil, accelerometer, resistance, etc.) could be married to the electromechanical deflection/optical fiber sensor mechanism700to make a sensor that can be interrogated passively in a similar manner. For example, the present disclosure includes non-passive sensing elements. The voltage-producing part of a sensor700could potentially be any sensor that produces a suitable voltage to drive the MEMS/optical portion600of the sensor700. This means that the sensor700can be modified to accommodate powered-drive elements if/when downhole power generation, long-life batteries, etc., are available to drive the sensors700. In that case, monitoring of the sensor700may still be accomplished with a standard passive fiber optic system. In other words, the MEMS/optical method disclosed herein may serve as a type of standard interface for many sensors, both passive and active, all being interrogated using the same fiber-optic methods disclosed herein.

In alternate embodiments of the sensor700, the pressure-transfer medium704can be eliminated. Instead of using a pressure transmitting medium704, the piezoelectric element706can be mounted within, for example, a foil, which can then be exposed directly to the pressure environment (e.g., production flow within a pipe). Alternatively, the piezoelectric element706could be mounted directly into the pressure environment, perhaps with an elastomer or glass seal between the outside diameter (“o.d.”) of the crystal and the inside diameter (“i.d.”) of the port within the drilling pipe. In the latter embodiment, it should be recognized that any seal may affect the response of the crystal. However, the response may be tuned in relation to the frequency range of interest.

In another embodiment, the piezoelectric element706can be mounted to the exterior of the high-pressure environment, such as, for example, the outside of a drilling pipe. This latter embodiment assumes that the perturbations of interest traveling down the pipe (e.g., a pressure pulse) are coincident with a hoop strain perturbation of the pipe itself, to which the piezoelectric crystal would be directly coupled.

In another embodiment, the piezoelectric element706can be placed in a fluid or in a rubber element, or any type of either flexible or fluid element that suitably transmits the pressure from the membrane to the piezoelectric element706. Thus, any fluids that are stable, not viscous or do not attenuate a pressure signal may be useful for transmitting pressure from the membrane702to the piezoelectric element706. Other elements or materials, such as rubber or other types of flex materials that do not attenuate pressure signals can also be used in lieu of the fluid in conjunction with the piezoelectric element706. In some cases, for example, some attenuation or other physical properties in the medium may be desirable to reduce the transmittance of large pressure signals before encountering the sensor. Similarly, there are multiple ways of connecting the piezoelectric element706to the optical elements of the low pressure section. Moreover, there are multiple arrangements for grounding the piezoelectric element706that are known to those skilled in the art. Still more embodiments are possible with the elements presented in this disclosure.

In some embodiments, it may be important to isolate the MEMS structure, or other elements of the interferometer, from the mechanical perturbation that is being detected by the piezoelectric element706, so that the optical path reading may be responsive only to the charge variations, and not the direct vibrations experienced by the sensor700. A suitable isolation can be accomplished by mounting the sensitive longitudinal axis610of the MEMS mirror600orthogonal to the anticipated perturbation axis. Alternatively, the MEMS mirror600could be mounted on a relatively heavy reaction mass, which itself could be mounted on a relatively soft spring system, in order not to pass any inputs above a certain frequency. In another embodiment, the MEMS mirror600structure could itself include appropriate mass and spring elements in order to have inherent isolation characteristics.

As illustrated inFIG. 7, the fiber interface can be a dead-end Fabry-Perot (interferometric) gap. It will be understood by those skilled in the art that there are other optical couplings that may have certain advantages. For example, an optical fiber might be mounted along the pertubating surface of the MEMS mirror600, or between the two capacitor plates714and602, such that the fiber experiences radial accelerations and motion, or radial stress (depending upon the rigidity of the mounting), which will cause a detectable perturbation in the light traveling through the fiber, e.g., via a microbending and/or macrobending effect, or an evanescent effect. By laying the fiber parallel, rather than perpendicular, to the capacitor plates, the fiber need not terminate (i.e., dead end), and can be used for multiple sensing stations. Different colors can be used, or optical time-domain reflectometry (“OTDR”) techniques, to discriminate between sensor stations.

Another embodiment of the sensor900is illustrated inFIG. 9. This embodiment employs a grating960, such as a Bragg grating that can be embedded into the filament922. As illustrated inFIG. 9, the piezoelectric device906can be made operative with the capacitor plate914via one or more feed-through conductors932to the low pressure section926. A moveable plate950can be placed in proximity to the capacitor plate914. The moveable plate950can be constructed and arranged to move along an axis970that is substantially perpendicular to the centerline of the filament922at the region between the clamps954. In operation, pressure fluctuations experienced by the membrane902cause the piezoelectric element906to emit an electrical current (signal) to the capacitor plate914which would then would attract (or repel), for example in a capacitive manner, the movable plate950along the axis970and thus impart a strain on the filament922within the grating960(between the clamps954) related to the electrical current generated by the piezoelectric device906. The tension perturbations experienced by the grating960have a corresponding affect on the interference pattern of the light signals that are transmitted through the grating960, which can be measured by the detector207as perturbations of the interference fringes that result from the grating960. The perturbations of the interference fringes can be correlated to the pressure fluctuations experienced by the piezoelectric device906, thereby making the sensor900useful for measuring pressure fluctuations downhole212within the system200.

FIG. 10illustrates an alternate embodiment of the sensor. In this embodiment, the sensor1000has a single filament1022in a pass-through arrangement so that this sensor may be arranged in series and/or parallel with other sensors. The casing1028can contain a low pressure section1026. The low pressure section1026can house a static plate1014that can be made operative with the piezoelectric device1006via the one or more feed-through conductors1032. The static plate can be capacitively coupled to a moveable plate1050. The moveable plate1050can be allowed to move along an axis1070via, for example, a slideable arrangement of guides1015although other arrangements are possible. A grating1060, such as a Bragg grating, can be inserted into the filament1022and placed between the static plate1014and moveable plate1050as illustrated inFIG. 10. The filament1022can be, for example, fixedly attached to the static plate1014and moveable plate1050such that movement of the moveable plate1050imparts a tension (or compression) on the filament1022in proximity to the grating1060in proportion to the sensing signal generated by the piezoelectric device1006. In operation, pressure fluctuations experienced by the membrane1002(seeFIG. 10) are transmitted to the piezoelectric device1006which generates electrical impulses that charge (or discharge) the static plate1014which in turn causes the moveable plate1050to move along the axis1070and impart a tension (or compression) at the grating region1060of the filament1022. The subsequent change in the interference signal due to the tension/compression change at the grating1060can be measured by the detector207at the surface208.

Another embodiment is illustrated inFIG. 11. In this embodiment, an evanescence interferometer may be employed in a filament1150(having core1151) that can be deformed by the imparter1160that is perturbed by the charged coupled plates in response to the state property fluctuations experienced by the piezoelectric device. As illustrated inFIG. 11, the filament1150is bent into a curve with clamps1154. A flat section1152can be made (typically by grinding) in the filament1150as illustrated inFIG. 11. Because the flat section1152extends to the core1151and because of the curvature of the filament1150, the reflection and/or transmission spectrum of the curved element depends upon the optical properties of the imparter and its distance from the curved element. Movement of the imparter1160changes the curvature of the filament1150, and thus, the evanescence pattern of the returned signal which may be detected by the detector207at the surface208.

FIG. 12illustrates the use of a long-period grating that can induce, for example, changes in transmission or reflection spectra that are detectable by the detector207. In this embodiment, the filament1150is again bent into a curve by clamps1154with a radius of curvature1270to form an arch along the filament centerline1262. However, unlike the previous example, no cutout is made and instead a long-period grating1202(between grating ends1204, seeFIG. 12) is made in the filament1150. The imparter1160is coupled to the piezoelectric device via the electrostatic plates and thus moves as indicated inFIG. 12due to changes in state properties experienced by the piezoelectric device. Because of the curvature of the filament1150, an interference pattern is created in the returned signal that is detectable by the detector207at the surface208. Movement of the imparter1160changes the radius of curvature1270of the filament1150and thus changes the character of the interference pattern detected by the detector207. The change in the interference pattern due to movement of the imparter1160is measurable and may be calibrated to reflect the change in state properties experienced by the piezoelectric device. The embodiments ofFIGS. 11 and 12may be placed in the low-pressure section of the sensor in lieu of the other grating embodiments that are illustrated in, for example,FIGS. 9 and 10.

In another embodiment, the optical path of the filament may be affected by the electromagnetic interference induced by the charge of the piezoelectric device.

The embodiments illustrated inFIGS. 9,10,11and12(and variations thereof) may be arranged in series, with the same filament, to measure pressure fluctuations at various points. A single filament may be employed by using different gratings at different points along the filament (e.g., at each sensor point). Each grating would have a characteristic internal distance that enables the resulting interference pattern to be distinguished from the other gratings.FIG. 13illustrates a series arrangement of the sensors. The arrangement1300has a single filament1322that can be looped through two or more sensors1302and1304as illustrated inFIG. 13. Each sensor1302,1304, has a corresponding grating1362,1364, respectively, as illustrated inFIG. 13. The grating of one sensor can be distinguished from the grating of any other sensor along the filament1322by, for example, adjusting the grating distance within the grating itself. Light from a light source206can be placed at one end1330of the filament1322and a detector207measures the resulting interferences at the other end1332of the filament1322. In alternate embodiments, the light source206and the detector207can be housed as a single device. While two sensors are illustrated inFIG. 13, it will be understood that any number of sensors may be placed a series so long as each grating can be distinguishable from any other grating along the filament1322.

The concept of a electromechanical device unit being driven by alternating voltage/charge can be extended to other electromechanical elements beyond simple capacitor plates. An electromechanical inductor (e.g., a coil and core) device might be coupled to a filament211(seeFIG. 2) by, for example, techniques similar the voltage/charge technique. For instance, a filament211could dead-end in a Fabry-Perot approach upon a reflective end surface of a core, which itself is perturbed within a coil responsive to a changing current from the piezoelectric device706/906or some other element responding to the environment. Alternatively, an filament211could be inserted down the middle of the borehole202(i.e., allowing for multiple sensors), with the core being induced to be perturbed axially along the fiber, creating axial strains detectable as discussed with the grating embodiments described above. Such alternative approaches for the electromechanical device may be constructed using MEMS techniques.

There may be certain formulations of glass or another optical medium that cause a change in the optical characteristics (e.g., the index of refraction) in response to an electrical charge. Such a change in characteristics would allow for a direct conversion of the charge or voltage perturbations into an optical response that can be detectable at the surface208.

The sensor described herein has other uses, such as a geophone, a shock sensor, and a pick-up for an acoustic telemetry system. Moreover, the sensor (and accompanying devices) has applications outside of downhole operations. For example, the sensor described herein can be used anywhere where remote power requirements are necessary and/or desirable. Remote power sources are often needed when the sensor must operate in a harsh environment (such as hot or high-pressure environments) or where radio-frequency interference (from the power generation and/or transmission process) is undesirable. The sensors disclosed herein may also be used for measuring properties of trapped annular fluids.

The light source206may be established downhole202, or at the surface208. Similarly, the detector207may be positioned at the surface208or downhole202. The embodiments disclosed herein can be used with filaments of short and/or long distances, e.g., thousands of feet. Those skilled in the art will appreciate what changes, if any, to the length of the fiber will have on performance.

The housing/casing elements720,724,728,730and736of the sensor700(and corresponding elements inFIGS. 9 and 10) can be made of stainless steel Inconel, MP35N, 4140 and other types of materials of thickness suitable for the operating environment to be encountered. The sensor700may be attached to other devices (such as test equipment, drill bits, etc.) by, for example, threads710. The piezoelectric element706can be a standard piezoelectric unit that is useful for the anticipated operating environment. The pressure vessels where the connectors and interferometers can be made of materials appropriate for the expected operating environment. The mirror section726can be separated from the piezoelectric element706by a high-pressure feed-through section734. The high-pressure feed-through section734can be made of plastic, glass, polyetheretherketone (“PEEK”), or other suitable material. These high-pressure feed-through sections can be press-fitted or actually embedded directly for the device to be useful for separating the high pressure piezoelectric section738from the low-pressure mirror section726.

In most embodiments, a proper seal, such as a hermetic seal, may be desirable so that there can be a vacuum or near vacuum in the low-pressure section of the sensor. The suitable wiring or grounding of the piezoelectric device is capable of wide variation, so long as the charge is sufficient to cause the desired movement of the mirror or strain-inducing elements within the low-pressure section of the sensor.

All of the embodiments of the sensors disclosed herein are useful for measuring/detecting changes in state properties, such as acoustic energy signals, seismic energy signals, transient waves, strain events, pressure pulses, and/or vibrations (e.g., seismic, acoustic and/or acceleration) and potentially other applications. The sensors disclosed herein may be useful for permanent monitoring of downhole conditions at one or more points.

The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. The foregoing description is not intended to be exhaustive, or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.