High precision locked laser operating at elevated temperatures

A system, method and apparatus for operating a laser at a downhole location is disclosed. A gas is configured to receive an output of a laser and to absorb a selected wavelength of the laser corresponding to a selected spectral line of the gas. A pressure device reduces broadening of the selected spectral line related to a temperature at the downhole location. A photodetector receives light from the gas chamber and provides a measurement related to the received light. A processor alters an operating parameter of the laser using the obtained measurement to operate the laser.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure is related to performing an operation in a wellbore and, in particular, to operation of a laser at a downhole location.

2. Description of the Related Art

In various drilling operations, it is useful to dispose a laser at a downhole location in order to perform a measurement or a downhole operation. Often, the accuracy of a laser-based downhole measurement is dependent on the wavelength of the laser. However, the wavelength of the laser may drift with temperature and/or other downhole conditions. In order to maintain a laser operating at a selected frequency at a downhole location, it is necessary to lock the laser at a selected wavelength. One method of locking a laser includes the use of a Fabry-Perot etalon. The etalon needs to have a low coefficient of expansion and be transparent at laser wavelengths. Zerodur® is a material that meets these requirements but has bad temperature stability at downhole temperatures. However, in a downhole environment, the temperature can range between about 120° C. and 200° C. and the temperature of the Zerodur® needs to be controlled within a few millidegrees in order to achieve high locking precision. Additionally, downhole operations require that this etalon control be maintained for up to 12 hours or longer. Another method of laser locking uses a spectral line of a gas to provide a wavelength standard. This method can also be affected by downhole temperatures and other conditions encountered downhole. Therefore, there is a need to provide a method and apparatus for maintain operation of a laser downhole at a selected wavelength.

SUMMARY OF THE DISCLOSURE

In one aspect, the present disclosure provides a method of operating a laser at a downhole location that includes: directing a laser beam from the laser onto a gas disposed at the downhole location; reducing a broadening of a selected spectral line of the gas related to a temperature at the downhole location; obtaining a measurement related to absorption of the laser at the selected spectral line; and altering an operating parameter of the laser using the obtained measurement to operate the laser.

In another aspect, the present disclosure provides an apparatus for operating a laser at a downhole location that includes: a gas configured to receive an output of the laser and to absorb a selected wavelength of the laser corresponding to a selected spectral line of the gas; a pressure device configured to reduce broadening of the selected spectral line related to a temperature at the downhole location; a photodetector configured to receive light from the gas chamber and provide a measurement related to the received light; and a processor configured to alter an operating parameter of the laser using the obtained measurement to operate the laser.

In another aspect, the present disclosure provides a system for performing an downhole operation including: a drill string; a laser disposed on the drill string at a downhole location; a gas configured to receive an output of the laser and to absorb a selected wavelength of the laser corresponding to a selected spectral line of the gas; a pressure device configured to reduce broadening of the selected spectral line related to a temperature at the downhole location; a photodetector configured to receive light from the gas chamber and provide a measurement related to the received light; and a processor configured to alter an operating parameter of the drill string to perform the downhole operation.

Examples of certain features of the apparatus and method disclosed herein are summarized rather broadly in order that the detailed description thereof that follows may be better understood. There are, of course, additional features of the apparatus and method disclosed hereinafter that will form the subject of the claims.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1shows an exemplary well logging apparatus100according to an exemplary embodiment of the disclosure. The well logging apparatus100is shown disposed in a well borehole102penetrating an earth formation104for making measurements of properties of the earth formations104. The borehole102may be filled with drilling fluid to prevent formation fluid influx. The well logging apparatus100may include a logging tool string106lowered into the well borehole102by an electrical cable108. The tool string106may be centered within the well borehole102by a top centralizer122aand a bottom centralizer122battached to the logging tool string106at axially spaced apart locations. The centralizers122a,122bmay be of types known in the art such as bowsprings. The cable108may be spooled and unspooled from a winch or drum110to raise and lower the logging tool string100. The logging tool string106may include one or more logging devices120that may be electrically connected to surface equipment112by an optical fiber forming part of the cable108. The surface equipment112may include one part of a telemetry system114for communicating control signals and data to the tool string106and computer116. The computer116may also include a data recorder118for recording measurements made by the apparatus and transmitted to the surface equipment112.

Circuitry for operating the one or more logging devices120may be located within an electronics cartridge124of the logging tool string106. The circuitry may further be connected to the one or more logging devices120through a connector126. In several embodiments, the one or more logging devices120may incorporate a laser for use in various downhole operations and/or downhole measurements as well as a device for maintaining operation of the laser at a selected frequency in the downhole environment.

FIG. 2is an elevation view of a measurement-while-drilling (MWD) system200that may incorporate various embodiments of the disclosure. A well borehole202is drilled into the earth under control of surface equipment including a drilling rig204. In accordance with a conventional arrangement, drilling rig204includes a drill string206. The drill string206may be a coiled tube, jointed pipes or wired pipes as understood by those skilled in the art. The drill string206may include a bottom hole assembly (BHA)208having one or more logging devices210disposed thereon. The drill string206may further include a downhole drill motor226for rotating a drill bit222disposed at a bottom end of the drill string206.

The exemplary MWD system200may include a drilling fluid212circulated from a mud pit214through a mud pump216, past a desurger218, through a mud supply line220. The drilling fluid212may flow down through a longitudinal central bore in the drill string206, and through jets (not shown) in the lower face of the drill bit222. Return fluid containing drilling mud, cuttings and formation fluid flows back up through an annular space between the outer surface of the drill string206and the inner surface of the borehole202to be circulated to the surface where it is returned to the mud pit214.

The exemplary MWD system200may include a surface controller224for processing commands and other information used in the drilling operations. The surface controller224may include a processor, memory for storing data, data recorder and other peripherals. The surface controller224may also respond to user commands entered through a suitable device, such as a keyboard.

In one embodiment, the BHA226contains various sensors and logging-while-drilling (LWD) devices incorporating aspects of the disclosure to provide information about the formation, downhole drilling parameters and the mud motor. In several embodiments, the logging devices210may incorporate a laser for performing downhole operations and/or downhole measurements and a device for maintaining operation of the high-gain semiconductor laser at a selected frequency in the downhole environment, as disclosed herein.

The MWD system200may use any conventional telemetry methods and devices for communication between the downhole components and the surface, such as the surface In an exemplary embodiment, mud pulse telemetry techniques are used to communicate data from downhole to the surface during drilling operations. A telemetry system228may be located in a suitable location on the drill string206such as above the logging devices210. The telemetry system228may be used to receive commands from, and send data to, the surface via the mud pulse telemetry described above or by other communication techniques known in the art. Acoustic pipe telemetry and/or wired pipe telemetry may be used, for example.

FIG. 3shows an exemplary tool300for conducting a downhole operation using the exemplary apparatus and methods disclosed herein. The exemplary tool300may be disposed to a downhole location via carrier334that carries the tool300into a well borehole. The carrier334may be configured for conveying the tool100either on a wireline apparatus such as shown inFIG. 1or an MWD apparatus as shown inFIG. 2. In several examples, the carrier334may include a jointed pipe, a wired pipe, a coiled tube or a wireline. Some or all of these carrier examples may be combined. The tool300may include any number of devices for conducting downhole operations, and several devices may include a laser device306selected for operation in the high temperatures typical of the downhole environment. In one example, the tool300may include a spectrometer304. In another example, the tool300may include one or more of a temperature sensor318, a pressure sensor320, a stress sensor322and/or a distance sensor324. The stress sensor may also be acceleration and/or a vibration sensor. A downhole computing device328may include a processor330and a memory332. The downhole computing device328may be coupled to the spectrometer304when included in the tool300. In several examples, the downhole computing device328may be in communication with other sensors318,320,322,325, when included, and may further be in communication with a high-gain semiconductor306used with the several sensors. Power and data may be conveyed to and from the sensors, spectrometer and computing device using an electrical conductor cable336. In some cases, an optical fiber326may be used for communicating information between tool components.

Several tool devices according to the disclosure may be used to sample and/or test formation or well bore fluids. A port302may be used to convey fluid into the tool300through a fluid conduit312. In some cases, a sample chamber316may be included for holding or transporting fluid samples. Fluids may be expelled from the tool when desired by including a port314for directing the fluids into the annulus out side of the tool300.

The exemplary spectrometer304may include a laser device306, a sample region308and one or more detectors310. In several embodiments, the laser device306may include a high-gain semiconductor used as a laser light source. The laser device306may provide light having a broader emission band than that of a laser where such a light source is desired. In an exemplary embodiment, the laser device306may be selected for high-temperature operation. The several sensors318,320,322, and324described above may also include a laser device306emitting laser or other useful light. In some cases, sensors or other tool devices may use a high-gain semiconductor device such as a FET, LED, MOSFET, transistor, diode or the like where the semiconductor includes the high-temperature structure.

The spectrometer304may be used for measuring refractive index of the formation fluid. In this case, the light detector310may be located so as to receive light after reflection and refraction from a fluid sample in the fluid sample region308. In other examples, the detector310may be placed such that light emitted from the laser device306passes through the sample region308and is detected at the detector310.

Alternatively, the laser device306may supply a laser beam fro use in an interferometer or a gravimeter. The gravimeter may employ an interferometer as well. The precision of the wavelength of the laser beam allows for precise determination of interference fringe measurements. In an exemplary embodiment of the present disclosure, the precision may be carried out 10 decimal places.

While the laser device306of the present disclosure may be used at downhole temperatures without cooling, it is contemplated that temperature control devices338may be utilized for controlling a temperature of the laser devices306. Examples of temperature control devices338may include sorption cooling devices, Dewar and thermo-electric cooling devices. While the high-gain semiconductor device306is shown with respect to spectrometer304, it is to be understood that the laser device306may be used in any suitable apparatus or to perform any suitable operation that uses a laser having a wavelength maintained at a selected frequency, as disclosed herein.

FIG. 4shows a schematic view400of an exemplary laser device306in one embodiment of the present disclosure. The exemplary laser device306includes a laser402, a gas chamber404containing a gas, and a detector406. The laser402may be, for example, a tunable laser, such as a diode laser, a fiber laser, a quantum dot-based semiconductor diode laser, etc. In an exemplary embodiment, the operating wavelength of the laser may be affected by a temperature of the laser, an operating current of the laser and other parameters. The laser may drift up to about 50 kilohertz per degree Celsius. The laser402may be disposed in a temperature control device412that may be used to control an operating temperature of the laser, thereby controlling an operating wavelength of the laser. The gas chamber404may contain a gas having absorption spectral lines at selected wavelengths. In an exemplary embodiment, the spectral lines may be due to molecular rotational and vibrational modes of the gas. An exemplary gas may include H13C14N, which exhibits rotational and vibrational spectral lines in a spectral range from about 1530 nanometers (nm) to about 1565 nm.FIG. 5shows an exemplary rotational-vibrational absorption spectrum for H13C14N. Other gases having rotational-vibrational spectral lines may also be used. A beam splitter410splits a laser beam exiting the gas chamber404into a first beam415and a second beam417. The first beam415is directed to a detector406for detection and the second beam417is directed to an external device (not shown) for use in performing a downhole operation or obtaining a downhole measurement, for example. Detector406may be a photodetector that produces a current in response to light being captured at the photodetector. In an exemplary embodiment, a magnitude of the current at the photodetector406is related to an intensity of light in the first beam415. The intensity of light in the first beam415may be related to a difference between a wavelength of the laser and a wavelength of an absorption line (spectral line) of the gas in the gas chamber404. As the laser wavelength changes with respect to a selected spectral line of the gas, the intensity of light at the photodetector406changes. Thus, the current measurement at the photodetector406reflects this change in wavelength. Pressure chamber420may be used to alter a pressure of the gas at the downhole location to reduce broadening effects on the spectral lines of the gas due to downhole temperatures, thereby increasing a precision of the laser control system.

A processor408is coupled to the photodetector406and to the laser402. The processor may receive a current measurement from the photodetector406and use the current measurement to determine a wavelength of the laser. Additionally, the processor408may control an operational parameter of the laser402to correct for a wavelength drift of the laser402from a selected spectral line of the gas. The processor408may control a temperature of the laser402and/or an operating current of the laser402, among other operational parameters, in various embodiments, to control the wavelength of the laser402.

FIG. 6shows exemplary a selected spectral line of the exemplary gas under various conditions. Spectral line601represents a spectral line of the exemplary gas at approximately room temperature. Broadened spectral line603represents a spectral line of a gas at an elevated temperature such as encountered at a downhole location. As the temperature of the gas increases, the thermal velocities of the gas molecules increase, thereby broadening spectral line601and reducing the peak at the central wavelength to obtain broadened spectral line603. Thus, spectral line603has a broader line width and the absorption at the central wavelength of the spectral line603line is less than then absorption of the central wavelength of spectral line601. Spectral line605correspond to pressure-reduces gas in a downhole location. Pressure of the gas plays a dominant role in the spectral broadening. Reducing the pressure of the gas reduces spectral broadening of line603to obtain spectral line605. Therefore, in one embodiment, the pressure chamber420may be used to reduce a pressure of the gas in the gas chamber404. The central wavelength is the same for the spectral lines601,603and605. However, since the peak of spectral line605is less than the peak of spectral line601, spectral line605absorbs less light that spectral line601at the central wavelength. In order to provide additional absorption at the selected wavelength, the laser beam may be made to pass through more gas than it would for a gas at room temperature at a surface location. Thus, the gas chamber404may be longer than a gas chamber used at a room temperature to increase the path of the laser through the gas.

FIG. 7shows a relation between laser output power (optical power) and an operating current of an exemplary laser of the present disclosure. Power-current curves are displayed for several operating temperatures. Curve702shows a power-current curve at about 25° C. or at about room temperature. There is substantially no optical power output for currents below a cutoff current of about 70 milliamps (mA). However, above about 70 mA, the optical power increases with operating current in a substantially linear fashion. In an exemplary embodiment, the laser is operated in a range over which there is an approximately linear relation between power and current. Thus, at room temperatures, a suitable operational range of the laser is above about 70 mA. As the temperature increases to 70° C. (curve704) and 100° C. (curve706), the cutoff current decreases. Increasing the temperature further to 125° C. (curve708), 140° C. (curve710) and 150° C. (curve712), a peak appears in the relation between optical power and current. The power-current relation is generally non-linear at the peak. Therefore, the linear region of the power-current relation is reduced at these higher temperatures. For a temperature of 150° C. (curve712), this approximately linear region is between about 35 mA and about 60 mA. This linear region corresponds to less than about 3 mW of optical power. As shown inFIG. 7, a temperature of 160° C. (curve714) is approximately an operating limit of the laser, since no output power is provided at any operating currents. Therefore, in an exemplary embodiment, a temperature of the laser may be maintained at the about 150° C. (curve712) at the downhole location.

FIG. 8shows exemplary wavelengths of a laser beam that may be emitted using the laser described inFIG. 7. The wavelengths are show for the laser operated at 125° C. The central wavelength of the laser is about 1314 nm at 1 milliwatt power and is about 1316 nm at 10 milliwatt power. As seen inFIG. 7, the laser is therefore capable of operating at this temperature to provide a substantial laser beam. Since the central wavelength is about 1314 nm to 1316 nm, the laser beam passed through a suitable gas that has spectral absorption lines at those comparable wavelengths.FIG. 9shows a rotational-vibrational absorption spectrum for a hydrogen fluoride (HF) gas. The spectrum is in the same spectral region as the wavelength shown inFIG. 8and may therefore be used in the gas chamber404for downhole laser locking of this laser beam.

FIG. 10shows a relation between laser line width and optical output power of the exemplary laser that may be achieved using the exemplary methods disclosed herein. The laser line width is shown for an operating temperature of about 125° C. The laser line width is about 1 megahertz (MHz) for optical output power between 1 mW and 10 mW. Therefore, the laser line width shows exceptional precision at these output powers.

In alternate embodiments, the locked laser beam may be used as part of a heterodyne laser in which at least one laser beam is locked to a selected wavelength. In other alternate embodiments, the laser may be locked to several absorption lines and measurements, such as interferometry measurements, may be made using the laser locked at each of the several absorption lines. Interpolation of the measurements may be the used to increase a precision of the measurements.

While the foregoing disclosure is directed to the certain exemplary embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.