Evanescent wave downhole fiber optic spectrometer

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Descriptive Report of the Invention Patent for EVANESCENT WAVE BOTTOM-HOLE FIBER OPTIC SPECTROMETER.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention generally relates to wellbore tools and in particular to apparatus and methods for estimating properties of downhole fluids.

2. Description of Related Art

Oil and gas wells have been drilled to depths ranging from a few hundred meters to as deep as eight kilometers. A large portion of current drilling activity involves directional drilling which includes drilling boreholes drifted from vertical to a few degrees above the horizontal to increase hydrocarbon production from subterranean formations.

Information about the subsurface formations traversed by the borehole can be obtained by any number of techniques. Some techniques used to obtain formation information include obtaining one or more core samples from the subsurface formations and obtaining one or more samples of fluid produced from the subsurface formations. These samplings are collectively referred to herein as formation sampling. Modern fluid sampling includes several downhole tests and sometimes fluid samples are taken for surface laboratory testing. However, the high costs of oil and gas exploration are creating a strong trend toward improved methods and apparatus for downhole testing.

What is needed, therefore, are improved methods and apparatus for downhole fluid testing, preferably methods and apparatus that provide users with the ability to rapidly identify constituents of a fluid sample.

SUMMARY OF THE INVENTION

One embodiment of the invention includes an apparatus for estimating

2/20 a property of a downhole fluid, the apparatus includes: an optical fiber that receives a light source and that includes an uncoated portion adapted to come into contact with the fluid; a photodetector for receiving optical signals from the portion; and a spectrometer for obtaining an evanescent spectrum of the fluid from the portion.

Another embodiment of the invention includes a method for estimating a property of a downhole fluid, the method includes: selecting a downhole spectrometer that includes an optical fiber having at least an uncoated portion; that at least partially contacts the portion in the downhole fluid; receiving light emitted from a light source through the optical fiber; estimating an evanescent spectrum of the fluid from the received light; and estimate the property from the spectrum.

A further embodiment of the invention includes a system for characterizing a fluid in a downhole environment, the system including: at least one light source for inputting an optical fiber having an uncoated portion adapted to come into contact with the fluid; a photodetector for receiving optical signals from the portion; a spectrometer for obtaining an evanescent spectrum of the fluid from the portion; and an electronic unit adapted to receive evanescent spectrum information and characterize the fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the various non-limiting embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and in which:

Figure 1 represents aspects of a logging cable system for performing logging in a borehole;

Figure 2 represents aspects of a system for performing profiling during drilling;

Figure 3 represents aspects of a fiber optic spectrometer useful for characterizing oil;

3/20 Figure 4 represents aspects of another embodiment of the fiber optic spectrometer useful for characterizing petroleum; and Figure 5 is a flowchart providing an exemplary method for characterizing fluid(s) with the downhole spectrometer.

DESCRIPTION OF THE INVENTION

Methods and apparatus for at least one of analyzing a downhole fluid and identifying fluid constituents by collecting evanescent wave optical absorption spectra are described herein. The techniques provided make use of an optical fiber immersed in or at least partially near the fluid. Exemplary optical fibers include those made from silicon, germanium, and sapphire. The techniques are particularly well suited to mid-infrared spectroscopy and can be used to estimate issues such as the percentage of oil-based mud contamination as well as hydrogen sulfide (H₂S), carbon dioxide, (CO₂) and also concentrations of methane, ethane, propane and butane.

Now for context, an exemplary apparatus for oil and gas exploration is considered. Figure 1 schematically illustrates aspects of a cable logging apparatus used for oil and gas exploration. In the example shown, a wellbore 110 or borehole 110 traverses subsurface materials that may include various subsurface formations 102. The borehole 110 may be filled or at least partially filled with a fluid mixture that includes various gases, water, drilling fluid, and formation fluids that are native to the subsurface formations penetrated by the borehole 110. Such fluid mixtures are referred to herein as “wellbore fluids” or “borehole fluids.” The terms “innate fluid” and “natural fluid,” as used herein, refer to fluids that exist naturally in or are extracted from subterranean formations 102 and are exclusive of any admixture or substantial contamination by fluids not naturally present in the formation, such as drilling fluid.

In this non-limiting example, an assessment tool

4/20 formation 120 is driven into well borehole 110 using a logging cable 104. The placement and removal of the logging cable may be accomplished by a powered winch pulled by a service truck 108, for example. The logging cable 104 may be a shielded cable carrying data and power conductors to supply power to the formation evaluation tool 120 and to provide two-way data communication between the tool processor 112 and a controller 114 that may be carried by the service truck 108. In general, the logging cable 104 is carried from a reel 116 by a pulley 118 supported by a tower 122. The reel 116 may be loaded by truck 108 as shown for land operations, by an offshore platform for subsea operations, or by any other suitable mobile or fixed support structure. The controller 114 may include a processor, such as within a computer or a microprocessor, data storage devices, such as solid state memory and magnetic tapes, peripherals, such as data input devices and display devices, and other circuitry for controlling and processing data from the training assessment tool 120. The surface controller 114 may further include one or more computer programs embodied in a computer readable medium accessible to the processor in the controller 114 for executing instructions contained in the computer programs to perform the various methods and functions associated with processing the data from the formation assessment tool 120. It can also be useful in producing logs to identify fluid phases in the wellbore and to identify which phases (gas, water, oil) are being produced from a particular perforation in the casing.

A lower portion of the formation evaluation tool 120 may include an assembly of a plurality of tool segments that are joined end-to-end by helical sleeves or mutual compression joints 124. An assembly of tool segments suitable for the present invention may include a power unit 126 that may include one or more of a hydraulic power unit, ...

5/20 of electrical power or an electromechanical power unit. In the example shown, a formation fluid extractor 128 is coupled to the formation evaluation tool 120 below the power unit 126. A large displacement volume motor/pump unit 130 may be provided below the formation fluid extractor 128 for line drainage. A similar motor/pump unit 132 having a smaller displacement volume may be included in the tool at a suitable location, such as below the large volume pump, to quantitatively monitor fluid received by the formation evaluation tool 120. One or more sample tank compartment sections 134 may be included to hold fluid samples from the small volume pump 132. Each compartment section 134 may have multiple fluid sample tanks 136. In various embodiments to be described in more detail below, the formation evaluation tool 120 includes a downhole spectrometer 300. The downhole spectrometer 300 may be used in both during-drilling and on-line logging embodiments.

The formation fluid extractor 128 generally includes an extendable suction probe 138 that is opposed by borehole wall feet 140. Both the suction probe 138 and the opposed feet 140 may be hydraulically or electromechanically extendable to securely engage the borehole wall of the well.

Figure 2 schematically illustrates a non-limiting example of a drilling system 200 in a measurement-while-drilling (MWD) arrangement in accordance with various non-limiting embodiments of the disclosure. A derrick 202 supports a drill string 204, which may be a coiled pipe or a drill pipe. Drill string 204 may carry a bottom-hole assembly (BHA) 220 and a drill bit 206 at a distal end of drill string 204 to drill borehole 110 through various earth formations and subsurface materials.

Drilling operations according to different modalities

6/20 may include pumping drilling fluid or “mud” from a mud tank 222, and using a circulation system 224, which circulates the mud through an internal bore of the drill string 204. The mud exits the drill string 204 at the drill bit 206 and returns to the surface through an annular space between the drill string 204 and the internal wall of the borehole 110. The drilling fluid is designed to provide hydrostatic pressure that is greater than the formation pressure to prevent blowouts. The pressurized drilling fluid may further be used to drive a drilling motor 208 and may provide lubrication for various elements of the drill string 204.

In the non-limiting embodiment of FIG. 2, the BHA 220 includes a training assessment tool 120 substantially similar to the training assessment tool 120 described above and shown in FIG. 1.

A while-drilling formation evaluation tool 120 may carry a fluid extractor 128 that includes an extendable suction probe 138 and opposing feet 140. In various embodiments to be described in more detail below, the formation evaluation tool 120 includes the downhole spectrometer 300. The downhole spectrometer 300 may be used in both while-drilling and logging wireline embodiments. Of course, other embodiments can be implemented in the same way. For example, the downhole spectrometer 300 can be used as a part of a tractor (i.e., a device placed at the bottom of the well, independent of a logging cable and generally self-propelled).

Referring to FIG. 3, and with respect to evanescent wave spectroscopy and downhole spectrometer 300, an illustration is provided for better perspective. In FIG. 3, an optical fiber 310 generally includes a cladding 301 and a core 302. In this example, which depicts some aspects of downhole spectrometer 300, a portion of fiber 310 is bare. That is, core 302 is exposed and free of the surrounding cladding 301. This portion of fiber 310 may be referred to

7/20 as “uncoated”. The portion of the core 302 that is exposed is at least partially surrounded by a sample 303 and may be included in or pass through a sample chamber 305 (such as a channel, cell, sample line, volume, sampling area or similar device). It should be appreciated that the uncoated portion, although it may be within a sample chamber 305, may be in any area or volume where the sample 303 contacts at least part of a surface of the uncoated portion. The downhole fluid sample 303 may be a flowing fluid or a non-flowing fluid. In general, the fluid may be pumped, ported, added, flowed, carried, dropped, or otherwise moved into contact with the uncoated portion for analysis.

A beam of light is supplied to a fiber interface 310. As the light passes through the exposed portion of the core 302, the light that is incident on the core/sample interface is partially reflected and partially reflected onto the sample 303. This occurs if an angle of incidence is less than a critical angle. With an increase in the angle of incidence, the portion of light reflected is increased, and if the angle of incidence exceeds a critical angle, total internal reflection is accomplished. The critical angle depends on the refractive index of the core material, and the sample material, n₂. A harmonic wave is referred to as the “evanescent wave”. The evanescent wave penetrates the sample 303 to a penetration depth, d_p, and is described by Eq. (1):

C) where z represents a distance normal to the interface between sample 303 and core 302, AND_the represents the amplitude of a wave at z = 0. A penetration depth, d, in sample 303 is given by Eq. (2): ά_Ρ = ^λΐ{η^^2π^ϊη^θ~(ηχΙη^ (²):

where λ represents a wavelength of light in the core 302 of the fiber 310, ni represents a refractive index (RI) of the core 302, and en₂represents the refractive index of sample 303. Similarly, and as an example, for an input of θ = 85 (degrees), λ = 2,000 (nm), ni = 1.75 and n₂

8/20 = 1.5, the penetration depth, d_p, is estimated as 92.34 nm.

The fiber optic spectrometer currently includes several adaptations to provide operation in a downhole environment. For example, some materials previously used for mid-infrared fiber optics (such as silver halides) are not practical for downhole use since such materials are brittle and break easily. Furthermore, such materials have highly unacceptable solubility and reactivity in formation fluids such as brines or crude oils, and this solubility and reactivity increases at the high temperatures encountered in the downhole environment.

Thus, the teachings herein offer embodiments of a downhole spectrometer that make use of, among other things, fiber optic material with a high refractive index, little or no solubility, and little or no reactivity in high temperature formation fluids and little absorption over the wavelengths of interest.

Materials selected for the downhole spectrometer 300 generally exhibit a select group of properties, such as, but not limited to, non-stick surfaces, non-corrosive, high refractive index, insolubility, and other properties as may be desired for the downhole. Materials suitable for the downhole spectrometer 300 generally will not deteriorate at high temperature in the presence of water.

Thus, in some embodiments, a metalloid is used for the downhole spectrometer 300. A metalloid behaves as a mirror at shorter wavelengths and as a transparent window at longer wavelengths. The metalloid, in one example silicon, has an added advantage of being a nearly non-reactive and non-sticky surface, so it is resistant to both corrosion by the fluid and contamination by deposits from the fluid. Germanium is also relatively chemically unreactive and a suitable material. Silicon and germanium are both semiconductors, so they have a band gap, and therefore

9/20 These act as a mirror for photons with energy greater than the band gap energy and act as a transparent window for photons with energy less than the band gap energy. Silicon, for example, changes behavior from a mirror to a window at wavelengths longer than about 1100 nm, and germanium changes from a mirror to a window at wavelengths longer than about 1800 nm.

In order to provide some context for the present teaching, a review of the fundamentals of fiber optics as they relate to the downhole spectrometer is now offered. A principle of fiber optics is that light is totally reflected at a core-cladding interface if the light strikes that interface at an angle of arrival that exceeds a critical angle. The fiber core must have a higher refractive index than the cladding surrounding the core for the critical angle to exist. However, even in the case of total internal reflection, there is an evanescent field of reflected light that exponentially decays below the core. This means that a portion of the light penetrates the cladding slightly to a depth that is less than a few wavelengths. If the cladding happens to be so highly absorbent at wavelengths of light that significant light is absorbed within the evanescent penetration depth, d_p, then the attenuated reflection spectrum of the cladding can be obtained. If a material that exhibits a sufficiently high refractive index is used for the fiber core, then the forming fluid, which at least partially surrounds the inserted core, can act as the cladding. In this case, an absorption spectrum for the forming fluid could be obtained using the evanescent signal. The fundamental molecular vibrational bands, which occur in the mid-infrared region of the spectrum (about 2.5 to 11 microns), are several orders of magnitude more absorbent than the overtone and combination bands that occur in the near-infrared region (0.8 to 2.5 microns) of the spectrum. The effective path length for an evanescent wave spectrometer is quite short. It equals the product of the penetration depth of just less than a few microns and the number of internal reflections. Because the

10/20 measured absorbance is the product of the effective path length and absorbance, it is preferable to make evanescent wave measurements in the high absorbance region, mid-infrared to compensate for the short effective path length.

In order to provide a robust downhole spectrometer, the fiber core material must have high chemical and thermal resistance to withstand the harsh downhole environment. The core material must also have a refractive index that is higher than typical downhole gas brine (Rl < 1.3) (Rl in the range of 1.30 to 1.33) or crude oil (Rl in the range of 1.40 to 1.55). Preferably, it should also have the ability to transmit mid-infrared light. Examples of suitable materials include sapphire (Rl = 1.75, transmits at wavelengths as long as 5 microns), elemental silicon (Rl = 3.4, transmits at wavelengths as long as 11 microns), elemental germanium (Rl = 4, transmits at wavelengths as long as 20 microns), and other such materials. Elemental silicon and germanium appear as shiny metals in the visible region of the electromagnetic spectrum, but become transparent at longer wavelengths (1,100 nm for silicon and 1,800 nm for germanium). The mid-infrared region is of particular interest for use with a downhole spectrometer, and spans from about 2,500 nm (2.5 microns) to 11,000 nm (11 microns).

Thus, it can be seen that selection of optical fiber 310 must consider properties of materials in core 302 as well as properties of sample 303. Table 1 below provides non-limiting examples of materials suitable for use in core 302 of optical fiber 310 as compared to the property of sample materials.

Table 1 Refractive indices for Downhole Spectrometer, n_1t and

Sample, n₂

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<td colspan="2">/?! = Refractive Index of the Nucleus</td><td colspan="2">n₂ = Refractive index of the fluid</td>

<td>Sapphire (AI)₂THE₃)</td><td> 1,75</td><td>Brine</td><td>1.3 to 1.33</td>

<td>Silicon</td><td> 3,42</td><td>Crude oils</td><td>1.4a 1.55</td>

<td>Germanium</td><td> 4,00</td><td>Gas</td><td> &lt; 1,3</td>

<td>Boron</td><td> 3,0</td><td colspan="2"></td>

<td>Tellurium</td><td>1.7 to 2.7</td><td colspan="2"></td>

<td>Diamond film</td><td> 2,42</td><td colspan="2"></td>

<td>Gallium Lanthanum Sulfide</td><td> 2,40</td><td colspan="2"></td>

<td>Rutile (TiO₂)</td><td> 2,56</td><td colspan="2"></td>

<td>Yttrium Aluminum Garnet (YAG)</td><td> 1,82</td><td colspan="2"></td>

Specific aspects and considerations for various materials are not provided. Although metalloids are generally considered suitable, arsenic, polonium and antimony were not considered or evaluated for use in the 300 downhole spectrometer due to toxicity concerns. However, this does not suggest that these materials could not perform well. On the contrary, it is considered that there is a probability that such materials may probably function well since they are members of the metalloid family.

Regarding other materials and considerations for the construction of the 300 downhole spectrometer, thin films of tellurium have shown favorable properties, such as low band gap and transparency in the infrared region. Diamond exhibits a transmission range of about 300 nm to 2.5 microns and a refractive index of 2.4175 at 0.589 microns, and is insoluble in water. Germanium has a transmission range of about 1.8 to 23 microns, a refractive index of 4.0026 at 11 microns, and is insoluble in water. Lanthanum gallium sulfide is a chalcogenide glass, and an alternative to toxic arsenic-based glasses, that exhibits a transmission range of about 0.5 to 10 microns and a refractive index of 2.398 at 1.014 microns. Rutile (titanium dioxide) exhibits a transmission range of about 0.43 to 5.0 microns, a refractive index of 2.555 at 0.69 microns, and is insoluble in water. Sapphire exhibits a transmission range of about

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0.17 at 5.5 microns and a refractive index of 1.75449. Silicon exhibits a transmission range of about 1.2 at 15 microns, a refractive index of 3.4223 at 5 microns and is insoluble in water. YAG (yttrium aluminum oxide Y3AI₅THE₁₂) exhibits a transmission range of about 0.21 to 5.5 microns, a refractive index of 1.81523 at 1.06 microns, and is insoluble in water.

The downhole spectrometer 300 may be realized in a variety of forms. In one embodiment, the optical fiber 310 (such as a silicon fiber or a germanium fiber) is attached to a substrate. The substrate provides mechanical reinforcement that reduces brittleness, since embodiments using semimetals are somewhat brittle. In this manner, the optical fiber 310 mounted on a substrate generally provides users with a design that is tolerant to vibration. Some additional embodiments of vibration-tolerant designs include mounting on resilient devices, such as motion-damping springs and the like.

In some other embodiments, the downhole spectrometer includes a photonic wire waveguide (such as a silicon wire) on a substrate that is very sensitive to trace amounts of sample 303. In additional embodiments, the downhole spectrometer 300 includes attenuated reflection optical windows made of silicon or germanium (e.g., windows between about 2 mm and about 3 mm). More specifically, Finite Element Analysis shows that windows made of these semimetals can withstand downhole pressures and temperatures.

In addition, various other physical forms of optical fiber material may be used. More specifically, it is recognized that certain aspects of the teachings herein are offered in the context of an optical fiber. However, the downhole spectrometer 300, at least in some embodiments, may be realized in other forms, such as a thin film waveguide. That is, in some instances, thin films may provide material waveguides that may not meet all downhole performance criteria. For example, thin film waveguides may provide robust downhole spectrometer 300 embodiments when using materials such as boron, diamond, silicon, gero, etc.

13/20 manium, sapphire, brittle etc. In embodiments using thin film waveguides, the material used in the downhole spectrometer may be supported on a rigid substrate and may be prepared by standard wafer processing techniques. These thin films, “optical wires” or “silicon photonics” can act as optical waveguides in the same way that a round optical fiber could, even though they have a different cross section, such as a square or rectangular cross section. Accordingly, as used herein, the terms “fiber optics,” “downhole spectrometer,” and other such related terms should be construed to include thin film waveguides of a form useful for performing downhole evanescent spectroscopy.

Referring now to Figure 4, aspects of another embodiment of downhole spectrometer 300 are shown. In this example, downhole spectrometer 300 includes a infrared light source 401 in optical communication with a waveguide 402 (which may be hollow) for transmitting light into a fiber optic element 403. In some embodiments, at the distal end of the fiber optic element 403, there is a mirror 404, such as a gold mirror 404 at the tip end of the fiber that may be included and used to reflect light onto the fiber optic element 403. The crude oil, or other fluid to be analyzed, acts as a coating on the fiber and is denoted by the hatch marks between the indicators 403 and 404. The waveguide 402 transmits the reflected light to at least one photodetector 405. The photodetector 405 may include, for example, a plurality of sensing elements for specific wavelength(s) for oil or other selected downhole fluid(s). Electronics 410 may be included and used to control the light source 401, analyze signals from the photodetector 405, and to transmit signals to remote locations as desired.

Photodetector 405 is generally used to receive light emitted from light source 401 after the light interacts with the fluid via return optical fiber 403. In one or more embodiments, photodetector 405 comprises a single broadband photodetector responsive to emitt light.

14/20 taken from light source 401 and/or light reflected at a fluid-metalloid interface. In other non-limiting embodiments, photodetector 405 includes a double-layer photodetector responsive to light emitted from light source 401 and/or light reflected at the fluid-metalloid interface. A suitable embodiment for photodetector 405 includes a double-layer photodetector that is a Si and InGaAs double-layer photodetector. The photodetector 405 provides an output signal indicative of the light received at the photodetector 405 to the electronics unit 42.

In some cases, the photodetector output signal may be an analog electrical signal, so an analog-to-digital converter may be used to convert the photodetector output signal to a digital signal that is received by a downhole controller within electronics unit 410 or by surface controller 114. The light emitted from the light source 401 may be modulated by a processor within the same controller that receives the output from the photodetector 405 or by a modulator in a separate controller. In the example shown, one modulator/controller is coupled to the photodetector 405 and a second modulator/controller is coupled to the light source 401. These controllers may be implemented as a single controller without departing from the scope of the disclosure. In other embodiments, the controller or controllers may be located at the surface of the wellbore as described above and shown in FIGS. 1 and 2 at 114 using any of several communication methods. Cooling of one or more of these downhole components may be accomplished using a cooling device. The cooling device used may be any one or more of a variety of devices, examples of which include electrothermal, thermal tunneling, sorption cooling, evaporators, and a dewar. Cooling is optional where the components selected are compatible with the downhole temperature environment.

Cooling may be applied where a component's operating temperature is lower than the downhole environment and/or where cooling may improve downhole spectrometer performance.

15/20 well 300. In various embodiments, the light source 401 is compatible with the downhole temperature environment and the cooling device is optional. The cooling, in some cases, increases a signal-to-noise ratio of the photodetector 405 and increases laser brightness where the light source 401 includes one or more lasers.

Non-limiting examples of spectrometer tools described herein provide a small, lightweight, fiber optic downhole spectrometer that has substantially higher resolution than conventional spectrometers. In addition, the spectrometer 300 may include in situ calibration capabilities, such as through the use of a positionable reference material and a light source that generates both short and long wavelengths.

In various embodiments, the light source 401 may include one or more broadband light sources such as an incandescent light source along with an optical filter to provide selected wavelengths, or the light source 401 may include one or more light emitting diodes (LEDs). The light source may also use one or more laser diodes. In other embodiments, the downhole spectrometer 300 may include one or more light sources 401 that include a combination of light source types. Some embodiments may be used as multi-wavelength implementations. For example, an incandescent light and a dual bandpass filter may be used as a light source 401 to generate two selected wavelengths. Alternatively, dual wavelength diodes or laser diodes may be used. The dual wavelength light may be conducted via the optical fiber 403 to a silicon or germanium tip immersed in target fluid within a fluid cell. A double layer photodetector 405 above may be used to detect light reflected from the fluid interface. In one embodiment, the two wavelengths include a short wavelength light and a long wavelength light, where the short wavelength light is light having an energy less than the band gap of the nonmetal material and the long wavelength light is light having an energy less than the band gap of the nonmetal material.

16/20 long is light that has an energy greater than the band gap of the nonmetal material.

As one skilled in the art will appreciate, a “single wavelength” as discussed herein may actually appear as a band of wavelengths, such as a peak in a spectrum. Thus, a plurality of wavelengths may actually manifest as a plurality of wavelength bands as can be effectively discriminated by associated electronics. Furthermore, the use of various types or combinations of optical filters for refining light and wavelength groups as may be used with downhole spectrometer 300. In some embodiments, light source 32 may be adjusted to multiplex varying wavelengths per time.

The dual-wavelength photodetector can provide a short-wavelength detector and a long-wavelength detector to simultaneously detect the wavelengths reflected from the fluid interface. In some embodiments, when using a fiber having a Si or Ge material and a Si and InGaAs detector, the Si portion of the detector will detect the higher intensity reflections of short wavelength light while the InGaAs portion of the detector detects lower intensity reflections of long wavelength light.

With respect to material selection, one advantage of a silicon interface is that a silicon interface provides a smooth surface that is resistant to contamination by downhole fluid deposits. In addition, silicon acts as a mirror for wavelengths of light shorter than about 1,100 nm and is substantially transparent for wavelengths longer than about 1,100 nm. This feature is useful in the dual-wavelength modalities described above. As described above, germanium is another exemplary metalloid that offers properties suitable for use as an optical fiber.

In some embodiments, at least one of the light source

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401, the electronics unit 410 and the photodetector 405 may be maintained remotely. For example, at least one of the light source 401, the electronics unit 410 and the photodetector 405 may be provided on the production deck. In such embodiments, users and designers are offered the opportunity to employ a large component array to perform required tasks. In such embodiments, communication with components of downhole spectrometer 300 may occur via a variety of devices, including, without limitation, cabled tubing, fiber, telemetry, and the like. In other embodiments, at least one of light source 401, electronics unit 410, and photodetector 405 may be included in downhole spectrometer 300 proximate to sample chamber 305. In these latter embodiments, the downhole spectrometer 300 may make use of components such as downhole power supplies, cooling units, and the like.

In general, the downhole spectrometer 300 provides results “in real time.” That is, the downhole spectrometer 300 provides results at a rate that is useful to users and operators of downhole equipment during the use of such equipment. Generally, the rate is adequate for users and operators to make meaningful decisions regarding the downhole environment and other downhole sequences.

Referring now to FIG. 5, an exemplary method for estimating a property of a downhole fluid is shown. In a first stage 501, the method calls for selecting a downhole spectrometer comprising an optical fiber with at least a portion without a coating disposed therein. In a second stage 502, the method calls for at least partially surrounding the portion in the downhole fluid. In a third stage 503, the method calls for receiving light emitted from a light source through the optical fiber. In a fourth stage 504, the method calls for estimating an evanescent spectrum of the fluid from the received light. In a fifth stage 505, the method calls for estimating a property of the sample according to the evanescent spectrum.

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Having introduced the aspects of the invention, certain advantages of the teachings herein should become apparent. For example, the downhole spectrometer provided herein generally offers simplified fabrication over prior art designs. More specifically, the use of a single fiber avoids the incorporation of optical interface(s) other than those required between the light source and the photodetector. Furthermore, the use of a single fiber offers simplified component assembly. That is, the use of a single fiber also avoids complications that occur when using components such as reflection crystals that have accompanying geometry requirements. Thus, the downhole spectrometer provided herein is generally more physically robust as well as more economical to maintain or replace.

As provided previously, this invention provides for downhole fluid analysis by collecting evanescent wave optical absorption spectra of the fluid, but using silicon, germanium, sapphire or other high temperature and otherwise robust optical fiber submerged in or at least partially surrounded by the fluid sample. This approach is well suited for mid-infrared spectroscopy and for determining issues such as the percentage of oil-based mud contamination or H concentrations.₂S or CO₂, methane, ethane, propane and butane and other such petroleum constituents.

In support of the teachings of the present, several analysis components can be used, including a digital system and/or an analog system. The system(s) may have components such as a processor, storage media, memory, input, output, communications link (wired, wireless, pulsed llama, optical or other), user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and the like) to provide operation and analysis of the apparatus and methods described herein in any of several ways well appreciated in the art. It is contemplated that these teachings may be, but need not be, implemented in conjunction with a series of executable instructions

19/20 by computer stored on a computer-readable medium, including memory (ROMs, RAMs), optical (CD-ROMs) or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide equipment operation, control, data collection and analysis, and other required functions relevant to a system designer, owner, user, or other persons, in addition to the functions described in this disclosure.

Additionally, several other components may be included and required to provide aspects of the present teachings. For example, a sample line, sample storage, sample chamber, sample exhauster, pump, piston, power supply (e.g., at least one of a generator, a remote supply, and a battery), vacuum supply, pressure supply, refrigeration (i.e., cooling) unit or supply, heating component, motive force (such as a translational force, propulsive force, or a rotational force), magnet, sensor, controller, optical unit, electrical unit or electromagnetic unit may be included in support of the various aspects discussed herein or in support of other functions beyond this description.

One of ordinary skill in the art will appreciate that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, such functions and features, as may be necessary in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings hereof and a part of the described invention.

Although the invention has been described with reference to exemplary embodiments, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be perceived by those skilled in the art to adapt an instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Accordingly, it is not necessary to

20/20 intends that the invention be limited to the particular embodiment described as the best contemplated mode for carrying out this invention, but that the invention include all embodiments that fall within the scope of the appended claims.