JP2005069882A - Method and device for performing rapid isotope analysis by laser spectroscopy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure the composition of isotopes in a specimen in real time. <P>SOLUTION: A method for measuring the composition of isotopes in a fluid in a geological layer includes steps for: (a) introducing a reference fluid having a known isotope composition into a reference cell; (b) acquiring the specimen of the geological layer; (c) preparing at least one laser beam; (d) scaling at least one parameter of the reference cell; (e) causing a beam to pass through the reference fluid to measure the reference measuring beam before and after passing through the reference fluid; and (f) causing a beam to pass through the specimen to measure the beam before and after passing through the specimen to calculate a first isotope concentration from measured values thus acquired. From the measured values, information is acquired as to carbon isotope compositions of individual compounds in a gaseous hydrocarbon mixture. Methane, ethane, propane, isobutane or n-butane, isopentane or n-pentane, etc. can be listed as compounds of this kind. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention generally relates to the use of a laser to perform rapid isotope analysis, thereby obtaining information about the composition of a single substance or mixture. More specifically, the present invention relates to a novel method capable of quickly and easily measuring the relative amounts of isotopes in a mixture containing two or more isotopes of unknown amounts. Relates to the device. More specifically, in the present application, for example, but not limited to, carbon and hydrogen of individual compounds in hydrocarbon gas mixtures such as methane, ethane, propane, iso- and normal butane and isopentane and normal pentane. Describes a new process and a new apparatus for analyzing the isotopic composition of

Both academia and industry have energetically studied the stable isotope composition of various elements in petroleum. Elements that have been extensively studied include carbon, hydrogen, sulfur, nitrogen and oxygen. Isotopic compositions of individual components such as natural gas, crude oil, crude oil distillate and solids have been studied. Most became the subject of research is a carbon ⁽¹³ C / ¹² C), there is a great difference, but the following is a hydrogen (D / H).
Mass spectrometry

There are various methods for quantitatively or qualitatively measuring the isotopic composition. For example, certain types of isotope ratio measurements typically use a mass spectrometer specifically designed for that purpose. Such an instrument can measure the isotope ratio of a specific gas, such as H ₂ , N ₂ , CO ₂ ( ¹³ C / ¹² C ratio and ¹⁸ O / ¹⁶ O ratio), SO ₂ and SF ₆ . Therefore, when measuring isotope ratios in organic or inorganic compounds containing these elements, these compounds must first be quantitatively converted to an appropriate gas. To measure the ratio of HD / H ₂ and ¹³ C / ¹² C in an organic compound, the sample is burned (usually in the presence of high temperature CuO) to CO ₂ and water, and from CO ₂ It is necessary to separate the water at cryogenic temperatures and to convert the water on hot metal shavings (U, Zn, Cr or Mn). In some cases, hydrogen in an organic compound may be thermally decomposed on hot metal shavings (Cr or Mn) and converted directly to elemental hydrogen. The gas thus obtained can be analyzed using a mass spectrometer. In general, these steps require very laborious preparatory work, and expensive mass spectrometers specially designed for isotope ratio measurements must be used.

In particular, in the case of natural hydrocarbon gas, the gas mixture must first be separated into its individual compounds. Typically this is done by gas chromatography (GC) using helium as the carrier gas. The separated gases are flowed into a high temperature tubular furnace (900 ° C.) filled with copper oxide, where the individual gases are quantitatively converted to CO ₂ and water. Once the separated compounds emerge from the end of the tubular furnace, there are two methods for measuring the isotopic composition of those individual compounds.

In the first method, the old type method, the CO ₂ and water obtained from the individual compounds are introduced into individually separable cold traps where they are frozen. All of the components are collected and the cold trap is transferred to a purification system where helium is withdrawn to separate CO ₂ from the water at cryogenic temperatures. Then, after the trap pure CO ₂ obtained in the "transition pipe (transfer Tube)" cryogenic, transferred to a mass spectrometer (MS), measuring the ratio of ¹³ CO ^_2/12 CO _2. The water is transferred to a reducing tube where it is quantitatively converted to hydrogen gas, which is then analyzed with a mass spectrometer to obtain a ratio of HD / H ₂ . The whole process is laborious and only a few steps are automated. This mass spectrometer is typically “tuned” to analyze a CO ₂ sample. If a sufficient amount of water sample was obtained, the mass spectrometer were tuned for HD / H ₂ analysis. Attempting to tune to switch between CO ₂ and hydrogen requires at least half a day of interruption.

In the second method (referred to as GC IRMS-GC Isotope Ratio Mass Spectrometry), the system is tuned to analyze either CO ₂ or hydrogen. In the case of CO ₂ , the helium / CO ₂ / H ₂ O mixture is passed through a water trap at the end of the tube furnace and the resulting helium / CO ₂ mixture is then injected directly into the mass spectrometer to produce ¹³ CO _2. Quantify the ratio of ¹² CO ₂ . Although only requires a very small sample in this way, ¹³ CO ^_2/12 CO ₂ ratio time is very short (GC peak width) of the measurement, poor accuracy of the ratio for that. The time for complete analysis (methane to pentane) is determined by the time required for all these components to elute from the GC. To quantify the HD / H ₂ ratio, the water trap is removed and the copper oxide tube furnace is replaced with a pyrolysis tube furnace where the gas is converted to elemental carbon and hydrogen gas. Then, using an appropriately tuned MS, the hydrogen gas is analyzed in the same manner as for CO ₂ .

  This hydrogen isotope analysis is a relatively new method and is not yet commonly used. All mass spectrometers have a relatively narrow dynamic range, often narrower than the concentration range that occurs in natural gas mixtures. Therefore, in order to measure all the components using the GCIRMS apparatus, in many cases, the same sample is injected many times while changing the sample amount for each injection, and the concentration of each component is the dynamics of MS. Need to be in range.

  Furthermore, the mass spectrometer and the sample preparation system that accompanies it are expensive, heavy and large in size. Many glass and quartz parts are used for them, and high-purity helium and liquid nitrogen must be constantly flowing. Therefore, it is not easy to “harden” them for on-site operation. Also, the time required for complete compound analysis is long, and real-time data often required in field operations cannot be obtained.

Gas Chromatography Gas chromatographs that detect the presence of hydrocarbon gases encountered during drilling have been employed in drilling rigs for many years. However, because these devices are time consuming and inaccurate, it is not possible to obtain information on the hydrocarbon composition of the gas immediately. To date, there are no instruments that can be employed for rapid carbon and hydrogen isotope analysis at drilling rig sites, and so is the isotopic composition.

Isotope analysis using molecular vibration A method for measuring isotope ratio using quantum vibrational transition has existed for many years. The two main measuring methods are based on the emission spectrum and absorption spectrum. First, quantitative measurement of isotope ratio using the emission spectrum of molecular transition is inaccurate and cannot be used. At the same time, the absorption spectrum does not provide the resolution necessary to characterize molecular transitions that overlap but are isotopically different. This was mainly due to insufficient output at a monochromatic wavelength. With the advent of lasers in the late 1960s, the above limiting factors were eliminated and high-resolution characterization of polyatomic species with isotopic substitution became possible. With the completion of laser absorption studies in well-conditioned laboratory environments, quantitative luminescence studies have become useful. The main focus of laser isotope studies on short chain hydrocarbons, especially methane, was to characterize spectroscopic properties for astronomical purposes. These studies, for the first time, measured the carbon isotopic composition of methane in the extraterrestrial planet's atmosphere by luminescence detected near satellites in the late 1970s.

Laser isotope studies on carbon in methane and other short chain hydrocarbons continued from academic interest until the late 1980s. In the 1990s, there has been little scientific research in the field of carbon isotope measurement in hydrocarbon gases. Since the 1990s, academic interest has shifted to laser isotope studies of inorganic polyatomic molecules. To date, however, ¹³ C / ¹² C measurements have been put to practical use in medical research (measurement of carbon dioxide in breath) and sandstone / mudstone, pyrite, sphalerite, galena and calcite. Only in geological studies for the determination of inorganic characterization of water and carbon in it. In the analysis of CO ₂ in exhaled breath, the isotope ratio is measured using light emitted from a CO ₂ laser. It should be noted that to date, ¹³ C measurements for geological purposes have only been applied to carbonate rocks. Isotopes with inorganic bonds of sulfur, oxygen and hydrogen have also been studied using lasers for geological and environmental purposes.

Analysis in laser-based systems for pollution analysis Measuring the absolute concentration of trace levels (low ppb range) in atmospheric gases using laser technology has been the goal of recent academic and industrial research. Yes. Currently considered gases include H ₂ O, CO ₂ , CO, NO, NO ₂ , N ₂ O, SO ₂ , CH ₄ , C ₂ H ₂ , HF, HCl, HBr, HI, HCN, H _{2. S, O 3, NH 3,} H 2 CO, PH 2, O 2, OCS, C 2 H 6, C 3 H 8, C 4 H 10 there are such, studies of the gas above the C ₂ laboratory Limited to the environment. Among hydrocarbon gases, only CH ₄ has been studied for purely academic purposes with regard to isotopes. Although most research is conducted for environmental purposes, there is also interest in monitoring leaks from industrial processes by detecting these molecules, and commercial laser spectroscopy for that purpose. A vessel has been developed.

Field analysis in the field It is common to drill wells to extract hydrocarbons from deep underground. In many cases, there are various formations between the surface and the target formation. Some of these formations may contain hydrocarbons, which are preferred if the isotopic composition can be measured. In addition, it is often preferable to measure the isotopic composition of hydrocarbons in the target formation itself. Furthermore, in many cases, it is preferable if the isotope composition can be analyzed during production, as during excavation and after production. The environment of the downhole is very harsh, high temperature and high pressure, and the liquid and gas are often corrosive. The downhole and well site are harsh, exist in remote locations, and have space constraints, so it is difficult to devise equipment that can directly analyze the formation fluid.

  Historically, the instability of laser spectrometers has limited the usefulness of isotope measurements by laser spectroscopy, but it has a very high repetition rate and improved stability. A new laser system with it became available.

  Despite the existence of such techniques, there remains a need in the drilling context to analyze hydrocarbons very accurately in real time. It would also be desirable to have a system that can measure the structural position of isotopically substituted elements.

  The method and apparatus of the present invention can be advantageously used in various applications. Examples of their applications include, but are not limited to, analysis of formation fluids in the context of oil drilling operations, analysis of hydrocarbon streams in oil refining and other operations, and various biomaterials related to medical diagnosis and treatment. Such as accurate and precise measurement.

  The present invention includes a method and apparatus for quantifying carbon isotopes in a fluid. A preferred embodiment of the apparatus includes a laser light source that generates a laser beam, a beam splitter, a fluid volume, a downstream optical detector, a reference cell, and a second downstream optical detector. The beam splitter receives the laser beam and generates first and second beams. A first beam is passed through the volume of fluid. The downstream optical detector is arranged to detect the first beam after passing through the first volume. The reference cell contains a predetermined concentration of isotopes, through which the second beam is passed. The second downstream optical detector is arranged to detect the second beam after the second beam has passed through the reference cell. The first optical detector generates a first downstream signal corresponding to the intensity of the first beam after the first beam passes through the first volume. The second optical detector generates a second downstream signal corresponding to the intensity of the second beam after the second beam passes through the reference cell. The tool further includes a microprocessor. The microprocessor receives the first and second downstream signals and calculates parameters representing the presence of isotopes in the fluid from those signals.

  In a particularly preferred embodiment, the first fluid volume is placed in a cell or conduit. Among them, the fluid may be flowing or stationary. In this measuring instrument, the sample cell and the reference cell are configured to have substantially the same temperature, and the sample volume and the reference cell are located in the wells when the first and second beams pass, respectively. Alternatively, the sample volume and the reference cell may be located outside the well when the measurement beam passes through them. In yet another embodiment, the microprocessor calculates the isotope concentration in the fluid relative to the isotope concentration in the reference cell or quantitatively calculates the isotope concentration in the fluid. In yet another embodiment of the measuring instrument, the laser light source is a tunable laser light source. The apparatus of the present invention can also be used to measure the presence of carbon isotopes when there is a carbon isotope in part of the hydrocarbon.

  In yet another embodiment, the apparatus of the present invention includes a first upstream detector and a second upstream optical detector. The first upstream detector detects the first beam portion before the first beam portion passes through the first volume and generates a corresponding first upstream signal. The second upstream optical detector also detects the second beam portion before the second beam portion passes through the reference cell and generates a corresponding second upstream signal.

  In yet another embodiment, the microprocessor receives the upstream signal and uses it to calculate the aforementioned parameters representing the presence of isotopes in the fluid. In some embodiments, the downstream signal indicates the transmittance of the first and second measurement beams that pass through the sample and the reference cell, respectively.

  In other embodiments, the invention includes a method for providing real-time data indicative of the isotopic composition of a hydrocarbon fluid. The method includes: (a) introducing a reference fluid having a known isotope composition into the reference cell; (b) defining a sample of the hydrocarbon fluid; and (c) providing a laser beam. (D) splitting the beam into a reference measurement beam and a sample measurement beam; and (e) passing the reference measurement beam through the reference fluid and measuring the reference measurement beam after passing through the reference fluid. (F) passing the sample measurement beam through the sample, measuring the sample measurement beam after passing through the sample, and (g) using the measurement values obtained in steps (e) and (f) Calculating parameters representing the presence of isotopes in the fluid.

  In a particularly preferred embodiment of the method, step (b) comprises introducing a sample of hydrocarbon fluid into the cell or conduit. During the measurement, the fluid in the conduit may be flowing or stationary. The microprocessor in step (g) can calculate the isotope concentration in the fluid relative to the isotope concentration in the reference cell or the quantitative isotope concentration in the fluid. By using this measurement, it is possible to obtain information about the carbon isotopic composition of individual compounds in a hydrocarbon gas mixture, especially regarding the presence of methane, ethane, propane, isobutane and normal butane and isopentane and normal pentane. Information can be obtained.

  In yet another embodiment, the method of the present invention can be performed as follows. That is, steps (e) and (f) can be performed at substantially the same temperature, or can be performed in a well, or a sample is taken from a well, but steps (e) and (f) ) Can be carried out outside the well.

  One advance in laser spectroscopy is the emergence of mid-infrared and far-infrared tunable diode lasers. It has been developed and used primarily since the early 1990s in the telecommunications industry. These wavelengths overlap with many molecular vibrational absorption regions of carbon-hydrogen bonds in hydrocarbons and can be used to detect such molecules.

  Other advances include the improvement of these lasers for ultrashort pulses close to a single wavefront (femtosecond pulse). Recently, the development of high-speed firing performance for diode lasers has made it possible to fire thousands of times per second over a long period of time. New electronic methods have also been developed to stabilize the laser output internally and externally. Combining these advances, it is now possible to analyze steady state fluid systems quickly and with high resolution.

  According to the present invention, laser spectroscopy is facilitated by a new detector and output stabilization technique, and the molecular components of a fluid system are isotopically detected by monitoring rotational vibration transitions of a target chemical species in real time. be able to. The present invention is particularly useful in the petroleum industry because it provides a means for detecting carbon and hydrogen isotope compositions with an accuracy equivalent to or better than current industry standards. In addition to being able to measure in situ in real time, the laser technology of the present invention has also made it possible to measure the position within the structure of isotopically substituted elements.

  In a preferred embodiment, signal stabilization for precise quantification of the sample is achieved by splitting the direct output of one laser beam into multiple beams of predetermined intensity. As will be described in detail below, while one beam passes through the sample cell to the first detector, another beam directly reaches a similar second detector, or a reference cell Pass through to reach a similar third detector. Differences between detected beam intensities can be converted to absolute or relative concentrations of molecular species or to absolute or relative concentrations of isotope species.

  The profile of rotational vibration is determined by the spectral constant of the molecular state. One of the physical properties on which these constants depend is the mass of the molecule. Therefore, different absorption wavelengths are observed for chemical species that are isotopically different in the same elemental and structural composition. Different absorption properties are also observed for isotopically different species.

The system of the present invention can determine the isotopic composition of individual hydrocarbons in a fluid, which depends on the presence or absence of substitution of heavy molecules ( ¹³ C and D) in the molecule. Take advantage of the different nature. The instrument and process of the present invention do not require any manual sample preparation. The measurement can be carried out directly on the gas of interest, and it does not need to be first converted quantitatively to CO ₂ and / or H ₂ , and other hydrocarbons and non-hydrocarbons from the mixture There is no need to remove the gas. In a preferred embodiment, the instrument and process of the present invention allowed the ¹³ C / ¹² C ratio and / or the D / H ratio to be measured simultaneously for all relevant hydrocarbon components in the gas mixture. . Information regarding the position of isotope substitution elements in molecules larger than C ₂ is lost in the combustion method, but in some cases it can be obtained using this technique.

  By using a reference cell, the present invention allows the absolute concentrations of both isotope species to be quantified individually or their relative concentrations to be more accurately known. This can be done by detecting individual rotational vibration absorption lines (not necessarily the same quantum transitions) separately from chemical species that are chemically identical but isotopically different. According to Beer's law, the ratio of the absorption intensity of each line is proportional to the concentration ratio of the species and the multiplication constant associated with the different absorption characteristics of the two quantum transitions. By using a reference cell containing a gas of known isotope composition, it is not necessary to know the Beer's Law absorption constant because the constant is the same for any transition detected in the sample and reference. Because. By using the reference cell, it is possible to accurately measure whether there are more or fewer isotopes than the reference gas. In this case, it is not necessary to know the absolute ratio of reference gas isotopes. In order to make a more accurate measurement, it is only necessary to measure the entire rotational profile or multiple rotational vibration transition of the vibration transition.

  Referring to the drawings, a preferred embodiment of the system of the present invention includes an optical measurement system 10 and a sample processing system 11. The optical measurement system 10 preferably includes a tunable laser light source 10, a reference cell 14, a sample cell 12 and a pre-dilution cell cell 16. The reference cell 14 has a pair of optical detectors 42 and 44 provided at its inlet and outlet, respectively. Similarly, the sample cell 12 has optical detectors 52 and 54 and the predilution cell 16 has optical detectors 62 and 64. The optical detectors 42, 44, 52, 54, 62, 64 preferably include a beam splitter and a sensor for measuring the amount of incident light as necessary, which are well known to those skilled in the art. belongs to. Sample cell 12 may be either a stationary or steady state system. In some cases, the sample cell 12 may actually be inserted into the laser cavity or omitted.

  The gas-containing volume through which the laser beam (s) is passed is referred to herein as a “cell”, but it is understood that the cell does not necessarily have to be sealed or have a constant volume. I want to be. For example, the light used to measure the presence of an isotope can be passed through a conduit or pipe through which the substance to be measured flows continuously. Similarly, a measurement beam can pass through a sample volume that is not contained in a fixed volume, for example, it can pass a measurement beam through a gas-containing space to cause gas in the space to pass. This is the case when measuring.

  For each cell of interest or other volume, the amount of light absorbed by the material therein can be measured by comparing measurements at the cell inlet and outlet detectors. In another embodiment (not shown), it is also possible to measure by directing the entrance and exit beams of each cell all to a single optical measurement device. With the advent of femtosecond lasers and detectors, it has become possible to distinguish and receive individual signals by a single measuring device by using the travel distance and signal arrival time.

  Returning to the figure, the sample processing system 11 preferably includes a sample inlet 92, a pre-dilution line 94, a sample line 96, a dilution line 98 and a dilution source 100. Inlet 92 receives fluid for analysis with this device. In the predilution line 94, fluid is connected between the inlet 92 and the predilution cell 16. In the sample line 96, fluid is connected between the inlet 92 and the sample cell 12. In the dilution line 98, fluid is connected between the dilution source 100 and the sample line 96. The sample processing system 11 is also preferably provided with vacuum lines 97 and 99 to empty the predilution cell 16 and the sample cell 12, respectively.

  Although the flow to the apparatus is preferably continuous, it does not necessarily have to be continuous. Furthermore, it is preferable to provide one or more optical boxes 21 in the path of one or more beams. This optical box is well known to those skilled in the art, but can be used to expand and contract the beam and enlarge (or reduce) its diameter, thereby reducing (or increasing) intensity. .

  In operation, a fluid sample enters from the inlet 92, a portion of which branches off and enters the predilution cell 16. Detectors 62 and 64 are used for optical measurement of the fluid and information about the undiluted sample is sent to a microprocessor (not shown). Accordingly, the microprocessor calculates the necessity and degree of dilution of the fluid. As the pressure increases, the width of the transition profile of the measured compound increases, so the sample can be diluted to reduce the partial pressure of the compound being measured and thereby improve the resolution of the resulting measurement. It is necessary or preferable to do. This is particularly useful when the broadening of the line is large and the peak of the compound of interest cannot be distinguished unless the resolution is improved.

If it is preferred to dilute the sample, the microprocessor will act on the diluent source 100 to deliver a predetermined amount of diluent to the sample line 96 at a predetermined rate. Nitrogen (N ₂ ) is a particularly preferred diluent. Other diluents include but are not limited to argon, boron or other noble gases. Diluent and sample are mixed in the sample line 96 and the diluted sample flows into the sample cell 12. Since the purpose is to reduce the partial pressure of one or more components in the sample, an alternative to dilution is to expand the gas in the sample and thereby reduce the total pressure, and accordingly There is also a way to reduce the partial pressure of all components.

  The laser source 10 preferably generates a light beam 20 having a short duration, preferably from about 2 to about 10 nanoseconds (nsec). A series of beam splitters 32 and 34 split the beam 20 into three beams 22, 24 and 26, which are split into the reference cell 14, sample cell 12 and predilution cell 16, respectively. send. As used herein, beam splitters 32 and 34 may be any device that can split beams, including multiplexing. The beam for measuring the pre-diluted sample does not necessarily have to be separated from the beam used to measure the sample or reference. Part of the light incident on each cell is absorbed by the material inside it. The amount of light absorbed by the reference cell and the sample cell is measured and the amounts are compared. The ratio of absorption in the reference cell and sample cell can be easily and directly related to the isotope composition of the reference fluid and sample fluid.

  In another embodiment, the beam is not split. Instead, a single beam or part of it is passed through both the reference cell and the sample cell. Alternatively, the fluid volumes can be arranged such that each fluid volume hits a separate or separate portion of the beam. Measure and quantify the effect of each cell on the light passing through that cell, regardless of whether the beam is split and regardless of which cell the beam first passes through. Can be used to generate information regarding the presence of isotopes in the sample cell.

The apparatus according to the present invention is sufficiently robust to be used in the field or in the laboratory and is small and preferably has a size of less than about 25 cubic feet (0.7 m ³ ). If it does so, it can replace with the isotope analyzer of the natural gas conventionally used. An important use of the present invention is in drilling rigs exploring oil and natural gas. In that case, there is no need to collect samples or send them to the laboratory for analysis. Nevertheless, the apparatus of the present invention can be advantageously used in many other applications, and examples of such applications include, but are not limited to, environmental measurements, pipelines and transportation. Management, identification of discharged pollutants and “fingerprint method”. By using a laser and eliminating the need for expensive, complex and bulky preparation systems, the tuning time for analysis can be reduced from hours to seconds. In addition, this makes it possible to make decisions about various excavations efficiently because information can be obtained substantially immediately. For applications where sample density and sample size are an issue, the device of the present invention can easily collect large data sets, which can be used to distinguish naturally occurring natural gas from man-made contamination. In addition, it is possible to specify the contamination source in detail.

  This instrument has a very wide dynamic range of detection, so it is not necessary to concentrate the gas of interest and can analyze extremely lean hydrocarbon gases. Another advantage of the system of the present invention is that it has a built-in dilution system, which further increases the range of concentrations that can be analyzed. The reason is that because a very dilute reference volume can be used, a wide range of sampled fluids need only be diluted and reduced to that concentration level.

  The present invention uses a combination of high-sensitivity detectors and detection methods, precision electronic gating systems, computer technology and algorithms, so laser spectroscopy can be used with any simple molecule, especially short chain hydrocarbons. Rapid isotope analysis. A detector system using an algorithm having a self-correcting function is commercially available and can be suitably used in the system of the present invention. Other new technologies can also promote the practical application of the system of the present invention. For example, smaller time gates, more stable lasers, more stable time gating equipment, more powerful microprocessors, and so on, using fewer samples and faster sample effects compared to previously possible methods Analysis.

  Using two main laser techniques, simple isotope analysis and absolute quantification of molecules are possible. One detects laser induced fluorescence (LIF) and the other detects changes (absorption) in laser beam intensity. LIF has higher sensitivity but is difficult to use as a quantitative method. The reason is that multiple molecular quantum states must be evaluated simultaneously while maintaining high resolution. In addition, it is difficult to use the reference cell from a technical aspect. Accordingly, the system of the present invention will be described in the context of an absorption system.

  The laser used in the present invention may be a continuous wave or a pulse wave. Semiconductor diode lasers are the most common for industrial use and are therefore readily available, but gas lasers, excimer lasers and dye lasers have also been used. The laser is selected in consideration of characteristics such as output, polarization, coherence, bandwidth, stability, Q-switch repetition rate, wavelength region, and maintenance labor. A semiconductor diode laser is preferred because it requires the least maintenance. They are most attractive for industrial use due to their high output performance, narrow bandwidth and high stability. Also for the apparatus of the present invention, a laser having a high repetition rate (about 10 KHz), a stability between shots of 3% or less, and a wavelength variable in the absorption band of the hydrocarbon fundamental vibration mode is most preferable. Multi-beam lasers can be used if the combined wavelength can cover the entire wavelength range of interest. Although tunable infrared diode lasers meet common standards, they are the subject of the design of the present invention, but this design is applicable to existing lasers that meet these standards, or any laser that will appear in the future. is there.

  For gas detection, a single line of vibration transition (rotational vibration) is selected for monitoring. The laser wavelength is set to the energy of the transition. The amount of light absorbed by a chemical species at a particular wavelength is proportional to the concentration of that chemical species, the distance traveled by the emitted light through the absorbing medium, and the absorption constant (Beer's law). This relationship holds for chemical species that have high resolution spectral lines, no non-linear interference, and no effect of widening the line width below the laser saturation level for the system. Since the chemical species absorbs the emitted light, its concentration can be calculated from the amount of emitted radiation transmitted, the distance that the emitted light travels through the medium, and the absorption constant of a particular transition at that wavelength. Great efforts have been made to design laser spectroscopy systems such that Beer's law or its guidance law holds.

  The conditions that have the greatest impact on Beer's law application in quantifying concentration by EM synchrotron absorption are temperature and pressure. Beer's law measures the concentration difference of a chemical species between two specific quantum states. It can be assumed that the concentration difference between the quantum states is proportional to the concentration difference of the chemical species as a whole. The population in the vibrational state and the rotational state (a state mainly measured by absorption of infrared radiation) varies with temperature. Therefore, when applying Beer's law directly by external correction, the complete set of spectroscopic constants for the quantum state in question must be determined in advance or experimentally corrected for temperature. Either the correction curve must be derived or it must be calibrated at the same temperature as the sample. The last approach is used in the present invention. Temperature also has a higher order effect that can be ignored under normal operating conditions. One of the higher order effects is the Doppler width of the transition. The Doppler width increases with temperature. It is desirable to keep the sample at the natural Doppler limit, so that the measurement is not unnecessarily complicated. Therefore, it is desirable to keep the temperature of the sample as constant as possible in order to avoid errors due to changes in the Doppler width.

  The pressure has a great effect on increasing the width of the transition profile. For this reason, for concentration measurements according to Beer's law, a complete set of spectroscopic constants must be determined in advance, an experimental correction curve must be derived, or between the sample and the calibration set. It is necessary to keep the pressure at the same pressure condition. Also in this case, the latter is often adopted. Higher order pressure effects are often ignored in normal operation. Given the assumption that the vibrational population does not change significantly with temperature, the entire rotational vibration profile of the calibration set and the sample can be measured without the need for temperature or pressure correction. However, this process is not practical in real-time scenarios because it requires too much tuning.

  The present invention does not require complete spectral detection, nor does it require correction by splitting the direct output of the laser and applying one of the remaining beams to an additional reference cell. Under these conditions, the only requirement is that the temperature and total pressure in the reference cell must be the same as that in the sample cell. The underlying assumption is that the absorption ratio of the sample to the reference species at a monochromatic wavelength is proportional to the concentration ratio of the sample to the reference. This is in accordance with Beer's law. This is similar to a one-point external correction, but provides several advantages for simultaneous measurements. One advantage is that quantification errors due to slight changes in the laser wavelength are canceled out because the absorption coefficient of the bale changes simultaneously due to the simultaneous measurement of the sample and reference. In order to perform absolute concentration measurement, it is necessary to know the distance that light passes through the sample cell and the reference cell and the absorption constant. In reality, however, the two constants are usually combined and determined experimentally by replacing a known standard with the positions of the sample cell and the reference cell. As long as one or more standards are used for standardization, absolute and relative concentration measurements can be made with this device.

  In an alternative embodiment, the invention can be implemented without a reference cell, but is less preferred. In this embodiment, an external calibration curve is determined for concentration, temperature and pressure. Certain non-linear temperature effects can be counteracted separately by measuring the overall rotational vibration transition, as described above.

  Pressure changes the spectral nature of certain transitions. In particular, the transition wavelength varies with pressure, as does the transition bandwidth. Again, this bandwidth can be explained by measuring the entire spectral profile of the rotational vibration transition, but for this application it is too time consuming and impractical.

  There are more practical assumptions about the pressure effect, which can greatly simplify device design. It concerns the Doppler line width limit. This is the temperature-dependent limit for minimizing the line width expansion of the quantum transition due to pressure. The reason why the line profile becomes wide is that molecules and atoms of chemical species collide with each other. Therefore, if the number of collisions is reduced, the line width becomes narrower. In practice, collisions disrupt and mitigate quantum states in which chemical species are present. Thus, not all quantum states have the same Doppler limit and are not affected by collisions in the same species. The limit at which the line width narrows at a certain temperature is the Doppler limit.

  Not all species collide in the same way, nor does it have the effect of expanding the line profile to the same extent as other species. In general, however, lowering the absolute pressure of the system reduces the number of collisions, thereby narrowing the line profile of the chemical species. Similarly, if the pressure of a chemical species that tends to widen the line width is increased while keeping the total pressure constant, the line profile is also narrowed if the pressure of a chemical species that tends to widen the line width is increased. . This last mentioned technique is called buffering. In the case of a compressible fluid, the chemical species used to narrow the line profile while keeping the total pressure constant is often called a buffer gas. Some buffer gases have the ability to reduce the line width to the Doppler limit at any pressure. In some cases, it is most effective to combine buffer action and reduced pressure. There is some evidence that nitrogen is an effective buffer gas, at least in the case of methane, and that nitrogen can arbitrarily determine the pressure of methane gas. The same information has not been reported for other gases of interest. In any case, if the total pressure is lowered, the line profile for the analytical measurement of the gas becomes narrower.

  In order to reduce the line width of the transition of interest to be lower than the Doppler line limit width, either introducing a buffer gas or lowering the total pressure of the system, both the predilution of the preferred embodiment It is a stage design. This is necessary for two reasons. First, line broadening is a non-linear effect that affects the application of Beer's law. In contrast to introducing a pressure correction factor or making the unknown partial pressure the same as the reference cell, it is easiest to experimentally ensure that the system is below the Doppler limit. . Secondly, due to the molecular transition of the bond between the normal element and the isotope element compared to the molecular transition of the bond between the normal element and the isotope element in the chemically same chemical species. The line will shift. However, this shift is very small compared to the normal range where the line width increases with pressure. Therefore, if the line profile for the transition in question is too wide, it will overlap the isotopically shifted line, breaking the Beel's law condition that the transition is spectrally separated. End up. Line shifts have little effect and can usually be ignored.

Disclosure of the present invention is dealing with analysis of short-chain hydrocarbons, the technique of the present invention can also be applied to larger hydrocarbons and other molecules than C _4. However, as the size of the hydrocarbon increases, the original line width overlaps with the isotope shift transition. In addition, the vibrational structure of the spectrum becomes more and more complex. Thus, for species with increased molecular size, it becomes increasingly difficult to find an isotopically shifted molecular line with good resolution. Without experimentation, it is difficult to determine practical limits on the size of hydrocarbons that this technique is capable of. If further research continues, it will be possible to use this instrument to determine hydrocarbon isotope ratios greater than C ₅ while maintaining information on the substituted carbon. This information has not been collected so far, and applying this new type of data would be extremely useful for the oil industry. However, such devices must be redesigned, for example, to increase the temperature in the sample chamber or significantly reduce the pressure in order to force less volatile hydrocarbons into the gas phase. It must be certain.

Thus, enumerating the possible uses of the system of the present invention include, but are not limited to, drilling rigs that encounter natural gas while drilling, such as environmental tests such as marsh. For example, it may be desirable to detect leaks from gas transport lines and obtain information about the isotopic composition of the fluid almost immediately. In addition, the ability to locate the isotope substitution element in the C ₃₊ molecule will open up new avenues for research on the origin, thermal maturity and mixing of subsurface natural gas. .

1 is a schematic diagram of an apparatus constructed in accordance with the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser light source 12 Sample cell 14 Reference cell 16 Predilution cell 42,44 Optical detector 52,54 Optical detector 62,64 Optical detector

Claims (52)

A measuring instrument for measuring the amount of carbon isotopes in a fluid,
A laser light source for generating at least one laser beam;
A first fluid volume;
A first downstream optical detector;
A reference cell containing a predetermined concentration of isotopes;
A second downstream optical detector;
A microprocessor;
And the first fluid volume is arranged such that at least a first portion of the laser beam passes through the fluid,
The first downstream optical detector is arranged to detect the first beam portion after the first beam portion has passed through the first volume, and the first downstream optical detector is the first downstream optical detector. Generating a first downstream signal corresponding to the intensity of the first beam portion after passing through the volume of
The reference cell is arranged such that at least a second portion of the laser beam passes through a predetermined concentration of isotopes;
The second downstream optical detector is arranged to detect the second beam portion after the second beam portion passes through the reference cell, and the second downstream optical detector passes through the reference cell. Generating a second downstream signal corresponding to the intensity of the second beam portion after
The microprocessor receives the first and second downstream signals, calculates a parameter indicative of the presence of an isotope in the fluid from those signals, and the measurement has an isotope increment or decrement vessel.   The measuring instrument of claim 1, wherein the first and second beam portions comprise a single beam.   The measuring instrument according to claim 1, wherein the first and second beam portions comprise separate beams.   The meter of claim 1, wherein the first fluid volume is contained within the cell.   The meter of claim 1, wherein the first fluid volume is contained within a conduit.   6. A meter according to claim 5, wherein the conduit contains a flowing fluid.   The instrument of claim 1, wherein the microprocessor calculates the isotope concentration in the fluid relative to the isotope concentration in the reference cell.   The measuring instrument according to claim 1, wherein the microprocessor quantitatively calculates the isotope concentration in the fluid.   The measuring instrument according to claim 1, wherein the laser light source is a wavelength-tunable laser light source.   The measuring instrument according to claim 1, wherein the carbon isotope is part of a hydrocarbon.   The measuring instrument of claim 1, wherein the first volume and the reference cell are at substantially the same temperature.   The measuring instrument according to claim 1, wherein the first volume and the reference cell are in the well when the first and second beam portions pass through the first volume and the reference cell.   The measuring instrument according to claim 1, wherein the reference cell contains an isotope of unknown concentration. A first upstream detector;
A second upstream optical detector;
The first upstream detector detects the first beam portion before the first beam portion passes through the first volume, generates a corresponding first upstream signal, and The measurement of claim 1, wherein the second upstream optical detector detects the second beam portion before the second beam portion passes through the reference cell and generates a corresponding second upstream signal. vessel.   The microprocessor of claim 14, wherein the microprocessor receives the first and second upstream signals and uses the first and second upstream signals to calculate a parameter representative of the presence of an isotope in the fluid. Measuring instrument.   The measuring instrument of claim 1, wherein the first and second downstream signals represent the transmittance of the first and second beam portions that have passed through the sample and reference cell, respectively. A method for obtaining real-time data representing the isotopic composition of a hydrocarbon fluid, comprising:
(A) introducing a reference fluid having an isotopic composition into the reference cell;
(B) limiting the sample of hydrocarbon fluid;
(C) providing at least one laser beam;
(D) normalizing at least one parameter of the reference cell;
(E) passing at least a portion of the laser beam as a reference measurement beam through the reference fluid and measuring the reference measurement beam after passing through the reference fluid;
(F) passing at least a portion of the laser beam as a sample measurement beam through the sample and measuring the sample measurement beam after passing through the sample;
(G) calculating parameters representing the presence of isotopes in the fluid using the measurements obtained in steps (e) and (f);
Having a method.   The method of claim 17, wherein the reference measurement beam and the sample measurement beam comprise a single beam.   The method of claim 17, further comprising splitting the laser beam to form a reference measurement beam and a sample measurement beam, wherein the reference measurement beam and the sample measurement beam are formed to have separate beams. .   The method of claim 17, wherein step (b) comprises placing a sample of the hydrocarbon fluid in the sample cell.   21. The method of claim 20, wherein step (b) further comprises normalizing at least one parameter of the reference cell to at least one parameter of the sample cell.   The method of claim 17, wherein step (b) comprises providing a conduit for flowing hydrocarbon fluid.   18. The method of claim 17, wherein step (f) is performed while the hydrocarbon fluid is flowing through the conduit.   18. The method of claim 17, wherein in step (g), the microprocessor calculates the isotope concentration in the fluid relative to the isotope concentration in the reference cell.   18. The method of claim 17, wherein in step (g), the microprocessor quantitatively calculates the isotope concentration in the fluid.   18. The method of claim 17, wherein the measurement provides information regarding the carbon isotope composition of individual compounds in the hydrocarbon gas mixture.   27. The method of claim 26, wherein the individual compounds are selected from the group consisting of methane, ethane, propane, isobutane and normal butane, and isopentane and normal pentane.   The method according to claim 17, wherein the laser beam is obtained from a tunable laser light source.   The method of claim 17, wherein steps (e) and (f) are carried out at substantially the same temperature.   18. The method of claim 17, wherein step (b) is performed in a well and steps (e) and (f) are not performed in a well.   The method of claim 17, wherein at least one of the first and second beams passes through an upstream detector before passing through the cell.   18. The method of claim 17, wherein step (g) is performed using the transmittance of the reference measurement beam and the sample measurement beam that pass through the reference cell and the sample cell. A measuring instrument for measuring the amount of carbon isotopes in a fluid,
A laser light source for generating at least one laser beam;
A first fluid volume;
A first downstream optical detector;
A reference cell containing a predetermined concentration of isotopes;
A second downstream optical detector;
A pre-dilution cell;
A third downstream optical detector;
A microprocessor;
And the first fluid volume is arranged such that at least a first portion of the laser beam passes through the fluid,
The first downstream optical detector is arranged to detect the first beam portion after the first beam portion has passed through the first volume, and the first downstream optical detector is the first downstream optical detector. Generating a first downstream signal corresponding to the intensity of the first beam portion after passing through the volume of
The reference cell is arranged such that at least a second portion of the laser beam passes through a predetermined concentration of isotopes;
The second downstream optical detector is arranged to detect the second beam portion after the second beam portion passes through the reference cell, and the second downstream optical detector passes through the reference cell. Generating a second downstream signal corresponding to the intensity of the second beam portion after
The predilution cell includes a portion of the fluid and is arranged such that at least a third portion of the laser beam passes through that portion of the fluid;
The third downstream optical detector is arranged to detect the third beam portion after the third beam portion has passed through the pre-dilution cell, and the third downstream optical detector includes the pre-dilution cell. Generating a third downstream signal corresponding to the intensity of the third beam portion after passing;
The microprocessor receives the first, second and third downstream signals, calculates parameters representing the presence of isotopes in the fluid from the first and second downstream signals, and the microprocessor A meter that determines from 3 downstream signals whether and to what extent the fluid is diluted with diluent.   34. A measuring instrument according to claim 33, wherein the first and second beam portions comprise a single beam.   34. A measuring instrument according to claim 33, wherein the first and second beam portions comprise separate beams.   34. The meter of claim 33, wherein the first fluid volume is contained within the cell.   34. The meter of claim 33, wherein the first fluid volume is contained within the conduit.   34. The instrument of claim 33, wherein the microprocessor calculates the isotope concentration in the fluid relative to the isotope concentration in the reference cell.   34. The measuring device according to claim 33, wherein the microprocessor calculates the isotope concentration in the fluid quantitatively.   34. The measuring device according to claim 33, wherein the reference cell has an isotope of unknown concentration. A first upstream detector;
A second upstream optical detector;
A third upstream optical detector;
And the first upstream detector detects the first beam portion before the first beam portion passes the first volume, generates a corresponding first upstream signal, and The second upstream optical detector detects the second beam portion before the second beam portion passes through the reference cell, generates a corresponding second upstream signal, and a third upstream optical detector; 34. The measurement device of claim 33, wherein the detector detects the third beam portion and generates a corresponding third upstream signal before the third beam portion passes through the predilution cell.   A microprocessor receives the first, second and third upstream signals and uses the first and second upstream signals to calculate a parameter representing the presence of an isotope in the fluid; and the microprocessor 42. The instrument of claim 41, wherein the method uses a third upstream signal to determine whether and to what extent the fluid is diluted with a diluent.   34. The measuring device according to claim 33, wherein the diluent has nitrogen.   34. The meter according to claim 33, wherein the diluent comprises at least one of nitrogen and at least one noble gas. A measuring instrument for measuring the amount of carbon isotopes in a fluid,
A laser light source for generating at least one laser beam;
A first fluid volume;
A first upstream optical detector,
A first downstream optical detector;
A reference cell containing a predetermined concentration of isotopes;
A second upstream optical detector,
A second downstream optical detector;
A microprocessor;
And the first fluid volume is arranged such that at least a first portion of the laser beam passes through the fluid,
The first upstream optical detector is arranged to detect the first beam portion before the first beam portion passes through the first volume, and the first upstream optical detector is the first upstream optical detector. Generating a first upstream signal corresponding to the intensity of the first beam portion before passing through the volume of
The first downstream optical detector is arranged to detect the first beam portion after the first beam portion has passed through the first volume, and the first downstream optical detector is the first downstream optical detector. Generating a first downstream signal corresponding to the intensity of the first beam portion after passing through the volume of
The reference cell is arranged such that at least a second portion of the laser beam passes through a predetermined concentration of isotopes;
The second upstream optical detector is arranged to detect the second beam portion before the second beam portion passes through the reference cell, and the second upstream optical detector passes through the reference cell. Generating a second upstream signal corresponding to the intensity of the second beam portion before
The second downstream optical detector is arranged to detect the second beam portion after the second beam portion passes through the reference cell, and the second downstream optical detector passes through the reference cell. Generating a second downstream signal corresponding to the intensity of the second beam portion after
A microprocessor that receives the first and second upstream and downstream signals and calculates parameters representing the presence of isotopes in the fluid from those signals.   46. The measuring device of claim 45, wherein the first and second beam portions comprise a single beam.   46. The measuring device of claim 45, wherein the first and second beam portions comprise separate beams.   46. The meter of claim 45, wherein the first fluid volume is contained within the cell.   46. The meter of claim 45, wherein the first fluid volume is contained within the conduit.   46. The measuring device of claim 45, wherein the microprocessor calculates the isotope concentration in the fluid relative to the isotope concentration in the reference cell.   46. The measuring device of claim 45, wherein the microprocessor calculates the isotope concentration in the fluid quantitatively. 46. The measuring device of claim 45, wherein the reference cell has an isotope of unknown concentration.

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