Optimised method for detecting the formation gas hydrates

Abstract

The present invention relates to a method for detecting the presence of gas hydrates and/or ice in a medium. The method comprises at least the following steps: measuring at least at one measurement point in said medium two characteristic values of Raman spectra corresponding to two distinct vibration modes of the OH bonds of water, and determining the ratio of said two characteristic values, determining the temperature T in said medium at said measurement point of said spectra, comparing ratio with a value .sub.0 corresponding to a predetermined threshold of formation of said crystals for said temperature T, and determining the presence or not of hydrate and/or ice crystals from said comparison.

Claims

1. A method for detecting the presence of gas hydrates and/or ice in a water-containing medium likely to form solid crystals, the method comprising: measuring, at least at one measurement point in the medium, two characteristic values of Raman spectra corresponding to two distinct vibration modes of the OH bonds of water, a first mode of the two distinct vibration modes having a wavenumber at 3160 cm.sup.140 cm.sup.1 and a second mode of the two distinct vibration modes having a wavenumber at 3400 cm.sup.1150 cm.sup.1, and determining a ratio of the two characteristic values, determining a temperature T in the medium at the measurement point of the two characteristic values of Raman spectra, comparing the ratio with a value .sub.0 corresponding to a predetermined threshold of formation of the crystals for the temperature T, determining the presence or not of hydrate and/or ice crystals from the comparison of the ratio with the value .sub.0, and distinguishing between the presence of gas hydrates and ice by comparing the temperature T in the medium at the measurement point with an ice formation temperature.

2. The method as claimed in claim 1, wherein the two characteristic values of Raman spectra correspond to intensities of the two vibration modes, or to values directly related to the intensities of the two vibration modes.

3. The method as claimed in claim 1, further comprising varying the temperature in the vicinity of the measurement point of the two characteristic values of Raman spectra so as to anticipate the formation of solid crystals.

4. The method as claimed in claim 1, wherein the presence of hydrate crystals is deduced if the ratio is greater than a calibration value .sub.0 and if the temperature T in the medium at the measurement point is higher than an ice formation temperature Tf under measurement conditions.

5. The method as claimed in claim 4 wherein, in case of presence of hydrates, an anti-hydrate additive is injected into the medium.

6. The method as claimed in claim 1, wherein the two characteristic values of Raman spectra correspond to integrals of the Raman spectra centered on the vibration modes.

7. The method as claimed in claim 1, further comprising varying the temperature in the vicinity of the measurement point of the two characteristic values of Raman spectra so as to anticipate the formation of hydrates.

8. The method as claimed in claim 1, further comprising cooling the medium in the vicinity of the measurement point to anticipate formation of hydrates and/or ice.

9. The method of claim 8, further comprising detecting the presence of hydrates and/or ice at the measurement point after cooling the medium in the vicinity of the measurement point, and injecting an anti-hydrate additive into the water-containing medium when the presence hydrates and/or ice is detected.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other features and advantages of the present invention will be clear from reading the description hereafter of embodiments given by way of non-limitative examples, with reference to the accompanying figures wherein:

(2) FIG. 1 shows Raman spectra as a function of the temperature of the medium considered, and

(3) FIG. 2 shows the evolution of ratio of the Raman intensity of two different vibration modes of the OH bonds.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present invention relates to a method for detecting the appearance of hydrate crystals, and more generally of solid crystals in a water-containing medium, by coupling characteristic values obtained from Raman spectrometry in the spectral range of the vibration modes of OH bonds and from a temperature sensor. Solid crystals are understood to be gas hydrate crystals and/or ice crystals. Detection of the vibration modes of the OH bonds allows to qualify the OH bonds present in liquid water, in ice and/or in hydrates. It is thus possible to determine whether ice and/or hydrates have formed.

(5) It is reminded that Raman spectrometry is an optical method of observing and characterizing the molecular composition and the external structure of a material. Raman spectrometry exploits the physical phenomenon according to which a medium slightly modifies the frequency of the light circulating therein. Raman spectroscopy consists in sending a monochromatic light onto the sample and in analyzing the scattered light. The information obtained by measuring and analyzing this shift makes it possible to trace certain properties of the medium, by spectroscopy.

(6) The signal from the Raman spectrometer is transmitted to the medium by a probe known as Raman probe. The Raman probe also allows to lead the signal from the measurement point to the spectrometer. Advantageously, the Raman probe can be immersed in the water-containing medium. The Raman probe immersed in the water-containing medium can come in form of a cylindrical steel tube connected to two optical fibers, the outward fiber (or first fiber) leading the signal from the laser source to the measurement point and the return fiber (or second fiber) leading the Raman signal from the measurement point to the spectrometer.

(7) The immersed end of the probe consists of a window, generally made of sapphire, allowing light rays to pass.

(8) This end is directly immersed in the medium to be analyzed, thus enabling in-situ analysis. The immersed probe(s) can be arranged at different points of the unit, depending on the objective pursued.

(9) According to an embodiment of the invention, the Raman spectrometer used can be a dispersive Raman spectrometer with an excitation laser wavelength below 785 nm (a frequency-doubled Nd-YAG for example (=532 nm)), a toric input mirror (improving image quality on the detector by correcting optical aberrations, in particular astigmatism) and a CCD detector. Selection of the laser and of the detector is conditioned by the search for optimum conditions in terms of signal-to-noise ratio in the spectral range of the vibration modes of OH bonds.

(10) Near to the point of the unit where the Raman spectrum is measured, a temperature sensor (a thermocouple for example, or a third optical fiber allowing to offset the sensor, or any other temperature measuring means) can be installed so as to simultaneously have the Raman spectrum and the temperature of the sample zone. Thus, each measurement point of the Raman spectroscopy is associated with a temperature measurement in the vicinity of the measurement point, allowing to measure the temperature of the fluid at least in the vicinity of this measurement point.

(11) Alternatively, the temperature can be known by any other means, for example measurement at another point, conditions imposed on the medium, etc.

(12) Both data (Raman spectrum and temperature) can be sent to analysis means, notably computer means (a PC for example) controlling the analytical chain, for exploitation of these measurements.

(13) A mathematical spectral decomposition method is then implemented in order to evaluate, after baseline subtraction (a method known to the person skilled in the art), a characteristic value for each of the following two vibration modes of the OH bonds (also referred to as water vibration modes): a first water vibration mode (referred to as mode A hereafter), such as that with a wavenumber at 3160 cm.sup.140 cm.sup.1, and a second water vibration mode (referred to as mode B hereafter), such as that with a wavenumber at 3400 cm.sup.1150 cm.sup.1.

(14) By limiting the use of Raman spectra in the OH bond vibration mode zone, the hydrate detection method becomes less long and less expensive than current methods: indeed, it is not necessary to sweep the whole spectrum.

(15) A characteristic value is understood to be the intensity of the signal or a value directly related to the intensity, for example the area (obtained by integration of the spectrum on bands corresponding to the two water vibration modes).

(16) The position of the bands corresponding to vibrations modes A and B can be given in wavenumber (cm.sup.1) or in wavelength (nm). It is reminded that the wavenumber is a quantity inversely proportional to the wavelength. This position of the bands is always given in relative terms (Raman shift) in relation to the position of the incident laser (the position of the bands expressed in wavelength depends on the wavelength of the incident laser of the Raman spectroscope).

(17) Once the two characteristic values determined, a ratio of these two characteristic values is calculated. Preferably, the ratio corresponds to the ratio of the first water vibration mode (mode A) to the second water vibration mode (mode B).

(18) Ratio is then compared with limit values .sub.0 previously determined by calibration in the medium considered. Limit values .sub.0 can depend on the medium, the temperature, the pressure, etc. Ratio .sub.0 can depend on the temperature, hence the interest of using a temperature measurement coupled with the Raman measurement. If >.sub.0, then the system contains water in solid form (hydrates and/or ice). If <.sub.0, then the system contains no water in solid form (hydrates and/or ice). Furthermore, when >.sub.0, if temperature T measured in the vicinity of said measurement point is higher than ice formation temperature Tf under the measurement conditions, one can distinguish between a presence of ice or a presence of gas hydrates: if >.sub.0 and T>Tf, then we can highlight the presence of gas hydrates.

(19) Temperature Tf notably depends on the water-containing medium and on the pressure. In particular, temperature Tf can be high in the presence of an additive.

(20) According to an example embodiment of the invention, ratio .sub.0 can range between 1 and 1.4 for the detection of hydrate formation in a methane-containing medium.

(21) The calibration operation is possibly carried out at different temperatures, under conditions representative of industrial operations of the water-containing medium.

(22) In short, from the calibration procedure, the on-line measurement of the Raman spectrum and temperature T in the vicinity of the measurement point, a limit value allowing to decide on the formation or not of water in solid form, notably in gas hydrate form, is determined.

(23) According to an implementation of the invention, a device for cooling the medium at the measurement point can be added, so as to be able to control the temperature of the medium (by imposing a temperature range at the measurement point) in order to anticipate the formation of hydrates, or more generally of water in solid form.

(24) According to an embodiment of the invention, if the formation of hydrates and/or of ice is detected at the measurement point after cooling the medium at the measurement point, it is possible to prevent hydrate formation in the medium by injecting an anti-hydrate additive into the water-containing medium. It is thus possible to anticipate hydrate prevention in the water-containing medium.

(25) The method can comprise the following steps: sending at least to one point of the medium a light signal whose wavelength is below 785 nm, collecting the Raman spectrum at the point considered, processing the Raman spectrum according to the method described above (by measuring the characteristic values for the two OH bond vibration modes), obtaining at the end of this processing the value of intensity ratio , measuring temperature T in the vicinity of the measurement point, comparing the value of ratio with a reference value .sub.0, according to the difference between measured value and reference value .sub.0, and according to the measured temperature, we decide on the presence or not of solid ice or hydrate crystals.

(26) According to this information, it is possible to act on at least one action variable, for example temperature, pressure, additive injection or fluid flow rate, in order to prevent hydrate (or ice) formation in the water-containing medium.

(27) In a variant, temperature T in the vicinity of the measurement point is controlled. A preliminary step of cooling said measurement point can be added. The method then allows to anticipate a hydrate formation temperature under real conditions.

EXAMPLE

(28) Other features and advantages of the method according to the invention will be clear from reading the application example hereafter. In this example, the medium consists of methane in gas phase at a pressure of 70 bars and a small amount of water in an enclosure containing a temperature sensor and a Raman probe. The spectrometer used is a RXN2C marketed by the Kaiser company with an excitation length of 532 nm.

(29) The Raman spectra illustrated in FIG. 1 were recorded upon cooling the enclosure between 15 C. and 2 C. Each curve corresponds to a temperature of the enclosure.

(30) It can be seen in this figure that the major component is the methane in gas phase, with a main peak at 2917 cm.sup.1 corresponding to the symmetric stretching vibration of the CH bonds of methane. In the method provided, we do not seek to exploit the vibration modes of these CH bonds, but we rely on the analysis of the vibration modes of the OH bonds of water that can be seen in FIG. 1 in the 3100 cm.sup.1-3600 cm.sup.1 range approximately. In this zone, two water vibration modes can be seen: a first mode (denoted by A in FIG. 1) at 3160 cm.sup.1, and a second mode (denoted by B) at 3400 cm.sup.1. It is observed that the vibration range of the OH bonds undergoes changes during the temperature decrease. More precisely, it can be noted that the relative intensities of the two water vibration modes evolve as the temperature decreases, with a more significant intensity increase of mode A. This evolution is attributed to the formation of solid water in hydrate form when the temperature decreases from 15 C. to 2 C. (because the temperatures are higher than the melting point of ice).

(31) In this example, the intensities at wavenumbers 3173 cm.sup.1 and 3413 cm.sup.1 are measured. Ratio of the two intensities (I(3173)/I(3413)) is then calculated as a function of time (FIG. 2) or indifferently as a function of temperature since the temperature is lowered over time in this test. In FIG. 2, intensity ratio is represented by grey squares and the variation of temperature T in C. is illustrated by the dotted curve. The comparison of the values of ratio , ranging here between about 1 and 1.5, with a reference value .sub.0 set here at 1.2 after prior calibration, allows to decide on the presence of water in solid form. In the region <.sub.0, the system contains no water in solid form. In the region >.sub.0, the system contains water in solid form. The time corresponding to the transition from one region to another, i.e. corresponding to the time when solid crystals form, is denoted by t.sub.sol. The measurement of temperature T in the vicinity of the Raman spectra measurement point allows to convert this time t.sub.sol to a temperature T.sub.sol of solid particles appearance. In this example, the temperature ranges between 15 C. and 2 C., and temperature T.sub.sol of solid particles appearance is higher than the ice formation temperature, which additionally allows to conclude on the appearance of crystals of gas hydrate type rather than of ice type.