Raman spectrometry for determination of composition of natural gas

Abstract

Apparatus and method for Raman-spectrography based measurement of the composition of gas mixture in a high-temperature borehole. The method includes any of determining molar densities of individual alkanes of the mixture, introducing refractive index corrections, utilization of reference species internally to the measurement apparatus, correction for the effect of self-absorption and cross-absorption, as well as minimizing fluorescence when a liquid fraction is present in the borehole. The apparatus is configured to detect vibrational bands of CH, OH, CC, HS, NN and CO functional groups as well as collective modes in the fingerprint spectral region.

1 . A method comprising:
 determining, using an optical system, dependencies of intensities of Raman-scattered radiations by predetermined individual fluids in response to radiation from a light source, as functions of molar densities of the individual natural gases;   correcting the dependencies by including linear terms that represent variations of indices of refraction corresponding to the predetermined fluids with fluid densities, the linear terms depending on molar refractivities corresponding to the predetermined fluids;   directing light from a light source to a mixture of fluids of an unknown composition, thereby inducing a first Raman-scattered radiation from the mixture of fluids;   detecting, using an optical detector, the first Raman-scattered radiation to generate first data representing a first spectrum of the mixture of fluids, wherein the light is delivered to the mixture through an optical window sealingly separating the optical measurement system from the mixture; and   determining, using a computer processor, the composition of the mixture of fluids from the first data based at least in part on the determined dependencies of intensities.   
     
     
         2 . The method according to  claim 1 , further comprising:
 receiving second Raman-scattered radiation from a reference optic of the optical measurement system to generate second data representing a spectrum of the second Raman-scattered radiation,   wherein a spectral position of a peak of the spectrum of the second Raman-scattered radiation does not vary as a function of pressure, and   wherein the determining the composition is based at least in part on the second data.   
     
     
         3 . The method according to  claim 1 , wherein the detecting the first Raman-scattered radiation includes receiving the first Raman-scattered radiation in an environment and for which a spectral position of a peak of the first spectrum varies as a function of i) pressure and temperature of the environment, and ii) chemical composition of said mixture. 
     
     
         4 . The method according to  claim 1 , further comprising:
 correcting the first data to compensate for effects of cross-absorption and self-absorption in the mixture of fluids by configuring the optical measurement system such that a response of the optical detector to the first radiation remains monotonic as a function of molar density of a corresponding fluid.   
     
     
         5 . The method according to  claim 1 , further comprising:
 correcting the first data to at least in part compensate for effects of cross-absorption and self-absorption in said mixture by accounting for pre-measure absorption coefficients of gas constituents of said mixture.   
     
     
         6 . The method according to  claim 5 , wherein the correcting is achieved at least in part as a result of adjusting a focal length of radiation-collecting optics of the optical measurement system. 
     
     
         7 . The method according to  claim 1 , wherein the determining the composition includes calculating molar densities of fluid components of the mixture. 
     
     
         8 . A method comprising:
 determining, using an optical system, dependencies of intensities of Raman-scattered radiations by predetermined individual fluids in response to radiation from a light source, as functions of molar densities of the individual natural gases;   directing light from a light source to a mixture of fluids of an unknown composition, thereby inducing a first Raman-scattered radiation from the mixture of fluids;   detecting, using an optical detector, the first Raman-scattered radiation to generate first data representing a first spectrum of the mixture of fluids, wherein the light is delivered to the mixture through an optical window sealingly separating the optical measurement system from the mixture;   correcting the first data to at least in part compensate for effects of cross-absorption and self-absorption in the mixture by configuring the optical measurement system such that a response of the optical detector the first Raman-scattered radiation remains monotonic as a function of molar density of a corresponding fluid; and   determining the composition of the mixture from the first data based at least in part on the determined dependencies of intensities.   
     
     
         9 . The method according to  claim 8 , further comprising:
 correcting the dependencies by including corresponding linear terms that represent variations of corresponding indices of refraction of the fluids with fluid densities, the linear terms depending on corresponding molar refractivities.   
     
     
         10 . The method according to  claim 8 , further comprising
 receiving second Raman-scattered radiation from a reference optical component of the optical measurement system to generate second data representing a spectrum of the second Raman-scattered radiation, wherein a spectral position of a peak of the spectrum of the second Raman-scattered radiation does not vary as a function of pressure or temperature; and   wherein the determining the composition is based at least in part on the second data.   
     
     
         11 . The method according to  claim 8 , wherein the correcting is achieved at least in part as a result of adjusting a focal length of radiation-collecting optics of the optical measurement system. 
     
     
         12 . The method according to  claim 8 , wherein the determining the composition includes calculating molar densities of components of the mixture. 
     
     
         13 . A method comprising:
 using an optical system to direct light from a light source to a mixture of fluids comprising a first alkane and a second alkane, thereby inducing a first Raman-scattered radiation from the mixture of fluids;   using the optical system to detect the first Raman-scattered radiation from the mixture of fluids to generate first data representing a first spectrum of the first Raman-scattered radiation corresponding to a CH stretch modes of the first alkane and the second alkane and a second spectrum of the first Raman-scattered radiation corresponding to a CC stretch mode of the second alkane, wherein the first spectrum is formed at least in part by overlap between (i) a CH stretch mode of the first alkane, and (ii) a CH stretch mode of the second alkane, wherein the second alkane has a chain length greater than that of the first alkane;   using a computer processor to determine a ratio of peak intensities of the CC stretch mode of the second alkane to the peak intensity of the CH stretch mode of the second alkane;   using the computer processor to determine, based on the determined ratio, a contribution of the CH stretch mode of the second alkane to the first spectrum;   using the computer processor to subtract the determined contribution from the first spectrum to determine a CH stretch Raman band of the first alkane; and   using the computer processor to determine a molar density of the first alkane based on a peak intensity of the determined CH stretch Raman band.   
     
     
         14 . The method according to  claim 13 , wherein the data representing intensity ratios of different modes of the collective modes have been acquired by individually measuring Raman scattering of the light by only one chosen alkane for each alkane from the mixture. 
     
     
         15 . The method according to  claim 14 , wherein the individually measuring includes determining a dependency representing the Raman scattering of the light as a function of molar density of the only one alkane for each alkane from the mixture. 
     
     
         16 . The method according to  claim 15 , further comprising:
 correcting the dependency by including a linear term that represents a variation of index of refraction of the only one alkane density thereof, the linear term depending on corresponding molar refractivity of the only one alkane.   
     
     
         17 . The method according to  claim 13 , wherein (i) the single CH stretch mode is that of C 1 , and (ii) the collective modes include one or more of CH stretch modes of C 2 , CH stretch modes of C 3 , a CC vibrational stretch mode of an alkane having two or more carbon atoms. 
     
     
         18 . The method according to  claim 17 , further comprising:
 determining a molar density of the at least a second alkane from a corresponding CC stretch mode.   
     
     
         19 . The method according to  claim 13 , further comprising:
 correcting the first data to at least in part compensate for effects of cross-absorption and self-absorption in the mixture by configuring the optical measurement system such that a response of an optical detector of the system to radiation received at the optical detector remains monotonic as a function of molar density of an alkane present in said mixture.   
     
     
         20 . The method according to  claim 13 , further comprising:
 using the optical measurement system to detect second Raman-scattered radiation from a reference optic of the optical measurement system to generate second data representing a spectrum of the second Raman-scattered radiation, wherein a spectral position of a peak of the spectrum of the second Raman-scattered radiation does not vary as a function of any of pressure and temperature; and   wherein the removing a contribution of the collective modes includes removing the contribution based at least in part on the second data.   
     
     
         21 . An optical measurement system comprising:
 a housing defining an optical aperture and a closed volume that is fluidly sealed from a medium outside of said housing;   a laser source configured to generate light and disposed inside said closed volume; and   a light-delivery system configured to provide optical communication between the laser source and the medium and between the medium and an optical detector of the optical measurement system, wherein the light-delivery system includes a reference optic configured to generate Raman-scattered radiation of the light and having a Raman-scattering spectrum in which a spectral position of a spectral peak does not vary as a function of pressure.   
     
     
         22 . The system according to  claim 21 , wherein the reference optic is configured as an optical window disposed across the optical aperture and configured as a fluid-tight seal. 
     
     
         23 . The system according to  claim 21 , wherein the laser is configured to generate pulsed light. 
     
     
         24 . A system according to  claim 21 , wherein the light-delivery system further includes:
 an optical fiber disposed between the optical detector and the optical aperture, and light-collecting optics between the optical fiber and the optical aperture.