Patent Publication Number: US-2013247649-A1

Title: Device for Quantifying The Contents of at Least One Gaseous Constituent Contained in A Gaseous Sample from A Fluid, Related Assembly and Process

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of European patent application serial number 06291305.8, filed Aug. 16, 2006, and is a continuation of U.S. patent application Ser. No.: 12/309970, filed Aug. 10, 2007, both of which are incorporated herein by reference for all purposes. 
     DESCRIPTION 
     The present invention relates to a device for quantifying the content of at least one gaseous constituent contained in a gaseous sample from a fluid, of the type comprising:
         a means for forming a gaseous flow from the sample, comprising means for separation by means of selective retention of the or each gaseous constituent to be analysed;   a means for combustion of the gaseous flow, connected to the separation means in order to successively form a gaseous residue from the or each constituent;   a means for quantifying the content of the or each constituent to be analysed in the gaseous flow.       

     This device is used in particular to analyse the gases extracted from a petroleum fluid produced in an oil well or to determine the content of the hydrocarbon constituents contained in drilling mud. 
     In the last case, when an oil or other outflow well is drilled (in particular gas, vapour, water), it is known to carry out an analysis of the gaseous compounds contained in the drilling muds originating from the well. This analysis allows the geological sequence of the formations passed through during the drilling operation to be reconstructed and is used to determine the possible applications of the fluid deposits encountered. 
     This analysis, which is carried out in a continuous manner, comprises two main phases. The first phase consists in extracting the gases carried by the mud (for example, hydrocarbon compounds, carbon dioxide, hydrogen sulphide). The second phase consists in qualifying and quantifying the extracted gases. 
     In order to extract the gases from the mud, a degassing means with mechanical agitation of the type described in FR 2 799 790 is often used. The gases extracted from the mud, mixed with a carrier gas which is introduced into the degassing means, are conveyed by means of suction through a gas extraction pipe to an analysis device which allows the extracted gases to be quantified. 
     The analysis device comprises a gas-phase chromatograph (GPC) which allows the various gases collected in the degassing means to be separated in order to be able to quantify them. 
     In some cases, however, it is necessary to carry out a more precise analysis of the gaseous content of the extracted gases, using a device for measuring the relationship between the contents of carbon isotopes .sup.13C and .sup.12C in the gaseous hydrocarbon compounds extracted from the mud. 
     A device of this type comprises, in conjunction with the gas-phase chromatography, a combustion oven and an isotope radio mass spectrometer (IRMS) which is intended to analyse the outflow from the combustion oven. 
     A device of this type is unsatisfactory, in particular when the analysis must be carried out on a drilling site or on a production site. The IRMS must be kept under pressure and temperature conditions which are substantially constant in order to obtain precise and repetitive measurements. Consequently, it is necessary to carry out an “off-line” analysis of the samples in a climate-controlled laboratory. If it is desirable to carry out the analysis “on-line”, however, it is necessary to bring a large, fragile and complex climate control and IRMS control assembly close to the well in an environment which can be hostile and inaccessible. 
     An object of the invention is therefore to provide a device for quantifying the content of at least one gaseous constituent from a fluid, which device can readily be arranged in the vicinity of an oil well or a drilling site in order to obtain “on-line” measurements whilst maintaining an adequate level of measurement precision for the analysis. 
     To this end, the invention relates to a device of the above-mentioned type, characterised in that the quantification means comprise:
         an optical measurement cell which is connected to the combustion means in order to receive the gaseous flow from the combustion means;   a means for introducing a laser incident optical signal into the cell;   a means for measuring a transmitted optical signal resulting from an interaction between the optical signal and the or each gaseous residue in the cell; and   a means for calculating said content on the basis of the transmitted optical signal.       

     The device according to the invention may comprise one or more of the following features, taken in isolation or according to all technically possible combinations:
         the quantification means comprise means for emitting an optical signal, and means for optically transmitting this signal to the introduction means, and the emission means comprise means for adjusting the wavelength of the emitted signal, which means are able to scan a specific wavelength range for a predetermined period of time;   the measurement cell comprises:   at least two mirrors which delimit a measurement cavity;   means for transporting the gaseous flow to the measurement cavity; and the introduction means comprise means for injecting the incident optical signal into the measurement cavity;   at least a first mirror has a reflectivity of less than 100%, the measurement means being arranged at the rear of the first mirror outside of the measurement cavity;   the mirrors are arranged opposite to each other along a cavity axis;   the mirrors have reflective surfaces which are arranged along the same cavity axis, the device comprising means for generating a plurality of reflections of the optical signal in at least two separate points on each mirror during its travel in the cavity in order to create at least two separate optical signal segments in the measurement cavity;   the means for generating a plurality of reflections comprise means for inclining the injection means in order to incline the incident optical signal to be introduced into the measurement cavity relative to the cavity axis; and   the separation means comprise a gas-phase chromatograph.       

     The invention further relates to an assembly for analysing at least one gaseous constituent contained in a petroleum fluid, of the type comprising:
             a means for sampling the petroleum fluid;       a means for extracting a gaseous sample from the fluid, which means are connected to the sampling means; and   a device as defined above, the extraction means being connected to the formation means.       

     The invention also relates to a method for quantifying the content of at least one gaseous constituent contained in a sample from a petroleum fluid of the type comprising the following steps:
         the formation of a gaseous flow from the sample, comprising a separation phase by means of selective retention of the or each gaseous constituent to be analysed;   the combustion of the gaseous flow from the separation phase in order to successively form a gaseous residue from the or each constituent;   the quantification of the content of the or each constituent to be analysed in the gaseous flow;   characterised in that the quantification step comprises the following phases:   the introduction of the gaseous flow from the combustion step into an optical measurement cell; and for the or each residue successively introduced into the measurement cell:   the introduction of an incident optical signal into the cell;   the measurement of a transmitted optical signal resulting from an interaction between the optical signal and the or each gaseous residue in the cell; and   the calculation of said content on the basis of the transmitted optical signal.       

     The method according to the invention may comprise one or more of the following features, taken in isolation or according to all technically possible combinations:
         the quantification step comprises a phase for emitting a substantially monochromatic optical signal, and a phase for optically transmitting this signal to the measurement cell in order to introduce it into the cell, and the emission phase comprises the adjustment of the wavelength of the emitted signal, and the scanning of a specific wavelength range for a predetermined period of time;   it comprises a step for transporting the gaseous flow to a measurement cavity which is delimited by at least two mirrors and the introduction step comprises a phase for injecting the incident optical signal into the measurement cavity:   the mirrors are arranged opposite to each other;   at least a first mirror has a reflectivity of less than 100%, the measurement step being carried out at a point at the rear of the first mirror outside of the cavity;   the mirrors have reflective surfaces which are arranged coaxially on a cavity axis, the method comprising a step for generating a plurality of reflections of the optical signal in at least two separate points on each mirror during its travel in the cavity in order to create at least two separate optical signal segments in the measurement cavity; and   the step for generating a plurality of reflections comprises the inclination of the incident optical signal introduced into the measurement cavity relative to the cavity axis.       

    
    
     
       The invention will be better understood from a reading of the following description, given purely by way of example and with reference to the appended drawings, in which: 
         FIG. 1  is a schematic sectioned view of an analysis assembly according to the invention, arranged in an installation for drilling an oil well; 
         FIG. 2  is a detailed view of a first quantification device according to the invention in the analysis assembly of  FIG. 1 ; 
         FIG. 3  is a partially sectioned schematic view of the optical measurement means of the device of  FIG. 2  comprising in particular a laser and a sensor; 
         FIG. 4  is a view illustrating the emission line of the laser of  FIG. 3  as a function of time, and the reception line measured by the sensor of  FIG. 3  as a function of time when a method according to the invention is used; and 
         FIG. 5  is a view similar to  FIG. 3  of the optical measurement means of a second quantification device according to the invention. 
     
    
    
     A quantification device according to the invention is, for example, used in an analysis assembly  9  used for the on-line analysis of the gaseous content of drilling muds in an installation  11  for drilling an oil production well. 
     As illustrated in  FIG. 1 , this installation  11  comprises a drilling pipe  13  in a cavity  14  through which a rotating drilling tool  15  extends, and a surface installation  17 . 
     The drilling pipe comprises, in the region of the surface  22 , a well head  23  which is provided with a pipe  25  for discharging a drilling fluid, referred to as drilling mud. 
     The drilling tool  15  comprises a drilling head  27 , a drilling assembly  29  and a liquid injection head  31 . 
     The drilling head  27  comprises means  33  for drilling through the rocks of the sub-stratum  21 . It is assembled in the lower portion of the drilling assembly  29  and is positioned at the bottom of the drilling pipe  13 . 
     The assembly  29  comprises an assembly of hollow drilling tubes. These tubes delimit an internal space  35  which allows a liquid to be conveyed from the surface  22  to the drilling head  27 . To this end, the liquid injection head  31  is screwed onto the upper portion of the assembly  29 . 
     The surface installation  17  comprises means  41  for supporting and driving the drilling tool  15  in rotation, means  43  for injecting drilling liquid and a vibrating sieve  45 . 
     The injection means  43  are hydraulically connected to the injection head  31  in order to introduce and circulate a liquid in the inner space  35  of the drilling assembly  29 . 
     The vibrating sieve  45  collects the liquid charged with drilling residues which is discharged from the discharge pipe  25  and separates the liquid from the solid drilling residues. 
     As illustrated in  FIG. 2 , the analysis assembly  9  comprises means  51  for sampling the mud, which means are tapped into the discharge pipe  25 , a gas extraction device  53 , and a device  55  for analysing and quantifying the extracted gases. 
     The sampling means  51  comprise a liquid sampling head  57  which is tapped into the discharge pipe  25 , a connection tube  59  and a peristaltic pump  61  whose flow rate can be adjusted. 
     The extraction device  53  comprises a vessel  63 , a pipe  65  for conveying the mud into the vessel  63 , a pipe  67  for discharging the mud from the vessel  63 , an inlet  69  for introducing a carrier gas into the vessel  63 , and a pipe  71  for extracting the extracted gases from the vessel  63 . 
     The vessel  63  is formed by a sealed receptacle whose inner volume is, for example, between 0.4 and 3 litres. This vessel  63  comprises a lower portion  73  in which the mud circulates and an upper portion  75  which has a gaseous cap. The vessel  63  is further provided with an agitator  77  which is immersed in the mud. 
     The mud supply pipe  65  extends between the outlet of the peristaltic pump  61  and an inlet opening which is arranged in the lower portion  73  of the vessel  63 . 
     This supply pipe  65  may be provided with means for heating the mud (not illustrated) in order to bring the temperature of this mud to values of between 25 and 120 degree. C., preferably between 60 and 90 degree. C. 
     The discharge pipe  67  extends between an overflow passage  87  which is arranged in the upper portion  75  of the vessel  63  and a retaining vessel  89  which is intended to receive the mud which is discharged from the device  53 . It comprises a siphon in order to prevent gas from being introduced into the upper portion  75  of the vessel  63  via the discharge pipe  67 . Gas is therefore introduced into the vessel  63  only via the carrier gas introduction inlet  69 . 
     The mud which is collected in the retaining vessel  89  is recycled towards the injection means  43  via a mud recirculation pipe  98 . 
     The gas extraction pipe  71  extends between an extraction opening  101 , which is arranged in the upper portion  75  of the vessel  63 , and the analysis device  55 . It comprises a transport line  107  which is provided with volume flow control means and suction means  109 . 
     The transport line  107  connects the vessel  63  which is arranged in the vicinity of the well head  23 , in the explosive zone, to the analysis device  55  which is arranged with spacing from the well head  23  in a non-explosive zone, for example, in a pressurised cabin. 
     This transport line  107  can be produced on the basis of a polymer material, known to be inert versus hydrocarbons, such as PTFE or THV, and has, for example, a length of from 10 m to 500 m. 
     The suction means  109  comprise a vacuum pump which allows the gases extracted from the vessel  63  to be conveyed, by means of suction, to the analysis device  55 . 
     As illustrated in  FIG. 2 , the analysis device  55  according to the invention comprises a stage  111  for forming a gaseous flow to be analysed, a combustion oven  113  which is connected to an outlet of the formation stage  111 , and a stage  115  for quantifying the content of the gaseous constituents to be analysed in the drilling mud. 
     The formation stage  111  comprises a sampling pipe  117  which is tapped into the extraction pipe  71  in the vicinity of the pump  109 , upstream of this pump, and a gas-phase chromatograph  119  which is provided with a column  121  for separation by means of selective retention of the gaseous constituents to be analysed. 
     The chromatograph  119  is, for example, a device of the type as known by those skilled in the art with a gas injection system and a chromatographic separation column  127  to separate compounds to be analysed before their combustion in the oven  113 . 
     The separation column  121  has a length which is between 2 m and 25 m in order to ensure a mean passage time for the gases of between 30 s and 600 s. It is connected to the sampling pipe  117  in order to take a gaseous sample from the extraction pipe  71  and form a gaseous flow at the outlet of the column  121 , in which flow the sample constituents to be analysed are separated over time. 
     The oven  113  comprises combustion means for the gaseous flow discharged from the column  121  at a temperature of substantially between 900.degree. C. and 1100.degree. C. 
     In the combustion means, each constituent contained in the gaseous flow undergoes an oxidation in which the constituent reacts with oxygen to form carbon dioxide. 
     The quantification stage  115  comprises an optical measurement unit  123  which is connected to an outlet of the combustion oven  113 , and a control and calculation unit  125  which is connected electrically to the measurement unit  123 . 
     As illustrated in  FIGS. 2 and 3 , the optical measurement unit  123  comprises an optical measurement cell  127 , a laser  129  for emitting an incident optical signal, a mechanism  131  for introducing the incident optical signal into the cell  127 , and a sensor  133  for measuring an optical signal transmitted from the cell  127 . 
     The cell  127  comprises a confinement chamber  135 , two concave mirrors  137 A,  137 B which are fixed in the chamber  135  and means  139  for transporting the gaseous flow from the combustion oven in the chamber  135 . 
     With reference to  FIG. 3 , the chamber  135  comprises a cylindrical wall  141  which extends substantially along a longitudinal centre axis X-X′, and two planar end walls  143 A,  143 B which longitudinally close the cylindrical wall  141 . 
     The end walls  143 A,  143 B are transparent with respect to wavelengths of in the near infrared region such as 1100 nm, 1600 nm or 2100 nm region. 
     Each mirror  137 A,  137 B is fixed in the chamber  135  to a corresponding end wall  143 A,  143 B. The mirrors  137 A,  137 B are fixed coaxially along the axis X-X′. Each mirror  137 A,  137 B has a substantially spherical, concave reflective surface  145 A,  145 B which is directed towards the inner side of the chamber  135 . 
     The radius of curvature of the concave surfaces  145 A,  145 B is, for example, between 4 m and 8 m. The reflectivity of the mirrors  135 A,  137 B is greater than 50% and preferably greater than 99% for wavelengths in the near infrared regions as specified above. 
     The surfaces  145 A,  145 B extend opposite each other symmetrically relative to a vertical centre plane of the chamber  135 . Together they delimit, in the chamber  135 , an absorption measurement cavity  147  for the interaction between the optical signal and the constituents which are introduced into the cavity  147  by the transportation means  139 . The distance which separates the surfaces  145 A,  145 B is substantially between 50 cm and 90 cm. 
     The transportation means  139  comprise a pipe  149  for introducing the gaseous flow into the chamber and a discharge pipe  151 . Each pipe  149 ,  151  is provided with a flow rate control valve  149 A,  151 A. 
     The introduction pipe  149  is connected to an outlet of the combustion oven  133 . It opens into the chamber  135  through the wall  141 , in the vicinity of the upstream mirror  137 A. 
     The discharge pipe  151  also opens into the chamber  135  in the vicinity of the downstream mirror  137 . 
     The chamber  135  is provided with respective temperature and pressure control means  152 A,  152 B. 
     The laser  129  comprises a cavity  153  for emitting a light ray which forms a substantially monochromatic optical signal, means  155  for adjusting the mean wavelength of the signal, and means  157  for controlling the intensity of the signal. 
     A substantially monochromatic signal is understood to be a signal which has a width at mid-range of, for example, between 0.05 nm and 1 nm. 
     The means  157  for controlling the intensity can generate a signal having substantially constant intensity for a variable period of time. 
     The transmission and introduction mechanism  131  comprises a deflection mirror  159  which is arranged substantially opposite the emission cavity  153  and a mirror  161  for adjusting the angle of injection into the measurement cavity  147 , which mirror is arranged opposite the downstream mirror  143 B at the outer side of the chamber  135 , and is arranged opposite the deflection mirror  159 . 
     The adjustment mirror  161  is provided with means for adjusting the injection angle .alpha. formed by the longitudinal axis X-X′ and the axis of the segment  162  of the incident optical signal introduced into the cavity  147 , taken between the reflection point  162 B thereof on the mirror  161  and the introduction point  162 A thereof in the chamber  135 . 
     The mirror  161  is further provided with means for transverse displacement relative to the axis X-X′ in order to position the introduction point  162 A with spacing from the axis X-X′. 
     The sensor  133  for measuring the transmitted optical signal comprises a focusing lens  163  which extends perpendicularly relative to the axis X-X′ at the rear of the upstream mirror  137 A at the outer side of the chamber  135 , and a intensity detector  165  which is arranged at the focal point of the lens  163  located on the axis X-X′ opposite the chamber  135  relative to the lens  163 . The detector  165  is electrically connected to the control and calculation unit  125 . 
     A first method for quantifying a constituent which is contained in a gaseous sample taken from a drilling mud and which is carried out on-line when a well is drilled will now be described with reference to  FIG. 1 . 
     In order to carry out the drilling operation, the drilling tool  15  is driven in rotation by the surface installation  41 . A drilling liquid is introduced into the inner space  35  of the drilling assembly  29  by the injection means  43 . This liquid moves downwards as far as the drilling head  27  and passes into the drilling pipe  13  through the drilling head  27 . This liquid cools and lubricates the drilling means  33 . Then the liquid collects the solid debris resulting from the drilling operation and moves upwards again through the annular space which is defined between the drilling assembly  29  and the walls of the drilling pipe  13 , then is discharged via the discharge pipe  25 . 
     The peristaltic pump  61  is then activated in order to remove, in a continuous manner, a specific fraction of the drilling mud which is circulating in the pipe  25 . 
     This fraction of mud is conveyed as far as the chamber  63  via the supply pipe  65 . 
     The agitator  77  is driven in rotation in the lower portion  73  of the chamber  63  in order to bring about the extraction of the gases contained in the mud and the mixture of the extracted gases with the carrier gas drawn through the injection inlet  69 . 
     The gaseous mixture is extracted via the extraction pipe  71 , under the action of the suction produced by the vacuum pump  109 . This mixture is then conveyed as far as the analysis device  55 . 
     The gaseous mixture containing a plurality of constituents to be analysed is then injected into the chromatograph  119  through the sampling pipe  117 . A gaseous flow, in which the various constituents to be analysed in the gaseous mixture are separated over time, is then obtained at the outlet of the column  121 . This gaseous flow successively comprises, for example, C.sub.1 hydrocarbons, then C.sub.2 hydrocarbons and other heavier compounds. The gaseous flow then enters the oven  113  where the combustion of this flow is carried out. 
     The various constituents which are separated in the column  121  and contained in the gaseous flow are successively converted into combustion residues, by oxidation in the combustion means  113 . 
     If these constituents are hydrocarbons, they form residues which are constituted principally by carbon dioxide. These residues are then conveyed into the optical measurement unit  123 . 
     In the unit  123 , the combustion residues of the various constituents are successively introduced into the chamber  135  and circulate in the optical cavity  147  between the introduction pipe  149  and the discharge pipe  151 . 
     In the method according to the invention, immediately after the first component to be analysed has entered in the cavity  147 , the cavity  147  is isolated from the gaseous flow with valves  149 A and  151 A to perform quantification. Then the means  155  for adjusting the wavelength are controlled to scan a wavelength range in the near infrared regions such as 1100 nm, 1600 nm or 2100 nm region (line  172  in  FIG. 4 ) for a predetermined period of time. 
     A scanning operation of this type is repeated for each passage of the various constituents which are to be analysed and which circulate successively in the measurement cavity  147  after opening of valves  149 A and  151 A. 
     A scanning operation of this type is repeated for each passage of the combustion residues corresponding to the various constituents which are to be analysed and which circulate successively in the measurement cavity  147 . 
     During this scanning operation, the emission cavity  153  of the laser emits an optical signal whose intensity as a function of time is illustrated on the line  171  as a solid line in  FIG. 4(   a ) and whose line  172  of the wavelength as a function of time is illustrated as a dotted line in this Figure. 
     The incident optical signal  169  is conveyed as far as the optical cavity  147  by means of reflection on the deflection mirror  159  and the adjustment mirror  161  then by transmission through the wall  143 B and the mirror  137 B. 
     The incident optical signal is introduced into the cavity  147  at a point  162 A which is located with spacing from the axis X-X′. The injection angle .alpha. is different from zero. 
     The optical signal then travels along an optical path back and forth in the measurement cavity  147 , formed by successive segments  173  which are delimited by a plurality of discrete reflection points  174 B on each concave surface  145 A,  145 B. This plurality of reflections is generated by the control of the inclination of the mirror  161 . 
     The optical signal therefore covers an optical path which comprises at least  100  segments in the measurement cavity  147 , and preferably at least 1000 segments. 
     Given the weak interactions between the various segments  173  of the optical signal formed between the successive reflection points  174 A,  174 B of the signal on the mirrors  137 A,  137 B, the measurement cavity  147  has no selectivity with respect to the transmission wavelength and it is not necessary to modify the length of the cavity  147  in order to adapt to the wavelength. The optical measurement unit  123  therefore has no electronic components which are costly and difficult to use on an oil site. 
     The interaction of the various segments  173  and the combustion residues contained in the measurement cavity  147  generates an optical signal which carries an item of information characteristic of the content of these residues in the measurement cavity  147 . 
     The optical signal interacts with the molecular constituents of the measurement cell by means of vibrational excitation. The molecules absorb a portion of the optical signal resulting in a loss of optical intensity. This occurs in each segment  173  which is transmitted through the upstream mirror  137 A and which is not reflected on the surface  145 A. 
     This transmitted optical signal is focused through the lens  163  and detected by the sensor  165  in order to obtain the intensity  175  as a function of time illustrated in  FIG. 4(   b ). The content of combustion residues resulting from a constituent to be analysed is, for example, calculated by the calculation unit  125  on the basis of the decay time of the intensity  175  of the transmitted signal. 
     Furthermore, when the range of the wavelength of the incident signal is adjusted in order to scan a range in which two characteristic absorptions of two respective isotopes of the same element are produced, for example, carbon .sup.12C and carbon .sup.13C, the intensity  175  of the transmitted signal as a function of the wavelength shows two respective characteristic absorption regions  176  and  177  of these two isotopes. The relationship of the contents of two isotopes of the same constituent, for example, the C.sub.1 hydrocarbons, in the drilling mud is then calculated on the basis of the relationship between the depths of the regions  176  and  177 . 
     The method is then repeated during the successive passage of the residues which correspond respectively to each constituent to be analysed in the cavity  147 . 
     The second device according to the invention illustrated in  FIG. 6  differs from the first device owing to the structure of the optical measurement unit  123 . 
     In contrast to the unit  123  illustrated in  FIG. 3 , the reflective surfaces  145 A,  145 B of the mirrors  137 A,  137 B are planar. Furthermore, the injection mirror  161  is partially reflective so that it injects only a portion of the incident optical signal into the cavity  147 . 
     The distance between the mirrors  137 A,  137 B can be adjusted in order to generate a resonance in the cavity  147  when a specific wavelength of the optical signal is used. 
     Moreover, the unit  123  further comprises a calibration cell  201  which has a similar structure to that of the measurement cell  127  and which is optically connected to the mirror  161  by means of a secondary deflection mirror  203  located at the rear of the mirror  161 . The cell  201  contains a compound whose content is known. 
     A secondary detection sensor  205  is arranged opposite the cavity  201 , opposite the secondary deflection mirror  203 . This sensor  205  is also connected to the control unit  125 . 
     The operation of this second device differs from that of the first device in that a portion of the incident optical signal is reflected on the mirror  161  in order to be injected into the measurement cavity  147  along the axis X-X′, and another portion of this signal is transmitted to the secondary deflection mirror  203  through the mirror  161 . 
     The mirror  161  is arranged so that the angle of injection into the measurement cavity  147  is zero. The signal then carries out a plurality of reflections between the two intersection points between the axis X-X′ and the respective reflective surfaces  145 A,  145 B of the mirrors  137 A,  137 B in the cavity  147 . 
     Furthermore, the portion of the incident optical signal which is not reflected on the mirror  161  is transmitted to the secondary deflection mirror  203 , then injected into the secondary calibration cavity  201  along the axis Y-Y′ of this cavity. 
     An optical calibration signal is collected by the secondary detector  205  and is used as a reference by the calculation means to quantify the content of each combustion residue which circulates successively in the measurement cavity  147 . 
     In another variant (not illustrated), the chamber has no mirrors and the incident optical signal interacts with the components contained in the cavity only along a single segment in a straight line which connects the point at which it enters the measurement cavity to the point at which it leaves the cavity. 
     Owing to the invention which has been described above, it is possible to provide a device  55  for quantifying the content of at least one gaseous constituent in a sample from a petroleum fluid, which can be readily fitted in the vicinity of a drilling installation or a well for the exploitation of fluids. 
     The combination of means  111  for forming a gaseous flow comprising a column  121  for separation by means of selective retention with a combustion oven  113  for the gaseous flow and a unit  115  for optical measurement of the content of the residues from the oven  113  allows “on-line” analysis of the gaseous compounds extracted from the fluid, whilst retaining a significant level of selectivity for the analysis. This selectivity in particular allows isotopic measurements to be carried out. 
     Furthermore, the use of an optical measurement unit  115 , in particular when it comprises a reflective absorption cavity  147  in which the incidence of the signal injected into the cavity  147  is not zero, considerably simplifies the instruments required, which allows the quantification device  55  to be readily displaced and positioned in the vicinity of a drilling installation or an oil well. 
     In addition, with regards to the device shown in  FIGS. 2 to 4 , a single laser  129  having a unique cavity  153  is used in the optical measurement unit  123 . 
     The range of wavelengths generated by the laser  129  when the scanning of the constituents in the cavity  147  is performed is wide enough to obtain two distinguishing absorptions regions corresponding to the two distinct isotopes, e.g. for carbon .sup.12C and carbon .sup.13C, without the need for using two different laser sources, 
     Moreover, the laser incident signal  169  produced in the cavity  153  is fully conveyed towards the cavity  147  without significant absorption on its path towards the cavity  147 . The signal  169  is not split or passed through a reference cell containing a reference sample. 
     The device  55  is deprived of such a reference cell, which is not necessary for obtaining the isotopic ratios.