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BACKGROUND OF THE INVENTION 
   The present invention relates to an analyzer device for analyzing at least one gas contained in a liquid, in particular a drilling liquid, flowing in a drilling pipe in an installation for extracting fluid from a subsoil. The device comprises an analyzer for analyzing the gas and a sampling apparatus for sampling at least a fraction of the gas. The sampling apparatus has at least one porous membrane member, the member comprising a support and having a first face in contact with the liquid flowing in the drilling pipe and a second face opening into a pipe connected to the analyzer. 
   When drilling a well for oil or some other effluent (in particular gas, steam, water), it is known to analyze the gaseous compounds contained in the drilling muds emerging from the well. Such analysis is used to reconstruct the succession of geological formations through which the borehole is being drilled and it contributes to determining the working possibilities of the fluid deposits encountered. 
   Such analysis is performed continuously and comprises two main stages. The first stage consists in extracting the gas conveyed by the mud (for example hydrocarbon compounds, carbon dioxide, hydrogen sulfide). The second stage consists in qualifying and quantifying the extracted gases. 
   For this purpose, mechanically-stirred degassers are frequently used. However, because of their size, such degassers must be installed at a distance from the well, generally close to a vibrating screen downstream from the wellhead. Muds are conveyed from the wellhead to the degasser via a flow line that might be open to the atmosphere. Thus, a fraction of the gaseous compounds present in the mud is released into the atmosphere while the mud is traveling along the line. An analysis of the gas present in the mechanically-stirred degasser is therefore not representative of the gaseous content of the mud in the well. 
   To solve that problem, devices of the above-specified type have been implanted directly in the drilling pipe, upstream from the wellhead, as described in U.S. Pat. No. 5,469,917. Such devices include a capillary tubular membrane supported capillary membrane (SCMS). However, the muds flowing around the membrane are laden with pieces of rock. 
   In order to avoid degrading the tubular membrane under the effect of impacts against these pieces of rock, the membrane is wound on a threaded rod. The thread of the support then protects the membrane against pieces of rock of a size greater than the distance between two consecutive threads of the threaded rod. 
   Those devices do not give entire satisfaction. To wind the membrane around the threaded rod, and thus provide it with protection, certain stresses need to be applied to the membrane. Thus, a membrane of tubular shape must be used in order to be capable of winding between the threads of the threaded rod. Furthermore, the membrane must be relatively flexible. Consequently, only a membrane based on organic materials can be used in such a device. Unfortunately, organic membranes present abilities at withstanding high temperatures and chemical compatibilities that are not always satisfactory in certain applications. 
   SUMMARY OF THE INVENTION 
   A main object of the invention is thus to provide a device for analyzing gas contained in a liquid that contains debris of varying size, in particular a drilling fluid, the device being installed directly in a pipe of an installation for extracting fluids from the subsoil, without putting large stresses on the membrane, in particular stresses concerning the nature and the shape of the membrane. 
   To this end, the invention provides a device of the above-specified type, characterized in that the first face presents Vickers hardness greater than 1400 kilograms-force per square millimeter (kgf/mm 2 ), in particular Vickers hardness lying in the range 1400 kgf/mm 2  to 1900 kgf/mm 2 . 
   The device of the invention may comprise one or more of the following characteristics taken in isolation or in any technically feasible combination:
         the porous membrane member includes a coating covering the support over the first face;   the coating is based on silicon carbide;   the first face is also water- and oil-repellent;   the wetting angle of water on the first face is greater than 120°;   the first face includes fluorine-containing polymers incorporated by grafting;   the first face of the membrane member that is in contact with the liquid is substantially plane;   the device further comprises a regulator for regulating the pressure in the pipe in register with the second face of the membrane member; and   it includes a plurality of membrane members, and the second faces of the members open out in succession to the pipe connected to the analyzer.       

   The invention also provides an installation for extracting fluids from the subsoil, the installation being of the type comprising a drilling pipe connecting at least one point of the subsoil to the surface, and a delivery pipe connected to the drilling pipe at the surface. The installation is characterized in that it further comprises at least one device according to the above-described characteristics, and in that the sampling apparatus of the device is mounted on a tubular element constituted by the drilling pipe or by the delivery pipe. 
   The installation of the invention may comprise one or more of the following characteristics taken in isolation or in any technically feasible combination:
         the first face of the membrane member in contact with the liquid is disposed substantially parallel to the long axis of the tubular element;   the first face in contact with the liquid is disposed in a wall of the tubular element;   the first face is disposed set back in a wall of the tubular element;   the tubular element includes a branch connection and the sampling apparatus is placed in the branch connection; and   the sampling apparatus of the device is placed in the drilling pipe upstream from the delivery pipe; and   the installation further includes a filter downstream from the delivery pipe and it includes two devices as defined above, the respective sampling apparatus of the two devices being placed respectively upstream and downstream of the filter.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are described below with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic vertical section view of a drilling installation provided with an analyzer device of the invention; 
       FIG. 2  is a diagram showing the main elements of the analyzer device of the invention; 
       FIG. 3  is a diagram showing a detail of a variant of the installation shown in  FIG. 1 ; 
       FIG. 4  is a diagrammatic vertical section view of an installation including two analyzer devices of the invention; and 
       FIG. 5  is a diagrammatic vertical section view showing a detail of a variant of the device shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A device of the invention is used for example in an installation  11  for drilling an oil production well. As shown in  FIG. 1 , the installation  11  comprises a drilling pipe  13  in a cavity pierced by a rotary drilling tool  15 , a surface installation  17 , and an analyzer device  19  of the invention mounted on the drilling pipe  13 . 
   The drilling pipe  13  is placed in the cavity drilled in the subsoil  21  by the rotary drilling tool  15 . At the surface, the pipe  13  has a wellhead  23  provided with a delivery pipe  25 . 
   The drilling tool  15  comprises a drilling head  27 , a drill string  29 , and a liquid injector head  31 . 
   The drilling head  27  has means  33  for drilling rock in the subsoil  21 . It is mounted at the bottom end of the drill string  29  and it is positioned in the bottom of the drilling pipe  13 . 
   The drill string  29  comprises a set of hollow drilling tubes. These tubes define an inside space  35  enabling a liquid to be taken from the surface  37  to the drilling head  27 . For this purpose, the liquid injector head  31  is screwed onto the top portion of the drill string  29 . 
   The surface installation  17  includes means  41  for supporting and rotating the drilling tool  15 , means  43  for injecting drilling liquid, and a vibrating screen  45 . 
   The injector means  43  are hydraulically connected to the injector head  31  to inject and drive a liquid along the inside space  35  of the drill string  29 . 
   The vibrating screen  45  collects the liquid laden with drilling residue that leaves the delivery pipe  25  and separates the liquid from the drilling residue. 
   The analyzer device  19  has a sampling head  51  for taking at least a fraction of the or each gas, and analyzer means  53  for analyzing the or each gas. 
   As shown in  FIG. 2 , the sampling head  51  comprises a porous membrane member  55  having a plane first face  57  in contact with the liquid flowing in the pipe  13  and a second face  59  looking into a pipe  61  connected to the analyzer means  53 . 
   The porous membrane member  55  comprises a membrane support  63  and a coating  65  covering the support  63  beside the liquid on the first face  57 . 
   This first face  57  is disposed in the pipe  13  parallel to the long axis of the pipe  13 , i.e. parallel to the flow of liquid. This first face  57  is preferably disposed along a wall of the pipe  13  or else is set back a little from said wall. Thus, tools can be inserted or extracted into or from the drilling pipe  13  while minimizing any risk of damaging the membrane member  55  by mechanical contact or impact. Furthermore, having the liquid flow parallel to the first face  57  puts a limit on the abrasive forces that are applied to the coating  65 . 
   The membrane support  63  is made on the basis of a porous material, e.g. a ceramic. Preferably, the membrane support  63  is in the form of a disk. In the example shown in the drawings, the diameter of the support is substantially equal to 50 millimeters (mm) and its thickness is less than 10 mm. 
   Examples of materials suitable for use in making the membrane support  63  include sintered stainless steel, metal fibers, or alumina fibers. 
   The size of the pores in the membrane support  63  lies in the range 0.01 micrometers (μm) to 5 μm, depending on the intended application. Pore diameter is preferably selected to lie in the range 0.02 μm to 3 μm. 
   The coating  65  which constitutes the first face  57  of the membrane member  55  comprises a thin layer based on silicon carbide deposited on the support  63 . The thickness of this layer lies in the range 0.5 μm to 2 μm. This thin layer covers the surface of the support between the pores. 
   Thus, the membrane member  55  is permeable to all of the gas present in the mud. 
   Furthermore, the Vickers hardness of the first face  57  of the membrane member  55  is greater than 1400 kgf/mm 2 . In the example described in the figures, this Vickers hardness lies in the range 1400 kgf/mm 2  to 1900 kgf/mm 2 . 
   This thin layer thus protects the membrane member  55  against abrasion generated by pieces of rock and drilling debris. 
   In a variant, the coating  65  is modified by grafting fluid-containing polymer chains that are highly water- and oil-repellent. This grafting is preferably performed on the basis of a perfluoroalkylethoxysilane. This modification of the coating  65  enables the first face  57  of the membrane member  55  to be made water- and oil-repellent. Consequently, the wetting angle of water on the first face  57  of the membrane member  55  is greater than 120°, and is substantially equal to 130°. 
   The membrane member  55  is thus impermeable to the liquid flowing in the pipe, which contributes to limiting clogging of the pores in the support by solid residue coming from the liquid. 
   The pipe  61  connecting the porous membrane member  55  to the analyzer means  53  includes a gas-receiver chamber  71 , a pressure controller  73  for controlling pressure in the chamber, means  75  for conveying the extracted gas from the receiver chamber  71  to the analyzer  53 , and filter  77  for filtering the extracted gas. 
   The receiver chamber  71  covers the second face  59  of the membrane member, in register with the first face  57 . It comprises a bell having an inlet orifice  79  and an outlet orifice  81  connected, respectively to the conveying means  75  and to the pressure controller  73 . 
   The pressure controller  73  for controlling pressure in the chamber comprises elements  83  for measuring the pressure difference between the liquid in the pipe and the gas in the chamber, associated with a pressure regulator  85  mounted on the delivery pipe downstream from the chamber. 
   This regulator  85  is controlled in such a manner that when the device of the invention is used for analyzing the gases contained in mud, the pressure difference between the liquid flowing in the drilling pipe  13  and the gas present in the receiver chamber  71  is substantially zero. This substantially zero pressure difference prevents the liquid flowing in the drilling pipe  13  from penetrating into the membrane member  55 . 
   Nevertheless, if the porous membrane member  55  should become clogged, it is possible to control the pressure regulator  85  so that the pressure in the chamber  71  becomes much greater than the pressure in the drilling pipe  13  for a few seconds. The difference between these two pressures can then lie in the range 1 bar to 3 bar. It is thus possible to unclog the pores in the membrane member  55 . 
   The means  75  for conveying the extracted gas comprise means  87  for introducing a vector gas into the receiver chamber  71  via the inlet orifice  79 . By way of example, the vector gas is nitrogen or air. 
   A mass flow regulator  89  sets the rate at which the vector gas enters into the chamber  71 , and consequently the rate at which gas enters into the analyzer  53 . As a result, the rate of dilution of the extracted gas is constant over time. A volume flow meter  91  is mounted in the pipe  61  downstream from the filter means  77  in order to measure the flow of gas that results from the vector gas together with the extracted gases. 
   The filter  77  is disposed on the pipe downstream from the pressure regulator  85 . The filter  77  serves in particular to eliminate the water vapor present in the extracted gas. By way of example it is constituted by a desiccator based on silica gel filter cartridges, a molecular sieve, or a coalescing filter. 
   The analyzer  53  comprises instrumentation  93  for detecting and quantifying one or more extracted gases, together with a computer  95  for determining the gas concentration in the liquid flowing in the drilling pipe  13 . 
   By way of example, the instrumentation comprises infrared detector appliances for quantifying carbon dioxide, flame ionizing detector (FID) chromatographs for detecting hydrocarbons, or indeed a thermal conductivity detector (TCD), depending on the gases to be detected. It is thus possible with the device of the invention to detect and quantify a plurality of gases simultaneously. 
   This instrumentation  93  is placed in the explosive zone in the vicinity of the well head  23  ( FIG. 1 ) in order to avoid conveying the gases over a long distance, thereby improving measurement accuracy. 
   The analyzer further comprises a sensor  97  for measuring the temperature of the liquid flowing in the drilling pipe  13 . 
   The computer  95  has a memory  99  containing calibration charts and a processor  101  for implementing a calculation algorithm. 
   The calibration charts are established as a function of temperature, of flow rate, and of the characteristics of the mud. They contain data relating to the concentration of one or more gases in the mud to the concentration of the gases extracted from the mud through the membrane member, and as measured using the instrumentation. 
   The calculation algorithm determines the real quantities of the gases in the mud on the basis of the measurements performed by be instrumentation  93 , the temperature measured in the drilling pipe  13  by the sensor  97 , and the data contained in the memory  99 . 
   The concentration of gases in the mud is determined either individually or cumulatively. 
   The operation of the device of the invention while drilling a well is described below by way of example. 
   While drilling, the drilling tool  15  is rotated by the surface installation  41 . A drilling liquid is introduced into the inside space  35  of the drill string  29  by the injector means  43 . The liquid goes down to the drilling head  27  and passes into the drilling pipe  13  through the drilling head  27 . This liquid cools and lubricates the drill  33 . Thereafter the liquid collects the solid cuttings that result from the drilling, and it rises via the annular space defined between the drill string  29  and the walls of the drilling pipe  13 . This liquid flows substantially parallel to the walls. 
   The liquid thus flows continuously over the first face  57  of the membrane member  55 . A fraction of the gas present in the liquid is extracted through the membrane member  55  and penetrates into the extractor chamber  71 . The pressure controller  73  controlling the pressure in the chamber  71  is activated so that the pressure difference between the chamber  71  and the drilling pipe  13  is substantially zero. This prevents liquid penetrating into the membrane member  55 . 
   The extracted gases are then entrained by the vector gas from the extractor chamber  71  through the outlet orifice  81 , the pressure regulator  85 , and the filter  77  to the analyzer  53 . The extracted gases are then analyzed by the instrumentation  63  and the computer  95  determines the real concentration of each analyzed gas in the drilling mud as a function of time. 
   In the variant shown in  FIG. 3 , the sampling head  51  is installed in a branch connection  111  on the drilling pipe  13 . Isolation means, such as an inlet valve  113  and an outlet valve  115 , are provided at the ends of the branch connection  111  on either side of the head  51  to isolate the branch connection and make it easy to remove the sampling head  51 . In this configuration, the risk of the membrane member  55  being damaged by mechanical contact or impact when tools are being inserted into the drilling pipe  13  or are being moved therealong is minimized. 
   In the variant shown in  FIG. 4 , a recirculation pipe  121  is provided for conveying the liquid extracted from the vibrating screen  45  to the means  43  for injecting liquid into the inside space  35  of the drill string  29 . 
   Unlike the installation shown in  FIG. 1 , two devices of the invention  19  and  19 A are used. The measuring head  51  of the first device  19  is disposed on the delivery pipe  25  in the upstream portion of said pipe, i.e. at the wellhead  23 . The measuring head  51 A of the second device  19 A is disposed on the injection pipe  123  between the injector means  43  and the injector head  31 . It is thus possible to quantify the difference between the gaseous content of the liquid leaving the drilling pipe  13 , and the gaseous content of the liquid reinjected after being degassed by the filtering screen  45 . 
   In the variant shown in  FIG. 5 , unlike the device shown in  FIG. 1 , the sampling head  51  has two porous membrane members  55  and  55 A. Each porous membrane member  55 ,  55 A is associated with a respective receiver chamber  71 ,  71 A for receiving extracted gases, and each having an inlet orifice  79 ,  79 A and an outlet orifice  81 ,  81 A. The inlet orifice of the first chamber is connected to the conveyor means  75 . The outlet orifice  81  of the first chamber is connected to the inlet orifice  79 A of the second chamber  71 A by the pipe  61 . 
   Thus, the vector gas is brought into the first chamber  71  via the inlet orifice  79  of said first chamber  71 . This gas brings the gases extracted into the first chamber  71  up to the second chamber  71 A via the outlet orifice  81 , the pipe  61 , and the inlet orifice  79 A of the second chamber  71 A. The second chamber  71 A thus receives a mixture containing the gases extracted into the first chamber  71  and the vector gas. This mixture then receives the gases extracted into the second chamber  71 A, thereby enriching it in gas coming from the drilling pipe  13  and making it easier for the analyzer  53  to detect the extracted gases. 
   In a variant, the support  63  of the porous membrane member has a face that presents Vickers hardness greater than 1400 kgf/mm 2 , in particular lying in the range 1400 kgf/mm 2  to 1900 kgf/mm 2 , without it being necessary to have a coating based on silicon carbide. In an example, the membrane member of this type may be made of α alumina. 
   In another variant, the membrane support is made on the basis of an organic material such as polytetrafluoro-ethylene, for example, and it has a coating of silicon carbide. 
   In another variant, a heater means is implanted on the drilling pipe upstream from the device of the invention relative to the flow direction of the drilling fluid in order to make it easier to extract dissolved or free gases. Under such circumstances, the device and the heater are disposed in a branch connection through which the mud flows freely or under assistance. 
   The invention as described above provides a device for analyzing accurately and continuously the gases contained in an abrasive liquid flowing along an installation for drilling into the subsoil. 
   Membrane members of a variety of kinds and shapes can be used with the device, depending on the characteristics of the drilling fluid and on the configuration of the well being drilled. 
   In particular, the device can be made from membranes that are simple in shape and easily available such as membranes in the form of plane disks. 
   The device is not selective and can be used to analyze individual or accumulated concentrations of a plurality of gases that are dissolved or free in the drilling liquid. 
   The device also presents the advantage of minimizing any risks of the device being damaged when objects are inserted into the drilling pipe and moved therealong. 
   The device also makes it possible to limit to a very great extent any clogging of the membranes and to limit the resulting loses of efficiency.

Summary:
A device includes an analyzer ( 53 ) and a sampling apparatus ( 51 ) for taking a sample of at least one fraction of the gas that has at least one porous membrane element ( 55 ). The porous membrane element ( 55 ) has a support ( 63 ) and a first surface ( 57 ) which is in contact with liquid circulating in duct ( 13 ) and a second surface ( 59 ) which opens out into a duct ( 61 ) which is connected to the analyzer ( 53 ). The hardness of the first surface ( 57 ) is more than 1400 Vickers (kgf/mm 2 ), ranging more particularly between 1400 and 1900 Vickers (kgf/mm 2 ). The device can be used to analyze the gaseous content of oil well boring sludge.