Abstract:
A wellbore tool has an electrochemical sensor for measuring the amount of hydrogen sulphide or thiols in a fluid downhole in a wellbore. The sensor comprises a temperature- and pressure-resistant housing containing a flow path for the fluids. The fluids flow over one side of a gas permeable membrane the other side of the membrane being exposed to a chamber containing at least two electrodes and containing a reaction solution which together with the hydrogen sulphide or thiols create a redox reaction resulting in an electrical current dependent upon the amount of hydrogen sulphide or thiols in the fluid. Measurement is made by passing formation fluid along the flow path and repeatedly applying varying potential to one electrode and measuring the peak current flowing between that electrode and a second electrode.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/541,568 which is incorporated herein by reference and which was based on PCT application PCT/GB2003/002345 filed 28 May 2003. This application claims priority under 35 USC 119 from GB application 03/0008125.5 dated 15 Jan. 2003. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to methods and apparatus for measuring the amount of hydrogen sulphide and thiols in fluids, and is more particularly but not exclusively concerned with methods and apparatus for measuring the amount of hydrogen sulphide and thiols in formation fluids from an earth formation surrounding a wellbore. 
     BACKGROUND OF THE INVENTION 
     It is highly desirable to be able to determine at as early a stage as possible the amount of hydrogen sulphide in oil and gas deposits in the earth formations surrounding a wellbore, since the amount of hydrogen sulphide can seriously impact the economic value of the deposits, and affect the composition (and therefore the cost) of the metalwork used in the extraction of the deposits from the formations. Additionally, because hydrogen sulphide is toxic in even relatively low concentrations, the hydrogen sulphide content of the deposits has an important bearing on the health, safety and environmental aspects of their extraction. 
     Several methods and apparatuses for the measurement of the hydrogen sulphide content of wellbore fluids are described in International Application No. WO 01/63094 (now granted as UK Patent No. 2 395 631). Among these are a method and apparatus based on an electrochemical sensor in which the current created by a redox reaction involving the hydrogen sulphide is measured. More specifically, the sensor comprises a reaction chamber or cell containing a precursor or catalyst (hereinafter referred to simply as a precursor) in an aqueous reaction solution, the walls of the chamber including a gas permeable membrane over which the wellbore fluids flow and through which hydrogen sulphide in the wellbore fluids diffuses into the reaction chamber to initiate the redox reaction, at the surface of an electrode controlled at certain voltage. 
     However, as the search for hydrocarbons is extended, wellbores are becoming deeper, so that the environment, in which electrochemical sensors are required to operate, is becoming increasingly hostile. Typically, the sensors need to be able to operate at temperatures of up to 200 degrees Celsius and pressures of up to 20,000 psi. 
     It is an object of the present invention to provide new electrochemical sensors of the type in which the current created by a redox reaction involving the hydrogen sulphide is measured, and which are suitable for use in severe borehole environments. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided an electrochemical sensor for measuring the amount of hydrogen sulphide or thiols in a fluid, the sensor comprising a housing having a flow path for the fluid therethrough, a substantially rigid gas permeable membrane disposed in the housing and having one side exposed to the flow path, and a chamber disposed in the housing, the chamber being exposed to the other side of the membrane and containing reagents which together with the hydrogen sulphide or thiols entering the chamber via the membrane create a redox reaction resulting in an electrical current dependent upon the amount of hydrogen sulphide or thiols in said fluid. 
     Preferably, the housing is provided with pressure balancing means for reducing the difference between the respective pressures on each side of the membrane. 
     It will be appreciated that the pressure balancing means serves to reduce the stresses on the membrane resulting from the generally high pressure environment in which the sensor is used, and in particular from rapid variations in pressures, which can sometimes vary between 20,000 psi and atmospheric in just a few seconds. 
     Advantageously, the pressure balancing means comprises a movable partition, piston or bellow having a first pressure surface in pressure communication with the flow path and a second pressure surface in pressure communication with the chamber. Thus the first pressure surface of the movable piston may be directly exposed to the fluid path, and the second pressure surface of the movable piston may be directly exposed to the reagents. 
     Also, the membrane is preferably trapped between respective sealing means which extend around the periphery of the membrane on each side thereof. 
     Advantageously, the housing includes a first housing member which is generally cup-shaped and is provided with a centrally disposed aperture in its base, and a second housing member which is substantially cylindrical and screws coaxially into the cup-shaped housing member so as to trap the membrane between the end of the second housing member within the first housing member and the base of the cup shape of the first housing member, one side of the membrane completely covering said aperture, and the flow path extending transversely through both housing members and communicating with the other side of the membrane via a coaxially disposed conduit in the second housing member. Conveniently, the housing includes a third housing member having a generally cylindrical recess for coaxially receiving the first and second housing members so as to define therewith a cylindrical space between the base of the cup shape of the first housing member and the base of the recess, said cylindrical space forming at least part of the chamber. The sealing means on said one side of the membrane preferably comprises a substantially coaxially disposed O-ring trapped between said one side of the membrane and the base of the cup shape of the first housing member, while the sealing means on the other side of the membrane may comprise sealing engagement between said other side of the membrane and a planar surface formed on the end of the second housing member within the first housing member. A further coaxially disposed O-ring may be trapped between the base of the cup shape of the first housing member and the base of the recess. 
     The chamber preferably includes a working electrode, a counter electrode and a reference electrode, the electrodes being spaced apart in the chamber and arranged such that said current flows between the working and counter electrodes. Advantageously, the working electrode is made from boron-doped diamond, although it can also be made from glassy carbon or platinum. 
     The chamber is exposed to the other side of the membrane and contains a working electrode, a counter electrode, a reference electrode and reagents which together with the hydrogen sulphide or thiols entering the chamber via the membrane create the redox reaction resulting in the electrical current dependent upon the amount of hydrogen sulphide or thiols in the fluid between the working and counter electrodes, wherein the working electrode is made from boron-doped diamond, glassy carbon or platinum. 
     The counter electrode may be made of platinum, while the reference electrode may be made of silver coated with silver chloride or silver iodide, or platinum. The electrodes may be mounted on or in an insulating base, preferably made from polyetheretherketone (PEEK). The housing members may also be made from PEEK. The reagents may include dimethylphenylenediamine (DMPD) or its structural analogues, or an aqueous ferrocyanide or ferrocene solution. The membrane may be made from zeolite or a suitable ceramic material. 
     From another aspect, the invention also includes a method of measuring the amount of hydrogen sulphide or thiols in formation fluid from an earth formation surrounding a wellbore, the method comprising positioning a wellbore tool equipped with an electrochemical sensor in accordance with the first aspect of the invention in the wellbore adjacent to the formation, exposing the sensor to the formation fluid, and measuring the resulting redox current produced by the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic representation of a wellbore tool which is positioned in a wellbore and which is equipped with an electrochemical sensor in accordance with the present invention for measuring the amount of hydrogen sulphide or thiols in formation fluid from an earth formation surrounding the wellbore; 
         FIG. 2  is a partially cutaway perspective view of the electrochemical sensor of  FIG. 1 ; 
         FIG. 3  is a more detailed sectional view of the electrochemical sensor of  FIG. 2 ; 
         FIG. 4  shows an electrode assembly forming part of the sensor of  FIGS. 2 and 3 ; 
         FIGS. 5A ,  5 B,  5 C and  5 D are four different views of part of the housing of the sensor of  FIGS. 2 and 3 ; 
         FIGS. 6A ,  6 B and  6 C are three different views of another part of the housing of the sensor of  FIGS. 2 and 3 ; 
         FIG. 7  shows cyclic voltammograms for the sensor of  FIGS. 2 and 3  for various concentrations of hydrogen sulphide, using dimethylphenylenediamine (DMPD); 
         FIG. 8  shows cyclic voltammograms for the sensor of  FIGS. 2 and 3  for various concentrations of hydrogen sulphide, but using ferrocyanide; and 
         FIGS. 9A and 9B  show a modified version of the sensor of  FIGS. 2 and 3 , incorporating a pressure balancing feature. 
     
    
    
     The terms “upper” and “lower” used in relation to the embodiments of the sensor of the invention described below merely refer to the orientation of the sensor as viewed in the drawings, and have no significance to the orientation of the sensor in use or any other context. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The wellbore tool shown in  FIG. 1  is indicated at  10 , and is based on Schlumberger&#39;s well known modular dynamics tester (MDT), as described in Trans. SPWLA 34 th  Annual Logging Symposium, Calgary, Jun. 1993, Paper ZZ and in U.S. Pat. Nos. 3,780,575, 3,859,851 and 4,994,671. The tool  10  comprises an elongate substantially cylindrical body  12 , which is suspended on a wireline  14  in the wellbore, indicated at  16 , adjacent an earth formation  18  believed to contain recoverable hydrocarbons, and which is provided with a radially projecting sampling probe  20 . The sampling probe  20  is placed into firm contact with the formation  18  by hydraulically operated rams  22  projecting radially from the body  12  on the opposite side from the sampling probe, and is connected internally of the body to a sample chamber  24  by a conduit  26 . 
     In use, and prior to completion of the well constituted by the wellbore  16 , a pump  28  within the body  12  of the tool  10  can be used to draw a sample of the hydrocarbons into the sample chamber  24  via the conduit  26 . The pump is controlled from the surface at the top of the wellbore via the wireline  14  and control circuitry (not shown) within the body  12 . It will be appreciated that this control circuitry also controls valves (not shown) for selectively routing the sampled hydrocarbons either to the sample chamber  24  or to a dump outlet (not shown), but these have been omitted for the sake of simplicity. 
     In accordance with the present invention, the conduit  26  additionally communicates with an electrochemical sensor  30  also provided within the body  12  of the tool  10 , so that the hydrocarbons flow over a face of the sensor on their way through the conduit. The sampling probe is located close to the electrochemical sensor  30 , at a distance comprised between 8 and 30 cm from said electrochemical sensor, advantageously approximately equal to 15 cm. As will become apparent, the sensor  30  produces an output current, which is dependent on the amount of hydrogen sulphide or thiols in the hydrocarbons flowing through the conduit  26 . This output current is measured in known manner by a digital current measuring circuit  32  in the body  12  of the tool  10 , and the measurement is transmitted to the surface via the wireline  14 . 
     The sensor  30  is shown in more detail in  FIGS. 2 to 6 , and comprises a generally cylindrical housing  40 , which is made from polyetheretherketone (PEEK) and which comprises a main housing member  42  having an upper portion  44  (as viewed in the drawings), a reduced diameter lower portion  46 , and a stepped diameter cylindrical bore  48  extending coaxially through it from top to bottom. The bore  48  has a large diameter upper portion  50  wholly within the upper portion  44  of the main housing member  42 , an intermediate diameter portion  52  also wholly within the upper portion of the main housing member, and a reduced diameter portion  54  largely within the lower portion  46  of the main housing member. 
     A flowpath  56  for the fluid whose hydrogen sulphide content is to be sensed extends diametrically through the upper portion  44  of the main housing member  42 , intersecting the upper portion  50  of the bore  48 . 
     Disposed in the intermediate diameter portion  52  of the bore  48 , and resting on the shoulder defined between the reduced diameter portion  54  and the intermediate diameter portion, is a cylindrical electrode assembly  58 , best seen in  FIGS. 2 ,  3  and  4 . The electrode assembly  58  comprises an insulating body  60 , also made of PEEK, having three electrodes on its upper surface, namely a working electrode  62  made from boron-doped diamond, a reference electrode  64  in the form of a silver dot coated with silver chloride or silver iodide, and a counter electrode  66  comprising a printed platinum track. The electrodes  62 ,  64 ,  66  are connected via respective electrical conductors  68  moulded into and extending axially through the body  60  in a sealed manner to respective electrical leads  70 , which exit the main housing  30  via the reduced diameter portion  54  of the bore  48 . An O ring  72  made of VITON™ is disposed in a groove  74  extending coaxially round the body  60  to seal the electrode assembly  58  within the intermediate diameter portion  52  of the bore  48 . 
     Disposed in the large diameter upper portion  50  of the bore  48 , and resting on the shoulder defined between the intermediate diameter portion  52  and the large diameter portion is a cylindrical membrane retainer assembly  76 , which comprises a cup-shaped housing member  78  (best seen in  FIGS. 5A ,  5 B,  5 C and  5 D), a cylindrical housing member  80  (best seen in  FIGS. 6A ,  6 B and  6 C) which screws part of the way into the cup-shaped housing member  78 , and a gas permeable membrane  82  in the form of a circular plate made of zeolite or other suitable ceramic material coaxially located in the cup-shaped housing member  78 , in the space between the bottom of the inside of the cup shape of the housing member  78  and the bottom of the housing member  80 . The housing member  80  has a diametrically extending flow path  84  therethrough, and the housing member  78  has diametrically opposed ports  86  aligned with the opposite ends of the flow path  84 , the flow path  84  and the ports  86  being aligned with the flow path  56  in the upper part  44  of the main housing member  42 . The housing member  80  further includes a short duct  88  communicating between the flow path  84  and the bottom of the housing member, and therefore communicating with the upper surface of the membrane  82 . 
     The bottom of the housing member  80  is flat, and bears on the upper surface of the membrane  82 , pressing it towards the bottom of the inside of the housing member  78 . An O-ring seal  90  made of VITON™ is trapped between the lower surface of the membrane  82  and the bottom of the inside of the housing member  78  to provide sealing around the entire periphery of the lower surface of the membrane, while the flat bottom of the housing member  80  and the upper surface of the membrane  82  provides a seal around the entire periphery of the upper side of the membrane. A further O-ring seal  92  also made of VITON™ is disposed in a groove  96  formed coaxially in the shoulder defined between the intermediate diameter portion  52  and the large diameter portion of the bore  48 , and is trapped between the underside of the bottom of the housing member  78  and the shoulder. 
     The generally cylindrical space  94  beneath the underside of the membrane  82  and the top of the electrode assembly constitutes a reaction chamber, and is filled with a reaction solution containing a precursor or catalyst, for example, dimethylphenylenediamine (DMPD). 
     In operation, the sensor  30  fits in a cylindrical recess in a block (not shown) through which the conduit  26  passes, with the flow path  56  in alignment with the conduit  26 , and with sealing provided by a VITON™ O-ring (not shown) in a groove  96  in the upper portion  44  of the housing  40  of the sensor. The upper side of the membrane  82  in the sensor  30  is thus exposed via the flow path  56  the ports  86 , the flow path  84  and the duct  88  to the hydrocarbons in the conduit  26 , and suitable electronic measurement equipment is used to apply a cyclically varying potential between the working electrode  40  and the reference electrode  44 , and to measure the peak currents flowing between the working electrode  40  and counter electrode  42 . Cyclic voltammograms for the sensor  30  are shown in  FIG. 7 , which includes an inset graph showing the variation of the peak oxidation current with sulphide concentration. It can be seen that for concentrations of sulphide between 20×10 −6  molar (0.7 ppm) and 100×10 −6  molar (3.5 ppm), the oxidation current increases substantially linearly with increasing sulphide concentration. 
     The sealing of the membrane  82  in the housing members  78  and  80  using a surface-to-surface seal and the O-ring seal  90 , coupled with the sealing provided by the O-ring seal  92 , ensures that the reaction solution is not washed out of the chamber  94  by the hot, high pressure hydrocarbons in the flow path  56 , while the materials used, in particular for the membrane  82 , are also able to withstand the hostile borehole environment. 
     Many modifications can be made to the described implementation of the sensor  30 . 
     In particular, reagents other than DMPD can be used. For example, for higher concentrations of hydrogen sulphide, an aqueous solution of ferrocyanide ions, e.g. potassium ferrocyanide, or an aqueous ferrocene solution can been used. Cyclic voltammograms for the sensor  30  using an aqueous solution of ferrocyanide ions are shown in  FIG. 8 , which again includes an inset graph showing the variation of the peak oxidation current with sulphide concentration. 
     To further improve the high pressure capability of the sensor  30 , the pressures on both sides of the membrane  82  can be balanced, as achieved in the modified version of the sensor  30  indicated at  30   a  in  FIGS. 9A and 9B . The sensor  30   a  is substantially identical to the sensor  30  except for the addition of the pressure balancing feature, so corresponding elements have the same reference numbers, and only the differences, due to the pressure balancing feature, will be described. 
     In the sensor  30   a , the upper portion  44  of the housing  40  has a first cylindrical bore  100  drilled into it parallel to but partly below the flowpath  56  and offset from the chamber  94 , the bore having an initial larger diameter portion  102  containing a movable piston  104  sealed in the bore by a VITON™ O-ring  106 . The bore  100  continues with a coaxially aligned intermediate diameter portion  108 , and finishes in a small diameter duct portion  110  aligned with the bottom of the intermediate diameter portion and at the level of chamber  94 . 
     A second cylindrical bore  112  is drilled into the upper portion  44  of the housing  40  at an angle of about 70 degrees to the first bore  100 , this second bore having an initial larger diameter portion  114  containing a piston-like plug member  116  sealed in the bore by a VITON™ O-ring  118  substantially equal in length to the larger diameter portion  114 . The bore  112  finishes in a small diameter duct portion  120  aligned with the bottom of the portion  114  of the bore and at the level of the chamber  94 , this passage forming a chord through one side of the circular cross-section of the chamber  94  and intercepting the end of the passage  110 . 
     It will therefore be appreciated that the respective portions of the bores  100  and  112  disposed between the piston  104  and the piston-like plug member  116  effectively form extensions to the chamber  94 , so that in use the liquid reagents in the chamber also fill these portions of the bores. Additionally, the respective surfaces of the piston  104  and the plug-like piston member  116  facing out of the bores  100  and  112 , being above the level of the sealing ring in the groove  96  in the upper portion  44  of the housing  40  of the sensor  30   a , are effectively exposed to the pressure of the hydrocarbons in the flowpath  56 . The piston  104  therefore maintains the pressure of the liquid reagents in the chamber  94  substantially equal to the pressure of the hydrocarbons in the flowpath  56 , thus substantially eliminating the pressure differential across the membrane  82  and prolonging its useful life. The piston-like plug member  116 , to the extent that it is capable of very slight movement in response to pressure, assists in the pressure balancing function of the piston  104 , while the respective portions of the bores  100  and  112  disposed between the piston  104  and the piston-like plug member  116  effectively increase the volume or capacity of the chamber  94  and therefore increase the volume of the reagents available in the sensor  30   a.    
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.