Abstract:
This invention relates to a measurement tool and method of use, and in particular to a measurement tool for use in determining a parameter of a stationary or moving fluid. The measurement tool has been designed primarily for use in borehole formation testing. The measurement tool can measure the dielectric constant of a fluid within a pipe or surrounding the tool. The pipe or wall between the tool and the fluid is electrically insulating. The tool has pair of capacitor plates mounted adjacent to the pipe or wall, a signal generator which can deliver an alternating electrical signal to at least one of the capacitor plates, and a detector for measuring a signal dependent upon the electrical capacitance between the capacitor plates. The measurement tool can additionally measure the electrical resistivity of the fluid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/236,591 filed 24 Sep. 2008, which claims priority to Great Britain Patent Application No.GB0718851.9 filed on 27 Sep. 2007, the contents of each one incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a measurement tool and method of use, and in particular to a measurement tool for use in determining a parameter of a stationary or moving fluid. The measurement tool has been designed for use in borehole formation testing and the following description will therefore relate primarily to such applications, but the invention is not thereby limited. 
       BACKGROUND OF THE INVENTION 
       [0003]    Measurement tools are in widespread use in borehole formation testing, for example in boreholes drilled into the earth in order to test for or recover underground reserves of oil and/or gas. Some such tools are carried by the drill string and the measurements are carried out during the borehole drilling operation (so-called “measurement-while-drilling” (MWD) or “logging-while-drilling” (LWD) applications). Other measurement tools are used after the borehole has been drilled, the measurement tools being lowered into the borehole by a cable or wire. In highly deviated wells conveyance may be assisted by semi-rigid tubing or by drill-pipe. Still other measurement tools are deployed downhole for lengthy periods of time with or without a connecting cable and are referred to as permanent or retrievable gauges. These are usually for use in production after the exploration phase is complete. 
         [0004]    Tools deployed using cable having one or more electrical conductors are generally referred to as “electric wireline tools”. The present invention is most likely to be a part of an electric wireline tool, though its use in MWD/LWD or other downhole applications is not thereby excluded. 
         [0005]    One known electric wireline tool is a formation testing tool or “pump-out” tool, which is used to extract a volume of fluid from a formation surrounding a borehole, the fluid being tested in order to evaluate the likely productivity of the oil or gas well. 
         [0006]    It is a recognised problem of operating formation testing tools that during the borehole drilling operation the fluid within the formation can be contaminated with drilling fluid (or “mud”) filtrate typically comprising liquid and other materials. In order to obtain valuable test results it is of prime importance that the formation fluid used for analysis represents virgin formation fluid with little or no contamination from fluids used in the borehole drilling operation. 
         [0007]    Drilling fluid is generally divided into oil base mud (OBM) and water base mud (WBM). The drilling fluid pressure is maintained higher than that of the formation, and as a result the drilling fluid seeps into the formation, the seeping fluid being known as filtrate. Fine particles that cannot penetrate the formation are left behind on the borehole wall and build up to form a filter (or “mud”) cake. This is relatively impermeable and forms a skin substantially preventing further ingress of fluid. The filtrate displaces virgin formation fluid from the vicinity of the borehole wall, until a stable ‘invaded zone’ results. Depending on the virgin fluid, the type of mud and the formation composition and structure, different degrees and depth of invasion occur into the formation. 
         [0008]    The formation fluid may naturally contain a large percentage of water, of some salinity. Water base mud is predominantly water but need not have the same salinity. Although perfect oil base mud has very little water, in practice it may contain as much as 40% water. Filtrate may include formation water from other depths in the borehole that has mixed into the mud. 
       DESCRIPTION OF THE PRIOR ART 
       [0009]    Traditionally, operators wishing to extract a volume of fluid from a formation surrounding a borehole in order to evaluate the likely productivity of the well utilised drill stem testing, in which the formation fluid was allowed to flow or was pumped to the surface for testing. This practice has become less desirable primarily because of the harmful environmental impact of needing to flare-off excess gas. Also, there is difficulty in bringing the fluid to the surface from particular wells, especially sub-sea wells. Furthermore, the pressure and temperature of the fluid changes during its movement through the borehole to the surface, and these pressure and temperature changes can cause changes in the consistency of the fluid (i.e. the fluid may separate out into oil, water and gas or otherwise change its material characteristics) which may invalidate the subsequent test. 
         [0010]    To overcome the problems associated with flowing the formation fluid directly to the surface, formation testing tools have been developed which can undertake at least some of the tests downhole. One such formation testing tool is described in U.S. Pat. No. 5,602,334, the tool including measurement tools able to measure selected parameters of the formation fluid downhole. This formation testing tool also includes containers which can be filled with formation fluid for transportation to the surface for additional testing if desired. 
         [0011]    It is of course necessary that formation testing tools such as that of U.S. Pat. No. 5,602,334 be able to determine whether the fluid being pumped out of the formation is virgin formation fluid, or is contaminated formation fluid, so that the tests are conducted only upon virgin formation fluid, and only virgin formation fluid is collected in the containers. For present purposes “virgin” means having as little contamination as possible, and certainly below some threshold of acceptability. 
         [0012]    Many different parameters are desired to be tested downhole, some of which assist in determining whether the fluid is virgin or contaminated, and others which assist the operator in assessing the likely productivity of the formation. 
         [0013]    A parameter which can be measured downhole is the electrical resistivity of the fluid. This parameter is often used to determine whether the fluid is virgin or contaminated because the electrical resistivity of oil is significantly different to that of water base muds. US patent application 2007/0018659 discloses a measurement tool for use in a formation testing tool, the tool measuring the resistivity of the formation fluid flowing through it. 
         [0014]    In US patent application 2007/0018659 the resistivity of the formation fluid is tested as the fluid is flowing through a pipe, and this is a particularly desirable feature of measurement tools used in formation testing tools where the pipe can lie within the formation testing tool. The pipe should preferably be substantially linear and free from constrictions, bends or voids which would induce pressure changes into the fluid, which pressure changes may affect the consistency of the fluid and thereby lead to a different test result than would be obtained upon fluid within the formation. 
         [0015]    Another parameter which can be measured downhole is pressure, typically as part of a draw-down and build-up pressure test which can be used to determine the mobility (permeability divided by viscosity) of a formation and therefore help to assess the likely productivity of the formation. 
         [0016]    Yet another parameter is the chemical constituents of the fluid, which can be used to determine whether the formation fluid at one depth of the borehole is the same as that at another depth, any chemical dissimilarity between the formation fluids at different depths indicating that the formation is not contiguous and is instead made up of discrete reservoirs which will make the oil and/or gas more difficult and expensive to recover. Chemical dissimilarity can also be used to differentiate virgin fluid and filtrate. 
         [0017]    The likely productivity of an oil and/or gas reservoir is a very valuable assessment for operators to make as this determines the likely value of the reservoir to the operator. It is an object of this invention to provide a measurement tool which can be used in a formation testing tool and which is able to test more relevant parameters of the formation fluid and/or which is able to test the relevant parameters more accurately and reliably, so that the operator can make a more accurate assessment of the productivity of a particular reservoir. 
         [0018]    It is another object of the present invention to assist in distinguishing virgin formation fluid from invasion filtrate (“contamination”), recognising that both the water and oil components in the invaded zone are often a mixture of residual virgin fluid and filtrate, and that virgin fluid beyond the invaded zone may have water or oil similar to that of the mud filtrate. 
       SUMMARY OF THE INVENTION 
       [0019]    According to the first aspect of the invention, there is provided a measurement tool for measuring the dielectric constant of a fluid within a pipe, the pipe being electrically insulating, the tool having: 
         [0000]    a pair of capacitor plates mounted adjacent to the pipe,
 
a signal generator which can deliver an alternating electrical signal to at least one of the capacitor plates, and
 
a detector for measuring a signal dependent upon the electrical capacitance between the capacitor plates.
 
         [0020]    By suitable arrangement of the capacitor plates, the measured signal will depend upon the capacitance of the fluid within the pipe, and the capacitance of the fluid can be used to determine the dielectric constant of the fluid. 
         [0021]    Preferably, the pair of capacitor plates are mounted outside of the pipe, so that there is no direct contact between the plates and the fluid. The capacitance measured will therefore depend upon the capacitance of the fluid and the capacitance of the pipe. Using an insulating pipe with a high dielectric constant in an appropriate arrangement with the capacitor plates will result in the measured signal being dependent primarily upon the capacitance of the fluid. 
         [0022]    It has been recognised that the dielectric constant of predominantly oil or gas virgin formation fluid is significantly different to the dielectric constant of water base muds. The dielectric constant of virgin formation fluid is also often measurably different to the dielectric constant of oil base muds because of their differing water contents, so that the present measurement tool can be used to assist determination of whether the fluid within the pipe is contaminated or is virgin formation fluid suitable for further testing. Also, the dielectric constant of virgin formation fluid can provide valuable information to the operator. 
         [0023]    The capacitor plates may surround respective parts of the pipe so that the capacitance is measured “along” the pipe. Alternatively, the capacitor plates may be mounted to opposed sides of the pipe, so that the capacitance is measured “across” the pipe. In embodiments in which the pipe is of circular cross-section the capacitor plates can be annular or part-annular. 
         [0024]    Desirably, the dielectric constant of the pipe is at least eight. The pipe therefore has a much higher dielectric constant than oil (which typically has a dielectric constant of around two) and is acceptable in relation to water (which has a dielectric constant ranging from around twenty to around eighty one according to factors like temperature and contamination. 
         [0025]    There can be three capacitor plates. The use of three capacitor plates can enhance the signal strength of the apparatus and increase the volume of measured fluid. 
         [0026]    The three capacitor plates can be arranged along the length of the pipe, with the signal generator connected to the central, driven, capacitor plate, and with the other two capacitor plates connected to ground. 
         [0027]    Preferably, the signal generator is connected to its capacitor plate by a screened signal wire and the signal generator is connected to a metallic screen by a screening connector, the screening connector being connected to an operational amplifier configured as a “voltage follower”, so that the voltage upon the screening connector is matched to that of the signal wire. Ideally, the screening connector also surrounds the signal wire for at least part of its length, in the form of a coaxial or screened cable or the like. Because the voltages of the signal wire and the screening connector are matched, the capacitance of the coaxial or screened cable can be ignored, and yet the screening connector protects the signal wire from extraneous electrical signals. 
         [0028]    Preferably, at least the driven capacitor plate and pipe are surrounded by one or more metallic plate(s) acting as a focussing plate, connected by way of a voltage follower to the same potential as the driven capacitor plate. The focussing plate(s) acts to reduce the desensitising effects of portions of the pipe dielectric material exposed between the capacitor plates. The focussing plate(s) also shields the capacitor plate(s) from extraneous electrical signals. 
         [0029]    Desirably, the focussing plate(s) is connected to the metallic screen so that they may share the same voltage follower. Desirably the signal generator and voltage follower circuits are at one end of the screened wire and the capacitor plates and focussing plate(s) at the other end. 
         [0030]    The signal which is measured can be the electrical voltage upon, and the electrical current flowing through, the signal wire, which together can be used to determine the capacitance in known fashion. 
         [0031]    The frequency of the alternating signal is chosen to suit the application, it being understood that particular frequency ranges will be better suited to determining changes in the capacitance of particular fluids at particular temperatures. For a measurement tool for use in a formation testing tool the presently preferred frequency is 16 kHz, though other frequencies are expected to be suitable for particular configurations and apparatus. 
         [0032]    According to a second aspect of the invention there is provided a measurement tool for measuring the dielectric constant of a fluid, the measurement tool having a wall, the wall of the measurement tool being electrically insulating, the tool having: 
         [0000]    a pair of capacitor plates mounted adjacent to the wall,
 
a signal generator which can deliver an alternating electrical signal to at least one of the capacitor plates, and
 
a detector for measuring a signal dependent upon the electrical capacitance between the capacitor plates.
 
         [0033]    The arrangement according to the first aspect of the invention, with the measurement tool located around a pipe within which the fluid is located, is reversed in the second aspect, so that the measurement tool is located inside a sensing element which is immersed in the fluid. 
         [0034]    Thus, the inventors have realised that the invented tool can also be used in applications such as production logging, i.e. the determination of the fluid characteristics of the oil and gas being produced by a well, perhaps during the lifetime of the well, or at least for an extended period of time. Such ongoing testing of a production well is used in “intelligent wells”, in which data concerning the production fluid is continuously or regularly assessed. 
         [0035]    In such applications, it may be preferable to utilise the invention according to its second aspect, i.e. it may be more practical to immerse the measurement tool within the fluid in the well, rather than seek to pass some or all of the fluid along the pipe of a measurement tool, which may unnecessarily restrict the flow of fluid. 
         [0036]    Alternative, preferable and desirable features of the invention in its second aspect correspond to the alternative, preferable and desirable features of the invention in its first aspect. 
         [0037]    According to the first aspect of the invention there is also provided a method of measuring the dielectric constant of a fluid within a pipe, the method comprising the steps of: 
         [0000]    {i} providing an electrically insulating pipe and introducing the fluid into the pipe;
 
{ii} mounting a pair of capacitor plates adjacent to the pipe,
 
{iii} connecting a signal generator to at least one of the capacitor plates and delivering an alternating electrical signal to said at least one of the capacitor plates,
 
{iv} providing a detector to measure a signal dependent upon the electrical capacitance between the capacitor plates, and
 
{v} using the signal measured by the detector to determine the dielectric constant of the fluid. According to the second aspect of the invention there is provided a method of measuring the dielectric constant of a fluid, the method comprising the steps of:
 
{i} providing a measurement tool having an electrically insulating wall and a pair of capacitor plates mounted adjacent to the wall,
 
{ii} connecting a signal generator to at least one of the capacitor plates and delivering an alternating electrical signal to said at least one of the capacitor plates,
 
{iii} providing a detector to measure a signal dependent upon the electrical capacitance between the capacitor plates,
 
{iv} introducing the measurement tool to the fluid, and
 
{v} using the signal measured by the detector to determine the dielectric constant of the fluid.
 
         [0038]    The method steps need not be sequential and their order can be amended if required, and/or some of the steps can be concurrent. 
         [0039]    The measurement tool can also include an apparatus for determining a signal indicative of the electrical resistivity of the fluid. Electrical resistivity can be used to distinguish between filtrate and virgin formation fluid due to their different salinities and hydrocarbon-water ratios, and can be used to obtain valuable information in its own right. For example, if the virgin fluid is found to be water and not oil or gas, the operator can avoid subsequently producing it. Generally speaking, when water is the continuous phase a resistivity reading can be obtained. When oil or gas is the continuous phase a dielectric reading can be obtained. The measurements of dielectric constant and resistivity are thus complementary. Moreover, as fluid flows from the formation its composition may exhibit short term fluctuations and these may be used as a further differentiator and indicator of transition from filtrate to virgin fluid. 
         [0040]    Desirably, the measurement tool further includes: 
         [0000]    a first toroid surrounding a part of the pipe,
 
a second toroid surrounding a part of the pipe, separate from the first toroid,
 
a second signal generator connected to the first toroid for delivering an alternating electrical current to the coil of the first toroid,
 
a second detector connected to the second toroid for determining the current flowing through the coil of the second toroid, and
 
a return path conductor connected to the fluid in the pipe either side of the two toroids.
 
         [0041]    Conducting fluid in the pipe and the return path conductor together form a closed conducting loop threaded through the two toroids, thereby creating a coupled pair of transformers by acting as a resistive secondary turn to the first toroid and a primary turn to the second toroid. The resistance is principally due to the fluid as it can be arranged for the return conductor to be of relatively low resistance, such as by making it of metal. For convenience, the metalwork of the tool which necessarily surrounds the toroids can be used as the return path conductor, although a direct wired connection (perhaps in addition to the metalwork of the tool) may be preferred in some applications. 
         [0042]    The current flowing through the first toroid induces an electrical current to flow within the pipe and within the fluid inside the pipe. Because the pipe is an electrical insulator the current induced in the pipe is very small or effectively zero. The current flowing in the fluid is directly dependent upon the driving current and the resistivity of the fluid in the pipe. Any current flowing within the fluid induces a current to flow within the coil of the second toroid, the induced current being directly dependent upon the current flowing within the fluid. A comparison of the current flows through the first toroid and the second toroid will therefore provide a direct measure of the resistivity of the fluid within the pipe. 
         [0043]    Reference is made above to a second signal generator to distinguish this from the first signal generator used in the dielectric constant measurement. Thus, it is recognised that the optimum frequency range of the alternating signal for the dielectric constant measurement will not necessarily be the same as the optimum frequency range for the resistivity measurement. In certain applications, however, these ranges may overlap in which case the first signal generator and the second signal generator can be the same component. Similarly, the term “second detector” is used to distinguish from the first detector used in the dielectric constant measurement, since these detectors will in most applications be different components. 
         [0044]    It will be noted that it is desirable that the pipe containing the fluid (according to the first aspect of the invention) for both of the dielectric constant measurement and the resistivity measurement is the same pipe, and it is a benefit of the present invention that both of these measurements can be performed (perhaps continuously) on substantially the same volume of fluid, if desired. Thus, changes in the material consistency or constituents of the fluid which affect both its dielectric constant and its electrical resistivity can be determined by measurement of both of these parameters at substantially the same time, whereas changes in consistency and/or constituents which affect only one of these parameters will be determined only by measurement of that particular parameter. This will provide valuable additional information to operators over two unrelated measurement tools. 
         [0045]    Accordingly, in its first aspect the invention can provide a method of measuring the dielectric constant and the electrical resistivity of a fluid within a pipe, the method comprising the steps of: 
         [0000]    {i} providing an electrically insulating pipe and introducing the fluid into the pipe;
 
{ii} mounting a pair of capacitor plates adjacent to the pipe,
 
{iii} connecting a first signal generator to at least one of the capacitor plates and delivering an alternating electrical signal to said at least one of the capacitor plates,
 
{iv} providing a first detector to measure a signal dependent upon the electrical capacitance between the capacitor plates,
 
{v} using the signal measured by the first detector to determine the dielectric constant of the fluid,
 
{vi} locating a first toroid adjacent to a part of the pipe,
 
{vii} locating a second toroid adjacent to another part of the pipe,
 
{viii} connecting a second signal generator to a coil of the first toroid and delivering an alternating electrical current to the coil of the first toroid,
 
{ix} connecting a second detector to a coil of the second toroid for measuring the current flowing through the coil of the second toroid,
 
{x} providing a return path conductor connected to the fluid in the pipe to either side of the two toroids,
 
{xi} using the current measured by the second detector to determine the electrical resistivity of the fluid.
 
         [0046]    In its second aspect the invention can provide a method of measuring the dielectric constant and the electrical resistivity of a fluid, the method comprising the steps of: 
         [0000]    {i} providing a measurement tool having an electrically insulating wall and a pair of capacitor plates mounted adjacent to the wall,
 
{ii} connecting a signal generator to at least one of the capacitor plates and delivering an alternating electrical signal to said at least one of the capacitor plates,
 
{iii} providing a detector to measure a signal dependent upon the electrical capacitance between the capacitor plates,
 
{iv} locating a first toroid adjacent to a part of the wall,
 
{v} locating a second toroid adjacent to another part of the wall,
 
{vi} connecting a second signal generator to a coil of the first toroid and delivering an alternating electrical current to the coil of the first toroid,
 
{vii} connecting a second detector to a coil of the second toroid for measuring the current flowing through the coil of the second toroid,
 
{viii} providing a return path conductor to either side of the two toroids,
 
{ix} introducing the measurement tool to the fluid,
 
{x} using the signal measured by the detector to determine the dielectric constant of the fluid,
 
{xi} using the current measured by the second detector to determine the electrical resistivity of the fluid.
 
         [0047]    The method steps need not be sequential and their order can be amended if required, and/or some of the steps can be concurrent. 
         [0048]    In preferred methods the pair of capacitor plates are located between the two toroids and the dielectric constant and the electrical resistivity are measured simultaneously or substantially simultaneously. This enables the two parameters to be measured on the same or substantially the same body of fluid, even if the fluid is flowing. 
         [0049]    The alternating electrical signal and the alternating electrical current could be of sinusoidal or square waveform but this is not necessary for the performance of the invention, and any suitable alternating waveform can be used. 
         [0050]    In the embodiments according to the first aspect in which the same pipe is used for the measurement of both parameters, it must satisfy the separate requirements for each parameter, as above indicated. A ceramic pipe made from silicon nitride (Si 3 N 4 ) has been found to have a dielectric constant and a resistivity which matches the requirements of the measurement tool, and a suitable material is obtainable from Ceradyne Inc., of 3169 Red Hill Avenue, Costa Mesa, Calif. 92626, USA, and sold under the trade name CERALLOY 147-31N. In addition, a pipe of this material having an internal cross-sectional diameter of around 6.4 mm (¼ inch) and a wall thickness of 3.2 mm (⅛ inch) can withstand internal pressures of more than 1.7×10 8  Pa (25,000 p.s.i.). Since these are the pressures typically encountered at borehole depths of around 10 km it is possible to surround the pipe by air rather than requiring some incompressible material which might adversely affect the measurement of dielectric constant or resistivity, or render construction more difficult and less reliable. 
         [0051]    The measurement tool has additional benefits in multi-flow formation testing tools such as that described in U.S. Pat. No. 7,805,988. In this formation testing tool two (or more) fluid flows from the formation are kept separate and are tested separately, and a measurement tool of the present invention could be arranged in each flow line and direct comparisons between the two fluids could be made as desired. In particular a first fluid flow can be the primary flow for measurement and sampling purposes and a second fluid flow can be arranged to come from a different (but ideally adjacent) part of the formation to that of the first flow. For different fluid flows which leave the formation at the same time it is usually desirable to minimise any difference in the time at which the testing is undertaken, and ideally the different fluid flows should be tested at exactly the same time so that measured differences over time in the first and second fluid flows can be used as an indicator of the first fluid changing to virgin fluid (for example). It is possible to minimise (or eliminate) any differences in the time of testing using a multi-flow testing tool such as that of U.S. Pat. No. 7,805,988 by arranging the pipes for the different fluid flows to lie alongside one another, and to be of substantially identical lengths. 
     
    
     
       DESCRIPTION OF PREFERRED EMBODIMENTS 
         [0052]    The invention will now be described in more detail, by way of examples, with reference to the accompanying drawings, which show: 
           [0053]      FIG. 1  a longitudinal cross-section through the pipe of a measurement tool according to an embodiment of the first aspect of the invention; 
           [0054]      FIG. 2  a schematic representation in transverse cross-section of the capacitance of the fluid and pipe as measured by the tool of  FIG. 1 ; 
           [0055]      FIG. 3  a representation as  FIG. 2  showing the effect of a metallic focussing plate; 
           [0056]      FIG. 4  a transverse cross-section through a measurement tool according to the second aspect of the invention; 
           [0057]      FIG. 5  a schematic representation in transverse cross-section of the capacitance of the fluid and pipe as measured in an alternative embodiment of the first aspect of the invention; and 
           [0058]      FIG. 6  a schematic representation as  FIG. 5  showing the effect of a focussing plate. 
       
    
    
     DETAILED DESCRIPTION 
       [0059]    According to the first aspect of the invention, the measurement tool  10  has a pipe  12 . The pipe  12  is made of an electrically insulating material. The pipe  12  is also substantially linear and has a substantially uniform cross-section along its length, so that the pipe does not induce unwanted pressure changes into a fluid flowing therealong. The ends of the pipe are not shown, but in known fashion the ends are fitted with connectors by which the pipe may be sealingly connected to adjacent pipes or couplings. When used in a formation testing tool for example the pipe  12  may be connected to adjacent pipes within the body of the formation testing tool, the adjacent pipes perhaps being parts of other measurement tools for measuring other parameters of the fluid. 
         [0060]    In this embodiment the pipe is of circular cross-section, with an outer diameter of approx. 12.7 mm (approx. ½ inch), and an inner diameter of approx. 6.4 mm (approx ¼ inch). 
         [0061]    The tool  10  also has three capacitor plates  14 ,  16 ,  18 , the capacitor plates in this embodiment comprising conductive sleeves surrounding respective parts of the pipe  12 . In this embodiment the capacitor plates  14 ,  16  and  18  are of identical dimensions, but this is not necessarily so. 
         [0062]    A signal generator  20  is connected to the central capacitor plate  16  by a signal wire  22 , whereby the signal generator  20  delivers an alternating electrical signal to the capacitor plate  16 . The capacitor plates  14  and  18  are connected to ground, and the voltage which builds up on the capacitor plate  16 , and the current which flows onto and off from the capacitor plate  16  during each cycle, is directly dependent upon the capacitance of the system. 
         [0063]    A detector  24  is able to measure the voltage upon the signal wire  22  (relative to ground), and also the current flowing along the signal wire  22 , and can use these signals to determine the electrical capacitance of the system. The detector can incorporate a phase-sensitive detector to enhance signal to noise ratio. 
         [0064]    The capacitance of the system, namely the capacitance between the capacitor plate  16  and the capacitor plates  14  and  18 , is dependent upon the dielectric constant of the material therebetween. In an arrangement such as that shown in  FIG. 1 , with the capacitor plates arranged along the pipe, the electric field is generated between the facing ends of the capacitor plates  16  and  14 , and also between the facing ends of the capacitor plates  16  and  18 . Part of the electric field lies within the wall of the pipe  12 , part within the fluid  26  inside the pipe  12 , and part within the material surrounding the pipe, and so the capacitance of the system depends upon the dielectric constant of the pipe, the fluid, and the material surrounding the pipe. 
         [0065]    In this embodiment the pipe  12  is made from silicon nitride which has a dielectric constant of approximately eight. Also, the pipe  12  is surrounded by air which has a dielectric constant of one. Accordingly, the capacitance of the system is highly dependent upon the dielectric constant of the fluid  26 , and changes in the dielectric constant of the fluid  26  caused by changes in the consistency or constituents of the fluid  26  will cause a change in the capacitance of the system. 
         [0066]    The tool  10  can be calibrated (either by calculation or more typically with known fluids  26  at known temperatures), so that the measurement tool  10  can determine the actual dielectric constant of the fluid  26 . This will allow the measurement tool  26  to be used quantitatively which will allow the operator to make detailed assessments of the fluid, including for example its chemical constituents. Alternatively, the tool can be used qualitatively to determine changes in the material characteristics (for example identifying the change from contaminated formation fluid to virgin formation fluid) which determinations can be utilised by other measurement tools. 
         [0067]    The capacitor plates  14 ,  16  and  18  are all surrounded by an electrically conductive sleeve  30 , usefully of metal. The sleeve  30  is arranged close to the capacitor plates  14 ,  16 ,  18  and is here referred to as a focussing plate since its action is to focus or concentrate the electric field within the fluid  26  (see the detailed description below of  FIGS. 2 and 3 ). The focussing plate  30  also provides a screening function by preventing extraneous electrical signals and the dielectric constant of material outside the plate from affecting the charge upon the capacitor plates  14 ,  16 ,  18 . 
         [0068]    In order to enable the capacitance between the focussing plate  30  and the capacitor plate  16  to be ignored, the voltage of the focussing plate  30  is matched to that of the capacitor plate  16 . This is achieved by connecting the focussing plate  30  to the signal generator  20  by way of a screening connector  32  and an operational amplifier  34  configured as a voltage follower. 
         [0069]    It will be understood that an operational amplifier  34  in voltage follower mode provides the same voltage at its output as that at its input and since in this embodiment its input is connected to the signal generator  20  the voltage at the output, and therefore the voltage upon the screening connector  32  and focussing plate  30 , matches that of the signal generator  20 . At all times therefore the voltage upon the focussing plate  30  matches that of the capacitor plate  16 . 
         [0070]    Also, at all times the voltage of the screening connector  32  matches that of the signal wire  22 , enabling the signal wire  22  and screening connector  32  to be respective parts of a coaxial or screened cable  36  for at least part of their length, with the screening connector  32  forming the shield surrounding the signal wire of the coaxial cable  36  in known fashion. Once again, because of their matched voltages, the capacitance between the signal wire  22  and the screening connector  32  can be ignored. 
         [0071]    The frequency applied by the signal generator  20  can be set as required, and can be varied during use of the measurement tool  10  if desired. The optimum frequency will depend upon the application, and may depend for example upon the range of dielectric constants expected for the fluid  26 , and the other variable parameters such as temperature of the fluid  26 . A suitable frequency for use in a one practical formation testing tool has been found to be 16 kHz. 
         [0072]    This measurement tool  10  is not only able to measure dielectric constant, but also resistivity. Importantly, the measurement tool  10  uses only one pipe  12  for the two measurements, so that the measurements can be carried out substantially simultaneously on the same volume of fluid  26 . 
         [0073]    To carry out a resistivity measurement upon the fluid, the measurement tool  10  has a first toroid  40  surrounding a part of the pipe  12  and a second toroid  42  surrounding another part of the pipe  12 , the toroids  40 , 42  being separated along the length of the pipe. The toroids are of conventional form, comprising a loop of iron (or other ferromagnetic material) surrounded by an electrical coil (not shown). 
         [0074]    A second signal generator  44  is connected to the electrical coil  38  of the first toroid  40  and delivers an alternating electric current to the electrical coil  38  (only a part of the coil  38  is shown in  FIG. 1 ). Passing an electric current through the coil induces a magnetic field in the first toroid which in turn induces an electric current to flow in any conductor located within the first toroid. The pipe  12  is located within the first toroid  40  and since the pipe  12  is an insulator a current will be induced to flow within the fluid  26 . The tool  10  includes a conductive return path comprising a pair of electrodes  46  and  48  connected by a wire  50 . The electrodes  46  and  48  are located within the pipe  12  so that they directly contact the fluid  26 . Preferably, the electrodes  46 ,  48  are embedded into the wall of the pipe  12  so that they do not induce turbulence or any pressure changes in the fluid  26  as it flows therepast. 
         [0075]    It will be understood that the position of the toroids in relation to the remainder of the tool does not matter and they can even be placed to either side of the focussing plate  30 . The metal housing of the cell or tool can in some embodiments provide part or all of the return path and this would avoid the requirement for a separate wire  50  and reduce the complexity of the tool. 
         [0076]    Desirably the capacitor plates  14 , 16 , 18  and focussing plate  30  are made from non-strongly magnetic materials so as not to increase the toroids&#39; leakage flux. 
         [0077]    Any current flowing through the fluid  26  between the electrodes  48  and  46  will induce a current to flow around the coil  54  of the second toroid  42  (only a part the coil  54  is shown in  FIG. 1 ). That current is detected by a second detector  52 , the current flowing through the coil around the second toroid being directly related to the current flowing within the fluid, and therefore directly related to the resistivity of the fluid  26 . The detector may incorporate a phase sensitive detector to enhance signal to noise ratio. 
         [0078]    The measurement tool  10  can be calibrated (again by calculation or by experimentation with fluids of known resistivities) so that it can be used quantitatively, or it may be used qualitatively to determine changes in the resistivity of the fluid  26 . 
         [0079]      FIGS. 2 and 3  show representations of the tool to demonstrate the advantage of a metallic focussing plate such as  30 . Specifically, the action of the focussing plate and the role of the pipe dielectric constant may be understood by reference to the simplified model of the capacitance distribution in the cell as shown in  FIG. 2 . It will be understood that these capacitances in reality represent the distribution of electric potential within the cell as may be calculated from electromagnetic theory by one practised in the art. It will also be understood that the present invention is not dependent upon the capacitance model. 
         [0080]    A pipe  100  surrounds fluid  101 . Annular capacitance plate  102  is connected to the signal generator M at  108  and annular capacitance plate  103  is connected to ground (the capacitance plates  102 ,  103  therefore replicating the capacitance plates  16 ,  14 , or  16 ,  18 , of  FIG. 1 ). The fluid capacitance to be measured is that represented by the notional capacitor  104 . This fluid capacitance is in parallel with the axial capacitance  106  of the pipe wall. There is also radial series capacitance  105  due to the pipe wall, and stray capacitance  107  exterior to the pipe between the electrodes. All capacitances other than  104  will affect the sensitivity and interpretation of the measurement. 
         [0081]    In order to maximise the sensitivity to the capacitance  104 , it is desirable to maximise series capacitance  105 , which can be achieved by maximising the pipe material&#39;s dielectric constant. On the other hand this will increase parallel capacitance  106  which is undesirable. 
         [0082]      FIG. 3  demonstrates the effect of adding an annular focussing plate  110 , the focussing plate  110  being held at the same potential as capacitor plate  102 , preferably by a voltage follower as described in relation to the focussing plate  30  of  FIG. 1 . With the focusing plate  110  present, the axial capacitance  106  is replaced by a radial capacitance  106 ′ and the stray capacitance  107  is eliminated. The measurement is now just of the desired capacitance  104  in series with the radial capacitance  105 . With a high dielectric constant pipe material, the measurement will be very sensitive to changes in fluid dielectric constant. 
         [0083]    It will furthermore be understood that the focussing action described requires only that the focussing plate  110  cover the dielectric material in the axial space between the capacitor plates  102  and  103 . Similarly for the arrangement of  FIG. 1 , so that if the screening function of the plate  30  is not required the focussing plate could by shortened to fill the axial space between plates  14  and  16  and the axial space between plates  16  and  18 , which would maximise the sensitivity to the fluid capacitance as desired. 
         [0084]      FIG. 4  demonstrates the invention according to its second aspect, it being appreciated that the dielectric (and resistivity) measurement can be made exterior to the wall  112  of the measurement tool  110  when the capacitor plates (and preferably also focussing plate(s)) are within the wall  112  and the fluid  126  to be tested is outside the wall. Such a configuration has wide applicability to measurement of fluid properties in boreholes such as during production in production logging tools and in permanent deployment as in an intelligent well. 
         [0085]    Capacitor plate  116  of the embodiment of  FIG. 4  performs the same function as capacitor plate  16  in the embodiment of  FIG. 1 , and similarly for the capacitor plates  114  and  14 , and also for the other components  130 ,  140 ,  142 ,  146  and  148  which perform the same functions as the components  30 ,  40 ,  42 ,  46  and  48  respectively. Because of the similarity of many of the components of the embodiment of  FIG. 4  to the components of the embodiment of  FIG. 1 , it is believed that a skilled person does not require a detailed description of  FIG. 4 . 
         [0086]    Whilst  FIG. 4  shows only two capacitor plates  114  and  116 , it will be understood that another embodiment could utilise three capacitors along the sensing element in a similar arrangement to that of  FIG. 1 , with the signal generator (M in  FIG. 4 ) being connected to the central capacitor and the other two capacitors being connected to ground. Whilst  FIG. 4  does not show the details of the signal generator M and related componentry, it will be understood that components identical or similar to the signal generator  20 , signal wire  22 , detector  24 , screening connector  32 , operational amplifier  34  and perhaps also the coaxial cable  36  of the embodiment of  FIG. 1  could be provided within the measurement tool  110 . 
         [0087]      FIG. 4  also shows the notional capacitance  104  of the fluid  126  which is desired to be measured, and also the notional series capacitance  105 , using the same reference numerals as  FIGS. 2 and 3 . 
         [0088]    In the embodiment of  FIG. 4  the wall  112  is tubular and the measurement tool  110  can be surrounded by the fluid  126 . In alternative embodiments the measurement tool is adapted to lie alongside the body of fluid, for example being mounted into the wall of a conduit for the fluid. 
         [0089]      FIGS. 5 and 6  represent a measurement tool in which the capacitance plates  202 ,  203  are arranged across the pipe  212 .  FIG. 5  corresponds to  FIG. 2  and again is a generalised approximation to the actual distributed field structure in the cell. The capacitance between the capacitor plates  202  and  203  is a series measurement comprising the capacitance through the wall material  205  and the capacitance of the fluid  204 . This capacitance is shunted by the capacitance around the wall material  106  and external stray capacitance  107 . 
         [0090]      FIG. 6  corresponds to  FIG. 3 , and shows the effect of a focussing plate  230 . The focussing plate  230  is driven by the signal generator M to be at the same potential as the capacitor plate  202 . This eliminates the effect of external capacitance and by altering the internal field largely reduces the shunt capacitance  206  to a capacitance  206 ′ between the focussing plate  230  and the grounded capacitor plate  203 . This results in a measurement closer to the simpler series structure of capacitances  205  and  204 . 
         [0091]      FIGS. 5 and 6  represent cross-sectional views through the measurement tool. The longitudinal length of the driven capacitor plate  202  (i.e. the length in the direction along the longitudinal axis of the tool) is chosen to suit the application. The longitudinal dimension of the focussing plate  230  is preferably greater than the longitudinal dimension of the driven capacitor plate  202 . 
         [0092]      FIG. 6  shows a plate  230  which provides both the focussing and screening functions as in the embodiments of  FIGS. 1 ,  3  and  4 . The region  230   a  of the plate  230  which lies between the capacitor plates  202  and  203  provides the focussing action, whilst the region  230   b  of the plate  230  which overlies the driven capacitor plate  202  provides the screening function. In addition, the plate  230  extends beyond the edges of the driven capacitor plate  202  in the longitudinal direction. 
         [0093]    It has been found to be beneficial that the grounded capacitor plate  203  extends longitudinally beyond the focussing plate  230  and the driven capacitor plate  202 , and in particular that the grounded capacitor plate  203  completely surrounds the measurement tool beyond the ends of the focussing plate  230 . Thus, in the orientation of  FIG. 6  the grounded capacitor plate  203  beneficially extends into and out of the paper beyond the driven capacitor plate  202  and the focussing plate  230 , and surrounds the pipe  212  both above and below the focussing plate  230 . 
         [0094]    It will also be noted that the focussing plate  230  is stepped around the driven capacitor plate  202  so that its region  230   a  lies closer to the pipe  212  between the capacitor plates  202  and  203 . This has been found to improve the focussing action. 
         [0095]    In a practical embodiment the capacitor plates  202 ,  203  and the focussing plate  230  are provided by a strip of flexible double-sided printed circuit board material, with the driven capacitor plate  202 , the region  230   a  of the focussing plate  230  and the grounded capacitor plate  203  being formed on one side thereof (with the region  230   a  of the focussing plate surrounding the driven capacitor plate  202 , and the grounded capacitor plate  203  surrounding the region  230   a  of the focussing plate). The screening region  230   b  of the focussing plate  230  is formed on the other side of the printed circuit board, and is connected to the region  230   a  of the focussing plate  230  by vias through the board. The flexible printed circuit board can be wrapped around the measurement tool with the driven capacitor plate  202 , the region  230   a  of the focussing plate  230  and the grounded capacitor plate  203  preferably in contact with the pipe. The region  230   b  of the focussing plate encloses the driven capacitor plate  202  and is separated from the driven capacitor plate  202  by the substrate material of the printed circuit board. 
         [0096]    Extending the grounded capacitor plate  203  beyond the longitudinal ends of the focussing plate  230 , and passing the grounded capacitor plate  203  circumferentially around the pipe  212  provides a defined environment for the driven capacitor plate  202 , within which the capacitance of the fluid is measured. This also helps to reduce or avoid any field distortion which might occur if the fluid being measured is conductive and is in engagement with electrically conductive parts of the housing of the measurement tool. 
         [0097]    It will be understood that the measurement tool represented by  FIGS. 5 and 6  uses capacitor plates  202 ,  203  mounted at radially spaced positions around the pipe  212  into which the fluid is introduced, so that this embodiment is according to the first aspect of the invention. In another embodiment a structure similar to that of  FIG. 4  could be provided, but with the capacitor plates mounted across the measurement tool for use in accordance with the second aspect. Measurement tools in which the capacitor plates are mounted across the tool have been found to be more suitable in certain applications since they have a larger response variation to different fluids in the range of interest, i.e. fluids with dielectric constants in the range of around two to around ten which are typical of downhole fluids in oil drilling applications. 
         [0098]    Also, whilst  FIGS. 5 and 6  do not show the toroids or other components for measuring the electrical resistivity of the fluid, it will be understood that such componentry could be provided. Alternatively considered, the radial arrangement of the two capacitors  202 ,  203  of  FIGS. 5 and 6  could replace the longitudinal arrangement of the three capacitors  14 ,  16  and  18  of  FIG. 1 , without altering the componentry of the electrical resistivity measurement. Furthermore, the signal generator  20 , detector  24  and related componentry of the embodiment of  FIG. 1  could be used with embodiments in which the capacitor plates are arranged across the measurement tool. 
         [0099]    It will also be understood that the invention will most often be utilised with fluid flowing along the pipe (or around the tool as applicable), but that the dielectric constant and electrical resistivity measurements could also be taken upon a stationary fluid if desired.